Bearing 1 Bearing It is the provided at

Bearing 1

Bearing It is the provided at the interface between two major components of bridge structure i. e. super structure and sub structure.

The factors causing movement in the bridge superstructure ● ● ● ● Bearings transfer the forces coming from superstructure of a bridge to substructure. These also allow necessary movements in the super- structure which are caused by various reasons. Thermal expansion and contraction Deformation under live load Longitudinal forces- tractive / braking Wind loads Settlement of supports Seismic forces Creep and shrinkage of concrete 3

Functions of bearing: : ● To transmit the vertical loads received from superstructure including self weight, superimposed loads like track, ballast, train loads, vertical components of seismic/ wind loads etc to the bed blocks of the sub-structure in a such a manner that the bearing stress induced are within the permissible limits. This can be accomplished if the area of the bed plate of bearing is adequate to ensure that no portion of the bearing/ substructure is over-stressed. Cont 4 …

Functions of bearing: ● Under vertical loads, the girders deflect, the value of which is higher in the middle. This causes rotation in the ends near supports. To cater for rotary movements known as articulation or rocking action cause by deflection of super-structure. ● To allow horizontal longitudinal movement of super structure on account of expansion and contraction. ● Restrict the lateral movement of super structure to ensure alignment of the track on the bridge. Bearings should transfer all the lateral loads like centrifugal/ raking/ wind/ seismic loads etc to the substructure.

Effect of Temperature on Girder if Bearing is Frozen UP

Translation of Bearing ● Translation can be permitted by the following modes of action – By sliding action – By rolling action – By shearing strain Rotation of Bearing ● Rotation can be permitted by the following modes of action – By rocking/hinge action – By differential compression (as in elastomeric pads) – By bending/flexure (as in tall piers, portals).

Classification ● Based on Degree of Freedom 1. 2. Fixed Sliding ● Based on material used Steel 2. Bronze 3. Synthetic material • Elastomeric pads • PTFE-Poly Tetra Fluoro Ethylene • Ultra High Molecular Weight Polyethylene (UHMWPE) 1.

Type Of Bearings ● Sliding Bearings ● Roller Bearing ● Knuckle Pin bearing ● Knuckle Leaf bearing ● Spherical Knuckle bearing ● Cylindrical Knuckle bearing ● Linear Rocker bearing

Type Of Bearings ● Point Rocker bearing ● Pendulum type bearing ● Pot Bearing ● Elastomeric Bearing ● Pot PTFE Bearing ● Spherical / Cylindrical Bearing

Type Of Bearings 11

Selection Of Bearing The selection depends upon a number of factors ● ● ● Functional requirement Expected life –compatible with life of bridge Maintenance efforts- should be minimum Cost Other factors – Height of the bearing – in case of regirdering – Management of horizontal force transferred to the substructure important in case of gauge conversation / upgrading load carrying capacity – Performance under seismic loads. – may alter the choice of bearing particularly in Zone IV & V

Even when bearing has been selected ● Choose the large components – (Rocker & Roller bearing). Small component are prone for accumulating dust & moisture. helps better inspection & Maintenance ● 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

SLIDING BEARINGS

Types Of Sliding Bearing � Shoe type bearing: � Flat sliding bearings: It has two flat steel surfaces only. � Centralized articulated bearings: Similar to flat bearing, but bearing having planing at the end (also called articulation or chamfer) to accommodate rotation of the girders. � Teflon (PTFE) Bearings: On one end of girder, which is the ‘free’ or ‘expansion’ a PTFE sheet is attached to bearing strip which slides over bed plate. Other end has steel to steel ungreased contact & acts as ‘fixed’ end. � Phosphor bronze Bearings : On one end of girder, which is the ‘free’ or ‘expansion’ a bearing strip slides over phosphor bronze plate fixed over bed plate. The other end has steel to steel ungreased contact and acts as ‘fixed’ end.

Different Sliding Bearing ● a) b) Common materials being used and their co -efficient of friction are Material μ Mild steel over mild steel 0. 2 to 0. 3 Mild steel over phosphor bronze c) PTFE over stainless steel 0. 15 Less than 0. 08

ROLLER BEARING

ROCKER & ROLLER BEARING Rocker & Roller bearings: These consist of a rocker which permits rotation & two or more rollers which permits translation and rotation of girder. � Lateral movements are prevented by guides/ sleeves in different components. These may be Single / Multiple Roller bearing. � Rollers are Forged steel of Class-3, as per IS: 2004 and Basic raw material is as per IS: 1875. These are made from C&W axles manufactured after 1931 & turned on machine to have an smooth rolling action. � USFD test is conducted to ensure that there are no internal flaws. � Fixed end has a rocker alone whereas the free end has a rocker as well as rollers. ●

21

• OIL BATH BEARINGS Oil Bath Bearing: Same as roller-rocker bearings. � Used for spans of 76. 2 m and above as mostly they are rail-cum-road bridges as lifting of the girders is difficult due to heavy dead load. � Oil bath meant for housing the roller immersed in oil. A leak proof MS box provided all around roller and filled with oil of proper quality. Rollers are not greased as these become self-lubricating under traffic movement. � Also dip gauge to check level of oil and drain outlet to bottom of MS box is provided to replace the oil. � Once in five year the oil bath is opened out to drain the oil and rollers are inspected and oil is changed. � The minimum size of rollers is 102 mm as per para 3. 1. 2. 3 of Steel Bridge code. 22

Oil Bath Bearing 23

ELASTOMERIC BEARING

Elastomeric bearings Synthetic rubber layers reinforced with steel layers in between which provide the sliding as well as rotation capabilities. These bearings have no moving parts and require least maintenance. There is no arrangement to prevent lateral movements or to provide fixed end in these bearings.

Elastomeric Bearing ● ● ● Elastomer is 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 UIC 772 -2 R 1989 and as per IRC 83 Part II 26

c c laminate b c l

Shear Translation Action

Rotation Bending Action

Action Under Vertical Load ● Vertical load gets converted to shear in the elastomer/ laminate interface

Shape Factor Compressive strength 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 3 mm steel reinforcement plates. ●

Properties of Elastomer (915. 2) Property Test method Value specified I. S. Specification reference 1. Physical Properties : 1. 1 Hardness* 1. 2 Mini. tensile strength 1. 3 Mini. elongation at break 2. 0 Max comp. set (comp strain of 25 % given for duration 24 (+0, -2) hr & temp 100 o C (+ 1 o C) IS: 3400(Part II) IS: 3400 (Part X) 60 + 5 IRHD 17 MPa 400% 35 % of initial

Properties of Elastomer (915. 2) Property Test method I. S. Specification reference Value specified IS: 3400 (Part IV) +15 IRHD 3. 0 Accelerated ageing (When it is heated for a duration of 70 h & at temp 100 ± 1 o C) 3. 1 Maximum change in hardness 3. 2 Maximum change in tensile strength -15 % 3. 3 Maximum change in elongation -30 % 4. 0 Ozone Test 20% strain, 40 o C, 50 pphm ozone conc. , for 96 hours There should be no cracking

Elastomeric Bearings ● The elastomeric bearings offer a number of advantages as listed below: 1. 2. 3. 4. 5. 6. Minimum maintenance- as no moving parts Installation is easy Permits movement in all directions Occupies small space Serves as a shock absorber due to anti-vibrations properties of elastomer As an aid to better management of longitudinal forces.

Anti slip devices

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. 36

POT BEARING 37

WEARING SURFACES PTFE PISTON ELASTOMER POT STAINLESS STEEL PLATE DUST SEAL BRASS SEAL 38

POT cum P. T. F. E. Bearings: ● ● These bearings has a cast steel pot which has non-reinforced rubber. This constitutes the POT that provides the rotation capability to the bearing. The sliding at the free ends is provided by Poly Tetra Fluoro Ethylene (PTFE) surface sliding over stainless steel surface. Slide guides provide restraint in the desired direction. The coefficient of friction in these bearings is quite low, at par with the friction in rollers. These are highly efficient bearings with minimum moving parts. Sliding arrangement of a ‘free’ bearing- stainless steel as well as PTFE surfaces are seen

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. 40

Pot Bearings ● 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 41

Pot Bearings ● Sliding component can be added at top. ● Friction between 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 42

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 43

SPHERICAL / CYLINDRICAL BEARINGS 44

Spherical/ Cylindrical Bearings �These bearings consist of steel concave/ convex surfaces having low friction sliding interface in between which permits rotation by incurve sliding. � These bearings allow rotation in all directions whereas cylindrical bearings allow rotation in one direction only. �The sliding arrangement in both the bearing is similar to the one in POT-PTFE bearings and material for sliding interface may be PTFE or Ultra High Molecular Weight Polyethylene (UHMWPE). �Slide guides provide restraint in the desired direction.

An opened spherical bearing with the rotation and sliding components both visible

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 No limit 20 -1780 47

Suitability of Bearing for Steel spans Type of bearing Bridge & Spans (m) Remarks Both end steel plates. Plate girders 9. 15, 12. 2, Elastomeric on local 18. 3, 24. 4, 30. 5 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 48

Suitability of Bearing for PSC spans Type of bearing Bridge & Spans Remarks (m) Elastomeric I section/ BOX As per Cl. 16. 9. 13 of Bearing section girders 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 49

Bearings recommended to be used for different types of spans S Type of Girders for which generally used N Bearing 1 Sliding 1. Plate Girders of spans upto 24. 4 m. bearings 2. Expansion end of stringers in open web girders wherever provided. 2 Phosphur 1. Composite girders for roads as well as Bronze railway loadings (Old Drawings). bearings 2. Underslung girders 30. 5 m (Old Drawings) 3 Roller. Rocker bearings 1. All open web girders. 2. Underslung 30. 5 m span latest drawings. 3. Composite girders 24. 4 m and 30. 5 m (Old Drawings).

Bearings recommended to be used for different types of spans S Type of Girders for which generally used N Bearing 4 Elastomeric 1. PSC Girders 9. 15 m to 30. 5 m spans (Not Bearings used under slabs). 2. Plate girders upto 30. 5 m (With anti-slip device, as an alternative to sliding bearings) 3. Composite girders upto 30. 5 m spans. 5 POT-PTFE PSC Girders for spans more than 30. 5 m Bearings 6 Spherical/ 1. Long span bridges (Span > 100 m) Cylindrical 2. Continuous girders Bearings 3. Cable stayed/ suspension bearings 4. Rail-cum-Road bearings

Pot Bearings used for the first time for Steel open web Girders of 120 m span of Indian Railways in Mandovi and Zuari Bridges under Konkan Railway. 52

THANKS