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bridge Bearings.ppt
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- 2. 2 Bearing It is theinterface between two major components of bridge structure i.e. super structure and sub structure.
- 3. 3 Functions of bearing: Load transmission Permit rotary or rocking movement caused by deflection of super structure. Allow horizontal movement of super structure due to expansion or contraction. Restrict lateral movement of super structure
- 4. 4 The factors causingmovement in the bridge superstructure Thermal expansion and contraction Deformation under live load Longitudinal forces- tractive / breaking Wind loads Settlement of supports Seismic forces Creep and shrinkage of concrete
- 5. 5 Load range &movement in Bearings Type of bearing Load (T) Movement (mm) one way Steel sliding plates. 20-133 25 Roller bearing 60-266 100 Elastomeric Bearing 30-220 60 Pot bearing 20-1780 No limit
- 6. 6 EFFECT OF TEMPERATUREON GIRDER IF BEARING IS FROZEN UP
- 7. 7 Translation canbe permitted by the following modes of action : – By sliding action – By rolling action – By shearing strain
- 8. 8 Rotation canbe permitted by the following modes – By rocking/hinge action – By differential compression (as in elastomeric pads) – By bending/flexure (as in tall piers, portals).
- 9. 9 Classification Based onDegree of Freedom 1. Fixed 2. Sliding 3. Rocker and roller Based on material used 1. Steel 2. Bronze 3. Synthetic material • Elastomeric pads • PTFE-Poly Tetra Fluoro Ethylene
- 10. 10 SELECTION OF BEARINGS Theselection depends upon a no. of factors Functional requirement Expected life –compatible with life of bridge Maintenance efforts- should be minimum Cost Other factors – Height of the bearing – Management of horizontal force transferred to the substructure – Performance under seismic loads.
- 11. 11 Even when bearinghas been selected Choose the large components Specify the highest grade of material Insist upon strictest tolerance possible – Because the trouble in bearings cause more problems for the maintenance engineer and the structure
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- 15. 15 DIFF. SLIDING BEARINGS Common materials being used and their co-efficient of friction are Material a) Mild steel over mild steel 0.2 to 0.3 b) Mild steel over phosphor bronze 0.15 c) PTFE over stainless steel Less than 0.08
- 16. 16 MAINTENANCE – GREASING Equipmentsrequired for greasing of sliding bearings : Jacks (appropriate capacity) 2 nos + 1 standby Hard wooden packing (below and above jack) Grease graphite grade 3 conforming to IS 508 Kerosene or released black oil for cleaning 6 mm thick steel scrapers Mortar pan Cotton waste.
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- 23. 23 Greasing – Careto be taken To be done after duly informing P-Way, OHE departments P Way/ OHE departments shall be available Proper train protection must be ensured Girder shall not be lifted excessively Tested equipment/ standby equipment shall be there Girder shall be handled from nominated locations only
- 24. 24 MAINTENANCE ASPECTS Guidestrips and Location strips breakage Holding down bolts corrosion Bed block shaking and cracking – Loose holding down bolts Sinking of bed plates into bed block Crack in weld/ flange near the bearing Corrosion in bearing/ bed plate
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- 36. 38 MAINTENANCE ASPECTS Excesstilting of rollers – Rollers not properly inclined at ambient temperature Flattening of rollers Cracks in rollers Corrosion in rollers – Seizure of rollers Problems in holding down bolts and bed plates
- 37. 39 Rollers Broken andPitted
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- 41. 43 ELASTOMERIC BEARINGS Elastomeris a form of rubber, a class of polymeric substance obtained after vulcanization. (making rubber stronger and more elastic by treating with sulphur at high temperature) As a result of vulcanization, rubber molecules are cross linked with sulphur, making the rubber stronger It possesses rubber like properties i.e. ability to regain shape almost completely even after large deformations These are very flexible in shear but very stiff in bulk compression Design as per UIC772-2R 1989 and as per IRC 83 Part II
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- 45. 47 ACTION UNDER VERTICAL LOAD Vertical load gets converted to shear in the elastomer/ laminate interface
- 46. 48 Shape Factor Compressivestrength of the bearing depends upon the ratio of loaded area to the area of bearing free to bulge. This is known as Shape Factor “S” which is a dimensionless parameter Greater compressive stiffness is ,therefore, obtained by dividing elastomer in to many layers by introducing very thin, usually 1 to 3mm steel reinforcement plates.
- 47. 49 PROPERTIES OF ELASTOMER (915.2) PropertyTest method I.S. Specification reference Value specified 1. Physical Properties : 1.1 Hardness* IS:3400(Part II) 60 + 5 IRHD 1.2 Mini. tensile strength IS:3400 (Part I) 17 MPa 1.3 Mini. elongation at break 400% 2.0 Max comp. set (comp strain of 25 % given for duration 24 (+0, -2) hr & temp 100o C (+ 1o C) IS:3400 (Part X) 35 % of initial
- 48. 50 PROPERTIES OF ELASTOMER (915.2) PropertyTest method I.S. Specification reference Value specifie d 3.0 Accelerated ageing (When it is heated for a duration of 70 h & at temp 100 ± 1o C) IS:3400 (Part IV) 3.1 Maximum change in hardness +15 IRHD 3.2 Maximum change in tensile strength -15 % 3.3 Maximum change in elongation -30 % 4.0 Ozone Test 20% strain, 40o C, 50pphm ozone conc., for 96 hours There should be no cracking
- 49. 51 ELASTOMERIC BEARINGS Theelastomeric bearings offer a number of advantages as listed below: 1. Minimum maintenance- as no moving parts 2. Installation is easy 3. Permits movement in all directions 4. Occupies small space 5. Serves as a shock absorber due to anti-vibrations properties of elastomer 6. As an aid to better management of longitudinal forces.
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- 51. 53 Limitations of Elastomeric Bearings Ordinary elastomeric bearing can’t be used as a fixed bearing Translation allowed by elastomeric bearing is restricted by its thickness – 0.5 to 0.6 of thickness Thick elastomeric pads are rather unstable Limit of vertical load which can be placed safely on elastomeric pads – It causes excessive compression & bulging.
- 52. 54 Maintenance Aspects Bulgingof neoprene Tearing of neoprene Tilting of bearing Disintegration of bearing Soft bearings (excess vibrations)
- 53. 55 General Guidelines for Inspection Shear deformation more than 50% of height of Elastomeric Pads Rotation leading to off loading of an edge Compression more than 5% of height of the pads
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- 59. 61 POT BEARINGS Potbearing – It take beneficial properties of elastomer in fixed bearings & design of large expansion bearings Rotational movement permitted by shear deformation of an elastomeric pad Translational movement restraint by completely encasing the elastomeric pad in a POT No compressive deflection of elastomer as it is encased Sliding component can be added at top.
- 60. 62 POT BEARINGS Frictionbetween PTFE and stainless steel is highly susceptible to intrusion of dust Hence Silicon grease is generally used as lubricant Dust seals are also provided around PTFE bearings
- 61. 63 PTFE It is a Linear chain polymer of high molecular strength Chemically inert Low coefficient of friction Not oxidized easily Remains stable at extreme atmospheric temperatures Resistant to all common solvent.
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- 69. 72 Pot Bearings usedfor the first time for Steel open web Girders of 120m span of Indian Railways in Mandovi and Zuari Bridges under Konkan Railway.
- 70. 73 Advantages of POT/PTFE When limitation of overall height of the bridge Large vertical force involved Large movements- in rotational as well as in translation anticipated More life Less maintenance efforts
- 71. 74 Load range &movement in Bearings Type of bearing Load (T) Movement (mm) one way Steel sliding plates. 20-133 25 Roller bearing 60-266 100 Elastomeric Bearing 30-220 60 Pot bearing 20-1780 No limit
- 72. 75 Suitability of Bearingfor Steel spans Type of bearing Bridge & Spans (m) Remarks Both end steel plates. Plate girders 9.15, 12.2, 18.3, 24.4, 30.5 Elastomeric on local conditions phosphor bronze Composite 9.15, 12.2, 18.3, 24.4, 30.5 For U/S O/W - 30.5 (With one end fixed) Rocker & Roller O/W through girders- 30.5, 45.7(2 rollers), 61.0, 76.2 (4 rollers) One end fixed & other free Rocker & Roller with oil bath O/W through girders more than 76.2 -do- POT PTFE bearings are being used for longer spans
- 73. 76 Suitability of Bearingfor PSC spans Type of bearing Bridge & Spans (m) Remarks Elastomeric Bearing I section/ BOX section girders As per Cl. 16.9.13 of Concrete Bridge Code, Elastomeric bearings shall be restricted for clear spans more than 30.5 m POT / PTFE bearing All PSC spans ≥ 30.5 M Earthquake restraint shall be provided for longer spans
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