11-BEARINGS



BEARINGS

1. GENERAL

Bearings are part of the bridge structure which bears directly all the forces from the superstructure and transmits the same to the substructure and are classified as Metallic, Elastomeric, POT/PTFE bearings. Since Metallic bearings are not commonly used by the department, only Elastomeric and POT/PTFE bearings are included under this section.

  1. The bearings shall be supplied and installed by the Contractor or otherwise specified in the Contract. The Contractor shall be solely responsible for the satisfactory supply and installation of the bearing. In the detailed description of the specification, a general reference shall be made about the manufacturer.
  2. The Contractor shall exercise the utmost care in setting and fixing all bearings in their correct positions and ensuring that uniformity is obtained on all bearing surfaces.
  3. Bearings shall be handled with care and stored under cover.
  4. For bridges with skew angle less than 20°, the bearings shall be placed at right angles to the longitudinal axis of the bridge. For bridges with skew angle greater than 20°, very wide bridges and curved bridges, the location of bearings shall be ensured as shown in the Drawing.
  5. Easy access to the bearing shall be made available for purposes of inspection and maintenance. Provision shall also be made for jacking up of the superstructure so as to allow repair/replacement of bearings.

2. ELASTOMERIC BEARINGS

Elastomeric bearings shall cater for translation and/or rotation of the superstructure by elastic deformation. A typical cross section of elastomeric bearing is shown in figure.

2.1 Materials

  1. Chloroprene Rubber (CR) only shall be used.
  2. Grades of raw elastomer of proven use in elastomeric bearings, with low crystallisation rates and adequate shelf life viz. Neoprene WRT Neoprene W, Bayprene 110, Bayprene 210, Skyprene B-5, Skyprene B-30, Denka S-40V and Denka M-40, shall be used.
  3. No reclaimed rubber or vulcanised wastes or natural rubber shall be used.
  4. The Polychloroprene content of the bearing shall not be lower than 60%. The ash content shall not exceed 5% of its weight. Polychloroprene content shall be determined in accordance with ASTM-D297 and ash content as per IS: 3400 (Part XXII).
  5. Use of synthetic rubber-like materials such as Ethylene Propylene Diene Monomer (EPDM) and similar candidate elastomers shall not be permitted.
  6. The elastomer for use in elastomeric bearings shall conform to the properties specified in the table.
  7. Shear modulus (G) is the apparent "conventional shear modulus" of the elastomeric bearing determined by testing. At nominal temperature of 23±2°C, the value of G shall comply with the values given in the table.

           Table: Properties of elastomer           

Sl.

No.

Property

Unit

Code

Value of the

characteristic

specified

1

Physical properties:

 

 

 

 

 

1.1

Hardness

IRHD

IS: 3400 (Part-2)

50±5

60±5

70±5

1.2

Minimum tensile strength

- Moulded test piece

- Test piece from bearing

 

MPa

MPa

 

IS: 3400

(Part-1)

 

17

14

 

17

14

 

17

14

1.3

Minimum Elongation at break

- Moulded test piece

- Test piece from bearing

 

%

%

 

IS: 3400 (Part-2)

IS: 3400 (Part-2)

 

450

400

 

400

350

 

300

250

2

Maximum compression:

- set (%) (24h, 100±1°C)

 

%

IS: 3400

(Part-9)

 

35

3

Accelerated aging:

(72 h, 100 ± 1°C)

(Maximum change from

un-aged value)

 

 

 

3.1

Maximum change in

hardness

IRHD

IS: 3400

(Part-4)

±5

3.2

Maximum change in

tensile strength

%

IS: 3400

(Part-4)

-15

3.3

Maximum change in

elongation

%

IS: 3400

(Part-4)

-30

 

Table:  Shear modulus (G) at nominal temperature

Hardness (IRHD)

G (MPa)

Tolerances of G (MPa)

50±5

0.70

±0.15

60±5

0.90

±0.18

70±5

1.15

±0.20

 

 

  1. The adhesion strength of elastomer to steel plates, determined according to IS:3400 (Part XIV) method A, shall not be less than 7 kN/m.
  2. For elastomeric bearings (CR) used in adverse climatic conditions, the ozone resistance of elastomer shall be proved satisfactory when assessed by test according to IS: 3400 (Part XX). The testing shall be carried out for a  duration of 96 hours at a temperature of 40±1°C, strain of 30% and ozone concentration of 100 pphm by volume.
  3. If any cracking is detected by visual observation at the end of the test, the material shall be considered unsatisfactory. No specific tests for assessment of low temperature resistance are deemed necessary.
  4. Laminates of mild steel conforming to IS: 2062/IS: 1079 or equivalent international grade, shall only be permitted. The yield stress of the material shall not be less than 250 MPa. Use of any other material like fibre glass or similar fabric as laminates, shall not be permitted.

2.2 Workmanship

  1. Each elastomeric bearing shall be clearly labelled or marked. The bearing shall be wrapped in a cover and packed in wooden crates with suitable arrangement to prevent movement and to protect corners and edges.
  2. Care shall be taken to avoid mechanical damage, contamination with oil, grease and dirt, undue exposure to sunlight and weather of the bearings during transport and handling prior to and during installation.
  3. Bearings shall be installed in the structure as per the Drawing or as directed by the Engineer to ensure that right bearing is being installed at the correct location.
  4. Bearings must be placed between true horizontal surfaces (maximum tolerance 0.2 percent perpendicular to the load) and at true plan position of their control lines marked on receiving surfaces (maximum tolerance ±3 mm).
  5. Concrete surfaces shall be free from local irregularities (maximum tolerance ±1 mm in height).
  6. Variation from common planarity of twin or multiple bearings shall be within such tolerance as may be specified or approved by the Engineer.
  7. Design shall be got checked for the actual inclination in seating, if larger inaccuracies than those specified are permitted.
  8. For cast in-situ concrete superstructure, where bearings are installed prior to concreting, the forms around the bearings shall be capable of easy removal.
  9. Forms shall also fit the bearings securely and prevent any leakage of mortar/grout. Any mortar contaminating the bearings during concreting shall be completely removed before setting.
  10. For precast concrete or steel superstructure elements, fixing shall be done by application of epoxy resin adhesive to interface, after specified surface preparation. The specification for adhesive material, workmanship and control shall be approved by the Engineer. Care shall be taken to guard against faulty application and consequent behaviour of the adhesive layer as a lubricant. The bonding by the adhesive shall be deemed effective only as a device for installation and shall not be deemed to secure bearings against displacement for the purpose of design.
  11. Lifting of a cast in-situ post-tensioned bridge deck for relieving time dependent deformation shortly after installation of bearings should be avoided. In case such lifting is unavoidable, the lifting arrangement, proper seating of the girder on the bearing etc. shall be rigidly controlled to avoid any risk of misalignment.

 2.3. Tolerances

The bearings shall be fabricated/manufactured with tolerances not exceeding the values specified in the table.

Table: Manufacturing Tolerances

No.

Items

Tolerances

1

Overall linear plan dimensions

-0 mm/+6 mm

2

Total mean bearing thickness:

(The mean thickness is the arithmetic average of

the thickness measured at five points on the

major surface as indicated for various shaped

bearings:

Rectangular-corners and centre,

Circular-corners of inscribed square and centre)

 

-0%/+5%.

3

i)

 

ii)

Parallelism:

Of top surface of bearing with respect to the

bottom surface as datum

Of one side surface with respect to the other as

datum

 

1 in 200.

 

1 in 100.

4

i)

ii)

iii)

Thickness of individual layer of elastomer:

Inner layer of elastomer

Outer layer of elastomer

Side cover

 

±10%.

-0 mm/+2 mm.

-0 mm/+3 mm.

5

i)

ii)

iii)

Dimension of laminates:

Plan dimensions of laminates

Thickness of laminate

Parallelism of laminate with respect to bearing

base as 1 in 100 datum (with respect to

diameter for plates circular in plan and shorter

side for plates rectangular in plan)

 

-3 mm/+0 mm.

±10%.

1 in 100.

6

 

i)

 

ii)

Flatness:

Flatness    shall    be    assessed    by    placing    a

straightedge along the diagonal or diameter. The

gap between the straight edge and the surface

shall not exceed the tolerances specified below:

Load bearing surface of the bearing

 

Steel laminate

 

0.3% of diameter or

diagonal or 2% of mean

bearing thickness

whichever is higher.

1% of diameter or

diagonal

(Max of 1.5 mm).

2.4 Testing

  1. The manufacturer shall have all test facilities required for process and acceptance control tests installed at his plant to the complete satisfaction of the Engineer. The test facilities and their operation shall be open to inspection by the Engineer on demand.
  2. All acceptance and process control tests shall be conducted at manufacturer’s plant. Cost of all materials, equipment and labour shall be borne by the manufacturer/Contractor.
  3. Acceptance testing shall be commenced with the prior submittal of testing programme by the manufacturer/Contractor to the Engineer and after obtaining his approval.
  4. Any acceptance testing delayed beyond 180 days of production shall require special approval of the Engineer and modified acceptance specification, if deemed necessary by him.
  5. Lot by lot inspection and acceptance shall be made. Size and composition of acceptance lot shall be got approved by the Engineer.
  6. A lot shall comprise all the bearings, including the pair of extra test bearings where applicable, of equal or near equal size produced under identical conditions of manufacture, to be supplied for a particular project.
  7. For the purpose of grading levels of acceptance testing lots will be classified as below:

i) A lot size of 24 or larger number of bearings shall be defined as a large lot.

ii) A lot size of less than 24 bearings shall be defined as a small lot.

  1. When the number of bearings of equal or near equal size for a single bridge project is large and phased production and acceptance is permitted, the number of bearings supplied in any single phase of supply shall comprise a lot under acceptance. When such phased supply is made, each such lot shall be considered as a large lot for the purpose of acceptance testing.
  2. The level of acceptance testing shall generally be graded into the following two levels depending on lot size:

(i) Level 1 acceptance testing.

(ii) Level 2 acceptance testing.

  1. Acceptance testing level 1 is a higher level inspection and shall be applicable to large lots only, unless otherwise specified. This shall involve manufacture of two extra bearings for each lot to be used as test bearing and eventually consumed in destructive testing.
  2. Acceptance inspection level 1 may be specified at the sole discretion of the Engineer, taking into account the special importance of bridge project, for small lots also under the purview of special acceptance inspection.
  3. Acceptance testing level 2 shall be applicable to small lots only unless otherwise specified and shall not involve any destructive testing of finished bearings.
  4. In acceptance testing level 2, all bearings of the lot shall be visually inspected for absence of any defects in surface finish, shape, hardness or any other discernible superficial defects. Also, all bearings shall be checked for overall dimensions, mean bearing thickness, parallelism of bearing surfaces and flatness of load bearing surfaces specified in table.
  5. Testing and inspection procedures shall be as per IRC: 83 (Part-2) and MoRTH Specifications.

2.4.1 Tests on specially moulded test pieces

  1. Test pieces shall be moulded by the manufacturer with identical compound and under identical vulcanising conditions as used in the manufacture of the bearings of the acceptance lot. The process shall be open to for inspection.
  2. Test pieces offered for inspection shall be identified by notable markings and duly certified by the manufacturer.
  3. The quality characteristics to be tested are listed below. Test specifications and acceptance criteria shall be as per IRC: 83 (Part-2).

i) Composition.

ii) Hardness.

iii) Tensile strength.

iv) Elongation at break.

v) Compression set.

vi) Accelerated ageing.

vii) Adhesion strength.

viii) Ozone resistance.

2.4.2 Tests on complete bearings or sections

  1. Two bearings shall be selected at random from the lot as test bearings. These bearings shall be excluded from the lot accepted.
  2. The following tests shall be conducted on test bearings:

i) Test for determination of shear modulus.

ii) Test for determination of compression stiffness.

iii) Test for determination of shear bond strength.

  1. Test specifications and acceptance criteria shall be as per IRC: 83 (Part-2).

2.4.3 Acceptance criteria

  1. The manufacturer shall furnish the following to the Engineer for the acceptance judgment:

i) Quality control certificate as laid down in IRC: 83 (Part-2).

ii) Inspection certificate as laid down in IRC: 83 (Part-2).

  1. The manufacturer shall furnish any supplementary information on the system of quality control and/or process and acceptance control testing as may be deemed necessary by the Engineer.
  2. In case of any evidence of process or acceptance control testing being deemed unsatisfactory by him, the Engineer at his sole discretion may call for a special acceptance inspection of the lot according to specifications laid down by him, without any prejudice to his right to reject the lot. The entire cost of such supplementary inspection shall be borne by the manufacturer/Contractor.
  3. Engineer shall be the sole authority for acceptance of a lot on scrutiny of the certificates according to IRC: 83 (Part-2), along with any supplementary evidence and complete satisfaction therewith.
  4. In case of rejection of a lot, the Engineer shall reserve the right to call for special acceptance inspection for the succeeding lots offered for inspection, according to the specifications laid down by him. The entire cost of such tightened inspection shall be borne by the manufacturer/Contractor.

2.5 Certification and marking

  1. Bearings shall be transported to bridge site after final acceptance by the Engineer and along with an authenticated copy of the certificate to that effect.
  2. Each bearing shall be uniquely and individually numbered on its external faces for identification. The identification number shall be unique and such as to enable other bearings manufactured at the same time, to be traced through the production control records, should the need arise.
  3. The manufacturer's name and unique identification number of the bearing should be vulcanised on the top or bottom of the bearing.
  4. An information card giving the following details for the bearings, duly certified by the manufacturer, shall also be appended:

i) Name of manufacturer.

ii) Date of manufacture.

iii) Bearing dimensions.

iv) Production batch number.

v) Acceptance lot number.

vi) Specific location of bridge, if any.

vii) Explanation of markings used on the bearing.

  1. All bearings shall have suitable index markings made in indelible ink or flexible paint identifying the information and should be visible after installation. The top of the bearing and direction of installation shall be indicated.

2.6 Maintenance

  1. The maintenance of bearings shall be carried out according to a planned schedule.
  2. The structure should be designed and detailed in such a way that the bearings are easily accessible after installation for inspection and maintenance. Arrangements for insertion of jacks to lift the bridge deck shall be made in detailing of structure.
  3. The exposed bearing surface shall be maintained clean and free from contamination with grease, oil or other deleterious matter.
  4. Annual routine maintenance inspection or special maintenance inspection of all bearings shall be made to check the aspects mentioned in (e) & (f) and the results reported.
  5. The top and bottom load bearing surfaces shall be in full contact with the plinth (bottom supporting surface) and the soffit (top supporting surface). If there is imperfect contact between the bearing and the soffit and plinth, the angle between the soffit and plinth shall be checked against the design.
  6. The magnitude of the shear deflection of each bearing shall be checked to ensure that it is within the design specifications.
  7. A visual inspection shall be made of all the accessible edges. A note shall be made of the size and position of any cracks, splits or uneven bulges.
  8. The plinth and soffit shall be examined for signs of displacement from original position of bearing which may be indicated by black marks left on the plinth and soffit.
  9. Where applicable, the sliding surfaces shall be examined for cleanliness and for any movements beyond the design range.
  10. Where applicable, protective coating and/or dust protection shall be examined for signs of deterioration.
  11. Damaged bearings shall be replaced immediately. To avoid differences in stiffness, all adjacent bearings on the same line of support shall also be replaced.

3. POT/POT CUM PTFE BEARINGS

  1. Pot bearings shall consist of a metal piston supported by a disc of unreinforced elastomer confined within a metal cylinder to take care of rotation. Typical cross sections of pot bearings are shown in figure.

                                      

                                   

 

 

a) Pot bearing (Fixed type)      b) Pot cum PTFE (Sliding type)

Fig: Typical cross sections of Pot bearings.

  1. Horizontal movement, if required, shall be provided by sliding surfaces of PTFE pads sliding against stainless steel mating surfaces, with a system of sealing rings.
  2. Pot bearings shall consist of cast steel assemblies or fabricated structural steel assemblies.

3.1 Materials

  1. Mild steel to be used for the components of the bearings shall generally comply with Grade B of IS: 2062.
  2. High tensile steel to be used for the components of the bearings shall comply with IS: 961.
  3. Cast steel shall comply with Grade 280-520W or 340-570W of IS: 1030.
  4. Stainless steel shall conform to AISI 316L or O2 Cr17 Ni12 Mo2 of IS: 6911.
  5. The raw material for PTFE used in bearings shall be pure polytetrafluoro ethylene, free sintered without regenerated materials or fillers. The mechanical and physical properties of unfilled PTFE shall comply with Grade A of BS: 3784 or equivalent. PTFE shall be either in the form of solid rectangular modules or large sheet with dimples formed by hot pressing or moulding. Sheet with dimples formed by machining or drilling from a solid PTFE sheet, shall not be permitted. The surface of PTFE sheets/modules which are to be in contact with metal backing plates shall be provided with suitable chemical treatment for proper bonding. Adhesives used for bonding PTFE to backing plates, shall produce a bond with minimum peel strength of 4 N/mm width when tested in accordance with BS: 5350      (Part C9).
  6. The elastomer including the confined elastomer to be used for the components of bearings shall comply with provisions of table.

Table: Properties of confined elastomer

Sl.

No.

Property

Unit

Code

Limits

specified

1)

Hardness

IRHD

IS: 3400 (Part-2)

50±5

2)

Min. tensile strength

MPa

IS: 3400 (Part-1)

15.5

3)

Minimum elongation

at break

 

As per table

(Ref: IRC: 83 Part-2)

 

4)

Max. compression set

 

-do-

 

5)

Accelerated aging

 

-do-

 

  1. For guide of pot bearings, composite material may be used for achieving lower coefficient of friction and higher strength. Such composite material shall consist of either (a) a bronze backing strip and a sintered inter-locking porous matrix impregnated and overlaid with a PTFE/lead mixture or (b) a mixture of PTFE, glass fibre and graphite embedded in a bronze mesh which is bonded to a galvanised steel backing strip.
  2. The internal seals shall be either of the following:
  1. Brass sealing ring made of metallic brass conforming to 1S: 410.
  2. Poly oxy methylene (POM) sealing chain of proven type consisting of individual interlocking elements made of moulded POM having properties as specified in IRC:83 (Part 3).
  1. External seals and wiper seals shall be made of elastomer conforming to respective provisions.
  2. Fasteners such as bolts, screws, nuts and lock nuts shall generally conform to 1S:1363, 1S:1364, 1S:1365, 1S: 2269, 1S: 3138, 1S: 6761 and 1S: 6639 as appropriate with mechanical properties conforming to 1S: 1367. Threads shall generally conform to 1S: 4218. Washers shall conform to 1S: 2016 and 1S: 6610 as appropriate.

3.2 Workmanship

The following points shall be carefully looked into while installing bearings.

  1. Bearings shall be so located as to avoid the accumulation of dirt and debris on or around them.
  2. Construction of the structure shall be such that water is prevented from reaching the bearings.
  3. In order to avoid contamination of moving surfaces, bearings should not normally be dismantled after leaving the manufacturer's workshop. However, if for any reason, a bearing is required to be dismantled, it shall be done only under expert supervision for which the manufacturer's help may be sought.
  4. Transfer of load from the superstructure to the bearings should not be allowed until the bedding material has developed sufficient strength.
  5. Temporary clamping devices should be removed at the appropriate time before the bearings are required to accommodate movement. The holes exposed on removal of temporary transit clamps should be filled with selected material. Where re-use of these fixing holes are required, the material used for filling the holes should be capable of being easily removed without damaging the threads.
  6. Suitable temporary supporting arrangements under bearing base plates should be made to accommodate thermal movement and elastic deformation of the incomplete superstructure. Such temporary supports, if provided, should be removed once the bedding material has reached its required strength. Any voids left as a consequence of their removal should be made good using the same bedding material. Steel folding wedges and rubber pads are suitable for use as temporary supports under bearing plates.
  7. The bedding material shall be selected keeping in view a number of factors such as the type and size of bearing, construction sequence, load on the bearing, required setting time, friction requirements, access around bearings, design and condition of surface in the bearing area and thickness, strength and shrinkage of bedding material.
  8. Commonly used bedding materials are cementitious or chemical resin mortar and grout. In some cases, it may be necessary to carry out trials to ascertain the most suitable material.
  9. The bedding material, whether above or below the bearing, should extend over the whole area of the bearing in order to ensure even loading. After installation, there shall be no voids or hard spots. The top surface of any extension of the bedding beyond the bearing shall have a downward slope away from the bearing.
  10. The bedding material shall be capable of transmitting the applied load to the structure without being damaged. Surfaces to receive bedding mortar shall be suitably prepared so as to be compatible with the mortar chosen.
  11. Bearings shall be anchored in order to counter vibration and accidental impact. Anchorage shall be accurately set into recesses cast into structure using templates. The remaining space in the recesses shall be filled with material capable of withstanding the loads.
  12. Bearings that are to be installed on temporary supports should firmly fixed to the substructure by anchorage or other means to prevent disturbance during subsequent operations. Voids beneath the bearings should be completely filled with bedding material using the appropriate method.
  13. Bearings may be fixed directly to metal bedding plates that may be in or bedded on top of the supporting structure to the correct level location.
  14. Threaded fasteners shall be tightened uniformly to avoid overstressing of any part of the bearing.
  15. Where bearings are installed prior to casting of an in-situ concrete deck, formwork around bearings should be properly sealed to prevent grout leakage. It is essential that the bearings and particularly the working surfaces are protected  during concreting operations. Sliding plates should be fully supported and care taken to prevent tilting, displacement or distortion of the bearings under the weight of green concrete. Mortar contaminating the bearings should be completely removed before it sets.
  16. For bearings supporting precast concrete or steel beams, a thin layer of synthetic resin mortar should be used between bearings and the beams. Bearings shall be bolted to anchor plates or sleeves embedded in precast concrete elements or to machined sole plates on steel elements.
  17. Bearings shall be located so that their centre lines are within ±3 mm of their correct position.
  18. The level of a bearing or the mean levels of more than one bearing at any support shall be within a tolerance of ±0.0001 times the sum of the adjacent spans of a continuous girder, but not exceeding ±5 mm.
  19. Bearings shall be placed in a horizontal plane within a tolerance of 1 in 200 in any direction, even under superstructure in gradient.

3.3 Inspection and testing

  1. Where any patented items are used, the manufacturer's certificate for the same with test proofs shall be submitted along with the design and got approved by the Engineer before their use in work.
  2. The manufacturer shall have all test facilities required for process and acceptance control tests, installed at his plant to the complete satisfaction of the Engineer.
  3. The test facilities and their operation shall be open for inspection by the Engineer at any time.
  4. The detailed procedure for inspection and testing shall be as stipulated in MoRTH Specifications and IRC: 83       (Part 3).

3.4 Maintenance

  1. Bearings shall be designed and manufactured to make them maintenance free so as to withstand undesirable effects caused by extreme atmosphere or aggressive environmental conditions/ unforeseen events.
  2. Suitable easy access to the bearings shall be provided for inspection and maintenance.
  3. Provision shall also be available for jacking up the superstructure so as to allow repair/replacement of bearings.
  4. The area surrounding the bearings shall be kept clean and dry to avoid damage to the bearings. The bearings shall also be periodically cleaned to remove deposits of salts, debris, dust or other foreign material.
  5. Periodic inspection and nominal maintenance of bearings shall be carried out in order to ensure their better performance and longer life. The bearings are required to be inspected at intervals of one year for the first five years after installation and at intervals of two years thereafter.
  6. The bearings shall also be examined carefully after unusual occurrences such as passage of heavy traffic/oversized loads, earthquakes and battering by floating debris in high floods.

RELATED CODES: 

1.IRC:83:-Standard specification and code of practice for road bridges:

 (Part 1):-Metallic bearings.

 (Part 2):-Elastomeric bearings.

(Part 3):-Pot, Pot-cum-PTFE, Pin and Metallic guide bearings.

2.IS: 1030:-Carbon steel castings for general engineering purposes- specification.

3.IS: 6911:-Stainless steel plate, sheet and strip - specification.

4.IS: 3400:-Methods of test for vulcanised rubber:

(Part 1):-Tensile stress-strain properties.

(Part 2):-Rubber, vulcanised or thermoplastic - determination of hardness.

(Part 4):-Accelerated ageing and heat resistance.

(Part 10):-Compression set at constant strain.

(Part 14):-Adhesion of rubber to metal.

5.BS 3784:-Specification for polytetrafluoroethylene (PTFE).

6.ASTM D297:-Standard test methods for rubber products- chemical analysis.

7.MoRTH:-Specifications for Road and Bridge works (5th revision).

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