EARTHQUAKE RESISTANT -1



EARTHQUAKE RESISTANT BUILDING WORKS

26 General

This section deals with construction procedures required to take care of effects of forces like earthquake, blast or other effects, which would ensure safety of the structure. As a first step, earthquake resistant structures are covered in Parts 1 and 2.

Part 1: Earthquake resistance of normal buildings

26.1 General

26.1.1 This Part deals with seismic resistant construction of normal buildings, using masonry, timber and precast building components.

In addition, requirements of buildings against earthquake forces using weaker materials, such as, low strength masonry are covered in Part 2. Some guidelines for improving earthquake resistance of earthen buildings and repair and seismic strengthening of buildings are also included in Part 2.

The basic requirements of seismic design shall conform to IS: 1893-1984 - Criteria for earthquake resistant design of structures.

26.1.2. Earthquake resistant construction of normal buildings:

Types of construction - The buildings covered include masonry construction using rectangular masonry units, timber construction and buildings with prefabricated flooring / roofing units.

26.1.3 General principles of seismic resistant construction:

26.1.3.1. Lightness - Since earthquake force is a function of the weight of the building, it shall be as light as possible.  Roofs and upper storeys of buildings should be as light as possible.

26.1.3.2 Continuity of construction:

  1. As far as possible, the parts of the building should be tied together in such a manner that the building acts as one unit.
  2. Additions and alterations to the structures shall be accompanied by provision of separation or crumple sections between new and old structures as far as possible, unless positive measures are taken to establish continuity between them.
  3. For parts of building, between separation or crumple sections or expansion joints, floor slabs shall be continuous throughout as far as possible. Concrete slabs shall be rigidly connected or integrally cast with support beams.

26.1.3.3 Projecting or suspended parts

  1. Projections shall be avoided as far as possible. If unavoidable, the parts projecting shall be properly reinforced and firmly tied to the main structure and their design shall be according to IS: 1893-1984 Ceiling plaster shall preferably be avoided; if unavoidable it shall be as thin as possible.
  2. Suspended ceiling shall be preferably avoided; if unavoidable it shall be as light as possible.

26.1.3.4 Building Configuration - In order to minimize torsion and stress consideration in buildings the following shall be adhered to:

  1. The building should be a simple rectangle in plan and symmetrical both with respect to mass and rigidly so that the centres of mass and rigidity coincide in which case no separation sections other than expansion joints are necessary.
  2. If symmetry is not possible in plan, elevation or mass provision shall be made for Torsional and other effects due to earthquake forces in structural design or through crumple sections. The length of such building between separation sections shall not preferably exceed three times the width.
Note - As an alternative to separations section to reduce the Torsional moment, the centre of rigidity of the building may be brought close or coincident with centre of mass by adjusting the locations and /or sizes of columns and walls.
  1. Buildings have plans with shapes like L, T, E and Y shall preferably be separated into rectangular parts by providing separation sections at appropriate places. Typical examples are shown in Fig. 1.
- 1) For buildings with small lengths of projections forming, L, T, E or Y need not be provided with separate section. In such cases, the length of projection may not exceed 15 to 20 percent of the total dimension of the building in the direction of projection..

26.1.3.5 Strength in various directions - The building shall be designed to have adequate strength against earthquake effects along both the horizontal axes. The design shall also be safe considering the reversible nature of earthquake forces.

26.1.3.6 Foundation - The structure shall not be founded on such loose soils which will subside or liquefy during an earthquake, resulting in large differential settlements.

26.1.3.7 Ductility - The main structural elements and their connections shall be designed to have a ductile failure. This will enable the structure to absorb energy during an earthquake to avoid sudden collapse of the structure.  Details of ductile joints for reinforced concrete structures are given in IS: 13920-1993

26.1.3.8 Non – structural parts - Suitable details should be worked out to connect the non-structural parts with the structural frame so that the deformation of the structural frame leads to minimum damage of non-structural elements.

26.1.4 Masonry structures – Special construction features

26.1.4.1 General - Normal construction work in masonry shall be as per prescribed specifications. The special features for earthquake resistant construction are as detailed below.

26.1.4.2. Mortars - The recommended mortar mixes are as in Table 1.

Table 1 Recommended mortar mixes (Clause 26.1.4.2)

Building Category

Proportions of Cement – Lime – Sand

A

M2 (cement-sand 1:6) or M3 (lime-sand 1:3) or richer

B and C

M2 (cement-lime-sand 1:2:9) or (cement-sand 1:6) or richer

D and E

H2 (cement-sand 1:4) or M1 (cement-lime-sand 1:1:6) or richer

Notes: 1) Mortar grades are given in section 0

  1. Building category based on µh values is given in IS: 4326-1993

Fig. 2 Plan and vertical irregularities

26.1.4.3 Cover - Where steel reinforcing bars are provided in masonry, the bars shall be embedded with adequate cover; in cement sand mortar not leaner than 1:3 a minimum cover of 10 mm shall be

provided or in cement concrete grade M15 a minimum cover of 15 mm or the bar diameter whichever is more shall be provided.

26.1.4.4 Seismic strengthening arrangements

  1. All masonry buildings shall be strengthened by the methods of as specified in Table 2.  Figure 3 and 4 show the overall strengthening arrangements to be adopted for category D and E buildings, which consist of horizontal bands of reinforcement at critical levels, vertical reinforcing bars at corners, junctions of walls and jambs of openings. For location of openings, which have a reducing effect on lateral load resistance (see Annex B).
  2. Lintel band – Lintel band is a band provided at lintel level on all load bearing internal, external longitudinal and cross walls. Lintel band if provided in panel or partition wall also, will improve their stability during severe earthquake.
  3. Roof band – Roof band is a band provided immediately below the roof or floors. Such a band need not be provided underneath slabs rests on bearing walls, provided that the slabs are continuous over the intermediate wall up to crumple sections, if any and cover the width of end walls, fully or at least ¾ of the wall thickness. Details of the specification of the band are given.
  4. Gable band – Gable band is a band provided at the top of gable masonry below the purlins. This band shall be made continuous with the roof band at eaves level. Details of the specification of the band are given.
  5. Plinth band – Plinth band is a band provided at plinth level of walls on top of the foundation wall. This is to be provided where strip footings of masonry (other than reinforced masonry or reinforced concrete) are used and the soil is either soft or uneven in its properties).
Table 2 Strengthening arrangements for masonry buildings (Clause 26.1.4.4)

Building Category

Number of storeys

Strengthening to be provided in all storeys

1

2

3

A

i) 1 to 3

ii) 4

Mortar (see 26.1.4.2)

Mortar (see 26.1.4.2), Lintel and roof band and where necessary gable band (see 26.1.4.4)

B

i) 1 to 3

 

ii) 4

Same as A (ii) plus bracing in plan at the level of roofs (see 26.1.4.4), where necessary plinth band (see 26.1.4.4).

Same as B (I) plus vertical steel at corners (see 26.1.4.4)

C

 

I) 1 and 2

ii) 3 and 4

Same as B (I)

Same as B(ii) plus vertical steel at jambs of openings (see 26.1.4.4)

D

i) 1 and 2

ii) 3 and 4

Same as C (ii)

Same as C (ii) plus dowel bars (see 26.1.4.4)

E

1 to 3   

Same as D (ii)

Note – Fourth storey not allowed in Category E buildings.

  1. Dowel bars – In category D and E buildings, to further integrate the box action of walls steel dowel bars may be used at corners and T junctions of walls, at sill level of windows, to a length of 900 mm from the inside corner in each wall. Such dowel may be in the form of U stirrups 9 mm dia the dowels shall be laid in 1:3 cement–sand mortars with a minimum cover of 10 mm.
  2. Vertical reinforcement – Vertical steel at corners and junctions of walls which are up to 340 mm (1 ½ brick) thick, shall be provided as in Table 3. For walls thicker than 340 mm, the area of the bars shall be proportionately being increased.
  3. Bracing in plan – At tie level all trusses and the gable end shall be provided with diagonal braces in plan so as to transmit the lateral shear due to earthquake force to the gable walls acting as shear walls at the ends.

26.1.4.4.1 Section and reinforcement of band

  1. The band shall be made of M 15 grade concrete or reinforced brick work in cement mortar 1: 3.  The band shall be full width of the wall and not less than 75 mm in depth and reinforced as given in Table 4.  In coastal areas the grade of concrete shall be M 20.
  2. In case of reinforced brickwork, the thickness of joints for steel bars shall be increased to provide a cover of 10 mm.  The area of steel shall be the same as for reinforced concrete work.
  3. For full integrity of walls at corners and junctions of walls and effective horizontal building resistance of bands, continuity of reinforcement is essential (see Fig.  6).

. 3 Overall arrangement of reinforcing masonry buildings

1. Lintel Band

4. Door

2. Roof/floor band

5. Window

3. Vertical band

 

Fig. 4 Overall arrangement of reinforcing masonry building having pitched roof

1

Lintel band

8

Holding down bolt

2

Eave level (Roof) band

9

Brick / stone wall

3

Gable band

10

Door lintel integrated with roof band

4

Door

a)

Perspective view

5

Window

b)

Details of truss connection with wall

6

Vertical steel bar

c)

Detail of integrating door lintel with roof band

7

Rafter

   

Table 3 Vertical reinforcement in masonry walls (Clauses 26.1.4.4 and 26.1.4.6)

Building Category

Storeys

Diameter of Bar (Deformed)

No.

Location

1

2

3

4

B

1 to 3

--

--

 

4

Top

10

 

 

Third

10

 

 

Second

10

 

 

Bottom

10

C

1, 2

--

--

 

3

Top

10

 

 

Middle

10

 

 

Bottom

12

 

4

Top

10

 

 

Third

10

 

 

Second

10

 

 

Bottom

10

D

1

--

--

 

2

Top

10

 

 

Bottom

12

 

3

Top

10

 

 

Middle

12

 

 

Bottom

12

 

4

Top

10

 

 

Third

12

 

 

Second

16

 

 

Bottom

20

E

1

--

12

 

2

Top

12

 

 

Bottom

16

 

3

Top

12

 

 

Middle

16

 

 

Bottom

16

Notes-1. Four story’s not permitted in Category E buildings.

2.   For precast components (see 5.)

  1. No strengthening arrangement for Category A buildings up to three storeys.

4.  Vertical reinforcement shall pass through the plinth masonry of foundations, roof slab, roof band, lintel bands in all storeys (see Fig. 5).

5.  The vertical bars will be covered with concrete M 20 or with cement mortar (1:3) in suitably created pockets, around the bars (see Fig. 5).

Fig. 5 Typical details of providing vertical steel bars in brick masonry

1 – One brick length;  ½ - Half brick length; V – Vertical steel bar with mortar/concrete filling in pocket (a) and (b) – Alternate courses in one brick wall; (c) and (d) – Alternate courses at corner junction of 1 ½ brick wall; (e) and (f) – Alternate courses at T-junction of 1 ½ brick wall.

Fig. 6 Reinforcement and bending detail in RC band

1

Longitudinal bars

2

Lateral ties

b1, b2

Wall thickness

 

 

a)

Section of band with two bars

b)

Section of band with four bars

c)

Structural plan at corner junction

d)

Sectional plan at T-junction of walls

Table 4 Recommended longitudinal steel in reinforced bands (deformed bars)

(Clauses 26.1.4.4, 26.1.4.4.1, 4.6 and 26.4.5.1)

Building Category

Span, m

Number of Bars

Diameter of Bar (Deformed)

1

2

3

4

B

5,6,7

2

8

 

8

2

10

C

5, 6

2

8

 

7

2

10

 

8

2

12

D

5

2

8

 

6

2

10

 

7

2

12

E

5

2

10

 

6

2

12

 

7

4

10

 

8

4

12

Notes-1. No strengthening arrangement for category A buildings up to 3 storeys.

2.  For spans greater than 8 m, plasters or buttresses may be introduced to reduce the span, or special calculations have to be made.

3.  For plain bars, the diameters should be 10, 12, 16, 20 and 25 respectively for deformed bars of 8,10,12,16, 20 given above.

4.  For RC bad a clear cover of 20 mm shall be provided for steel.

5.  For RC band the vertical depth shall be 75 mm for 2 bars and 150 mm for 4 bars.

6.  Stirrups shall be 6 mm dia spaced at 150 mm.       

1. Window, 2. Door,  3.  Brick pane, 4. Lintel band

Fig. 7 Framing of thin load–bearing brick walls

26.1.4.5 Framing of thin load bearing walls (see Fig.  7)

  1. For thin load bearing walls, 150 mm thick including plaster reinforced concrete columns shall be provided at all corners and junctions of walls, spaced at not more than 1.5 m apart.
  2. Horizontal bands should be located at all floor, roof and levels of the openings.
  3. The sequence of construction between walls and the columns shall be first to build the wall up to 4 to 6 courses height leaving toothed gaps (tooth projection being about 40 mm only) for the columns and then to pour M15 grade concrete to fill the columns against the walls using forms only on two sides. The band concrete should be cast on the wall masonry directly so as to develop full bond with it.
  4. Such construction should be limited only to two storeys.  The horizontal length of wall should not exceed 7 m and the storey height to 3 m.

26.1.4.6 Hollow Block Masonry - The horizontal and vertical steel for earthquake resistance shall be placed as described below. 

Horizontal Band – U –shaped blocks may be used for construction of horizontal bands at various levels of the storeys as shown in Fig.8.  The reinforcement shall be 2.5 per cent more than that recommended in Table 4 continuity of reinforcement shall be ensured.

Fig. 8 U-blocks for horizontal bands

  1. Vertical Reinforcement – Bars, as given in Table 3 shall be located inside the cavities of hollow blocks, one bar in each cavity (see Fig 9).  When more than one bar have to be located, this can be done in two or more adjacent cavities; these cavities should be filled with cement-coarse sand mortar 1:3 and compacted with rod.
  2. Splicing of reinforcement by welding or by overlap is permitted. To reduce the number of overlaps the block ends may be U-shaped as shown in Fig. 9 which will help in tieing the bars together by binding wire.

Fig. 9 Vertical reinforcement in cavities

26.1.5 Special construction features – Floors and roofs with small precast components - Special construction features for floor and roofs with small precast components, are given below:

a) Tie beam – Tie beam is a beam provided all round the floor or roof to bind together all the precast components to make it into a diaphragm. The beam shall be full width of the wall allowing for bearing of precast units; the depth shall be the depth of precast component.

  1. M 15 grade should be used for the beam and the reinforcement shall be as in Table 4.
  2. If depth of the beam is more than 75 mm, the reinforcement shall be provided at each corner with 8 mm bar.
  3. A typical detail is shown in Fig.  10.

b) Top reinforcement – The reinforcement of 6 mm dia bars of 150 mm centres on top of the channel core unit shall project out at both ends and tied to the tie beam reinforcement.

Fig. 10 Connection of channel/cored unit floor/roof (With deck concrete) with tie beam

  1. Deck concrete – Deck concrete over the precast units shall be of M 20 grade to act monolithically with the unit; it shall be at least 35 mm thick.
  2. In general for precast components, the principle is to make them act as a diaphragm to withstand seismic forces.

26.1.6 Timber structures – special arrangements

26.1.6.1 Foundation - Timber structures shall preferably start on masonry or concrete foundations except small buildings (50 m2 or less) may rest on ground.  The structure may be fixed to the foundation as shown in Fig. 11.

For small buildings of area less than 50 m2 resting on ground, they may be fixed to vertical poles embedded in the ground.  The superstructure has to be strengthened for earthquake resistance.

26.1.6.2 Stud wall or brick nogging construction:

It consists of timber studs and corner posts framed into sills, top plates and wall plates. Horizontal struts and braces are used to stiffen the frame against horizontal forces. Typical details are as in Fig.12.

Fig. 11 Details of connection of column with foundation

  1. There shall be at least one diagonal brace of minimum size 20 mm x 40 mm for every 1.5 m2 area of the wall. Diagonal braces shall be connected to the stud wall members with at least 4 nails.
  2. Horizontal bracing, 20 mm x 90 mm shall be provided at not more than 1 m apart. It shall be provided at T – junctions and corners of wall at sills, first floor and eaves level. They shall be connected to the wall plates by at least 6 nails.

26.2. PART II - Earthquake resistance of weaker buildings

26.2.1 Low strength masonry buildings 

26.2.1.1 General - The use and strengthening of low strength masonry buildings shall be restricted to Zones III and IV of IS: 1893-1984.  No special provisions are necessary for such buildings in Zones I and II. Low strength masonry includes brickwork laid in mud mortar, random rubble, uncoursed undressed or semi-dressed stone masonry with weak mortars. Other requirements of 26.1.4 and 26.1.6 of Part 1 shall apply.

Fig. 12 Timber framing in stud wall construction with opening in wall

This type of construction should not be permitted for important buildings with I > 1.5 and should preferably be avoided for building categories D and E.

a) To protect the weak mortar a damp-proof course may be laid as prevent rainwater from soaking the wall and softening the mortar. A waterproof plaster may be used.

b) Resistance to overturning under the action of horizontal force shall be designed for.

1. Door    2. Window     3. Ventilator     4. Cross wall

Fig. 13 Dimensions of openings and piers for recommendations given in Table 5

26.2.2 Brickwork in weak mortar

a) For this type of construction, the height of the building shall be restricted to the following :

Category – Three storeys with flat roof; two storeys plus attic for pitched roof.

A, B and C

Category D – Two storeys with flat roof; one storey plus attic for pitched roof.

b) Bond - Usual brick bonds should be followed. For perpendicular walls, a sloping (stepped) joint is necessary by making the corners to a height of 600 mm and then building the wall in between them.  Otherwise toothed joint should be provided in both the walls alternatively, it should be built in lifts of about 450 mm (see Fig 15).

c) The minimum wall thickness should be one brick for one storey construction; and one brick for top storey and 1 ½ brick of storeys below for 3 storey construction.

d) The mortar should be lime mortar 1:3 or mud mortar. When steel is provided horizontally it should be embedded in cement mortar 1:3 with a suitable cover of 6 mm.

26.2.3 Stone masonry (random rubble or semi-dressed):

a) Height of stone masonry wall should be restricted to 2 storeys for category A and B if built with lime mortar or mud mortar; another storey may be permitted if built with cement mortar.  For category C and D two storeys may be permitted with cement mortar; only one storey with lime or mud mortar. Attic may be permitted for two storey buildings.

W = Window               h  = Thickness of concrete

T =  Wall thickness     V = Vertical bar

t2 = Lintel thickness

Fig. 14 strengthening masonry around opening

b) Masonry should preferably be brought to neither courses at nor more than 600 mm lift. The wall thickness should be not larger than 450 mm.

c) Stones of the inner and outer Wythes should be interlocked. ‘Through’ stones of full length equal to the thickness of wall should be used in every lift of 600 mm and not more than 1.2 m apart, horizontally.  If full length stones are not available, stones in pairs each of about ¾ of the wall thickness may be used (see Fig. 16).

d) In place of through stones, ‘bonding elements’ of steel bars 8 to 10 mm dia bent to S shape or as hooked links may be used with a cover of 25 mm from each face of the wall (see Fig. 17).

A, b, c = Toothed joints in wall A, B and C

Fig. 15 Alternating toothed joints in walls at corner and T-junction

e) Also in place of through stones wooden cut size pieces 40 mm x 40 mm cross section or concrete piece of section of 50 mm x 50 mm with 8 mm bars may be inserted.

f) Bonding elements should also be used at corners and junctions of walls.

g) Mortar should be either lime mortar 1:3, mud mortar or cement mortar 1:6.

h) Buttresses may be provided for walls longer than 5 m.

26.2.4 Improving earthquake resistance of earthen buildings

26.2.4.1 General

Earthen walls may be constructed in the following four ways:

a) Hand formed layers using mud lumps to form walls are the weakest of all earthen walls. Use of straw will impart strength and reduce fissures.

b) Block or adobe constructions, cut from hardened soil or formed in moulds and compacted, are laid in courses using mud mortar from the same soil. Addition of straw in mud mortar in equal volume would make it non-shrinking; the mortar mix should be allowed to remain for 7 days before use. Normal breaking of joints and related masonry practices should be followed.

c) Rammed earth in which moist soil is filled between forms and compacted manually or mechanically.  The soil for rammed earth construction will generally have less clay than that used for blocks or adobes.  Small amounts of straw, not more than one-fourths the volume of soil water mix, shall be added for fissure control.

1. Through stone 2. Pair of overlapping stone 3. S-shape tie 4. Hooked tie 

5. Wood plank 6 – Floor level

Fig. 16 Through stone and bond elements

d) Wood, bamboo or cane structures plastered with mud (Ikra walling in N. E. Region)

The improvements are applicable to buildings in Zones III, IV and V of IS1893: 1984, and without the use of stabilizers for earthen elements.

26.2.4.2 General arrangements for seismic resistance are as below:

  1. The height of adobe building should be restricted to one storey plus attic only in Zones IV and V and to two storeys in Zone III. Important building, with l > 1.5, shall not be constructed with earthen walls in Zones IV and V; in Zone III they may be constructed but restricted only to single storey.
  2. Sites with sand, loose soils, poorly compacted clays and fill material should generally be avoided; also sites with high water table should be avoided.
  3. Foundation depth shall vary between one or two times the thickness of wall depending on number of storeys; depth shall be at least 0.4 m.
  4. Wall dimensions, openings shall be as in Fig. 17 and 18.

26.2.4.3 Strengthening Arrangements

In load bearing walls, two horizontal continuous reinforcing and bonding beams or bands should be placed one coinciding with lintels over doors and windows, the other just below the roof level.  If the wall height is less than 2.5 m, the band at the lintel level may be avoided.

1 – Cross wall   2. Pilaster 3 – Buttress 4 – Wall thickness

Fig. 17 Wall dimensions

D – Door   W – Window   V - Ventilators

Fig. 18 Wall dimensions, pilasters at corners

  1. The bands may be of timber as shown in Fig 19.
  2. The horizontal band should cover the buttresses and plasters.
  3. Vertical reinforcement in mesh form may be used in Zone V (see Fig.  2)

1 – Light roof; 2 – Light gable wall (matting or boarding); 3 – Rain protection overhang (about 500 mm); 4 – Stable plaster; 5 – Plinth height for flood protection; 6 – Stable foundation;      7 – Good mortar; 8 – Floor level; 9 – Ground level; 10 – Waterproof layer.

  1. For construction with wood/cane, strengthening arrangements are bracings as shown in Fig.  20.
  2. For higher seismic intensities as in Zones VIII and IX, internal bracing as shown in Fig. 21 may be done.

26.2.5 Repair and seismic strengthening of buildings -

Reference may be made to IS 13935: 1993, IS: 4326-1993, IS: 13827-1993 & IS: 13828-1993 which are enclosed as Annexure 26-A.2, Annexure 26-A.3,

Annexure 26-A.4 and Annexure 26-A.5 respectively.

1 – Adobe;   2 – Mud mortar;   3 – Wooden band, 4 – Diagonal brace.

Fig. 19 Wooden band in walls at lintel and roof levels

1. Clay mud covering over framing.    2. Mud plaster on matting.

3. Cane/bamboo/wood framing   4. Cane/bamboo/knitting

1. Diagonal brace        2. Mud plaster on matting

3. Cane/bamboo/wood framing  4. Cane/bamboo/ikra knitting

Fig. 20 Bracing of earthen construction with canes/bamboo or wooden structure Minimum dimensions

1

Column

100x75 or 100j

2

Sill

100 x 75

3

Beam

100x100 or 75 j

4

Diagonal

100 x 50

5

Strut

100x50

6

Ceiling Beam

75x125 or 100 j

 

Corner

100 x 100

7

Holdfast

 

Fig. 21 Braced wood frame for adobe and other walls in mud mortar

Annex A

Building categories based on earthquake resisting features

A.1 For the purpose of specifying the earthquake resisting features in masonry and wooden buildings, the buildings may be categorised on the basis of value of µh = µ0.  I . b.

Where

µh = design seismic coefficient for the building.

µ0 = basic seismic coefficient for the zone in which it is located as per IS: 1893 : 1984.

I = importance factor applicable to the building as per IS 1893: 1984, and

b = soil foundation factor as per IS 1893: 1984.

A-2 The building categories, A to E are listed below:

Building Categories

Range of µh

A

0.04 to 0.05

B

0.05 to 0.06

C

0.06 to 0.08

D

0.08 to 0.12

E

0.12 and above

Annex B

Openings in load bearing walls (Clause26.1.4.4)

B-1 Door and window openings in walls reduce their lateral load resistance, should preferably by small and more centrally located. The guidelines on the size and position of openings are given in Fig. 13 and Table 5.

Table 5 Size and position of openings in load bearing walls(Clauses B-1 and B-3)

Sl. No.

Position of Opening     

Details of Opening for Building Category

 

 

A and B

C

D and E

1

2

3

4

5

I

Distance of b5 from the inside corner of outside wall, min

Zero

230 mm

450 mm

Ii

For total length of openings ; the ratio (b1 + b2 + b3)/l1, or (b6 + b7)/ l2 shall   not exceed

a) for one – storeyed building

b) for two – storeyed building

  c) 3 or 4 storeyed building

 

 

 

0.60

0.50

0.42

 

 

 

0.55

0.46

0.37

 

 

 

0.50

0.42

0.33

Iii

Pier width between consecutive openings b4, Min

340 mm

450 mm

560 mm

Iv

Vertical distance between two openings one above the other h3, Min

600 mm

600 mm

600mm

B-2 Openings in any storey shall preferably have their top at the same level so that a continuous band could be provided over them, including lintels throughout the building.

B-3 Where openings do not comply with the guide lines of Table 5, they should be strengthened by providing reinforced concrete or reinforced brickwork as shown in Fig 14 with high strength deformed (H.S.D) bars of 8 mm diameter. Quantity of steel shall be measured at the jambs. Vertical reinforcement shall be as given in Table 3

B-4 Projecting windows/ventilators shall be suitably anchored.

B-5 If a tall opening from top to bottom were to split wall into two portions, these portions shall be reinforced with horizontal reinforcement of 6 mm dia bars at nor more than 450 mm spacing, one on each face, properly tied to the vertical steel at jambs, corners or junctions of walls.

B-6 The use of arches over openings is a source of weakness.  When used, steel ties should be provided.

Annexure 26–A.1

CRITERIA FOR EARTHQUAKE RESISTANT DESIGN OF STRUCTURES

(Extract of BIS: 1893-1984)

1. Scope

This annexure deals with earthquake resistant design of structures and is applicable to buildings; elevated structures; bridges, concrete, masonry and earth dams; embankments and retaining walls.

2. Terminology

2.1 Centre of mass – The point through which the resultant of the masses of a system acts.  This corresponds to centre of gravity of the system.

2.2 Centre of rigidity – The point through which the resultant of the restoring forces of a system acts.

2.3 Critical DampingThe damping beyond which the motion will not be oscillatory.

2.4 Damping – the effect of internal friction, imperfect elasticity of material, slipping, sliding, etc, in reducing the amplitude of vibration and is expressed as a percentage of critical damping.

2.4. EpicenterThe geographical point on the surface of earth vertically above the focus of the earthquake.

2.6 FocusThe originating source of the elastic waves, which cause shaking of ground.

2.7 Intensity of Earthquake The intensity of an earthquake at a place is a measure of the effects of the earthquake, and is indicated by a number according to the Modified Mercalli Scale of Seismic Intensities (see Appendix D).

2.8 LiquefactionLiquefaction is a state in saturated cohesionless soil wherein the effective shear strength is reduced to negligible value for all engineering purposes due to pore pressures caused by vibrations during an earthquake when they approach the total confining pressure.  In this condition the soil tends to behave like a fluid mass. 

2.9 Lithological FeaturesThe nature of the geological formation of the earth’s crust above bedrock on the basis of such characteristics as colour, structure, mineralogic composition and grain size.

2.10 Magnitude of Earthquake (Richter’s Magnitude)The magnitude of an earthquake is the logarithm to the base 10 of the maximum trace amplitude, expressed in microns, with which the standard short period torsion seismometer (with a period of 0.8 second, magnification 2,800 and damping nearly critical) would register the earthquake at an epicentral distance of 100 km.  The magnitude M is thus a number, which is a measure of energy released in an earthquake.

2.11 Mode shape coefficient – When a system is vibrating in a normal mode, the amplitude of the masses at any particular instant of time expressed as a ratio of the amplitude of one of the masses is known as mode shape coefficient.

2.12 Normal mode – A system is said to be vibrating in a normal mode or principal mode when all its masses attain maximum values of displacements simultaneously and also they pass through equilibrium positions simultaneously.

2.13 Response spectrum – The representation of the maximum response of idealized single degree freedom systems having certain period and damping, during that earthquake.  The maximum response is plotted against the undamped natural period and for various damping values, and can be expressed in terms of maximum absolute acceleration, maximum relative velocity or maximum relative displacement.

2.14 Seismic coefficients and seismic zone factors –

2.14.1 Basic seismic coefficient (a0) – A coefficient assigned to each seismic zone to give the basic design acceleration as a fraction of the acceleration due to gravity.

2.14.2 Seismic zone factor (F0) – A factor to be used for different seismic zone along with the average acceleration spectra.

2.14.3 Importance factor (I) – A factor to modify the basic seismic coefficient and seismic zone factor, depending on the importance of a structure.

2.14.4 Soil-foundation system factor (b) – A factor to modify the basic seismic coefficient and seismic zone factors, depending upon the soil foundation system.

2.14.5 Average acceleration coefficient – Average spectrum acceleration expressed as a fraction of acceleration due to gravity.

2.14.6 Design horizontal seismic coefficient (ah) – The seismic coefficient taken for design.  It is expressed as a function of the basic seismic coefficient (a0) or the seismic zone factor together with the average acceleration coefficient, the importance factor (I) and the soil-foundation system factor (b)

2.15 Tectonic featureThe nature of geological formation of the bed rock in the earth’s crust revealing regions characterized by structural features, such as dislocation, distortion, faults, folding, thrusts, volcanoes with their age of formation which are directly involved in the earth movement or quakes resulting in the above consequences.

3. General principles and design criteria

3.1 General principles

3.1.1 Earthquakes cause random motion of ground, which can be resolved in any three mutually perpendicular directions.  This motion causes the structure to vibrate.  The vibration intensity of ground expected at any location depends upon the magnitude of earthquake, the depth of focus, distance from the epicentre and the strata on which the structure stands.  The predominant direction of vibration is horizontal.  Relevant combinations of forces applicable for design of a particular structure have been specified in the relevant clauses.

3.1.2 The response of the structure to the ground vibration is a function of the nature of foundation soil; materials, form; size and mode of construction of the structure; and the duration and the intensity of ground motion.  This standard specifies design seismic coefficient for structures standing on soils or rocks which will not settle or slide due to loss of strength during vibrations.

3.1.3 The seismic coefficients recommended in this standard are based on design practice conventionally followed and performance of structures in past earthquakes.  It is well understood that the forces which structures would be subjected to in actual earthquakes would be very much larger than specified in this standard as basic seismic coefficient.  In order to take care of this gap, for special cases importance factor and performance factor (where necessary) are specified in this standard elsewhere.

3.1.4 In the case of structures designed for horizontal seismic force only, it shall be considered to act in any one direction at a time.  Where both horizontal and vertical seismic forces are taken into account, horizontal force in any one direction at a time may be considered simultaneously with the vertical force as specified in 3.4.5.

3.1.5 The vertical seismic coefficient shall be considered in the case of structures in which stability is a criterion of design or, for overall stability, analysis of structures except as otherwise stated in the relevant clauses.

3.1.6 Equipment and systems supported at various floor levels of structures will be subjected to motions corresponding to vibrations at their support points.  In important cases, it may be necessary to obtain floor response spectra for design.

3.2 Assumptions – The following assumptions shall be made in the earthquake resistant design of structures:

a) Earthquake causes impulsive ground motion which is complex and irregular in character, changing in period and amplitude each lasting for small duration.  Therefore, resonance of the type as visualized under steady state sinusoidal excitations will not occur, as it would need time to build up such amplitudes.

b) Earthquake is not likely to occur simultaneously with wind or maximum flood or maximum sea waves.

c) The value of elastic modulus of materials, wherever required, may be taken as for static analysis unless a more definite value is available for use in such condition.

3.3 Permissible increase in stresses and load factors

3.3.1 Permissible increase in material stresses - Whenever earthquake forces are considered along with other normal design forces, the permissible stresses in materials, in the elastic method of design, may be increased by one-third.  However, for steels having a definite yield stress, the stress be limited to the yield stress; for steels without a definite yield point, the will stress will be limited to 80 percent of the ultimate strength or 0.2 percent proof stress whichever is smaller and that in prestressed concrete members, the tensile stress in the extreme fibres of the concrete may be permitted so as not to exceed 2/3 of the modulus of rupture of concrete.

3.3.2 Load factors – Whenever earthquake forces are considered along with other normal design forces, the following factors may be adopted:

a) For ultimate load design of steel structures:

UL = 1.4 (DL + LL + EL)

Where

 UL= the ultimate load for which the structure or its elements should be designed according to the relevant Indian Standards for steel structures.

 DL= the dead load of the structure;

 LL= the superimposed load on the structure considering its modified values as given in the relevant clauses of this standard; and

EL = the value of the earthquake load adopted for design.

The partial safety factors for limit states of serviceability and collapse and the procedure for design as given in relevant Indian Standards (see IS: 456-1978 and IS: 1343-1980) may be used for earthquake loads combined with other normal loads.  The live load values to be used shall be as given in the relevant clauses of this standard.

Note 1 – The members of reinforced or prestressed concrete shall be under reinforced so as to cause a tensile failure.  Further, it should be suitably designed so that premature failure due to shear or bond may not occur subject to the provisions of IS: 456-1978 and IS: 1343-1980.

Note 2 – the members and their connections in steel structures should be so proportioned that high ductility is obtained avoiding premature failure due to elastic or inelastic buckling of any type.

Note 3 – Appropriate details to achieve ductility are given in IS: 4326-1976.

3.3.3 Permissible increase in allowable bearing pressure of soils – When earthquake forces are included, the permissible increase in allowable bearing pressure of soil shall be as given in Table 1, depending upon the type of foundation of the structure.

APPENDIX

(Clause 2.7)

EARTHQUAKE INTENSITY SCALES

D-1.  MODIFIED MERCALLI INTENSITY SCALE (ABRIDGED)

Class of Earthquake

Remarks

I.

Net felt except by a very few under specially favorable circumstances

II.

Felt only by a few persons at rest, specially on upper floors of buildings; and delicately suspended objects may swing

III.

Felt quite noticeably indoors, specially on upper floors of buildings but many people do not recognize it as an earthquake; standing motor cars may rock slightly; and vibration may be felt like the passing of a truck

IV.

During the day felt indoors by many, outdoors by a few, at night some awakened; dishes, windows, doors disturbed; walls make creaking sound, sensation like heavy truck striking the building; and standing motor cars rocked noticeably

V.

Felt by nearly everyone; many awakened; some dishes, windows, etc, broken; a few instances of cracked plaster; unstable objects overturned; disturbance of trees, poles and other tall objects noticed sometimes; and pendulum clocks may stop

VI.

Felt by all, many frightened and run outdoors; some heavy furniture moved; a few instances of fallen plaster or damaged chimneys; and damage slight

VII.

Everybody runs outdoors, damage negligible in buildings of good design and construction; slight to moderate in well built ordinary structures; considerable in poorly built or badly designed structures; and some chimneys broken, noticed by persons driving motors cars

VIII.

Damage slight in specially designed structures; considerable in ordinary substantial buildings with partial collapse; very heavy in poorly built structures; panel walls thrown out of framed structures; falling of chimney, factory stacks, columns, monuments and walls; heavy furniture overturned, sand and mud ejected in small amounts, changes in well water; and disturbs persons driving motor cars

IX.

Damage considerable in specially designed structures; well designed framed structures; thrown out of plumb; very heavy in substantial buildings with partial collapse; buildings shifted off foundations; ground cracked conspicuously; and underground pipes broken

X.

Some well built wooden structures destroyed; most masonry and framed structures with foundations destroyed; ground badly cracked; rails bent; landslides considerable from river banks and steep slopes; shifted sand and mud; and water splashed over banks

XI.

Few, if any, masonry structures remain standing; bridges destroyed; broad fissures in ground, underground pipelines completely out of service; earth slumps and landslips in soft ground; and rails bent greatly

XII.

Total damage; waves seen on ground surfaces; lines of sight and levels distorted; and objects thrown upward into the air.

 

D-2 COMPREHENSIVE INTENSITY SCALE

The scale was discussed generally at the inter-governmental meeting convinced by UNESCO in April 1964.  Though not finally approved, the scale is more comprehensive and describes the intensity of earthquake more precisely.  The main definitions used are as follows:

Structure A

Buildings in fieldstone, rural structures, unburnt brick houses, clay houses.

Structure B

Ordinary bricks buildings, buildings of the large block and prefabricated type, half-timbered structures, buildings in natural hewn stone.

Structure C

Reinforced buildings,  well built wooden structures.

b) Definition of Quantity:

Single, few

About 5 percent

Many

About 50 percent

Most

About 75 percent

c) Classification of damage to buildings:

Grade 1

Slight damage

Fine cracks in plaster; fall of small pieces of plaster

Grade 2

Moderate damage

Small cracks in walls; fall of fairly large pieces of plaster, pantiles slip off; cracks in chimneys; parts of chimney fall down

Grade 3

Heavy damage

Large and deep cracks in walls; fall of chimneys

Grade 4

Destruction

Gaps in walls; parts of buildings may collapse; separate parts of the building lose their cohesion; and inner walls collapse

Grade 5

Total damage

Total collapse of buildings

d) Intensity scale:

I.

Not noticeable

The intensity of the vibration is below the limit of sensibility; the tremor is detected and recorded by seismographs only

II.

Scarcely noticeable (very slight)

Vibration is felt only by  individual people at rest in houses, especially on upper floors of buildings

III.

Weak, partially observed only

A few people feel the earthquake indoors, outdoors only in favorable circumstances.  The vibration is like that due to the passing of a light truck.  Attentive observers notice a slight swinging of hanging objects, somewhat more heavily on upper floors.

IV.

Largely observed

The earthquake is felt indoors by many people, outdoors by few.  Here and there people awake, but not one is frightened.  The vibration is like that due to the passing of a heavily loaded truck.  Windows, doors and dishes rattle.  Floors and walls crack.  Furniture begins to shake.  Hanging objects swing slightly liquids in open vessels are slightly disturbed.  In standing motor cars the shock is noticeable.

V

Awakening:

a) The earthquake is felt indoors by all, outdoors by many.  Many sleeping people awake.  A few run outdoors.  Animals become uneasy.  Buildings tremble throughout.  Hanging objects swing considerably.  Pictures knock against walls or swing out of place.  Occasionally pendulum clocks stop.  Unstable objects may be overturned or shifted.  Open doors and windows are thrust open and slam back again.  Liquids spill in small amounts from well-filled open containers.  The sensation of vibration is like that due to heavy object falling inside the buildings.

b) slight damages in buildings of Type A are possible

c) sometimes change in flow of springs

VI.

Frightening:

a) Felt by most indoors and outdoors.  Many people in buildings are frightened and run outdoors.  A few persons lose their balance.  Domestic animals run out of their stalls.  In few instances dishes and glassware may break, books fall down.  Heavy furniture may possibly move and small steeple bells may ring.

b) Damage of Grade 1 is sustained in single buildings of Type B and in many of Type A.  Damage in few buildings of Type A is of Grade 2.

c) In few cases cracks up to widths of 1 cm possible in wet ground; in mountains occasional landslips; change in flow of springs and in level of well water are observed.

VII.

Damage of buildings:

a) Most people are frightened and run outdoors.  Many find it difficult to stand.  Persons driving motor cars notice the vibration.  Larger bells ring.

b) In many buildings of Type C damage of Grade 1 is caused; in many buildings of Type B damage is of Grade 2.  Most buildings of Type A suffer damage of Grade 3, few of Grade 4.  In single instances landslips of roadway on steep slopes; cracks in roads; seams of pipelines damaged; cracks in stone walls.

VIII.

Destruction of  buildings:

a) Fright and panic; also persons driving motor cars are disturbed.  Here and there branches of trees break off.  Even heavy furniture moves and partly overturns.  Hanging lamps are damaged in part.

b) Most buildings of Type C suffer damage of Grade 2, and few of Grade 3.  Most buildings of Type B suffer damage of Grade 3, and most buildings of Type A suffer damage of Grade 4.  Many buildings of Type C suffer damage of Grade 4.  Occasional breaking of pipe seems.  Memorials and monuments move and twist.  Tombstones overturn.  Stone walls collapse.

c) Small landslips in hollows and on banked roads on steep slopes; cracks in ground up to widths of several centimeters.  Water in lakes becomes turbid.  New reservoirs come into existence.  Dry wells refill and existing wells becomes dry.  In many cases change in flow and level of water is observed.

IX.

General damage to buildings:

a) General panic considerable damage to furniture.  Animals run to and fro in confusion and cry.

 

b) Many buildings of Type C suffer damage of Grade 3, and a few of Grade 4.  Many buildings of Type B show damage of Grade 4, and a few of Grade 5.  Many buildings of Type A suffer damage of Grade 5.  Monuments and columns fall.  Considerable damage to reservoirs; underground pipes partly broken.  In individual cases railway lines are bent and roadway damaged.

 

 

c) On flat lad overflow of water, sand and mud is often observed.  Ground cracks to widths of up to 10 cm, on slopes and river banks more than 10 cm; furthermore a large number of slight cracks in ground; falls of rock, many landslides and earth flows; large waves in water.  Dry wells renew their flow and existing wells dry up.

X.

General destruction of buildings:

a) Many buildings of Type C suffer damage of Grade 4, and a few of Grade 5.  Many buildings of Type B show damage of Grade 5; most of Type A have destruction of Grade 5; critical damage to dams and dykes and severe damage to bridges.  Railway lines are bent slightly.  Underground pipes are broken or bent.  Road paving and asphalt show waves.

b) In ground, cracks up to widths of several centimeters, some times up to 1 metre.  Parallel to watercourses occurs broad fissures.  Loose ground slides from steep slopes.  From riverbanks and steep coasts, considerable landslides are possible.  In coastal areas, displacement of sand and mud; change of water level in wells; water from canals, lakes, rivers, etc, thrown on land.  New lakes occur.

XI.

Destruction:

a) Severe damage even to well built buildings, bridges, water dams and railway lines; highways become useless; underground pipes destroyed.

b) Ground considerably distorted by broad cracks and fissures, as well as by movement in horizontal and vertical directions; numerous landslips and falls of rock.  The intensity of the earthquake requires to be investigated specially.

XII.

Landscape Changes:

a) Practically all structures above and below ground are greatly damaged or destroyed.

b) The surface of the ground is radically changed.  Considerable ground cracks with extensive vertical and horizontal movements are observed.  Falls of rock and slumping of riverbanks over wide areas, lakes are dammed; waterfalls appear, and rivers are deflected.  The intensity of the earthquake requires to be investigated specially.

Annexure 26-A.2

SECTION 26

EARTHQUAKE RESISTANT BUILDINGS

GUIDELINES TO REPAIR AND SEISMIC STRENGTHENING OF BUILDINGS

(Extract of IS: 13935-1993

1. Scope

This standard covers the selection of materials and techniques to be used for repair and seismic strengthening of damaged buildings during earthquakes and retrofitting for upgrading of seismic resistance of existing buildings.

The repair materials and techniques described herein may be used for all types of masonry and wooden buildings, and the concrete elements used in buildings.

The provisions of this standard are applicable for buildings in seismic Zones III to V of

IS:  1893-1984 which are based on damaging seismic intensities VII and more on Modified Mercalli or M.S.K. scales. The scheme of strengthening should satisfy the requirements stipulated for the seismic zone of IS: 1893-1984, building categories of IS: 4326-1993 and provisions made in IS: 13827-1993 for earthen buildings and IS: 13828-1993 for low strength masonary building. No special seismic resistance features are considered necessary for buildings in seismic Zones I and II.

2.  References

The Indian Standards listed below are the necessary adjuncts to this standard:

IS No.

Title

456 : 2000

Code of practice for plain and reinforce concrete

1893 : 1984      

Criteria for earthquake design of structures

4326 : 1993

Code of practice for earthquake resistant design and construction of buildings (third revision)

13827 : 1993    

Guidelines for improving earthquake resistance earthen buildings

13828 : 1993

Guidelines for improving earthquake resistance of low strength masonry buildings

3.  Terminology - For the purpose of this guide, the following definitions shall apply.

3.1 Separation section - A gap of specified width between adjacent buildings or parts of the same building, to permit movement, in order to avoid hammering due to earthquake.

3.2 Crumple section - A separation section filled with appropriate material which can crumple or fracture in an earthquake.

3.3 Centre of rigidity - The point in a structure, where a lateral force shall be applied to produce equal deflections of its components, at any one level in any particular direction.

3.4 Shear wall - A wall designed to resist lateral force in its own plane. Braced frames, subjected primarily to axial stresses, shall be considered as shear walls for the purpose of this definition.

3.5 Space frame - A three-dimensional structural system composed of interconnected members without shear or bearing walls, so as to function as a complete self-contained unit, with or without the aid of horizontal diaphragms or floor bracing systems.

3.6 Moment resistant frame - A space frame capable of carrying all vertical and horizontal loads by developing bending moments in the members and at joints.

3.7 Moment resistant frame with shear walls  - A space frame with moment resistant joints use in combination with shear walls to resist the horizontal loads.

3.8 Box system - A bearing wall structure without a space frame, the horizontal forces being resisted by the walls acting as shear walls.

3.9 Band - A reinforced concrete, reinforced brick or wooden runner provided horizontally in the walls to tie them together and to impart horizontal bending strength in them.

3.10 Seismic zone, and seismic coefficient - Classification of seismic Zones I to V and the corresponding basic seismic coefficients, a0 shall be as specified in IS 1893: 1984.

3.11 Design seismic coefficient - The value of horizontal seismic coefficient computed taking into account the soil foundation system and the importance factor, as only specified in 3.4.2.3(a) of IS 1893: 1984.

3.12 Concrete grades - 28 days crushing strength of concrete cubes of 150 mm side, in MPa, for example, for M15 grade of concrete (see IS 456 : 1978), the strength = 15 MPa.

4. General principles and concepts

4.1 Non-structural/Architectural repairs

4.1.1     The buildings affected by earthquake may suffer both non-structural and structural damages. Non-structural repairs may cover the damages to civil and electrical items including the services in the building. Repairs to non-structural components need to be taken up after the structural repairs are carried out. Care should be taken about the connection details of architectural components to the main structural components to ensure their stability.

4.1.2     Non-structural and architectural components get easily affected / dislocated during the earthquake. These repairs involve one or more of the following:

  1. Patching up of defects such as cracks and fall of plaster;
  2. Repairing doors, windows, replacement of glass panes;
  3. Checking and repairing electric conduits/wiring;
  4. Checking and repairing gas pipes, water pipes and plumbing services;
  5. Re-building non-structural walls, smoke chimneys, parapet walls, etc;
  6. Replastering of walls as required;
  7. Rearranging disturbed roofing tiles;
  8. Relaying cracked flooring at ground level; and
  9. Redecoration – white washing, painting etc.

The architectural repairs as stated above do not restore the original structural strength of structural components in the building and any attempt to carry out only repairs to architectural/non-structural elements neglecting the required structural repairs may have serious implications on the safety of the building. The damage would be more severe in the event of the building being shaken by the similar shock because original energy absorbtion capacity of the building would have been reduced.

4.2 Structural repairs

4.2.1     Prior to taking up of the structural repairs and strengthening measures, it is necessary to conduct detailed damage assessment to determine:

  1. The structural condition of the building to decide whether a structure is amendable for repair; whether continued occupation is permitted; to decide the structure as a whole or a part require demolition, if considered dangerous;
  2. If the structure is considered amendable for repair then detailed damage assessment of the individual structural components (mapping of the crack pattern, distress location; crushed concrete, reinforcement bending/yielding, etc). Non-destructive testing techniques could be employed to determine the residual strength of the members; and
  3. To work out the details of temporary supporting arrangement of the distressed members so that they do not undergo further distress due to gravity loads.

4.2.2     After the assessment of the damage of individual structural elements, appropriate repair methods are to be carried out component wise depending upon the extent of damage. The repair may consist of the following:

  1. Removal of portions of cracked masonry walls and piers and rebuilding them in richer mortar. Use of non-shrinking mortar will be preferable.
  2. Addition of reinforcing mesh on both faces of the cracked wall, holding it to the wall through spikes or bolts and then covering it, suitably, with cement mortar or micro-concrete.
  3. Injecting cement or epoxy like material which is strong in tension, into the cracks in walls.
  4. The cracked reinforced cement elements may be repaired by epoxy grouting and could be strengthened by epoxy or polymer mortar application like shotcreting, jacketting, etc.

4.3 Seismic strengthening - The main purpose of the seismic strengthening is to upgrade the seismic resistance of a damaged building while repairing so that it becomes safer under future earthquake occurrences. This work may involve some of the following actions:

  1. Increasing the lateral strength in one or both directions by increasing column and wall areas or the number of walls and columns.
  2. Giving unity to the structure, by providing a proper connection between its resisting elements, in such a way that inertia forces generated by the vibration of the building can be transmitted to the members that have the ability to resist them. Typical important aspects are the connections between roofs or floors and walls, between intersecting walls and between walls and foundations.
  3. Eliminating features that are sources of weakness or that produce concentration of stresses in some members. Asymmetrical plan distribution of resisting members, abrupt changes of stiffness from one floor to the other, concentration of large masses and large openings in walls without a proper peripheral reinforcement are examples of defects of this kind.
  4. Avoiding the possibility of brittle modes of failure by proper reinforcement and connection of resisting members.

4.4 Seismic retrofitting - Many existing buildings do not meet the seismic strength requirements of present earthquake codes due to original structural inadequacies and material degradation due to time or alterations carried out during use over the years. Their earthquake resistance can be upgraded to the level of the present day codes by appropriate seismic retrofitting techniques, such as mentioned in 4.3.

4.5. Strengthening or retrofitting vs. reconstruction

4.5.1 Replacement of damaged buildings or existing unsafe buildings by reconstruction is, generally, avoided due to a number of reasons, the main ones among them being:

  1. Higher cost than that of strengthening or retrofitting,
  2. Preservation of historical architecture, and
  3. Maintaining functional social and cultural environment.

In most instances, however, the relative cost of retrofitting to reconstruction cost determines the decision. As a thumb rule, if the cost of repair and seismic strengthening is less than about 50 percent of the reconstruction cost, the retrofitting is adopted. This may also require less working time and much less dislocation in the living style of the population. On the other hand reconstruction may offer the possibility of modernization of the habitat and may be preferred by well-to-do communities.

4.5.2 Cost wise the building construction including the seismic code provisions in the first instance, works out the cheaper in terms of its own safety and that of the occupants. Retrofitting and existing inadequate building may involve as much as 4 to 5 times the initial extra expenditure required on seismic resisting features. Repair and seismic strengthening of a damaged building may even be 5 to 10 times as expensive. It is therefore very much safe as well as cost-effective to construct earthquake resistant buildings at the initial stage itself according to the relevant seismic IS codes.

5 Selection of materials and techniques

5.1 General - The most common materials for repair works of various type buildings are cement and steel. In many situations suitable admixture may be added to cement mortar/cement concrete to improve their properties, such as, non-shrinkage, bond, etc. Steel may be required in many forms like bolts, rods, angles, beams, channels, expanded metal and welded wire fabric. Wood and bamboo are the most common material for providing temporary supports and scaffolding, etc, and will be required in the form of rounds, sleepers, planks, etc.

Beside the above, special materials and techniques are available for best results in the repair and strengthening operations. These should be selected appropriately depending on the nature and cost of the building that is to be repaired, materials availability and feasibility and use of available skills, etc. Some special materials and techniques are described below.

5.2 Shotcrete- Shotcrete is cement mortar or cement concrete (with coarse aggregate size maximum 10 mm) conveyed through a hose and pneumatically placed under high velocity on to a prepared concrete or masonry surface. The force of the jet impingement on the surface compacts the shotcrete material and produces a dence homogeneous mass. Basically there are two methods shotcreting; wet mix process and dry mix process. In the wet mix process, all the ingredients including water are mixed together before they enter the delivery hose. In the dry mix process, the mixture of damp sand and cement is passed through the delivery hose to the nozzle where the water is added. The dry mix process is generally used in the repair of concrete elements. The bond between the prepared concrete surface of the damaged member and the layer of shotcrete is ensured with the application of suitable epoxy adhesive formulation. The shear transfer between the existing and new layer of concrete is ensured with the provision of shear keys.

5.3 Epoxy resins - Epoxy resins are excellent binding agents with high tensile strength. These are chemical preparations the compositions of which can be changed as per requirements. The epoxy components are mixed just prior to application. Some products are of low viscosity and can be injected in fine cracks too. The higher viscosity epoxy resin can be used for surface coating or filling larger cracks or holes. The epoxy resins may also be used for gluing steel plates to the distress members.

5.4 Epoxy mortar - For larger void spaces, it is possible to combine the epoxy resins of either low viscosity or higher viscosity with sand aggregate to form epoxy mortar. Epoxy mortar mixture has higher compressive strength, higher tensile strength and a lower modulus of elasticity than cement concrete. The sand aggregate mixed to form the epoxy mortar increases its modulus of elasticity.

5.5 Quick-setting cement mortar - This material is a non-hydrous magnesium phosphate cement with two components, that is, a liquid and a dry powder, which can be mixed in a manner similar to cement concrete.

5.6 Mechanical anchors - Mechanical type of anchors employ wedging action to provide anchorage. Some of the anchors provide both shear and tension resistance. Such anchors are manufactured to give sufficient strength.

Alternatively, chemical anchors bonded in drilled holes through polymer adhesives can be used.

6. Techniques to restore original strength

6.1 General - While considering restoration of structural strength, it is important to realise that even fine cracks in load bearing members which are unreinforced like masonry and plain concrete reduce their resistance very largely. Therefore all cracks must be located and marked carefully and the critical ones fully repaired either by injecting strong cement or chemical grout or by providing external bandage. The techniques are described below along with other restoration measures.

6.2 Repair of minor and medium cracks - For the repair of minor and medium cracks (0.50 mm to 5 mm), the technique to restore the original tensile strength of the cracked element is by pressure injection of epoxy. The procedure is as follows (see Fig. 1A):

‘The external surfaces are cleaned of nonstructural materials and plastic injection ports are placed along the surface of the cracks on both sides of the members and are secured in place with an epoxy sealant. The centre-to-centre spacing of these ports may be approximately equal to the thickness of the element. After the sealant has cured, a low viscosity epoxy resin is injected into one port at a time beginning at the lowest part of the crack, in case it is vertical, or at one end of the crack, in case it is horizontal.

The resin is injected till it is seen flowing form the opposite sides of the members at the corresponding port or from the next higher port on the same side of member. The injection port should be closed at this stage and injection equipment moved to the next port and so on.

The smaller the crack higher is the pressure or more closely spaced should be the ports so as to obtain complete penetration of the epoxy material throughout the depth and width of member. Larger cracks will permit larger port spacing depending upon width of the member. This technique is appropriate for all types of structural elements – beams, columns, walls and floor units in masonry as well as concrete structures. In the case of loss of bond between reinforcing bar and concrete, if the concrete adjacent to the bar has been pulverised to a very fine powder (this powder will block the epoxy from penetrating the region). It should be cleaned properly by air or water pressure prior to injection of epoxy.

6.3 Repair of major cracks and crushed concrete - For cracks wider than about 5 mm or for regions in which the concrete or masonry has crushed, a treatment other than injection is indicated. The procedures may be adopted as follows:

  1. The loose material is removed and replaced with any of the materials mentioned earlier, that is, expansive cement mortar quick setting cement (see Fig. 1B).
  2. Where found necessary, additional shear or flexural reinforcement is provided in the region of repairs. This reinforcement their strength as well as protection to the reinforcement (see Fig. 1C).
  3. In areas of very severe damage, replacement of the member or portion of member can be carried out as discussed later.
  4. In the case of damage to walls and floor diaphragms, steel mesh could be provided on the outside of the surface and nailed or bolted to the wall. Then it may be covered with plaster or micro-concrete (see Fig. 1C).

6.4 Fractured excessively yielded and buckled reinforcement - In the case of severely damaged reinforced concrete member it is possible that the reinforcement would have buckled or elongated or excessive yielding may have occurred. This element can be repaired by replacing the old portion of steel with new steel using butt welding or lap welding.

Splicing by overlapping will be risky. If repair has to be made without removal of the existing steel, the best approach would depend upon the space available in the original member. Additional stirrup ties are to be added in the damaged portion before concreting so as to confine the concrete and enclose the longitudinal bars to prevent their buckling in future.

In some cases, it may be necessary to anchor additional steel into existing concrete. A common technique for providing the anchorage uses the following procedure:

‘A hole larger than the bar is drilled. The hole is filled with epoxy expanding cement or other high strength grouting material. The bar is pushed into place and held there until the grout has set’.

6.5 Fractured wooden members and joints - Since wood is an easily workable material, it will be easy to restore the strength of wooden members such as beams, columns, struts, and ties by splicing additional material. The weathered or rotten wood should first be removed. Nails wood screws or steel bolts will be most convenient as connectors. It will be advisable to use steel straps to cover all such splices and joints so as to keep them tight and stiff.

7 Seismic strengthening techniques

7.1 Modification of roofs or floors

7.1.1     Slates and roofing tiles are brittle and easily dislodged. Where possible, they should be replaced with corrugated iron or asbestos sheeting.

7.1.2 False ceilings of brittle material are dangerous. Non-brittle material, like hessian cloth, bamboo matting or light ones of foam substances, may be substituted.

7.1.3 Roof truss frames should be braced by welding or clamping suitable diagonal bracing members in the vertical as well as horizontal planes.

7.1.4 Anchors of roof trusses to supporting walls should be improved and the roof thrust on walls should be eliminated.

7.15 Figures 2 and 3 illustrate one of the methods for pitched roofs without trusses.

Where the roof or floor consists of prefabricated units like RC rectangular T or channel units or wooden poles and joists carrying brick tiles, integration of such units in necessary. Timber elements could be connected to diagonal planks nailed to them and spiked to an all round wooden frame at the ends. Reinforced concrete elements may either have 40 mm cast-in-situ-concrete topping with 6 mm dia bars 150 mm c/c both ways or bounded by a horizontal cast-in-situ-reinforced concrete ring beam all round into which the ends of reinforced concrete elements are embedded. Fig. 4 shows one such detail.

7.1.6 Roofs or floors consisting of steel joists flat or segmental arches must have horizontal ties holding the joists horizontally in each arch span so as to prevent the spreading of joists. If such ties do not exist, these should be installed by welding or clamping.

7.2 Inserting new walls

7.2.1     Unsymmetrical buildings which may produce dangerous torsional effects during earthquake the center of masses can be made coincident with the centre of stiffness by separating parts of buildings thus achieving individual symmetric units and/or inserting new vertical resisting elements such as new masonry or reinforced concrete walls either internally as shear walls or externally as but-tresses.    Insertion of cross wall will be necessary  for providing transverse supports to longitudinal walls of long barrack-type buildings used for various purpose such as schools and used for various purposes such as schools and dormitories.

7.2.2     The main problem in such modifications is the connection of new walls with old walls. Figures 5, 6 and 7 show three examples of connection of new walls to existing ones. The first two cases refer to a T-junction whereas the third to a corner junction. In all cases the link to the old walls is performed by means of a number of keys made in old walls. Steel is inserted in them and local concrete infilling is made. In the second case, however, connection can be achieved by a number of steel bars inserted in small length drilled holes filled with fresh cement-grout which substitute keys.

7.3 Strengthening existing walls - The lateral strength of buildings can be improved by increasing the strength and stiffness of existing individual walls, whether they are cracked or uncracked, can be achieved.

  1. By grouting
  2. By addition of vertical reinforced concrete coverings on the two sides of the walls, and
  3. By prestressing wall

7.3.1 Grouting - A number of holes are drilled in the wall (2 to 4 in each square metre) (see Fig. 8). First water is injected in order to wash the wall inside, and to improve the cohesion between the grouting mixture and the wall elements. Secondly, a cement water mixture (1:1)   is grouted at low pressure (0.1 to 0.25 MPa) in the holes starting from the lower holes and going up.

Alternatively, polymeric mortars may be used for grouting. The increase of shear strength which can be achieved in this way is considerable. However, grouting can not be relied on as far as the improving or making a new connection between orthogonal walls is concerned.

NOTE – The pressure need for grouting can be obtained by gravity flow from superelevated containers.

7.3.2 Strengthening with wire mesh - Masonry walls with concentration of multiple cracks in the same portion and appearing on both sides on the wall or weak wall regions may be repaired with a layer of cement mortar or micro concrete layer 20 to 40 mm thick on both sides, reinforced with galvanized steel wire fabric (50 mm X 50 mm size) forming a vertical plate bonded to the wall. The two plates on either side of the wall should be connected by galvanized steel rods at a spacing of about 300 to 400 mm (see Fig. 9).

7.3.3 Connection between existing stone walls - In stone buildings of historic importance, consisting of fully dressed stone masonry in good mortar, effective sewing of perpendicular walls may be done by drilling inclined holes through them inserting steel rods and injection cement grout (see Fig. 10).

7.4 Achieving integral box action - The overall lateral strength and stability of bearing wall buildings is very much improved, if the integral box like action of room enclosures is ensured. This can be achieved by (a) use of prestressing (b) providing horizontal bands. Strength of shear walls is achieved by providing vertical steel at selected locations as described in 7.4.1 and 7.4.2.

7.4.1 Prestressing - A horizontal compression state induced by horizontal tendons can be used to increase the shear strength of walls. Moreover, this will also improve, considerably, the connections of orthogonal walls (see Fig. 11). The easiest way of affecting the precompression is to place two steel rods on the two sides of the wall and stretching them by turnbuckles. Note that, good effects can be obtained by slight horizontal prestressing (about 0.1 MPa) on the vertical section of the wall. Prestressing is also useful to strengthen spandrel beam between two rows of openings in the case no rigid slab exists.

Opposite parallel walls can be held to internal cross walls by prestressing bars as illustrated above the anchoring being done against horizontal steel channels instead of small steel plates.

The steel channels running from one cross wall to the other will hold the walls together and improve the integral box like action of the walls.

7.4.2 External binding - The technique of covering the wall with steel mesh and mortar or microconcrete may be used only on the outside surface of external walls but maintaining continuity of steel at the corners. This would strengthen the walls as well as bind them together.

As a variation and for economy in the use of materials, the covering may be in the form of vertical splints located between the openings and horizontal ‘bandages’ formed over spandrel walls at suitable number of points only (see Fig. 12).

7.5 Masonry arches - If the walls have large arched openings in them, it will be necessary to install tie rods across them at springing levels or slightly above it by drilling holes on both sides and grouting steel rods in them (see Fig. 13a). Alternatively, a lintel consisting of steel channels or I-shapes could be inserted just above the arch to take the load and relieve the arch as shown at Fig. 13b. In jack-arch roofs, flat iron bars or rods shall be provided to connect the bottom flanges of I-beams connected by bolting or welding (see Fig. 13c).

7.6 Random rubble masonry walls - Random rubble masonry walls are most vulnerable to delimitation and complete collapse and must be strengthened by internal impregnation by rich cement mortar grout in the ratio of 1 : 1 as explained in 7.3.1 or covered with steel mesh and mortar as in 7.3.2. Damaged portions of the wall, if any should be reconstructed using richer mortar. In thick walls, ‘through; stones or bonding elements shall be installed, if not present originally, at each one-third point along the length and height of wall (see Fig. 14)

7.7 Strengthening long walls - For bracing the longitudinal walls of long barrack type buildings a portal type framework may be inserted transverse to the walls and connected to them. Alternatively masonry buttresses or pillasters may be added externally as shown in Fig. 14.

7.8 Strengthening reinforced concrete members

7.8.1 Columns - Reinforced concrete columns can best be strengthened by casing, that is, by providing additional cage of longitudinal and lateral tie reinforcement around the columns and casting a concrete ring (see Fig. 15). The desired strength and ductility can thus be built-up.

7.8.2 Beams - A reinforced concrete beam can be encased as shown in Fig. 16(A). For holding the stirr-up in this case, holes will have to be drilled through the slab. Alternatively it can be jacketed as shown in Fig. 16(B), and Fig.16(C) wherein holes will need to drilled through web of existing beam for the new stirr-ups. Desired quantity of longitudinal and transverse steel may be added in each case.

Reinforced concrete beams can also be strengthened by applying prestress to it so that opposite moments are caused to those applied. The wires will run on both sides of the web outside and anchored against the end of the beam through a steel plate. Loss of prestress due to creep relation and temperature fall shall be duly considered.

7.8.3 Shear walls - The casing technique could be used for strengthening reinforced concrete shear walls.

7.8.4     Inadequate section of beams, columns and walls could be strengthened by adding a layer of reinforced concrete (outershell) around the members with the addition of new reinforcements. Also to the existing steel, new steel reinforcement bars could be welded to increase the carrying capacity of the members. In cases of adding new concrete to old concrete, effective bond should be ensure. Such bond should be created by the application of suitable epoxy adhesive formulations on the prepared old concrete surface. In addition to this, suitable shear connectors in the form of steel rods placed in predrilled holes in the old concrete at required spacing should be provided. These rods should also be dipped in epoxy adhesive formulations before placing in position.

7.8.5     In case of adding new concrete to old concrete the original surface should be roughened, grooves made in the appropriate direction for providing shear transfer. The ends of the additional steels are to be anchored in the adjacent beams or columns as the case may be. 

7.9 Strengthening of foundations- Strengthening of foundations before or after the earthquake is the involved task it may require careful underpinning operations. Some alternatives are given below for preliminary consideration of the strengthening scheme:

  1. Introducing new load bearing members including foundations to relieve the already members. Jacking operations may be needed in this process.
  2. Improving the drainage of the area to prevent saturation of foundation soil to oviate any problems of liquefaction which may occur because of poor drainage.
  3. Providing apron around the building to prevent soaking of foundation directly and drainage.
  4. Adding strong elements in the form of reinforced concrete strips attached to the existing foundation part of the building. These will also bind the various wall footings and may be provided on both side of the wall or only one side of it. In any case, the reinforced concrete strips and the wall have to be linked by a number of keys inserted into the existing footing.

Note: To avoid disturbance to the integrity of the existing wall during the foundation strengthening process proper investigation and design is called for.

Annexure 26-A.3.

SECTION 26

EARTHQUAKE RESISTANT DESIGN AND CONSTRUCTION OF BUILDINGS

(Extract of IS:  4326-1993)

1. Scope

This standard deals with the selection of materials, special feature of design and construction for earthquake resistance buildings including masonry construction using rectangular masonry units, timber construction and buildings with prefabricated flooring/roofing elements.

Guidelines for earthquake resistance buildings constructed using masonry of low strength and earthen buildings are covered in separate Indian Standards.

2.  REFERENCES

The Indian Standards listed below are the necessary adjuncts to this standard:

IS No Title
456-1978 Code of practice for plain and reinforcement concrete (third revision)
883-1992 Code of practice for design of structural timber in buildings (forth revision)
1597-1992  (Part 2) Code of practice for construction of stone masonry : part 2 Ashlar masonry (First revision)
1641-1988 Code of practice for fire safety of buildings (general) : General principles of fire grading and classification (first revision)
1642-1989 Code of practice for fire safety of buildings  (first revision)
1644-1988 Code of practice for fire safety of buildings (general): Exit requirements and personal hazard (first revision)
1646-1982 Code of practice for fire safety of buildings (general) : Electrical installations (first revision)
1893-1984 Criteria for earthquake resistant design of structures (fourth revision)
1904-1986 Code of practice for design and construction of foundations in soils: General requirements (third revisioin)
1905-1987 Code practice for structural use of unreinforced masonry (third revision)
2212-1991  Code of practice for brickwork (first revision)
275-1979 Recommended practice of welding mild steel plain and deformed bars for reinforced construction (first revision)
3414-1968 Code of practice for design and installation of joints in buildings
9417-1989 Recommendations for welding cold worked bars for reinforced steel construction (first revision)
13920-1993 Code of practice for ductility detailing of reinforced concrete structures subjected to seismic forces.

3 Terminology - For the purpose of this standard, the following definitions shall apply.

Separation section - A gap of specified width between adjacent buildings or parts of the same building, either left uncovered or covered suitably to permit movement in order to avoid hammering due to earthquake.

Crumple section - A separation section filled with appropriate material which can crumple or fracture in an earthquake.

Centre of rigidity - The point in a structure where a lateral force shall be applied to produce equal deflections of its components at any one level in any particular direction.

Shear wall - A wall designed to resist lateral force in its own plane. Braced frames, subjected primarily to axial stresses, shall be considered as shear walls for the purpose of this definition.

Space frame - A three-dimensional structural system composed of interconnected members, without shear or bearing walls, so as to function as a complete self-contained unit with or without the aid of horizontal diaphragms or floor bracing systems.

Vertical load carrying frame - A space frame designed to carry all the vertical loads, the horizontal loads being resisted by shear walls.

Moment resistant frame - A space frame capable of carrying all vertical and horizontal loads, by developing bending moments in the members and at joints.

Moment resistant frame with shear walls - A space frame with moment resistant joints and strengthened by shear walls to assist in carrying horizontal loads.

Box system - A bearing wall structure without a space frame, the horizontal forces being resisted by the walls acting as shear walls.

Band - A reinforced concrete or reinforced brick runner provided in the walls to tie them together and to impart horizontal bending strength in them.

Seismic zone and seismic coefficient - The seismic zones I to V as classified in IS 1893: 1984 and corresponding basic seismic coefficient a0 as specified in 3.4 of IS: 1893-1984.

Design seismic coefficient - The value of horizontal seismic coefficient computed taking into account the soil-foundation system and the importance factor as specified in 3.4.2.3(a) of IS 1893: 1984.

 Concrete grades - 28 day crushing strength of concrete cubes of 150 mm size, in MPa; for example, for Grade M15 of IS: 456-1978, the concrete strength = 15 MPa.

4 General principles

The general principles given 4.1 to 4.9 shall be observed in construction of earthquake resistance buildings.

4.1 Lightness - Since the earthquake force is a function of mass, the building shall be as light as possible consistent with structural safety and functional requirements. Roofs and upper storeys of buildings, in particular, should be designed as light as possible.

4.2 Continuity of construction

4.2.1     As far as possible, the parts of the building should be tied together in such a manner that the building acts as one unit,

4.2.2 For parts of buildings between separation or crumple sections or expansion joints, floor slabs shall be continuous throughout as far as possible. Concrete slabs shall be rigidly connected or integrally cast with the support beams.

4.2.3 Additions and alterations to the structures shall be accompanied by the provision of separation or crumple sections between the new and the existing structures as far as possible, unless positive measures are taken to establish continuity between the existing and the new construction.

4.3 Projecting and suspended parts

4.3.1 Projecting parts shall be avoided as far as possible. If the projecting parts cannot be avoided, they shall be properly reinforced and firmly tied to the main structure, and their design shall be in accordance with IS: 1893-1984.

4.3.2 Ceiling plaster shall preferably be avoided. When it is unavoidable, the plaster shall be as thin as possible.

4.3.3 Suspended ceiling shall be avoided as far as possible. Where provided they shall be light, adequately framed and secured.

4.4 Building configuration - In order to minimize torsion and stress concentration, provisions given in 4.4.1 to 4.4.3 should be complied with as relevant.

4.4.1 The building should have a simple rectangular plan and be symmetrical both with respect to mass and rigidity so that the centres of mass and rigidity of the building coincide with each other in which case no separation sections other than expansion joints are necessary. For provision of expansion joints reference may be made to IS: 3414-1968.

4.4.2 If symmetry of the structure is not possible in plan, elevation or mass, provision shall be made for torsional and other effects due to earthquake forces in the structural design or the parts of different rigidities may be separated through crumple sections. The length of such building between separation sections shall not preferably exceed three times the width.

Note – As an alternative to separation section to reduce torsional moments, the centre of rigidity of the building may be brought close or coincident to the centre of mass by adjusting the locations and/or sizes of columns and walls.

4.4.3 Buildings having plans with shapes like, L, T, E and Y shall preferably be separated into rectangular parts by providing separation sections at appropriate places. Typical examples are shown in Fig. 1.

Notes-1. The buildings with small lengths of projections forming L, T, E or Y shapes need not be provided with separation section. In such cases the length of the projection may not exceed 15 to 20 percent of the total dimension of the building in the direction of the projection (see Fig. 2).

Fig. 2 Plan and vertical irregularities

2.  For buildings with minor asymmetry in plan and elevation separation sections may be omitted.

4.5 Strength in various directions - The structure shall be designed to have adequate strength against earthquake effects along both the horizontal axes. The design shall also be safe considering the reversible nature of earthquake forces.

4.6 Foundations - The structure shall not be founded on such loose soils which will subside or liquefy during an earthquake, resulting in large differential settlements (see also 5.3.3)  

4.7 Ductility - The main structural elements and their connection shall be designed to have a ductile failure. This will enable the structure to absorb energy during earthquakes to avoid sudden collapse of the structure. Providing reinforcing steel in masonry at critical sections, as provided in this standard will not only increase strength and stability but also ductility. The detail for achieving ductility in reinforced concrete structures is given in IS 13920: 1993.

4.8 Damage to non-structural parts - Suitable details shall be worked out to connect the non-structural parts with the structural framing so that the deformation of the structural frame leads to minimum damage of the non-structural elements.

4.9 Fire safety - Fire frequently follows an earthquake and therefore, buildings shall be constructed to make them fire resistant in accordance with the provisions of following Indian Standards for fire safety, as relevant; IS: 1641-1988, IS: 1642-1989, IS: 1643-1988, IS: 1644-1988 and IS: 1646-1986.

5 Special construction features

5.1 Separation of adjoining structures

5.1.1. Separation of adjoining structures or parts of the same structure is required for structures having different total heights or storey heights and different dynamic characteristics. This is to avoid collision during an earthquake.

5.1.2. Minimum width of separation gaps as mentioned in 5.1.1, shall be as specified in Table 1. The design seismic coefficient to be used shall be in accordance with IS 1893-1984.

Table 1 Gap width for adjoining structures

Sl.No.

Types of construction

Gap width/storey, in mm for design seismic coefficient at 0.12

(1)

(2)

(3)

i)

Box system for frames with shear walls

15.0

ii)

Moment resistant reinforced concrete frame

20.0

iii)

Moment resistant steel frame

30.00

Note – Minimum total gap shall be 25 mm. For any other value of the gap width shall be determined proportionately.

5.1.2.1 For buildings of height greater than 40 meters, it will be desirable to carry out model or dynamic analysis of the structures in order to compute the drift at each storey, and the gap width between the adjoining structures shall not be less than the sum of their dynamic deflections at any level.                           

5.1.3 Where separation is necessary, a complete separation of the parts shall be made except below the plinth level. The plinth beams, foundation beams and footings may be continuous. Where separation sections are provided in a long building, they shall take care of movement owing to temperature changes also.

5.2 Separation or crumple section

5.2.1 In case of framed construction, members shall be duplicated on either side of the separation or crumple section. As an alternative, in certain cases, such duplication may not be provided, if the proportions on either side can act as cantilevers to take the weight of the building and other relevant loads.

5.2.2. Typical details of separation and crumple sections are shown in Fig.3 for other types of joint details reference may be made to IS: 3414-1968.

5.3 Foundations

5.3.1 For the design of foundations, the provisions of IS: 1893-1984 shall generally be followed.

5.3.2. The sub grade below the entire area of the building shall preferable be of the same type of the soil. Wherever this is not possible, a suitably the located separation or crumple section shall be provided.

5.3.3 Loose fine sand, soft slit and expansive clays should be avoided. If unavoidable, the building shall rest either on a rigid raft foundation or on piles taken to a firm stratum. However, for light construction the following measures may be taken to improve the soil on which the foundation of the building may rest:

  1. Sand piling, and
  2. Soil stabilization.

5.3.4 Isolated footing for columns - All the individual footings or pile caps where used in Type III Soft soils (Table 3 of IS: 1893-1984), shall be connected by reinforced concrete ties at least in two directions approximately at right angles to each other.

For buildings with no basement the ties may be placed at or below the plinth level and for buildings with basement they may be placed at the level of basement floor. They may be designed to carry the load of the panel walls also.

Note – The ties will not be necessary where structural floor connects the columns at or below the plinth level.

5.3.4.1 Where ties are used, their sections shall be designed to carry in tension as well as in compression, an axial load not less than the earthquake force acting on the heavier of the columns connected, but the sections shall not be less than 200 mm X 200 mm with M15 concrete reinforced with 4 bars of 12 mm dia plain mild steel bars or 10 mm dia high strength deformed bars, one at each corner, bound by 6 mm dia mild steel stirrups not more than 150 mm apart.

Note – In working out the buckling strength of ties, the lateral support provided by the soil may be taken into account. Calculations show that for such buried ties, lateral buckling is not a problem and the full section of the tie may be taken effective as a short column.

5.3.4.2. In the case of reinforced concrete slabs. The thickness shall not be less than 1/50 th of the clear distance between the footings, but not less than 100 mm in any case. It shall be reinforced with not less than 0.15 percent mild steel bars or 0.12 percent height strength deformed bars in each direction placed symmetrically at top and bottom.

5.4 Roofs and floors

    1.  

Flat roof or floor shall not preferably be made of terrace of ordinary bricks supported on steel, timber or reinforced concrete joists, nor they shall be of a type which in the event of an earthquake is likely to be loosened and parts of all of which may fall. If this type of construction cannot be avoided, the joists should be blocked at ends and bridged at intervals such that their spacing is not altered during an earthquake.

5.4.1.1 For pitched roofs, corrugated iron or asbestos sheets shall be used in preference to country, Allahabad or Mangalore tiles or other loose roofing units. All roofing materials shall be properly tied to all supporting members. Heavy roofing materials shall generally be avoided.

5.4.2 Pent roofs

5.4.2.1   All roof trusses shall be supported on reinforced concrete or reinforced brick band (see 8.4.3). The holding down bolts shall have adequate length as required for earthquake forces in accordance with IS: 1893-1984.

Where a trussed roof adjoins a masonry gable, ends of the purlin shall be carried on and secured to a plate or bearer which shall be reinforced brick band at the top of gable end masonry (see 8.4.4).

Note – Hipped roof in general have shown better structural behavior during earthquakes than gable ended roofs.

Fig. 3.1 Typical details of separation or crumple section

Fig. 3.2 Typical details of separation or crumple section

5.4.2.2          At tie level all the trusses and the gable end shall be provided with diagonal braces in plan so as to transmit the lateral shear due to earthquake force to the gable walls acting as shear walls at the ends as specified in 8.4.

5.4.3 Jack arches - Jack arched roofs or floors, where used shall be provided with mild steel ties in all spans along with diagonal braces in plan to ensure diaphragms actions.

5.5 Staircases

5.5.1     The interconnection of the stairs with the adjacent floors should be approximately treated by providing sliding joints at the stairs to eliminate their bracing effect on the floors (see 4.5.4). Large stair halls shall preferably be separated from the rest of the building by means of separation crumple sections.

5.5.2     Three types of stair construction may be adopted as described below:

i)  Separated Staircases- One end of the staircase rests on a wall and the other end is carried by columns and beams which have no connection with the floors. The opening at the vertical joints between the floor and the staircase may be covered either with a tread plate attached to one side of the joint and sliding on the other side, or covered with some appropriate material which could crumple or fracture during an earthquake without causing structural damage. The supporting members, columns or walls, are isolated from the surrounding floors by means of separation or crumple sections. A typical example is shown in Fig.4.

Fig. 4 Separated staircase

ii)  Built-in Staircase – When stairs are built monolithically with floors, they can be protected against damage by providing rigged walls at the stair opening. An arrangement, in which the staircase is enclosed by two walls, is given in Fig.5. In such cases, the joints, as mentioned in respect of separated staircase, will not be necessary.

Fig. 5 Rigidly built-in staircase

The two walls mentioned above, enclosing the staircase, shall extend through the entire height of the stairs and the building foundations.

iii)  Staircases with Sliding Joints – In case it is not possible to provide rigid walls around stair openings for built-in staircase or to adopt the separated staircase, the staircases shall have sliding joints so that they will not act as diagonal bracing.

6. Types of construction

6.1 The types of construction usually adopt in buildings are as follows:

a)  Framed construction, and

b)  Box type construction.

6.2 Framed construction - This type of construction is suitable for multistoried and industrial buildings as described in 6.2.1 and 6.2.2.

6.2.1 Vertical load carrying frame construction - This type of construction consist of frames with flexible (hinged) joints and bracing members. Steel multistoried building or industrial frames and timber construction usually are of this type.

6.2.1.1    Such buildings shall be adequately strengthened against lateral forces by shear walls and / or other bracing systems in plan, elevation and sections such that earthquake forces shall be resisted by them in any direction.

6.2.2 Moment resistant frames with shear walls - The frames may be of reinforced concrete or steel with semi-rigid joints. The walls may be of capable of acting as shear walls and may be of reinforced concrete or of brickwork reinforced or unreinforced bounded by framing members through shear connectors.

6.2.2.1   The frame and wall combination shall be designed to carry the total lateral force due to earthquake acting on the building. The frame acting alone shall be designed to resist at least 25 percent of the total lateral force.

6.2.2.2    The shear walls shall preferably be distributed evenly over the whole building. When concentrated at one point, forming what is called a rigid core in the building, the design shall be checked for torsional effects and the shear connection between the core and the floors conservatively designed for the total shear transfer.

6.2.2.3  The shear should extend from the foundation either to the top of the building or to a lesser height as required from design consideration. In design, the interaction between frame and the shear walls should be considered properly to satisfy compatibility and equilibrium conditions.

Note – Studies show that shear walls of height about 85 percent of total height of building are advantageous.

6.3 Box type construction - This type of construction consists of prefabricated on in situ masonry, concrete or reinforced concrete wall along both the axes of the building. The walls support vertical loads and also act as shear walls for horizontal loads acting in any direction. All traditional masonry construction falls under \ this category. In prefabricated construction attention shall be said to the connections between wall panels so that transfer of shear between them is ensured.

7 Categories of buildings

7.1 For the purpose of specifying the earthquake resisting features in masonry and wooden buildings have categories A to E based on the value of ah given below:

ah = a0 I, b

Where, ah   = design seismic coefficient for the building.

a0 = basic seismic coefficient for the seismic zone in which the building is located (See 8.4 and Table 2 of IS:  1893-1984)

I = importance factor application to the building (see 3.4.2.3 and Table 4 of

IS:  1893-1984), and

b = Soil foundation factor (see 3.4.2.3 and Table 3 of IS: 1893-1984)

7.1.1     The building categories are given in Table 2.      

8 Masonry constructions with rectangular masonry units

8.1 The design and construction of masonry walls using rectangular masonry units in general shall be governed by IS: 1905-1987 and IS: 2212-1991.

Table 2 Building categories for earthquake resisting features (Clause 7.1.1)

Building categories

Range of ah

A

Less than 0.05

B

0.05 to 0.06 (both inclusive)

C

More than 0.06 and less than 0.08

D

0.08 to less than 0.12

E

Equal to or more than 0.12

8.1.1     Masonry units

8.1.1.1    Well burnt bricks and soil concrete blocks having a crushing strength not less than 35 MPa shall be used. However, higher strength of masonry units may be required depending upon number of storeys and thickness of walls (see IS: 1905-1987).

      1.  

Squared stone masonry, stone block masonry for hollow concrete block masonry, as specified in IS: 1597 (part 2)-1992 of earthquake strength may also be used.

8.1.2. Mortar

8.1.2.1 Mortars, such as those given in Table 3 or of equivalent specification, shall preferably be used for masonry construction for various categories of buildings.

8.1.2.2 Where steel reinforcing bars are provided in masonry the bars shall be embedded with adequate cover in cement sand mortar not leaner than 1:3 (minimum clear cover 10mm) or in cement concrete of grade M15 (minimum clear cover 15 mm or bar diameter whichever more), so as to achieve good bond and corrosion resistance.

8.2 Walls

8.2.1 Masonry bearing walls built in mortar, as specified in 8.1.2.1 unless rationally designed as reinforced masonry shall not be built or greater height than 15 m subject to a maximum of four storeys when measured from the mean ground level to the roof slab or ridge level. The masonry bearing walls shall be reinforced in accordance with 8.4.1.

8.2.2 The bearing walls built direction shall b straight and symmetrical in plan as far as possible.

8.2.3 The wall panels formed between cross walls and floors or roof shall be checked for their strength in bending as a plate or as a vertical strip subjected to the earthquake forced acting on its own mass.

Note – For panel walls of 200 mm or larger thickness having a storey height not more than 3.5 meters and laterally supported at the top, this check need not be exercised.

Table 3 Recommend mortar mixes (Clause 8.1.2.1 and 8.2.6)

*Category of construction

Proportion of cement-lime-sand†

A

M2 (cement-sand 1:6) or M3 (Lime-cinder ‡1:3) or richer

B, C

M2 (cement-l;ime-sand 1:2:9 or cement-sand 1:6) or richer

D, E

H2 (cement-sand 1:4) or M1 (cement-lime-sand 1:1:6) or richer

Note – Thought the equivalent mortar with lime will less strength at 28 days, their strength after one year will be comparable to that of cement mortar.

  • Category of construction is defined in Table 2.

= Mortar grades and specification for types of limes etc, are given in IS 1905:1987.

‡ In this case some other pozzolanic material like Surkhi (burnt brick fine powder)may be used in place of cinder.

8.2.4 Masonry bond - For achieving full strength of masonry, the usual bonds specified for masonry should be followed so that the vertical joints are broken properly from course ton course. To obtain full bond between perpendicular walls, it is necessary to make a slopping (stepped) joint by making the corners first to a height of 600 mm and then building the wall in between them.  Otherwise, the toothed joint should be made in both the walls alternatively in lifts of about 450 mm (see Fig.6.).

a, b, c = Toothed joints in wall and A,B,C

Fig. 6 Alternating toothed joints in walls at corner and T-junction

8.2.5     Ignoring tensile strength, free standing walls shall \be checked against overturning under the action of design seismic coefficient ah allowing for a factor safety of 1.5.

8.2.6     Panel or filler walls in framed buildings shall be properly bonded to surrounding framing members by means of suitable mortar (see Table 3) or connected through dowels. If the walls are so bonded they shall be checked according to 8.2.3 otherwise as in 8.2.5.

8.3 Opening in bearing walls

8.3.1 Door and window openings in walls reduce their lateral load resistance and hence, should preferably be small and more centrally located. The guidelines on the size and position of opening are given in Table 4 and Fig.7.

8.3.2 Opening in any storey shall preferably have their top at the same level so that a continuous band could be provided over them, including the lintels throughout the building.

8.3.3 Where openings do not comply with the guidelines of Table 4, they should be strengthen by providing reinforced concrete or reinforcing the brickwork, as shown in Fig.8 with high strength deformed (H.S.D) bars of 8 mm dia but the quantity of steel shall be increased at the jambs to comply with 8.4.9, if so required.

8.3.4 If a window or ventilator is to be projected out, the projection shall be in reinforced masonry or concrete sand well anchored.

8.3.5 If an openings is tall from bottom to almost top of a storey, thus dividing the wall into two portions, these portions shall be reinforced with horizontal reinforcement of 6 mm diameter bars at not more than 450 mm intervals, one on inner and one on outer face, properly tied to vertical steel at jambs, corners or junction of walls, where used.          

Fig. 7 Dimensions of openings and pipes for recommendations in Table 4

8.3.6 The use of arches to span over the openings is a source of weakness and shall be avoided. Otherwise, steel ties should be provided.

Table 4 Size and position of openings in bearing walls (Clause 8.3.1 and fig 7)

Sl No

Position of opening

Details of opening for building category

1

Distance b2 from the inside corner Of outside wall, Min

A and B

Zero mm

C

230 mm

D and E

450 mm

2

For total length of openings, the Ratio (b1 + b2 + b3) / l­1, or (b6 + b7) / lShall not exceed:

 

 

 

 

a) One-storied building

0.60

0.55

0.50

 

b) Two-storied building

0.50

0.46

0.42

 

c) 3 or 4-storeyed building

0.42

0.37

0.33

3

Pier width between consecutive opening B4, Min

340 mm

450 mm

560 mm

4

Vertical distance between two openings One above the other h3,Min

600 mm

600 mm

600 mm

5

Width of opening of ventilator 

900 mm

900 mm

900 mm

Fig. 8 strengthening masonry around

8.4 Seismic strengthening arrangements

8.4.1 All masonry buildings shall be strengthened by the methods, as specified for various categories of buildings, as listed in Table 5, and detailed in subsequent clauses.  Figures 9 and 10 show, schematically, the overall strengthening arrangements to be adopted for category D and E buildings which consists of horizontal bands of reinforcement at critical levels, vertical reinforcing bars at corners, junctions of walls and jambs of opening.

8.4.2 Lintel band is a band is a band (see 3.6) provided at lintel level on all load bearing internal, external longitudinal and cross walls. The specifications of the band are given in 8.4.5.

Note – Lintel band if provided in panel or partition walls also will improve their stability during severe earthquake.

8.4.3 Roof band is a band (see 3.6) provided immediately below the roof or floors. the specifications of the band are given in 8.4.5.such a concrete or brick-work slabs resting on bearing walls, provided that the slabs are continuous over the intermediate will up to the crumple sections, if any, and over the width, fully or at lea  ¾ of the wall thickness.

Fig. 9 Overall arrangement of reinforcing masonry building

1

Lintel band

4

 Door

2

Roof/floor band

5

Window

3

Vertical bar

 

 

Fig.10 Overall arrangement of reinforcing masonry building having pitched roof

1

Lintel band

8

Holding down bolt

2

Eave level (Roof) band

9

Brick/stone wall

3

Gable band

10

Door lintel integrated with roof band

4

Door

a)

Perspective view

5

Window

b)

Details of truss connection with wall

6

Vertical steel bar

c)

Detail of integrating door lintel with roof band

7

Rafter

   

Table 5 Strengthening arrangements recommended for masonry buildings

(Rectangular masonry units) (Clause 8.4.1)

Building category

Number of stories

Strengthening to be provided in all storeys

(1)

(2)

(3)

A

i) 1 to 3

A

 

ii) 4

a,b,c

B

i) 1 to 3

a,b,c,f,g

 

ii) 4

a,b,c,d,f,g

C

i) 1 and 2

a,b,c,f,g

 

ii) 3 and 4

a to g

D

i) 1 and 2

a to g

 

ii) 3 and 4

a to h

E

1 to 3*

a to h

Where

a – Masonry Mortar (see 8.1.2),

b – Lintel band (see 8.4.2)

c – Roof band and gable band where necessary (see 8.4.3 and 8.4.4)

d – Vertical steels at corners and junctions of walls (see 8.4.8),

e – Vertical steels at jambs of openings (see 8.4.9),

f – Bracing in plan at the level of roofs (see 8.4.6),

g – Plinth band where necessary (see 8.4.6), and

h – Dowel bars (see 8.4.7).

4th storey not allowed in category E.

Note – In case of four storey buildings of category B, the requirements of vertical steel may be checked through a seismic analysis using a design seismic coefficient equal to four times the one given in (a) 3.4.2.3 of IS: 1893-1984. (This is because the brittle behavior of masonry in the absence of vertical steel results in much higher effective seismic force than that envisaged in the seismic coefficient provided in the code). If this analysis shows that vertical steel is not required the designer may take the decision accordingly.

8.4.4 Gable band is band provided at the top of gable masonry below the purlins. The specifications of the band are given in 8.4.5. this band shall be made continuous with the roof band at the eaves level.

8.4.5  Section and reinforcement of band - The band shall be made of reinforced concrete of grade not leaner than M15 or reinforced brick-work in cement mortar not leaner than 1: 3 .the bands shall be of the full width of the wall, not less than 75 mm in depth and reinforced with steel, as indicated in Table 6.

Fig. 11 Reinforcement and bending detail in R.C. band

1

Longitudinal bars

a)

Section of band with two bars

2

Lateral ties

b)

Section of and with four bars

 

B1,  b2  wall thickness

c)

Structural plan at corner junction

 

 

d)

Section plan at T-junction of walls

Note – In coastal areas, the concrete grade shall be M20 concrete and the filling mortar of 1:3 (cement – sand with water proofing admixture).

In case of reinforced brickwork, the thickness of joints containing steel bars shall be increased so as to have a minimum mortar cover of 10 mm around the bar. In bands of reinforced brickwork the area of steel provided should be equal to that specified above for reinforced concrete bands.

    1.  

For full integrity of walls at corners and junctions of walls and effective horizontal bending resistance of bands continuity of reinforcement is essential. The details as shown in Fig.11 are recommended.

Plinth band is a band provided at plinth level of walls on top of the foundation wall. This is to be provided where strip footings of masonry (other than reinforced concrete or reinforced masonry) are used and the soil is either soft or uneven in its properties, as frequently happens in hill tracts. Where used, its section may be kept same as in 8.4.5. this band will serve as damp proof course as well.

In category D and E buildings, to further iterate the box action of walls steel dowel bars may be used at corners and T-junctions of walls at the still level of windows to length of 900 mm from the inside corner in each wall. Such dowel may be in the form of U stirrups 8 mm dia. Where used, such bars must be laid in 1:3 cement-sand-mortars with a minimum cover of 10 mm on all sides to minimize corrosion.

Table 6 Recommended longitudinal steel in reinforced concrete bands Clause 8.4.5)

Span

Building category

Building category

Building category

Building category

 

B

C

D

E

 

No.of  Bars

Dia

No.of Bars

Dia

No.of Bars

Dia

No.of Bars

dia

(1)

(2)

(3)

(4)

(5)

(6)

(7)

(8)

(9)

M

 

Mm

 

Mm

 

mm

 

mm

5 or less

2

8

2

8

2

8

2

10

6

2

8

2

8

2

10

2

12

7

2

8

2

10

2

12

4

10

8

2

10

2

12

4

10

4

12

Notes-1. Span of wall will be less the distance between centre lines of its cross walls or buttresses. For spans greater than 8 m it will be desirable to insert pilasters or buttresses to reduce the span or special calculations shall be made to determine the strength of wall and section of band.

2. The number and diameter of bars given above pertains to high strength deformed bars. If plain mild-steel used keeping the same number, the following diameters may be used:

High Strength Def. Bar dia : 8    10   12   16   20

Mild steel plain bar dia        10   12   16   20   25

3.  Width of R.C band is assumed same as the thickness of the wall. Wall thickness shall be 200-mm minimum. A clear cover or 20 mm from face of wall will be maintained.

4.  The vertical thickness of RC band be kept 75 mm minimum, where two longitudinal bars are specified, one on each face; and 150 mm, where four bars are specified.

5. Concrete mix shall be of grade M15 of IS 456:1978 or 1:2:4 by volume.

6. The longitudinal steel bars shall be held in position by steel links or stirrup 6 mm dia spaced at 150 mm apart.

8.4.8     Vertical reinforcement - Vertical steel at corners and junctions of walls, which are up to 340 mm (1½-brick) thick, shall be provided as specified in Table 7. For walls thicker than 340 mm the area of the bars shall be proportionately increased. For earthquake resistant framed wall construction, see 8.5. No vertical steel need be provided in category a building.

Table 7 Vertical steel reinforcement in masonry walls with rectangular masonry units

No. of

Storeys

Storey

Diameter of HSD single bar in mm at each critical section

Category B

Category C

Category D

Category E

One

-

Nil

Nil

10

12

Two

Top

Nil

Nil

10

12

 

Bottom

Nil

Nil

12

16

Three

Top

Nil

10

10

12

 

Middle

Nil

10

12

16

 

Bottom

Nil

12

12

16

Four

Top

10

10

10

Four storeyed

 

Third

10

10

12

Building not

 

Second

10

12

16

Permitted

 

Bottom

12

12

20

 

Notes–1. The diameters given above are for H.S.D. bars. For mild-steel plain bars, use equivalent diameters as given under Table 6 Note2.

2.  The vertical bars will be covered with concrete M15 or mortar 1:3 grade in suitably created pockets around the bars (see Fig. 12). This will ensure their safety from corrosion and good bond with masonry.

3.  In case of floors/roofs with small precast components, also refer 9.2.3 for floor/roof band details.

8.4.8.1 The vertical reinforcement shall be properly embedded in the plinth masonry of foundations and roof slab or roof band so as to develop its tensile strength in bond. It shall be passing through the lintel bands and floor slabs or floor level bands in all storeys.

Bars in different storeys may be welded (see IS: 2751-1979 and IS: 9417-1989, as relevant) or suitably lapped.

Note – Typical details of providing vertical steel in brick work masonry with rectangular solid units at corners and T-junctions are shown in Fig.12

8.4.9 Vertical reinforcement at jambs of window and door openings shall be provided as per Table 7. It may start from foundation of floor and terminate in lintel band (see Fig. 8).

8.5 Framing of thin load bearing walls (see fig.13) - Load bearing walls can be made thinner than 200 mm say 150 mm inclusive of plastering on both sides. Reinforced concrete framing columns and collar beams will be necessary to be constructed to have full bond with the walls. Columns are to be located at all corners and junctions of walls and spaced not more than 1.5 m apart but so located as to frame up the doors and windows.

Fig. 12 Typical details of providing vertical steel bar in brick masonry

1 – One brick length, ½ - Half brick length V – Vertical steel bar with mortar/concrete filling in pocket.

(a) and (b) – Alternate courses in one brick wall. (c) and (d) Alternate courses at corner junction of 1 ½ brick wall. (e) and (f) Alternate courses at T-junction of 1 ½ brick wall.

1. Window    3.  Brick panel

2. Door         4.  Lintel band

Fig. 13 framing of thin load-bearing brick walls

1.Window, 2. Door, 3. Brick panel, 4. Lintel band

Fig. 13 Framing of thin load-bearing brick walls

The horizontal bands or ring beams are located at all floors roof as well as lintel levels of the openings. The sequences of construction between walls and columns will be first to build the wall up to 4 to 6 courses height leaving toothed gaps (tooth projection being about 40 mm only) for the columns and second to pour M15 (1: 2: 4) concrete to fill the columns against the walls using wood forms only on two sides. The column steel should be accurately held in position all along. The band concrete should be cast on the wall masonry directly so as to develop full bond with it.

Such construction may be limited to only two storeys maximum in view of its vertical load carrying capacity. The horizontal length of walls between cross walls shall be restricted to 7 m and the storey height to 3 m.

8.6 Reinforcing details for hollow block masonry - The following details may be followed in placing the horizontal and vertical steel in hollow block masonry using cement-sand or cement-concrete blocks.

8.6.1 Horizontal band - U-shaped blocks may be used for construction of horizontal bands at various levels of the storeys as shown in Fig. 14, where the amount of horizontal reinforcement shall be taken 25 percent more than that given in Table 6 and provided by using four bars and 6 mm dia stirrups. Other continuity details shall be followed, as shown in Fig. 11.

8.6.2 Vertical reinforcement - Bars, as specified in Table 7 shall be located inside the cavities of the hollow blocks, on bar in each cavity (see Fig. 15). Where more than one bar is planned these can be located in two or three consecutive cavities. The cavities containing bars are to be filled by using micro-concrete 1: 2: 3 or cement-coarse sand mortar 1: 3, and properly rodded for compaction. The vertical bars should be spliced by welding or overlapping for developing full tensile strength. For proper bonding, the overlapped bars should be tied together by winding the binding wire over the lapped length. To reduce the number of overlaps, the blocks may be made U-shaped as shown in Fig 15 which will avoid lifting and threading of bars into the hollows.

9 Floors/roofs with small precast components

9.1 Types of precast floors/roofs - Earthquake resistance measures for floors and roofs with small precast components, as covered in this standard, have been dealt with as typical examples.

9.1.1 Precast reinforced concrete unit roof/floor - The unit is a precast reinforced concrete component, channel (inverted trough) shaped in section (see Fig.16). The nominal width of the unit varies from 300 to 600 mm, its height from 150 to 200 mm and a minimum flange thickness of 30 mm. Length of unit shall vary according to room dimensions, but the maximum length is restricted to 4-2 m from stiffness considerations. Horizontal corrugations are provided on the two longitudinal faces of the units so that the structural roof/floor acts monolithic after concrete grouted in the joints between the units attains strength (see Fig. 17)

Fig. 14 U-blocks for horizontal bands

Fig. 15 Vertical reinforcement in cavities

Fig. 16 Channel units

9.1.2 Precast reinforcement concrete cored unit roof/ floor - The unit is a reinforced concrete component having a nominal width of 300 to 600 mm and thickness of 130 to 150 mm having two circular hollows 90 mm diameter, throughout the length of the unit (see Fig.18). The minimum flange/web thickness of the unit shall be 20 mm. Length of unit varies according to room dimensions, but the maximum length shall be restricted to 4.2 m from stiffness considerations. Horizontal corrugations are provided on the two longitudinal faces of the units so that the structural roof/ floor acts monolithic after concrete grouted in the joints between the units attains strength (see Fig.19.).

Fig. 17 Channel unit floor

9.1.3 Precast reinforced concrete plank and joints scheme for roof/floor - The scheme consists of precast reinforced concrete planks supported on partially precast reinforced concrete joints. The reinforced concrete planks are 300 mm wide and the length varies according to the spacing of the joists, but it shall not exceed 1.5 m (see Fig.20). To provide monolithicity to the roof/floor and to have T-beam effect with the joints, the planks shall be made partially 30 mm thick and the partially 60 mm thick and in-situ concrete shall be filled in the depressed portions to complete the roof/floor structurally (see Fig.21).

Fig. 18 Core units

9.1.4 Prefabricated brick panel system for roof / floor - It consists of prefabricated reinforced brick panels (see Fig. 22) supported on precast reinforced concrete joists with nominal reinforced 35 mm thick structural deck concrete over the brick panel and joists (see Fig.23). The width of the brick panels shall be 530 mm for panels made of bricks of conventional size and 450 mm for panels made of bricks of modules size. The thickness of the panels shall be 75 mm or 90 mm respectively depending upon whether conventional or modular bricks are used. The length of the panels shall vary depending upon the spacing of the joists, but the maximum length shall not exceed 1.2 m.

9.1.5 Precast reinforced concrete waffle unit roof / floor - Waffle units are of the shape of inverted troughs, square or rectangular in plan, havi8ng lateral dimensions upto 1.2 m and depth depending upon the span of the roof/floor to be covered (see Fig.24 and 25). The minimum thickness of flange /web shall be 35 mm. Horizontal projections may be provided on all the four external faces of the unit and the unit shall be so shaped that it shall act monolithic with in-situ concrete to ensure load transfer Vertical castallations, called shear keys, shall be provided on all the four external faces of the precast units to enable them to transfer horizontal shear force from one units to adjacent unit through in-situ concrete filled in the joints between the units. The waffle units shall be laid in a grid pattern with gaps between two adjacent units, and reinforcement, as per design, and structural concrete shall be provided in the gaps between the units in both the directions. The scheme is suitable for two way spanning roofs and floors of buildings having large spans.

Fig. 19 Cored, unit floor

9.2 Seismic resistance measures

9.2.1 All floors and roofs to be constructed with small precast components shall be strengthened as specified for various categories of buildings in Table 8. The strengthening measures are detailed in 9.2.3 and 9.2.8.

Fig. 20 Precast reinforced concrete plank

9.2.2. Vertical castallations, called shear keys, shall be provided on the longitudinal faces of the channel, cored and waffle units to enable them to transfer horizontal shear force from one unit to the adjacent unit through the in-situ concrete filled in the joints between the units. The minimum percentage of area of shear keys as calculated below, on each face of the unit, shall be fifteen.

Fig. 21 Precast reinforced concrete plank floor

Shear keys shall have a minimum width of 40 mm at its root with the body of the components and shall be to the full height of the component and preferably at uniform spacing. Percentage of area of shear keys shall be calculated as:

Fig. 22 Prefab brick panel

9.2.3 Tie beam (a in Table 8) is a beam provided all round the floor or roof to be bind together all the precast components to make it a diaphragm. The beams shall be top the full width of the supporting wall or beam less the bearing of the precast components. The depth of the beam shall be equal to the depth of the precast components plus the thickness of structural deck concrete, where used over the components. The beam shall be made of cement concrete of grade not leaner than M15 and shall be reinforced as indicated in Table 6. If depth of tie is more than 75 mm, equivalent reinforcement shall be provided with one bar of minimum diameter 8 mm at each corner. Tie beams shall be provided on all longitudinal and cross walls. Typical details of the beams are shown Fig.26 to 30.

Fig. 23 Brick panel floor

Table 8 Strengthening measures for floors /roofs with small precast components

(Clauses 9.2.1, 9.2.3, 9.2.4.9.2.5, 9.2.6, 9.2.7 and 9.2.8)

Building category

No.of storeys

Strengthening to be provided in floor/roof with

Channel/

Cored

Units

R.C planks

And joists

Brick panels

And joists

Waffle

units

(1)

(2)

(3)

(4)

(5)

(6)

A

1 to 3

Nil

Nil

Nil

Nil

 

       4

A

a

a

a

B

1 to 3

A

a

a

a

 

        4

a,c

A,c

a,d

a

C

1 & 2

a,b

a

a

a

 

 3 & 4

a,b,c

A,c

a,d

a,e

D

1 to 4

a,b,c

A,c

a,d

A,c,e

E

1 to 3

a,b,c

A,c

a,d

A,c,e

Where, 

a = Tie beam as per 9, 2,3,

b = Reinforcing bars of precast unit and tied to beam reinforcement as per  9.2.4

c = Reinforced deck concrete as per 9.2., 5,

d = Reinforced deck concrete as per 9.2.6, and

e = Reinforcement bars in joint between precast waffle units tied to

Tie beam reinforcement as per 9.2.7.       

Fig. 24 Waffle units

Fig. 25 Wapple unit floor

Fig. 26 Connection of precast cored/channel unit with tie beam

9.2.4 Top reinforcement in the channel or cored units (termed B in Table 8) shall be projected out at both the ends for full anchorage length and tied to tie beam reinforcement.

9.2.5 Structural deck concrete (c in Table 8) of grade not leaner than M15 shall be provided over precast components to act monolithic with wherever, deck concrete is to be provided, the top surface of the components shall be finished rough. Cement slurry with 0.5 kg of cement per sq.m of the surface area shall be applied over the components immediately before laying over the deck concrete and the concrete shall be compacted using plate vibrators. The minimum thickness of deck concrete shall be 35 or 40 mm reinforced with 6 mm dia bars @ 150 mm apart both ways and anchored into the tie beam placed all round. The maximum size of coarse aggregate used in deck concrete shall not exceed 12 mm.

 Fig. 27 Connection of channel/cored unit floor/roof (with deck concrete) with tie beam

Fig. 28 Connection of precast reinforced concrete plank and precast brick panel floor/ Roof (with deck concrete) with tie beam

Fig. 29 Connection of precast waffle unit floor / roof (With deck concrete) with tie beam

Fig. 30 Provision of reinforcement in concrete floor finish

Note – Under conditions of economic constraints, the deck concrete itself could serve as floor finish. The concrete is laid in one operation (see Fig. 30) without joints.            

9.2.6 The deck concrete normally used over the brick panel with joist floor shall be reinforced with 6 mm dia bars spaced 150 mm apart both ways (d in Table. 8)

9.2.7 For floors/roofs with precast waffle units, two 16 mm dia high strength deformed bars be provided as top reinforcement in the joints between waffle units, in addition to reinforcement required for taking bending moment for vertical loads. This reinforcement (e in Table 8) shall be fixed to tie beam reinforcement.

9.2.8 In case of floors/roofs with precast components other than those indicated in Table 8, the buildings shall be analyzed for maximum expected seismic forces and the floor/roof shall be designed to act as diaphragm and take care of the resulting forces.

10 Timber construction

10.1 Timber has higher strength per unit weight and is, therefore, very suitable for earthquake resistant construction. Materials, design and construction in timber shall generally conform to IS: 883-1992.

10.2 Timber construction shall generally be restricted to two storeys with or without the attic floor.

10.3 In timber construction attention shall be paid to fire safety against electric short-circuiting, kitchen fire, etc.

10.4 The superstructure of timber buildings shall be made rigid against deformations by adopting suitable construction details at the junctions of the framing members and in wall panels as given in 10.6 to 10.10 so that the construction as a whole behaves as one unit against earthquake forces.

10.5 Foundations

10.5.1 Timber construction shall preferably start above the plinth level, the portion below being in masonry or concrete.

10.5.2 The superstructure may be connected with the foundation in one of the two ways as given in 10.5.2.1 to 10.5.2.2.

10.5.2.1 The superstructure may simply rest on the plinth masonry, or in the case of small buildings of one storey having plan area less than about 50 m², it may rest on firm plane ground so that the building is free to slide laterally during ground motion.

Notes-1. Past experience has shown that superstructure of the buildings not fixed with the foundation escaped collapse even in a severe earthquake although they were shifted sideways.

2.  Where fittings for water supply or water borne sanitation from the house are to be installed, proper attention should be given to permit movement so as to avoid fracture or damage to pipes.

10.5.2.2 The superstructure may be rigidly fixed into the plinth masonry or concrete foundation as given in Fig. 31 or in case of small building having plan area less than 50 m², it may be fixed to vertical poles embedded into the ground. In each case the building is likely to move along with its foundation. Therefore, the superstructure shall be designed to carry the resulting earthquake shears.

10.6 Types of framing - The types of construction usually adopted in timber buildings are as follows;

  1. Stud wall construction, and
  2. Brick nogged timber frame construction.

Fig. 31 Details of connection of column with foundation

Fig. 32-A Stud wall construction

10.7 Stud wall construction

10.7.1 The stud wall construction consists of timber studs and corner posts framed into sills, top plates and wall plates. Horizontal struts and diagonal braces are used to stiffen the frame against lateral loads. The wall covering may consist of EKRA, timber or like. Typical details of stud walls are shown in Fig. 32. Minimum sizes and spacing of various members used are specified in 10.7.2 to 10.7.10

10.7.2 The timber studs for use in load bearing walls shall have a minimum finished size of 40 X 90 mm and their spacing shall not exceed those given in Table 9.

Fig. 32 B Stud wall construction

10.7.3 The timber studs in non-load bearing walls shall not be less than 40 X 70 mm in finished cross section. Their spacing shall not exceed 1 m.

10.7.4 There shall be at least one diagonal braces for every 1.6 X 1 m area of load bearing walls. Their minimum finished sizes shall be in accordance with Table 10.

10.7.5 The horizontal struts shall be spaced not more than one metre apart. They will have a minimum size of 30 X 40 mm for all locations.

10.7.6 The finished sizes of the sill, the wall plate and top plate shall not be less than the size of the studs used in the wall.

10.7.7 The corner posts shall consists of three timbers, two being equal in size to the studs used in the walls meeting at the corner and the third timber being of a size to fit so as to make a rectangular section (see Fig.32).

10.7.8 The diagonal braces shall be connected at their ends with the stud wall members by means of wire nails having 6 gauge (4.88 mm dia) and 10 cm length. Their minimum number shall be 4 nails for 20 mm X 40 mm braces and 6 nails for 30 mm X 40 mm braces. The for end of nails may be clutched as far as possible.

10.7.9 Horizontal bracing shall be provided at corners or T-junctions of walls at sill, first floor and eave levels. The bracing members shall have a minimum finished size of 20 mm X 90 mm and shall be connected by means of wire nails to the wall plates at a distance between 1.2 m and 1.8 m measured from the junction of the walls. There shall be a minimum number of six nails of 6 gauge (4.88 mm dia) and 10 cm length with clutching as far ends.

10.7.10 Unsheathed studding shall not be used adjacent to the wall of another building. The studding must be sheathed with close jointed 20 mm or thicker boards.

10.8 Brick nogged timber frame construction

10.8.1 The brick nogged timber frame consists of intermediate verticals, columns, sills, wall plates, horizontal nogging members and diagonal braces framed into each other and the space between framing members filled with tight-fitting brick masonry in stretcher bond. Typical details of brick nogged timber frame construction are shown in Fig.33. minimum sizes and spacing of various elements used are specified in 10.8.2 to 10.8.9.

10.8.2 The vertical framing members in brick nogged load bearing walls will have minimum finished sizes as specified in Table.10.

10.8.3 The minimum finished size of the vertical members in non-load bearing walls shall be 40mm X 100mm-spaced not more than 1.5m apart.

Fig. 33 Bricknogged timber frame construction

10.8.4 The sizes of diagonal bracing members shall be the same as in Table 10.

10.8.5 The horizontal framing members in brick nogged construction shall be spaced not more than 1m apart. Their minimum finished sizes shall be in accordance with Table 12.

10.8.6 The finished sizes of the sill, wall plate and top plate shall be not less than the size of the vertical members used in the wall.

10.8.7 Corner posts shall consist of three vertical timbers as described in 10.7.7.

10.8.8 The diagonal braces shall be connected at their ends with the other members of the wall by means of wire nails as specified in 10.7.8.

10.8.9 Horizontal bracing members at corners or T-junctions of wall shall be as specified in 10.7.9.

Table 9 Maximum spacing of 40mm x 90mm finished size studs in stud wall construction(Clause 10.7.2)

Group of Timber

(Grade I*)

Single storied or first floor

Of the double storeyed buildings

Ground floor of double storeyed buildings

 

Exterior

Wall

Interior

Wall

Exterior

wall

Interior

wall

(1)

(2)

cm

(3)

cm

(4)

cm

(5)

cm

Group A,B

100

80

50

40

Group C

100

100

50

50

*Grade I timbers as defined in Table 5 of IS:  883-1992.  

Table 10 Minimum finished sizes of diagonal braces (Clauses 10.7.4 and 10.8.4)

Building category

(see Table 2)

Group of timber

(Grade 1*)

Single storied or First floor of double storied

Buildings

Ground floor

of double storied

buildings

Exterior

wall

Interior

wall

Exterior

wall

Interior

wall

(1)

(2)

(3)

(4)

(5)

(6)

 

 

Mm x mm

Mm x mm

mm x mm

mm x mm

A,B,C

All

20 x 40

20 x 40

20 x 40

20 x 40

D and E

Group A

and B

20 x 40

20 x 40

20 x 40

30 x 40

Group C

Group C

20 x 40

30 x 40

30 x 40

30 x 40

* Grade I timber defined in Table 5 of IS: 883-1992.

Table 11 Minimum finished sizes of vertical in brick nogged timber frame construction

(Clause 10.8.2)Spacing

Group of timber

(Grade I*)

Single storied or first floor orf double storied buildings

Ground floor of double storied buildings

Exterior wall

Interior wall

Exterior wall

Interior wall

(1)

(2)

(3)

(4)

(5)

(6)

M

 

mm x mm

mm x mm

mm x mm

mm x mm

1

Group A,B

50 x 100

50 x 100

50 x 100

50 x 100

 

Group C

50 x 100

70 x 100

70 x 100

90 x 100

1.5

Group A,B

50 x 100

70 x 100

70 x 100

80 x 100

 

Group C

70 x 100

80 x 100

80 x 100

100 x 100

 * Grade I timbers as defined in Table 5 of IS: 883-1992

Table 12 Minimum finished size of horizontal nogging members (Clause 10.8.5)

Spacing of verticals

Size

(1)

(2)

m

m

1.5

70 x 100

1

50 x 100

0.5

25 x 100

10.9 Notching and cutting

10.9.1 Timber framing frequently requires nothing and cutting of the vertical members. The notching or cutting should in general be limited to 20 mm in depth unless steel strips are provided to strengthen the notched face of the member. Such steel strips, where necessary, shall be at least 1.5 mm thick and 35 mm wide extending at least 15 cm beyond each side of the notch of cut and attached to the vertical member by means of bolts or screws at each end.

10.9.2 The top plate, the wall plate or the sill of a wall may be notched or cut, if reinforcing strip of ion is provided as specified in 10.9.1. in case the member is notched or cut not to exceed 40 mm in depth, such reinforcing strip may be placed along the notched edge only. Where the notch or cut is more than 40 mm in depth or the member is completely cut through, such reinforcing strips shall be placed on both edges of the member. The details of notching sand cutting are shown in Fig.34.

10.9.3 Joints timber shall preferably be bound by metallic fasteners.

Fig. 34 Notching and cutting

10.10 Bridging and blocking - All wooden joists shall have at least one row of cross bridging for every 3.5 m length of span. The cross section of the bridging member shall be a minimum of 40 X 70 mm and the member shall be screwed or nailed to the joists.

All spaces between joists shall be blocked at all bearing with solid blocks not less than 40 mm thick and the full depth of the joists. The block shall be screwed or nailed to the joists as well as to the bearings.

* * * *