EARTHQUAKE RESISTANT -2



EARTHQUAKE RESISTANT BUILDING WORKS

Annexure 26-A.4

GUIDELINES TO IMPROVE EARTHQUAKE RESISTANCE OF EARTHEN BUILDINGS

(Extract of IS: 13827-1993)

1 Scope

The guidelines covered in this standard deal with the design and construction aspects for improving earthquake resistance of earthen houses, without the use of stabilizers, such as cement, lime, asphalt, admixtures, etc. The provisions of this standard are applicable for seismic zones III, IV and V. no special provisions are considered necessary in zones I and II (see IS: 1893-1984 for seismic zones).

Notes-1 Earthen buildings are inherently weak against water and earthquakes, and should preferably be avoided in flood prone, high rainfall areas and seismic zones IV and V

Attention is hereby drawn to the fact that earthen construction as dealt with herein will neither qualify as engineered construction nor totally free from collapse in severe seismic intensities VIII and IX on MMI1) scale. However, inclusion of special design and construction features as recommended in this standard will raise their weather and seismic resistance appreciably reducing greatly the chances of collapse even in such seismic intensities.

2 References - The following Indian Standards are the necessary adjuncts to this standard:

IS No. Title
883-1993 Code of practice for design of structural timber in building (fourth revision)
1893-1984 Criteria for earthquake resistance design of structures
2720-1980 (Part 7) Methods of test for soils. Part 7 Determination of water content – dry density relation using light compaction (second revision)

3 Terminology - For the purposes of this standard, the definitions given in Section 26 shall apply.

4 General considerations

4.1 For the safety of earthen houses, appropriate precautions must be taken against the actions of rain and flood waters and earthquakes. Minimum precautions are recommended in this standard.

4.2 Whereas dry clay block is hard and strong in compression and shear, water penetration will make it soft and weak, the reduction in strength could be as high as 80 to 90 percent. Hence, once built, ingress of moisture in the walls must be prevented by the protection, roof projection and waterproof of mud plastering.

4.3 These recommendations are low-cost and do not include the use of stabilizers, which are rather costly though effective in increasing the strength and water-resistance of the clay units or walls. Where feasible time-stabilized compacted clay blocks or cement-stabilized sandy soil blocks may be used with compatible stronger mortars.

4.4 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 of buildings should, in particular, be made of lightweight type.

4.5 Height - Experience in intensity areas of VIII has shown the high vulnerability of two-storeyed houses, hence only one storey construction should preferably be adopted in seismic zone IV and V.

4.6 Shape of building - For better earthquake resistance, the building should have a simple rectangular plan and be symmetrical, as far as possible about both the axes. The load bearing walls should run continuously in both directions. Large houses may have an inner courtyard for light and ventilation with proper drainage outlets, instead of having projections giving rise to L, T shape plans.

5 Construction of earthen walls - Earthen walls may be constructed in the following four ways.

5.1 Hand-formed in layers using mud-lumps to form walls.

5.2 Built by using sun-dried blocks or adobe which may be cut from hardened soil, or formed in moulds, or moulded and compacted and laid in courses using clay mud as mortar.

5.3 Built by using rammed earth in which moist soil is filled between wall forms and compacted manually or mechanically.

5.4 Constructed using wood, bamboo or cane structure with wood, bamboo, cane or ikra mesh enclosures plastered with mud..

5.5 Whereas system 5.1, 5.2 and 5.3 depend on the strength of earthen walls for stability, the system 5.4 behaves like wood frame and its construction has been dealt with under clause 12.

6 Suitability of soil - The following qualitative tests may be used for determining the stability of a soil for earthen construction.

6.1 Dry strength test - Five or six small balls of soil of approximately 2 cm in diameter are made. Once they are dry (after 48 hours), each ball is crushed between the forefinger and the thumb. If they are strong enough that none of them breaks, the soil has enough clay to be used in the adobe construction, provided that some control over the mortar micro-fissures caused by the drying process is exercised.

Note – If some of the balls break, the soil is not considered to be adequate, because it does not have enough clay and should be discarded.

6.2 Fissure control test - At least eight folded units are made with mortars made with mixtures in different proportions of soil and coarse sand. It is recommended that the proportion of soil to coarse sand vary between 1:0 and 1:3 in volume. The unit having the least content of coarse sand which, when opened after 48 hours, does not show visible fissures in the mortar, will indicate the most adequate proportion of soil/sand for adobe constructions, giving the highest strength.

6.3 Strength test of adobe - The strength of adobe may be qualitatively ascertained as follows: After 4 weeks of sun drying the adobe, it should be strong enough to support in bending the weight of a person 60-70 kg.  If it breaks, more clay and fibrous material is required to be added.

6.4 Quantitatively, the compressive strength may be determined by testing 100 mm cubes of clay after completely drying them. A minimum value of 1.2 N/mm² will be desirable.

7. Hand-formed layered construction 

7.1 Walls built by hand forming are the most primitive and weakest of all earthen walls, since enough moisture for full dispersion of clay is horizontally and vertically. Use of straw is recommended in the clay, so as to impart strength and reduction of fissures.

7.2 The quality of construction will improve if the clay-water-straw mixture was allowed to rest for 7 days (minimum 3 days) before use in walls so that thorough dispersion of moisture in clay and decomposition of straw into fibres takes place.

7.3 The area of the lower layer should be moistened well before adding the new layer so as to minimize the horizontal fissures at the joints.

8 Block or Adobe constructions

8.1 Suitable soil should be used for making the blocks, by using uniform size of moulds, after keeping the soil-water mix for 24 hours. The blocks should be allowed to dry out of the moulds so as to allow ‘free’ shrinkage without developing fissures.

8.2 Block sizes are not standardized yet and various sizes are used in the country and the world. The following sizes of blocks are recommended for making 380 mm thick walls:

Rectangular:  380 mm X 250 mm X 110 mm

(Overlap of about 125 mm)

Square: 380 mm X 380 mm X 110 mm

(Overlap of about 190 mm)

8.2.1     The square type will be better for stronger construction in view of less vertical joints between units and better breaking of vertical joints.

8.3 The mud ‘mortar’ used to join the blocks together should be the same soil as used in making blocks. However, to make it non shrinking, straw in to ratio 1:1, by volume, should be mixed. The wet mix should be allowed to rest for 7 days (minimum 3 days) before use. The lower layer of adobes should be moistened before the ‘mortar’ is laid. Also, the surface of the adobes to be laid should be moistened for a few minutes before the adobe is laid. If the mortar is seen to fissure on drying some sand could be added to the mixture, as indicated by the ‘fissure control test’ in 6.2.

8.4 The usual good principles of bonds in masonry should be adopted for construction of adobe walls, that is:

  1. all courses should be laid level,
  2. the vertical joints should be broken between the consecutive courses by overlap of adobes and should be fully filled with mortar, and
  3. the perpendicular joints between walls should be made in such a way that through vertical joint is avoided.

9 Rammed earth constructions - Rammed earth construction is also known as ‘Pise’ or ‘Tapial’ construction in some countries.

9.1 To construct walls, in this method, most soil is poured in long wooden forms of the walls and compacted to achieve the desired density. The soil suitable for rammed earth construction will generally have less clay than that used for making adobes. The moisture content should be kept less but close to optimum moisture content determined by Proctor Compaction Test [see IS: 2740 (Part 7)-1980].

9.2 To control shrinkage fissure on drying, prior testing may be required for determining the quantity of sand to be added to the clayey soil, based on the moisture, the layering and the amount of compaction to be used in the construction.

9.3 The soil should be placed in layers of about 100 mm thickness and fully compacted, then water should be sprinkled on the compacted layer before placing the next layer of 100 mm. The total height of this block achieved this way may be kept 500 to 800 mm. Before starting the new block, sufficient water should be poured on the completed layer to ensure its connection with the new layer.

9.4 Higher compaction leads to higher strength but up to a limit only. Compaction should be standardized. The following procedure is recommended:

‘50 strokes per 1000 cm² of wall area using a wooden mallet of about 8 to 10 kg weight.’

9.5. Small amount of straw, in the ratio of not more than one-fourth of the volume of soil water mixture, may be used in the soil for fissure control.

10 Recommendations for seismic areas

10.1 Walls

The height of the adobe building should be restricted to one storey plus attic only in seismic zones V and IV and to two storeys in zone III. Important building (I > 1.5) should not be constructed with earthen walls in seismic zones IV and V and restricted to only one storey in seismic zone III.

The length of a wall, between two consecutive walls at right angles to it, should not be greater than 10 times the wall thickness t nor greater than 64 t²/h where h is the height of wall..

When a longer wall is required, the walls should be strengthened by intermediate vertical buttresses.

The height of wall should not be greater than 8 times its thickness..

The width of an opening should not be greater than 1.20 m..

The distance between an outside corner and the opening should be not less than 1.20m.

The sum of the widths of openings in a wall should not exceed 33¯ percent the total wall length in seismic zone V and 40 percent in zones IV and III.

The bearing length (embedded) of lintels on each side of an opening should not be less than 300 mm. For an adequate configuration for an earthen house, see 10.5

Hand-formed walls could preferably be made tapering upwards keeping the minimum thickness 300 mm at top and increasing it with a batter of 1:12 at bottom..

Providing outside pilasters at all corners and junctions of walls are recommended as these increase the seismic stability of the building a great deal..

Special seismic strengthening features may be done as specified in 11.

10.2 House site

Sites with sandy loose soils, poorly compacted clays, and fill materials should generally be discarded due to their excessive settlements during seismic vibrations. Also, sites with very high water table should be avoided. These recommendations are particularly important for seismic zones V and IV

Sites shall be above high flood level or the ground shall be raised to this effect.

10.3 Foundation

10.3.1 Width of strip footings of the walls may be kept as follows:

i) One storey on firm soil - Equal to wall thickness

ii) 1.5 or 2 storeys on firm soil - 1.5 times the thickness

iii) One storey on soft soil - 1.5 times the wall thickness

iv) 1.5 or 2 storeys on soft soil - 2 times the wall thickness

10.3.2   The depth of foundation below existing ground level should at least be 400 mm.

10.3.3 The following should preferably be built by using stone, fired brick using cement or lime mortar. Alternatively, it may be made in lean cement concrete with plums (cement: sand: gravel: stones as 1:4.6:10) or without plums as 1:5:10. Lime could be used in place of cement in the ratio lime: sand: gravel as 1:4:8.

10.3.4 Plinth masonry - The wall above foundation up to plinth level should preferably be constructed using stone or burnt bricks laid in cement or lime mortar. Clay mud mortar may be used only as a last resort.

The height of plinth should be above the flood water line or a minimum of 300 mm above ground level. It will be preferably to use a waterproofing layer in the form of waterproof mud (see 13.3) or heavy blank polythene or polyethylene sheet at the plinth level before starting the construction of super structure wall. If adobe itself is use for plinth construction, the outside face of plinth should be protected against damage by water by suitable fascia or plaster. A water drain should be made slightly away from the wall to save it from seepage.

10.4 Roof

The roofing structure must be light, well connected and adequately tied to the walls. Trusses are superior to sloping roofs consisting of only rafters or frames.                  

The roof covering should preferably be of light material, like sheeting of any type. Heavy roofs consisting of wood joists and earth topping are dangerous and should not be used in zones V and IV. Tiled and slate roofs are also heavier and shall be avoided in zones V and IV.

If thatch is used for roof covering, it should better be made waterproof and fire resistant by applying waterproof mud plaster (see 13.3).

The proof beams, rafters or trusses should preferably the rested on longitudinal wooden elements for distributing the load on walls.

The slopes and the overhanging will depend on local climate conditions. In zones subjected to rain and snow, wall protection must be ensured by protection the roof by about 500 mm beyond the walls.

The roof beams or rafters should be located to avoid their position above door or window lintels. Otherwise, the lintel should be reinforced by an additional lumber..

10.5 Adequate configuration - Summarizing most of the recommendations contained in this standard a configuration which will, in general, be adequate for seismic areas including zone V and IV. Additional seismic strengthening features are presented in clause 12.

11 Seismic strengthening of bearing wall buildings

11.1 Collar beam or horizontal band - Two horizontal continuous reinforcing and binding beams or bands should be placed, one coinciding with lintels of door and window opening, and the other just below the roof in all walls in seismic zones III, IV and V. Proper connection of ties placed at right angles at the corners and junctions of walls should be ensured. Where the height of wall is not more than 2.5 m, the lintel band can be avoided, but the lintels should be connected to the roof band (see 11.2). The bands could be in the following forms:

  1. Unfinished rough cut or sawn (70 X 150 mm in section) lumber in single pieces provided diagonal members for bracing at corners.
  2. Unfinished rough cut or sawn (50 X 100 mm or 70 X 70 mm in section) lumber two pieces in parallel with halved joints at corners and junctions of walls placed in parallel.

In each case, the lengthening joint in the elements shall be made using iron-straps with sufficient nails/screws to ensure the strength of the original lumber at the joint.

11.2 Pilasters and buttresses - Where pilasters or buttresses are used, as recommended earlier at corner or T-junctions, the collar beam should cover the buttresses as well, as shown in Fig. 10. Use of diagonal struts at corners will further stiffen the collar beam.

11.3 Vertical reinforcement in walls - In seismic zone V, mesh form of reinforcing is recommended. Here the whole walls are reinforced by a mesh of canes or bamboos as shown in Fig. 11 along with the collar beams which may in this case be made from canes or bamboos themselves. The vertical canes must be tied to the horizontal canes as well as the collar beam at lintel and the roof beam at eave level (see 11.1)

12 Earthen constructions with wood or cane structures

12.1 The scheme of earthen construction using structural framework of wood or cane, consists of vertical posts and horizontal blocking members of wood or large diameter canes or bamboo, the panels being filled with cane, bamboo or some kind of reed matting plastered over both sides with mud. The construction could be done in situ, building element-by-element or by using prefabricated panels.

12.2 For the satisfactory behavior of this type of this type of construction the following fundamental rules given in 12.2.1 to 12.2.6, should be observed.

12.2.1 Good connection between the wood or cane elements, so as to ensure an integral behavior of the structure. The connections are normally fixed with nails. Their number and dimensions should be enough but not excessive so as to split the elements. The connections can also be tied with wires, ropes, leather straps, etc.

12.2.2  Preservation of the wood or cane elements by charring the surface or painting with coal tar, especially in the part embedded in the foundation, which should preferably be of concrete, stone or bricks laid with cement, lime or gypsum mortar.

12.2.3 In houses, built as a continuous system as in those made with pre-fabricated panels, an upper ring beam should be placed to ensure the integral behavior of all walls, and to distribute evenly the roofing load (see 11.1)

12.2.4 The panel filling material should consist of wood or cane mesh, over which a layer of mud and straw (1:1 in volume) is placed on each face in the form of plaster. Very often, the meshes are knit in themselves and around the structure.

12.2.5 The mud filling should be placed only after fixing this upper ring beam and the roof (after completing the nailing). This will avoid fissuring caused by the strokes of the nailing operation.

12.2.6 In the case of pre-fabricated panels, the frames could have economical sections 25 X 50 mm or 25 X 75 mm or larger. The connection between panels is made through nails, but the wood or cane knit mesh over which the mud filling is placed may be fixed without the use of nails.

12.3 Bracing and braced frames - For achieving adequate seismic resistance in zones V and IV, it will be desirable to provide diagonal bracing members in the planes of walls as well as horizontally at the top level of walls. This can be done by using canes or bamboos nailed to the framing members at the ends and intermediate points of intersection, before fixing the panel meshes and applying plaster to them..

Schemes for providing internal bracing systems in earthen houses, holdfast to the walls and other alternatives are explained in Annex A

13 Plastering and painting - The purpose of plastering and painting is to give protection and durability to the walls and thatch roof, in addition to obvious aesthetic reasons.

13.1 In dry areas, plastering based on natural additives could be formed in two layers. The first one of about 12 to 15 mm, is a mixture of mud and straw (1 : 1 in volume), plus a natural additive like cow dung used to increase the moisture resistance of the mud, thus preventing the occurrence of fissures during the drying process. The second and last layer is made with fine mud which when dried, should be rubbed with small, hard, rounded pebbles.

13.2 In wet areas, the walls should be covered with waterproof mud plaster. To obtain this, the following procedure may be followed:

‘Cut-back should be prepared by mixing bitumen 80/100 grade and kerosene oil in the ratio 5:1. For 1.8 kg cut-back, 1.5 kg bitumen is melted and is poured in a container having 300 milliliters kerosene oil, with constant stirring, till complete mixing. This mixture can now be mixed with 30 litres of mud mortar to make it both, water repellent and fire resistant.’

13.3   For improving water and fire resistance of thatch roof, the water proof plaster may be applied on top surfaces of the thatch, 20 to 25 mm thick, and allowed to dry. It may then be coated twice with a wet mixture of cow dung and waterproof plaster in the ratio of 1:1, and allowed to dry again.

13.4   The exterior of walls after plastering and thatch roof after treatment as explained in 13.3 may be suitably painted using a water-insoluble paint or washed with water solutions of lime or cement or gypsum.

Annexure-A

INTERNAL BRACING IN EARTHEN HOUSES

(Clause 12.3)

A-1       Internal bracing system

A-1.1    Earthen houses are intrinsically very weak under lateral load, hence require very special techniques to make them collapse proof in seismic intensities VIII and IX areas such as vertical tension members as well as diagonal braces. Calculations for single storied buildings with flat heavy flexible roofs (for example, wooden beams with clay topping) show that even the soft timbers (Group C in IS: 883-1993) when suitably framed using nail joints can serve the purpose of holding the roof in place in the event when weak walls give way partially. The frames will also restrain the walls from disintegrating completely.

A-1.2    In using the method described in A-1.1, the following three systems can be adopted:

  1. System A – The whole building plan may be framed as one piece and the external walls built keeping the wooden frame as the inner face of external walls and the internal walls built keeping the frame on one of its faces (preferably on the bed room side). Such a frame will have the advantage of redundancy, and use of less number of columns. But the frame can be subjected to torsional stress under the earthquake motions.
  2. System B – Each room may be framed individually, thus the external walls will have the frame only on their inner face, the internal walls will have the frames on both faces, preventing the fall of the inner wall either way. This system will have the advantage of permitting any plan shape without the problem of torsion of frames and much greater safety of cross walls. It will, however, consume more timber since all frames on the inner walls will be doubled.
  3. System C – In the third system, the frames of system B may be joined across walls making it a stronger whole building frame. Such a system will have the advantages of both A and B systems and can be adopted for the more important buildings such as those built for community services.

As a general guidance, system A may be adopted for near symmetrical plans and system B for general unsymmetrical plans.

A-2       Holdfasts to the walls - The earthen walls may be kept no more than 400 mm thick. To improve their behavior, steel holdfasts of Z-shape may be screwed to the wooden posts at least one for each triangle and be built into the cladding earthen wall.

A-3       Other alternatives and applications

A-3.1    As an alternative to wooden frames. Steel pipe or angle iron frames of equal strength may be used.

A-3.2    The internal bracing system will also be appropriately suitable for the seismic safety of random rubble or brick work in mud mortar constructions.

A-3.3    Such frames could also be inserted in existing low strength masonry houses for retrofitting them against collapse in future earthquakes.

Annexure 21-A.5

SECTION 21

GUIDELINES TO IMPROVE EARTHQUAKE RESISTANCE OF LOW STRENGTH MASONRY BUILDINGS

(Extract of IS: 13828-1993)

1 Scope

This standard covers the special features of design and construction for improving earthquake resistance of buildings of low-strength masonry. The provision of this standard are applicable in seismic zones III to V. No special provisions are considered necessary for buildings in seismic zones I and II.

The various provisions of IS: 4326-1993 regarding general principles, special construction features, types of construction, categories of buildings and masonry construction with rectangular masonry units are generally applicable to the masonry buildings of low strength dealt with in this standard. There are however certain restrictions, exceptions and additional details which are specially included herein. For completeness however all necessary portions are repeated here.

Note – Attention is here drawn to the fact that low-strength masonry as dealt with herein will neither qualify as engineered construction nor totally free from collapse in the severe seismic intensities VIII or IX. However, inclusion of special seismic design and construction features provided herein will raise their seismic resistance appreciably, reducing greatly the chances of collapse even in such seismic intensities.

2 References - The following Indian Standards are necessary adjuncts to this standard:

IS No.  

Title

1597(Part 1)-1967           

Code of practice for construction of masonry : part 1 rubble stone masonry

1893-1984

Criteria for earthquake resistant design of structures(first revision)

1893-1984

Criteria for earthquake resistant design of structures(first revision)

1904-1984

Code of practice for design and construction of foundations in soils: General requirements

4326-1993

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

3 Terminology - For the purpose of this standard, the definitions in Section 26 shall apply.

4 General principles

General - The general principles given in 4.1 to 4.5 should be observed in the construction of buildings for improving their earthquake resistance. –

4.1 Lightness - Since the earthquake force is a function of mass, the building should 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 - As far as possible, all parts of the building should be tied together in such a manner that the building acts as one unit.

For integral action of building, roof and floor slabs should be continuous throughout as far as possible.

Additions and alterations to the structures should be accompanied by the provision of positive measures to establish continuity between the existing and the new construction.

4.3 Projecting and suspended parts

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

Note – In cases where stability of projecting parts against overturning is achieved by counterweight in the form of wall, slab etc, the overturning should be checked by increasing the weight of the projecting part and decreasing the weight of stabilizing mass simultaneously in accordance with the vertical seismic coefficient specified in 4.4.2 of IS:  1893-1984.

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

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

4.4 Shape of building - In order to minimize torsion, 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. It will be desirable to use separate blocks of rectangular shape particularly in seismic zones V and IV.

NOTE – For small buildings, minor asymmetry in plan and elevation may be ignored. Designing such buildings against torsion may be difficult and uncertain.

4.5 Fire safety - Fire frequently follows an earthquake and therefore buildings should be constructed to make them fire resistant in accordance with the provisions of relevant Indian Standards for fire safety.

5 Special construction features

5.1. Foundations

5.1.1     For the design of foundations, the provisions of IS: 1904-1986 in conjunction with IS:  1893-1984 shall generally be followed.

5.1.2 The sub grade below the entire area of the building should preferably be of the same type of the soil. Wherever this is not possible, the buildings should preferably be separated into units and then the units should be located separately.

5.1.3 Loose fine sand soft silt and expansive clays should be avoided. If unavoidable the following measures may be taken to improve the soil on which the foundation of the building may rest:

(1). Sand piling/under reamed piling/stone columns, etc.

(2). Soil stabilization.

5.2 Roofs and floors

5.2.1 Flat roof or floor should not preferably be made of tiles or 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 or 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.

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

5.2.2 Pent roofs

All roof trusses should be supported on and fixed to timber band reinforced concrete band or reinforced brick band. The holding down bolts should have adequate length as required for earthquake and wind forces.

Where a trusses roof adjoins a masonry gable, the ends of the purlins should be carried on and secured to a plate or bearer which should be adequately bolted to timber reinforced concrete or reinforced brick band at the top of gable end masonry.

At tie level, all the trusses and the gable end should 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.

5.2.3 Jack arches

Jack arched roofs or floors where used should be provided with mild steel ties in all spans along with diagonal braces in plan to ensure diaphragm actions.

5.3 Staircases

5.3.1 The interconnection of the stairs with the adjacent floors should be appropriately treated by providing sliding joints at the stairs to eliminate their bracing effect on the floors. Ladders may be made fixed at one end and freely resting at the other.

5.3.2 Built-in Staircase - When stairs are built monolithically with floors, they can be protected against damage by providing rigid walls at the stair opening. The walls enclosing the staircase should extend through the entire height of the stairs and to the building foundations.

6 Box type construction - This type of construction consists of prefabricated or in-situ masonry wall along with 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 wall construction, attention should be paid to the connections between wall panels so that transfer of shear between them is ensured.

7 Categories of buildings - For the purpose of specifying the earthquake resisting features, the buildings, have been categorized in five categories A to E, as given in Table 1, based on the value of ah, given by:

ah = a0Ib

Where

ah = design seismic coefficient for the building,

a0 = basic seismic coefficient for the seismic zone in which the building is located

(See Table 2 of IS: 1893-1984),

I = importance factor applicable to the building (see 3.4.2.3 of IS: 1893-1984),

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

8 Low strength masonry constructions

8.1 General

Two types of construction are included herein, namely: Brick construction using weak mortar, and random rubble and half-dressed stone masonry construction using different mortars such as clay mud lime-sand and cement sand.

Table 1 Building categories for earthquake resisting features (Clause 7)

Category

Range of ah    

A

0.04 to less than 0.05

B

0.05 TO 0.06 (BOTH INCLUSIVE)

C

More than 0.06 but less than 0.08

D

0.08 to less than 0.12

E

More than 0.12

These constructions should not be permitted for important buildings with I ³ 1.5 and should preferably be avoided for building category D (see Table 1).

It will be useful to provide damp-proof course at plinth level to stop the rise of pore water into the superstructure.

Precautions should be taken to keep the rain water away from soaking into the wall so that the mortar is not softened due to wetness. An effective way is to take out roof projections beyond the walls by about 500 mm.

Use of a water-proof plaster on outside face of walls will enhance the life of the building and maintain its strength at the time of earthquake as well.

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

8.2 Brickwork in weak mortars

8.2.1 The fired bricks should have a compressive strength not less than 3.5 MPa. Strength of bricks and wall thickness should be selected for the total building height.

8.2.2 The mortar should be lime-sand (1:3) or clay mud of good quality. Where horizontal steel is used between courses, cement-sand mortar (1:3) should be used with thickness so as to cover the steel with 6 mm mortar above and below it. Where vertical steel is used, the surrounding brickwork of 1 X 1 or 1½ X 1½ brick size depending on wall thickness should preferably be built using 1: 6 cement-sand mortar.

8.2.3 The minimum wall thickness shall be one brick in one storey construction and one brick in top storey and 1½ brick in bottom storeys of up to three storey construction. It should also not be less than 1/16 of the length of wall between two consecutive perpendicular walls.

8.2.4 The height of the building shall be restricted to the following, where each storey height shall not exceed 3.0 m:

For Categories A, B and C – three storeys with flat roof; and two storeys plus attic for pitched roof.

For Category D -  two storeys with flat roof; and one storey plus attic for pitched roof       

8.2.5     Special bond in brick walls - 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 to course. To obtain full bond between perpendicular walls, it is necessary to make a sloping (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, alternately in lifts of about 450 mm.

8.3 Stone masonry (random rubble or half-dressed)

8.3.1 The construction of stone masonry of random rubble or dressed stone type should generally follow IS: 1597 (Part 1)-1967.

8.3.2 The mortar should be cement-sand (1: 6), lime-sand (1: 3) or clay mud of good quality.

8.3.3  The wall thickness ‘t’ should not be larger than 450 mm. Preferably it should be about 350 mm, and the stones on the inner and outer wythes should be interlocked with each other.

Note – If the two wythes are not interlocked, they tend to delaminate during ground shaking bulge apart and buckle separately under vertical load leading to complete collapse of the wall and the building.

8.3.4 The masonry should preferably be brought to courses at not more than 600 mm lift.

8.3.5 Through’ stones of full length equal to wall thickness should be used in every 600 mm lift at 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 in place of one full length stone so as to provide an overlap between them.

8.3.6  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. Alternatively, wood bars of 38 mm X 38 mm section, Cross concrete bars of 50 mm x 50 mm section  with an 8 mm dia rod placed centrally may be used in place of ‘through’ stones. The wood should be well treated with preservation so that it is durable against weathering and insect action.

8.3.7 Use of ’bonding’ elements of adequate length should also be made at corners and junctions of walls to break the vertical joints and provide bonding between perpendicular walls.

8.3.8  Height of the stone masonry walls (random rubble or half-dressed) should be restricted as follows, with storey height to be kept 3.0 m maximum, and span of walls between cross walls to be limited to 5.0 m:

a)  For categories A and B – Two storeys with flat roof or one storey plus attic, if walls are built in lime-sand or mud mortar; and one storey higher if walls are built in cement-sand 1: 6 mortar.

b)  For categories C and D – Two storeys with flat roof or two storeys plus attic for pitched roof, if walls are built in 1 : 6 cement mortar; and one storey with flat roof or one storey plus attic, if walls are built in lime-sand or mud mortar, respectively.

8.3.9 If walls longer than 5 m are needed, buttresses may be used at intermediate points not farther apart than 4.0 m. The size of the buttress be kept of uniform thickness. Top width should be equal to the thickness of main wall, t, and the base width equal to one sixth of wall height.

8.4 Opening in bearing walls

8.4.1 Door and window openings in walls reduce their lateral load resistance and hence should preferably be small and more centrally located. The size and position of openings shall be as given in Table 2.

8.4.2 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.

8.4.3 Where openings do not comply with the guidelines of table 2, they should be strengthened by providing reinforcement concrete lining with 2 high strength deformed (H S D) bars of 8 mm dia.

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

8.5 Seismic strengthening arrangements

All buildings to be constructed of masonry shall be strengthened by the methods as specified for various categories of buildings and detailed in subsequent clauses. Schematically, the overall strengthening arrangements to be adopted for category D buildings, which consist of horizontal bands of reinforcement at critical levels and vertical reinforcing bars at corners and junctions of walls.

Table 2 – Size and position of openings in bearing wall. (Clauses 8.4.1 and 8.4.3)

Description

Building category

A, B & C

D

i)

Distance b5 from the inside corner of outside wall, Min

230 mm

600 mm

ii)

Total length of openings. Ratio, Max:

(b1 + b2 + b3) / 1,

(b6 + b7)/12

 

0.46

0.37

 

0.42

0.33

iii)

Pier width between consecutive openings b­4

450 mm

560 mm

iv)

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

600 mm

600 mm

Strengthening method

Lintel band (see 8.5.2)

Roof band and gable band where necessary (see 8.5.3 and 8.5.4)

Vertical steel at corners and junctions of walls (see 8.5.7)

Bracing in plan at tie level of pitched roofs (see 5.2.2.2)

Plinth band where necessary (see 8.5.6)

Note – For building of category B in two storeys, constructed with stone masonry in weak mortar, it will be desirable to provide vertical steel of 10 mm dia in both storeys.

8.5.2 Lintel band is a band (see 3.5) provided at lintel level in all internal and external longitudinal as well as cross walls except partition walls. The details of the band are given in 8.5.5.

8.5.3 Roof band is a band (see 3.5) provided immediately below the roof or floors. The details of the band are given in 8.5.5. Such a band need not be provided underneath reinforced concrete or reinforced brick slabs resting on bearing walls, provided that the slabs cover the width of end walls fully.

8.5.3 Gable band is a band provided at the top of gable masonry below the purlins. The details of the band are given in 8.5.5. This band shall be made continuous with the roof band at the eave level.

8.5.5 Details of band

8.5.5.1.  Reinforced bond - The band should be made of reinforced concrete of grade not leaner than M15 or reinforced brickwork in cement mortar not leaner than 1:3. The bands should be of the full width of the wall, not less than 75 mm in depth and should be reinforced with 2 HSD bars 8mm dia and held in position by 6 mm dia bar links, installed at 150 mm apart.

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

2. In case of reinforced brickwork, the thickness of joints containing steel bars should be increased to 20 mm so as to have a minimum mortar cover of 6 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.

3. For full integrity of walls at corners and junctions and junctions of walls and effective horizontal bending resistance of bands, continuity of reinforcement is essential.

8.5.5.2 Wooden band - As an alternative to reinforced band, the lintel band could be provided using wood beams in one or two parallel pieces with cross elements.

8.5.6 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 8.5.5.1. This band serves as damp proof course as well. 

8.5.7 Vertical reinforcement - Vertical steel at corners and junctions of walls which are up to 350 mm thick should be provided as specified in Table 3. For walls thicker than 350 mm, the area of the bars should be proportionately increased.

The vertical reinforcement should 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 should pass through the lintel bands and floor slabs or floor level bands in all storeys. Bars in different storeys may be welded or suitably lapped.

For providing vertical bar in stone masonry, use of a casing pipe is recommended around which masonry be built to height of 600 mm. The pipe is kept loose by rotating it during masonry construction. It is then raised and the cavity below is filled with M15 (or 1:2:4) grade of concrete mix and rodded to compact it.

Table 3 Vertical steel reinforcement in low strength masonry walls (Clause 8.5.7)

No. of

Storeys

Storey

Diameter of HSD single bar; in mm, at each critical section for

Category A

Category B

Category C

Category D

One

-

Nil

Nil

Nil

10

Two

Top

Bottom

Nil

Nil

Nil

Nil

10

10

10

12

Three

Top

Middle

Bottom

Nil

Nil

Nil

10

10

12

10

10

12

10

12

12

 Notes - 1     The diameters given above are for HSD (High Strength Deformed) bars with yield strength 415 MPa.  For mild steel plain bars, use equivalent diameters.

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

Annexure 26.A.6

CRITERIA FOR BLAST RESISTANT DESIGN OF STRUCTURES FOR EXPLOSIONS

ABOVE GROUND

(Extract of IS: 4991-1968)

Introduction

Structures designed to resist blast loads are subjected to completely different type of load than that considered in conventional design. Here they are hit with a rapidly moving shock wave which may exert pressures many times greater than those experienced under the greatest of hurricanes. However, in blast phenomenon, the peak intensity lasts for a very small duration only.

The blast wave loads the exposed surface of the structure and then the load is transmitted to the other elements. Thus, the response of each individual element is important unlike the ground motion where in the whole structural system is simultaneously causing inertial effects on all parts.

To design a structure capable of resisting these intense but short duration loads, members and joints are permitted to deflect and strain much greater than is allowed for usual static loads. This permitted deflection is, ordinarily, well into the plastic range of the material. Large amounts of energy are absorbed during this action, thus reducing the required design strength considerably below that required by conventional design within elastic range. Moreover, under higher rates of loading the strength developed by the material, increases with the rate of loading, and may often be adequately described as a function of time within a certain range.

Whereas, if the location of the ground zero (see 2.8), and the size of bomb are known, the corresponding blast loading for an existing structure may be found by the methods explained in this standard. However, it will never be possible to have exact data for specifying the expected ground zero and bomb size. Therefore, three different standard blast loadings (see 12.2), are recommended in this standard. Nevertheless, this standard also contains necessary information for evaluating various parameters of a blast wave generated by any other size of explosion at a given distance and the design of a structure for the same. It may be mentioned that if a building receives a shock stronger than the one for which it is designed, it is likely to suffer a higher category of damage. Fires may generally follow an air raid, it is, therefore, important to design structures against fire. Attention to the relevant Indian Standards on fire safety is, therefore, invented.

Flying splinters from a bomb may also cause considerable damage to the portion of the structure exposed to them. Wall thickness considered safe against flying splinters from a bomb are given in Appendix C.

For the purpose of deciding whether a particular requirement of this standard is complied with, the final value, observed or calculated, expressing the result of a test or analysis, shall be rounded off in accordance with IS: 2-1960*. The number of significant places retained in the rounded off value should be the same as that of the specified value in this standard.

1 Scope - This standard covers the criteria for design of structures for blast effects of explosions above ground. This standard does not cover the design for blast effects of nuclear explosions.

2 Definitions - For the purpose of this standard, the following definitions shall apply.

Blast wind – It is the moving air mass along with the over-pressures resulting from pressure difference behind the shock wave front. The blast wind movement during the positive phase of the overpressures is in the direction of shock front propagation.

Clearance time – this is the time in which the reflected pressure decays down to the sum of the side on overpressure and the drag pressure.

Decay parameter – It is the coefficient of the negative power of exponent e governing the fall of pressure with time in the pressure-time curves.

Drag force – It is the force on a structure or structural element due to the blast wind. On any structural element, the drag force equals dynamic pressure multiplied by the drag coefficient of the element.

Ductility ratio – It is the ratio of the maximum deflection to the deflection corresponding to the elastic limit.

Dynamic pressure – It is the pressure effect of air mass movement called the blast wind.

Equivalent bare charge – It is the weight of a bare high explosive charge geometrically similar to any given chased charge, which produces the same blast field as the given cased charge.

Ground zero – It is the point on the earth surface vertically below the explosion.

Impulse – Impulse per unit of projected area is the pressure-time product given by the area under the pressure-time curve considered for the positive phase only unless otherwise specified.

Mach number – It is the ration of the speed of the shock front propagation to the speed of sound in standard atmosphere at sea level.

Overpressure – It is the rise in pressure above atmospheric pressure due to the shock wave from an air blast.

Reflected overpressure – It is the overpressure resulting due to reflection of a shock wave front striking any surface. If the shock front is parallel to the surface, the reflection is normal.

Shock wave front – It is the discontinuity between the blast wave and the surrounding atmosphere. It propagates away from the point of explosion in all directions at a speed greater than the speed of sound in the undisturbed atmosphere.

Side-on overpressure – It is the overpressure if it is not reflected by any surface.

Transit time – It is the time required for the shock front to travel across the structure or its element under consideration.

Yield – It is a measure of the size of the explosion expressed in equivalent weight of reference explosive.

3.  Notations - For the purpose of this standard, the following notations shall apply.

a : Velocity of sound in air

B: Span or width of structure across the direction of shock wave propagation

Cd: Drag coefficient

E : Modulus of elasticity of material

H: Height of the structure

I: Moment of inertia of member

j : Number of concentrated load points

K: Coefficient of earth pressure

KLM : Load mass factor

KL  : oad factor

KM: Mass factor     

k : Ratio of resistance required to peak overpressure

K&  K2 Values of K for m and time ratios   td1   and   td2              

KE: Effective stuffiness of equivalent single spring-mass system

L : Length of structure in the direction of motion of blast wave  (see fig. 2.7 & 9)

M : Mach number for incident shock front

Mt: Total actual mass

M: Concentrated mass at point r

m : Distributed mass intensity per unit length

n : Number of concentrated masses

P: Total dynamic load (at any instant of time)                                                         

P: Concentrated dynamic force at point r

P(x) : Intensity of distributed dynamic load per unit length

P: Ambient atmospheric pressure

Pro : Peak reflected overpressure

P: Side-on overpressure

Pso : Peak side-on overpressure

q : Dynamic pressure

qo: Peak dynamic pressure

Rm: Resistance required by a structural member

S : ½  B or H Whichever is less

T : Effective time period of structural member

t: Clearance time

td : Duration of the equivalent triangular pulse

t: Time for positive phase of side-on overpressure

t: Duration of the equivalent triangular pulse for drag loading only

tt  : Transit time

U: Shock front velocity

W : Yield of explosion

Y: Deflection/deformation at yield point of idealized resistance deflection

Yrm : Maximum deflection/deformation permitted in the design of the structure

Z : Tension steel ratio

Z: Compression steel ratio

Decay parameter

jr : Deflection at point r of an assumed deflection shape for concentrated loads

j(a) : Deflection at point x of an assumed deflection shape for distributed load          

4 General characteristics of blast and effects on structures

4.1 The source – The conventional chemical charge is considered spherical. The shock front at the ground surface from a contact burst is approximately vertical. The effective yield of a contact burst is almost double of on equal explosion high in the air. This condition is assumed to give most serious effects.

4.2 Shock wave – As result of explosion, a shock wave is generated in the air which moves outward in all directions from the point of burst with high speed causing time-dependent pressure and suction effects at all points in its way. The shock wave consists of an initial positive pressure phase followed by a negative (suction) phase at any point as shown in Fig.1. The shock wave is a accompanied by blast wind causing dynamic pressures due to drag effects on any obstruction coming in its way. Due to diffraction of wave at an obstructing surface reflected pressure is caused instantaneously which clears in a time depending on the extent of obstructing surface.

Fig. 1 Shock wave produced by blast

4.3. Pressure and durationAt any surface encountered by the shock wave, the pressure rises almost instantaneously to the peak values of side-on. The peak values depend upon the size of explosion, the distance of the surface from the source, and other factors like ambient pressure and temperature in air.

The incident blast wave characteristics are described by the peak initial overpressure Pso, the overpressure Ps versus time t curve; the maximum dynamic pressure qo the dynamic pressure q verses time   t curve and the duration of positive phase to.

The peak positive intensity quickly drops down to zero: the total duration of the positive phase being a few milliseconds. The maximum negative overpressure is much smaller than the peak positive overpressure is much smaller than the peak positive overpressure, its limiting value being one atmosphere. But the negative phase duration is 2 to 5 times as long as that of the positive phase.

4.4 General principlesAs in the case normal loads, members subjected to blast pressure resist the applied force by means of internal stresses developed in them. However, the effective load due to blast, for which resistance should be developed in the member, depends upon the dynamic properties of the member itself. Longer the natural time period of the member smaller is the effective load for design.

The duration of positive phase of blast is generally small as compared with the natural period of the structural elements, hence may be treated as an impulse problem.

Considering the probability of occurrence of blast loading to be small, structures may be permitted to deform in the plastic range for economical design. Permitting plastic deformations increases the energy absorption and has the further advantage that the effective time period of the structural element is elongated, thereby reducing the effective load for its design.

Most severe blast loading on any face of a structure is produced when the structure is oriented with the face normal to the direction of propagation of the shock front. However, for lack of known orientation of future explosion, every face of the structure shall be considered as a front face. When the blast field surrounds the structure, the difference of pressures on front and rear faces tends to tilt and overturn the structure as a whole.

5 Blast force

5.1 Maximum values for reference explosionThe maximum values of the positive side-on overpressure Pso, reflected overpressure Pro and dynamic pressure qo, as caused by the explosion of one tonne explosive at various distance from the point of explosion, are given in Table.1. The duration of the positive phase of the blast to and the equivalent time duration of positive phase td are also given Table 1.

5.2 Decay of pressure with timeThe pressure varies with time according to the following relations:

t                       t

Ps   = Pso          1 -                  e - a 

 to                                            to

t                         t        

q   =     qo           1 -                  e - 2a 

to                                                  to                       

Note 1 – As will be noted from the above equations the dynamic pressure q decays much faster with time than the side-on overpressure Ps.

Note 2 – Since the use of these relations in design problem would involve tedious calculations, the pressure time relations in the positive phase are idealized by using a straight line starting with the maximum pressure value but terminating at a time td or tq such that the impulse value remains the same. This criteria has been adopted in this standard                                                    Table 1 Blast parameters from ground burst of 1 tonne explosive (Clause 5.1)

Distance

m

x

Peak side on overpressure ratio Pso/Pa

Mach

No.

M

Positive phase duration

tm milli secs

Duration of equivalent triangular  pulse td, milli secs.

Dynamic pressure ratio qo/ Pa

Peak reflected overpressure ratio Pro/Pa

(1)

   (2)

(3)

  (4)

    (5)

     (6)

    (7)

15

8.00

2.80

9.50

5.39                 10.667

41.60

 

18

5.00

2.30

11.00

7.18

5.208

22.50

21

3.30

1.96

16.38

9.33

2.643

12.94

24

2.40

1.75

18.65

11.22

1.532

8.48

27

1.80

1.60

20.92

13.30

0.920

5.81

30

1.40

1.48

22.93

15.39

0.583

4.20

33

1.20

1.42

24.95

16.31

0.439

3.45

36

1.00

1.36

26.71

17.94

0.312

2.75

39

0.86

1.32

28.22

19.20

0.235

2.28

42

0.76

1.28

29.74

20.20

0.186

1.97

45

0.66

1.25

31.25

21.60

0.142

1.66

48

0.59

1.23

32.26

22.70

0.115

1.46

51

0.53

1.20

33.52

23.70

0.093

1.28

54

0.48

1.19

34.52

24.70

0.077

1.14

57

0.43

1.17

35.53

26.40

0.062

1.01

60

0.40

1.16

36.29

26.60

0.054

0.93

63

0.37

1.15

37.30

27.80

0.046

0.85

66

0.34

1.14

38.05

28.76

0.039

0.77

69

0.32

1.13

38.81

29.25

0.035

0.72

72

0.30

1.12

39.56

29.87

0.031

0.67

75

0.28

1.11

40.32

30.71

0.027

0.62

78

0.26

1.104

40.82

31.85

0.023

0.58

81

0.25

1.100

41.58

31.92

0.022

0.55

84

0.24

1.098

42.34

32.00

0.020

0.53

87

0.23

1.095

42.84

32.26

0.018

0.50

90

0.22

1.086

43.60

33.39

0.016

0.47

93

0.26

1.082

44.35

34.70

0.014

0.43

96

0.19

1.077

45.46

35.37

0.013

0.41

99

0.18

1.072

45.61

36.22

0.012

0.40

Note 1 – The value of Pa the ambient air pressure may be taken as 1 kg/cm² at means sea level.

Note 2 – One tonne of explosive referred to in this table is equivalent to 1.5 X 10² calories.

Note 3 – Velocity of sound in m/s may be taken (331.5 + 0.607 T) where T is the ambient temperature in centigrade.                    

5.3 Scaling lawsFor any explosion other than reference explosion, the peak pressure and time duration may be found from the peak values given in Table 1 buy the cube root scaling laws as given below:

                                     Actual distance           

Scaled distance x =                           ....... (1)

                                     W¯

 

                                 Actual time

Scaled time t0  =                          .......(2)

                                W¯

Where

W = Yield of explosion in equivalent weight of the reference explosive measured in tonnes,

X = scaled distance for entering the Table 1 for reading peak values, and

T0 = scaled time read from Table 1 against scaled distance.

Note - Actual distance is measured from the ground zero to the point under consideration. Actual time is the time for actual explosion.

6 Blast load on above ground structures

6.1 Types of structuresThere are mainly the following two types of structures:

a)  Diffraction Type structures - There are the closed  structures without openings, with the total area opposing the blast. These are subjected to both the shock wave overpressure Ps and the dynamic pressure q caused by blast wind.

b)  Drag Type Structures - These are the open structures composed of elements like beams, columns, trusses, etc, which have small projected area opposing the shock wave. These are mainly subjected to dynamic pressure q.

6.2 Closed rectangular structures – (see fig. 2.)

Fig. 2 Above ground rectangular structure

6.2.1 Front face - As the shock wave strikes the vertical face of a structure normal reflection occurs and the pressure on the front face instantaneously increases to the reflected overpressure Pro given by the following equation:

  6 Pso

Pro    =  Pso (  2  +                                   )  ...... (3)

Pso + 7 Pa

Where

Pa = the ambient atmospheric pressure. Taking   Pa = 1 kg/cm², the value of Pro are given in Table 1.

The net pressure acting on the front face at any time t is the reflected overpressure Pr or (Ps + Cd q), whichever is greater:

Where

Cd = drag coefficient given in Table 2, and

Pr = the reflected overpressure in which drops from the peak value (Ps + Cd q)  in clearance time tc given by:

         3S       

tc  =          or td   whichever is less (see Fig. 2)  .... (2)

           U                                                                     

Where

S = H or B/2 whichever is less (see Fig. 2)

U = Shock front velocity = M.a

Where

A  = velocity of sound in air which may be taken as 344 m/s at mean sea level at 20 degree centigrade, and

M = Mach number of the incident pulse given by   4 1 + 6 Pso / Pa. The values of M for various conditions are also tabulated I Table1.

The net average loading on the front face (B XH) as a function of time is shown in Fig.3A or 3B depending on whether tc is smaller than or equal to td. The pressure Pro, Pso and qo and time td are for actual explosion determined according to the scaling laws given in 5.3.

6.2.2 Rear face – using the pressure for the actual explosion, the average loading on the rear face (B X H in Fig.2) is taken as shown in Fig. 4 where the time has been reckoned from the instant the shock first strikes the front face. The time intervals in interest are the following:

= the travel time of shock from front to rear face, and

4S

= pressure rise time on back face,        

U

Table 2 Drag coefficient Cd (Clause 6.2.1.1)

Sl

No

Shape of Element

Drag coefficient Cd

Remarks

(1)

(2)

(3)

(4)

 

For closed rectangular structures

 

 

i)

Front vertical face

1.0

 

ii)

Roof, rear and side faces for

qo = 0 to 1.8 kg/cm2

qo = 1.8 to 3.5 kg/cm2

qo = 3.5 to 9.0  kg/cm2

 

-0.4

-0.3

-0.2

 

For above ground

structrures

iii)

Front face sloping

4  to 1

1 ½ to 1

 

 

Zero

0.4

 

For semi-buried

structures

 

For open, drag type structures

 

 

iv)

Sphere

0.1

 

v)

Cylinder

1.2

This covers steel tubes used as columns, truss members etc.

vi)

Structural shapes

2.0

This covers flats, angles, tees, I sections, etc.

Vii)

Rectangular projection

1.3

This covers beam projections below or above slabs, cantilever walls standing freely above ground, etc.

6.2.3 Roof and side wallsAs for rear face in 6.2.2, the average pressure versus time curve for roof and side walls is given in Fig. 5A, when td is greater than the time tt = L/U. When tt is greater than td the load on roof and side Walls may be considered as a moving triangular pulse having the peak value of overpressure (Pso + Cd.qo) and time td as shown in Fig.5B.

6.2.4  Overturning of structure – The net average load as a function of time which tends to cause sliding and overturning of the building is obtained by subtracting the loading on back face from that on the front face.

6.2.5 An example of calculation of pressure-time curves on a rectangular above-ground building is given in Appendix A.

Fig. 3 Pressure versus Time for front page

Fig. 4 Pressure versus Time for rear face

Fig. 5 Pressure versus Time for roof and side walls

6.3 Structures with openings

6.3.1     Opening or drag type structures - The net translational pressure on the obstructing areas of elements may be taken as shown in Fig.6 where Cd is the drag coefficient depending upon the shape of the structural element given in Table 2 and tq is given as follows

tq = ½  to ..... (5)

6.3.2     Partly open structuresWhen the area of openings is more than 50 percent of the area of walls, the structure may be considered drag type (see 6.3.1). When thew area of openings less than 5 percent of the area of walls, the structure may be considered closed type, (see 6.2). For intermediate conditions, direct interpolation may be made between the two conditions of both maximum pressures and time duration.

Fig. 6 Pressure versus Time for open structures

6.4 Closed cylindrical arch-shape structures

6.4.1     Gable ends – The loading may be taken same as for front and rear faces of above ground rectangular structure.

6.4.2     Curved surface – The direction of shock wave propagation is taken transverse to the ridge of the structure and since the usual arch spans are large so that the transit time tt is greater than the positive phase time td, average loading condition can not be assumed. Therefore, the loading on curved surface may be taken as a moving triangular pulse as shown Fig. 5B.

6.5 Closed dome structures – The loading on a domical structure may be taken as a moving triangular pressure pulse as shown in Fig.5B. The variation of pressure transverse to the direction of propagation of the pulse may be considered symmetrical varying according to Cos q where the angle q is measured from longitudinal vertical section of the dome (see Fig.5C).

7 Blast load on below – ground structures

7.1 Types of structuresThe below-ground structures are classified into buried and semi-buried structures depending upon the earth berms. The buried structure is subjected only to the general overpressure Pso, the reflected and dynamic pressure being neglected. The semi-buried structure is subjected to partial dynamic pressures besides the general overpressure. Both are acted upon by air-induced ground shock also.

7.2 Buried rectangular structures

A rectangular structure is considered buried if it satisfies the condition shown in Fig.7.

The pressure versus time diagrams for the various faces of the structure Are shown inFig.8. The values of the coefficient of earth pressure Ka appearing in Fig.8. are given in      Table 3.

7.3 Buried arch or dome structures

An arch or dome structure is considered buried if it satisfies the condition shown in Fig. 9. The pressure versus time relations at crown and wall are the same as that on the roof and walls of a buried rectangular structure respectively. The pressure on the slope of the arch or dome may gradually be reduced from crown towards the springings such that the pressure at springing becomes equal to that on the wall. If the semi-central angle is less than 45°, the arch or dome may be designed for the same pressure as the roof of a buried rectangular structure.

Fig. 7 Buried rectangular structure

Table 3 Coefficient of earth pressure Ka   (Clause 7.2.2)

Sl No

Type of soil

Coefficient Ka

i)

Cohesionless soil, dry or damp

¼

ii)

Cohesive soil:

  1. Stiff unsaturated
  2. Medium unsaturated
  3. Soft unsaturated

iii

?

½

¾

iv)

Fully saturated soil

1

7.4 Semi-buried structures

Semi-buried structures are those which do not have the minimum earth cover or the slopes are steeper than those specified in Fig. 7 and 9. These are subjected to dynamic pressures besides the general overpressures.

A minimum slope of 1½ to 1 may be used for the earth cover. The reflected pressure effect may be neglected and dynamic pressure may be considered on the front face with drag coefficient may be reduced making it zero when the slope becomes 4 to 1.

Fig. 8 Pressure versus Time diagrams for various faces of buried structure

The loading on the earth beam surface may be computed as for the above ground structures.

8 Response of structural elements

8.1 Significant factors

The significant factors, on which the response of a structural element subject to blast forces depends, are the pressure versus time diagram acting on the element, the effective time period of the element, the resistance versus deflection diagram of the element, and the maximum permissible deflection.

When the ratio of time duration td or to  to the natural period of the element is less than 0.1, the problem may be considered as an impulse problem taking the area under the pressure versus time curve as impulse per unit area. In such a case, the shape of pressure-time curve is not important.

Fig. 9 Buried arch or dome structure

Fig. 10 Idealized pressure time diagrams

In most cases, the pressure-time diagram can be idealized without loss of accuracy as a triangular pulse (Fig. 10A) having the same maximum pressure as in the original diagram and the time so adjusted that the area under the curve remains unchanged. However, where the load acting on the front face has the shape of the diagram of Fig. 10B, it may be treated as such (see also 8.2).

For simplicity the resistance versus deflection diagram of an element is idealized as elasto-plastic as shown in Fig. 11 by keeping the area under the actual and idealized curves about the same up to the maximum permissible deflection.

Fig. 11 Idealized resistance deflection diagram

The maximum permissible deflection defines the energy absorption capacity of the element which is equal to the area under the resistance versus deflection curve. For a given blast, greater the permissible deflection, lesser will be the maximum resistance required in the member.

8.2. Elastic and elasto-plastic response

Based on the triangular pressure-time curve shown in Fig. 10A and elasto-plastic resistance-deflection curve shown in Fig.11, the ratio of resistance Rm required to the peak overpressure P0  is given in Fig. 12 for various values of td /T and m.

Figure 12 also shows the time tm at which maximum deflection occurs, as a ratio of time T for various values of td /T and m.

Fig. 12 Response chart for triangular pressure pulse

When the rime ratio td /T is less than 0.1, the ratio k may be computed from the following equation.

p      td

K =   .

2 m - 1 T

 

When the pressure-time diagram is given by Fig. 10b, the following equation shall be satisfied:

P01                    P02

K1 +                  K2  = 1

 Rm                     Rm

Where

Rm  = the required resistance, and

k1, k2   = the values of ratios k for ductility ratio m and time ratios td1 /T and td2 /T respectively

8.3. Elastic rebound – After attaining the maximum response, the structure is found to oscillate and may have an elastic rebound equal and opposite to the maximum deflection. Therefore, members shall be designed to have the same strength for the reversal of the effective design load.

9 Time period of structural members

9.1 The structural element or frame-work may be replaced by an equivalent single spring-mass system having effective stiffness kE and effective mass equal to KLM .Mt where Mt  is the actual mass of the member under consideration and KLM is a load-mass factor depending upon the stiffness and mass distribution in the member and its boundary conditions. The equivalent system is defined so that the deflection of the equivalent single mass is the same as that of some significant point in the given structure. The effective stiffness kE is defined with respect to the deflection of this point

9.2 The load-mass factor KLM is equal to the ratio of mass factor KM to the load factor KL.  The factors are evaluated on the basis of an assumed deflected shape of the structure as given below:

KLM = KM / KL

 

1          n

KM =  S         Mr jr2  +   m  j2(x)  dx

Mt      r = 1

 1          n

KL =                   S         Pr jr2  +   p  j2(x)  dx

Pt           r = 1

Where

Mt = total actual mass,

n = number of concentrated masses,

Mr = concentrated mass at point r,

jr  = deflection at point r of an assumed deflected shape for concentrated loads,

m = distributed mass intensity per unit length,

j(x) = deflection at point x of an assumed deflected shape for distributed loads

Pt   = total dynamic load (at any instant of time),

J    = number of concentrated load points,

Pr   = concentrated dynamic force at point r, and

P(x) = intensity of distributed dynamic load per unit length.

Note 1 – Values of the factors kE and KLM for certain structural members are given in Tables 4 to 6.

Note 2 – The deflected shape is suitably chosen to resemble as far as possible the true deflected shape taking into consideration whether the structure or member remains elastic or goes into the plastic range. It may be taken the same as due to static application of the dynamic load on the structure. The deflected shape has to be normalized such that j = 1 at the point with respect to which the effective stiffness kE is defined.

9.2.1     Two typical examples explaining the evaluation of KLM are given in Appendix B.

9.3. The time-period T of the structural member may be calculated from the equation

T = 2 p            KLM X Mt

KE

9.4        For elastic analysis (ductility ratio m £ 1.0) of structures, the effective stiffness kE  and load mass factor KLM are to be used as given in Tables 4 to 6. For elasto-plastic analysis kE  is to be used as given in Tables 4 to 8 but value of KLM may be chosen in between the elastic and plastic cases depending upon the ductility factor.

For elasto-plastic design of fixed slabs, the modified value of KE is to be worked out in accordance with Fig. 11B using the stiffness values of slab in elastic and elasto-plastic cases as given in Table 6 and KLM is to be suitably chosen depending upon the ductility factor.

9.5        For calculating moment of inertia I of reinforced concrete sections, the effective transformed area may be used. The value of modular ratio shall be taken the same as in static design for calculating EI.

10 Dynamic strength of materials, design stresses

10.1. GeneralPlastic deformation of the structural elements should be permitted except where the functioning of the structure would be adversely affected by their permanent displacements.

Under rapid rates of straining as associated with blast loading, materials develop higher strengths than in statically loaded members. Under such conditions the dynamic strength may be taken greater than the minimum specified static strength as indicated in 10.2 to 10.5.

10.2      Design stresses for structural steel

The average dynamic yield stress of structural carbon, mild, weldable or rivet steels may be assumed to exceed the minimum specified static yield stress by 25 percent and that of high strength alloy steels by 10 percent.

For elastic design, the resistance Rm of various elements of a framework may be assumed to have just been reached. The dynamic yield stress of steel as given in 10.2.1 shall be used for calculating Rm .

For elasto-plastic design, the ductility ratio may be assumed as follows:

Types of member Members of roof trusses

Ductility Ratio m

Slenderness ratio (l/r) = 180

1.0

Slenderness ratio (l/r) = 60

5.0

Members subjected to bending and direct stresses

Minor damage

5.0

Moderate damage

10.0

Considerable damage

20.0

Note – For intermediate values of l/r, linear interpolation may be done.

10.3. Design stress for reinforced concrete

The dynamic strength of material may be assumed as follows:

  1. Reinforcing Steel – Dynamic yield stress 25 percent higher than the minimum specified static yield stress.
  2. Concrete – The dynamic cube compression strength may be assumed to be 25 percent higher than the minimum static cube strength at 28 days.

For dynamic shear in reinforced concrete members, no increase over the static shear strength shall be permitted. For dynamic bond stress in reinforced concrete members, an increase of 25 percent may be permitted over the static strength.

In elastic design, for calculating resistance Rm the ultimate flexural strength on the basis of ultimate load theory shall be assumed to have just been reached. For ultimate strength calculations a reference may be made to IS: 456-1964 *, and the dynamic strength of materials as given in 10.3.1 should be used.

For elasto-plastic design of reinforced concrete members the ductility ratio may be assumed as follows, provided that the steel ratio remains less than balanced steel ratio for plastic design:

Members subjected to Bending and direct stresses

Ductility ratio µ

Minor damage

0.04   but > 5

Z-Z1

Moderate damage

0.07    but > 10

Z-Z1

Considerable damage

0.1     but  > 15

Z-Z1

Where

Z’ = compression steel ratio, and

Z = tension steel ratio.

10.4 Design stress for masonry or plain concrete

The dynamic flexural strength of plain brick and stone masonry may be assumed to be the same as the corresponding static strength. The compressive strength shall be taken 25 percent higher than the corresponding static strength.

For unreinforced brickwork the ductility ratio may be limited to 1.5

For reinforced brickwork, with not less than 0.05 percent steel on each face and not more than balanced percentage, the ductility factors as for reinforced concrete may be used.

10.5 Design pressures on foundation material

The following allowable bearing pressures may be used for design under blast loading:

Rock

Static crushing strength

Granular soil

Static load per unit area for 4.0 cm settlement of the structure

Cohesive soil

¾ of static failure load per unit area determined by quick undrained test.

In the absence of test data on soils as envisaged under 10.5, the design bearing pressure may be taken equal to twice the allowable static bearing pressure.

Note – In working out the pressure under foundation, the direct loading of ground due to blast over pressures may be neglected.

For raft foundation, the raft area need not be more than the roof area.

11.  Load combinations for design

Wind or earthquake forces shall not be assumed to occur simultaneously with blast effects. Effects of temperature and shrinkage shall be neglected.

 Loads on floors shall be considered as per IS: 875-1964* depending upon the class of building. No live load shall be considered on roof at the time of blast.

12 Recommended values of blast pressures for design

The design parameters that is the yield of explosion and its distance from the structure will depend upon the importance of the structure and conditions prevailing in a particular time and should be considered by the designer in each specific case.

For general guidance the buildings may be designed for a bare charge of 100 kg at distances given in Table 7.

General recommendations for planning blast resistant buildings are given in Appendix C.

Table 7 Building design for a charge of 100 kg

Building category

Type

Distance M

A

Residential buildings

40

B

Community buildings, such as schools, offices, cinemas, etc. and industrial buildings with continuous human occupancy.

30

C

Buildings provided for accommodating essential service which would be of post bombing importance, such as hospitals, emergency relief stores, power stations, water works, communication centers, etc.

20

APPENDIX A

(Clause 6.2.5)

AN EXAMPLE OF CALCULATION OF PRESSURE-TIME CURVES ON A RECTANGULAR ABOVE GROUND BUILDING  

A-1 EXAMPLE

A-1.1 Blast parameters due to the detonation of a 0.1 tonne explosive are evaluated on an above ground rectangular structure, 3 m high, 10 m wide and 8 m long, situated at 30 m from ground zero.

Characteristics of the Blast

30

Scaled distance x =    = 64.65 m

(0.1) 1/3           

From Table 1 assuming pa = 1.00 kg/cm² and linearly interpolating between 63 m and 66 m for the scaled distance 64.65 m, the pressures are directly obtained:

Ps0 = 0.35 kg/cm²

Pr0   = 0.81 kg/cm²

q0   = 0.042 kg/cm²

The scaled times to and td obtained from Table 1 for scaled distance 64.65 m are multiplied by (0.1)1/3 to get the values of the respective quantities for the actual explosion of 0.1 tonne charge.

to = 37.71 (0.1) 1/3 = 17.5 milliseconds

td = 28.32 (0.1) 1/3 = 13.15 milliseconds                  

6          pso

M=   1+  = 1.14

7          pa

a = 344 m/s   U = 392 m/s = 0.392 m/millisecond

b) Pressures on the Building

Here H = 3 m, B = 10m, and L = 8m

Then S = 3m

3S            3 X 3   

tc =       =   = 23.0 milliseconds > td

U             0.392

L             8

tt =             =                    = 20.4 milliseconds > td           

U             0.392

4S           4 X 3

tr =              =                 = 30.6 milliseconds > td

 U             0.392  

As tr > td no pressure on the back face are considered.

For roof and sides Cd = 0.4

Pso + Cd qo = 0.35 – 0.4 X 0.042 = 0.33 kg/cm2

The pressure diagrams are as shown below:

APPENDIX B

(Clause 9.2.1)

TYPICAL EXAMPLES OF EVALUATION OF KLM

B-1. A Typical example of evaluation of KLM for a cantilever column subjected to uniform distributed dynamic load p

B-1.1 The deflected shape of the column in elastic range when p is considered to be acting statically is given by:

                                                                   4

                                     4   x                    x

                    j (x)  = 1 -   + 1/

                                     3   l                     l

 

                    where the deflection at the free end has been taken as unity.

                   

                                 1         1                       2  

                    KL =                       j(x) dx  =

                                 L        0                                        5  

                    If mass is considered to be uniformly distributed along the height of the column:

                                  l                   104 

                    KM  = 1 j²(x)  dx =                [ .. K LM = KM /KL = 52/81]

 

                                o                     405

 

 

Effective stiffness of the cantilever will be defined with respect to the deflection at the free end where j(x) = 1

                                                8 EI

                                 KE =

                                                L3

Period of the cantilever is thus obtained as:

 

 
 

 

                                                KLM   Mt

                                 T = 2pb    KE

                                                           

 

                                                            Mt    L3

                                    = 1.78              EI                                                              

B-2.   A Typical example of evaluation of klm for a single storey frame

B-2.1 A single storey rigid frame with distributed masses on the roof and sides is subjected to a concentrated dynamic force F(t) at the roof level plus a distributed dynamic load P(t) on one wall surface. Considering only horizontal motion, the equivalent one degree system is

defined such that its displacement is equal to the displacement at the roof. If the walls are assumed to remain straight then the displacement Yx of a point at a distance x from the base is given by:

                         

                                                 X                                                                      x

                    Yx   =               Y                           ;           j (x) =      

                                   H                                                           h

The mass factor KM is given by the following equation:

 

                                  1                   h           x    2           

                     K M =             m 1 + 2    m2                  dx          

                                 Mt                 0           h

 

                                         2    

                        = (m1l +        m2h)/ (m1l + 2 m2h)

                                            3                                  

Where m1 and m2 are the masses per unit length (along the frame) of the roof and walls respectively.

The equivalent load is:

                                                            h            x

                                 Fe (t) = F(t) +      P(t)                    dx

                                                            O             h                              

                                       = F(t) + ½ P(t) h

                                 And the load factor is:

                    F(t)  + ½ P(t) h

KL     =                                     ,           then KLM            = kM / kL

                    F(t)  +  P(t) h

 

 

 

 

 

 

 

 

The effective stiffness KE will be given by the total load [F(t) and P(t) h] causing unit displacement at the top of the frame. If F(t) and P(t) have different time variation, numerical analysis is necessary.

 

APPENDIX C

(Clause 12.3)

GENERAL RECOMMENDATIONS FOR PLANNING BLAST RESISTANT BUILDINGS

C-1 Size of rooms

C-1.1 Small size of rooms generally confines the blast damage to a limited area of the structure, because of the screening action of the partition walls.

C-2 Corridors

C-2.1 Long narrow corridors should be avoided as they tend to increase the extent of damage along the length of the corridors because of ‘multiple reflections’.

C-3 Projections

C-3.1 All slender projections like, parapets and balconies specially those made of brittle materials should be avoided as far as possible.

C-4 Chimneys

C-4.1 Masonry chimneys on factory buildings and boiler houses are a potential hazard and should be avoided.

C-5 Roofing and cladding materials

C-5.1 Brittle roofing materials, such as tiles and corrugated asbestos sheets are especially prone to blast damage. When corrugated galvanized iron sheets are used for roofing and / or cladding, particular attention should be paid to the fixtures fastening the corrugated galvanized iron sheets to the framework.

C- 6 Use of timber and inflammable materials

C–6.1.  These are especially prone to catch fire in a strafing or incendiary attack and should be best avoided in strategic structures where such attacks might be expected.

C–7 Electric Wiring

C-7.1 Conduit wiring is preferable to open wiring, as in case of large movement of the walls the conduct will give an added protection to the wiring inside and prevent them from getting cut thus preventing fire hazards due to short circuits.

C-8 Glass panes

C-8.1. The most widespread damage due to blast is the breaking of glass panes. The splinters from shattered glass window are dangerous to personnel safety. It is preferable to use non-splintering type glass panes wherever their use cannot be avoided.

C-9 Doors

C-9.1 Doors should be designed for the front face load.

C-10 Walls thickness against flying splinters

C-10.1 For protection against splinters from bombs with equivalent bare charges exploding at a distance of 15 m, the wall thickness given in Table 8 will be adequate.

Table 8 Minimum wall thickness against flying splinters

Material of wall

Wall thickness, cm

 

For bomb with equivalent bare charge of

50 kg

For bomb with equivalent bare charge of

100 kg

Reinforced concrete

30

38

Plain concrete or brickwork

34

45

* * * *