A bridge is designed to allow slight movement at a support. Usually a provision is made so that movement of 1 mm can take place per 30 m. It is also necessary to make provision to allow for changes in length due to live load stresses. In spans over 90 m long it is necessary to make allowances for expansions and contractions in the floor system.
Expansion bearings are needed to allow such movements. In addition, in order to control the movements, it is necessary to provide at least one fixed bearing in each single or continuous span. A fixed bearing is a bearing which is firmly anchored to prevent horizontal and vertical movement but can allow only end rotation in a vertical plane. An expansion bearing permits only end rotation and also longitudinal movement of the supported member.
Where the spans exceed 15 m, the bearings should be designed to allow end rotation. For this purpose curved bearing plates, elastomeric pads or pin arrangements may be used, elastomeric bearings being generally preferred. At expansion bearings, the spans should be provided with rollers, rockers or sliding plates. In the case of short spans, the spans may slide on smooth metal plates.
The supports should be free from accumulation of dirt which may obstruct the free movement of the spans. The supports should also be free from trapping of water which may cause corrosion.
ADVERTISEMENTS:
Self-lubricating bronze or copper alloy sliding plates are also used in expansion bearings in place of elastomeric pads, rollers or rockers. These sliding plates should be at least 12 mm thick and provided with chamfered ends. Sole plates and masonry plates shall have a thickness of at least 20 mm.
Elastomeric pads are bearings which are made of elastomer. These are used to transmit loads from a structural member to a support permitting movement between the bridge and the support. Pads may consist of alternate layers of steel or fabric reinforcement, bonded to the elastomer. In addition to this, the bearings may also have external steel plates well bonded to the elastomeric bearings.
PTFE pads are bearings having sliding surface made of polytetra fluoroethylene (PTFE) consisting of filled or unfilled sheet, fabric with PTFE fibres, interlocked bronze and filled PTFE structures. The sliding surfaces of the pads allow translation as well as rotation by sliding of the PTFE surfaces over a smooth hard contact surface.
This may preferably be of stainless steel or any corrosion resistant material. In order to avoid local stresses on the sliding surface, an expansion bearing should allow a rotation of at least 1° due to live load or any misalignment of the bearing. This condition can be achieved with the use of hinges, curved sliding surfaces or elastomeric pads.
ADVERTISEMENTS:
Pot Bearings:
These are provided mainly for long span bridges. These bearings are available as fixed, guided expansion and non-guided expansion bearings. These are designed to allow for thermal expansion and contraction, rotation, camber changes and, creep and shrinkage of structural components.
A pot bearing consists of an elastomeric rotational element which is confined and sealed by a steel piston and steel base pot. In effect the supported end of a structure, supported on a pot bearing floats on a low profile hydraulic cylinder or pot containing the elastomer as the liquid medium.
ADVERTISEMENTS:
These are very commonly provided at the supports for a bridge. Often it becomes necessary to make provision for the end supports of a structure so as to allow certain small movements. For example, it is necessary that one end of a bridge should be supported on rollers or rockers to allow for changes in the span length due to temperature variations.
When the end of a girder rests on masonry or other support, provision should be made to distribute the load on sufficient area of masonry such that the intensity of the bearing stress is less than the safe stress (4 N/mm2). For example, if the maximum end reaction is 1000 kN, the required area of bearing equals –
A bed plate of this area will be provided to distribute the load on the masonry. A bed plate 500 mm x 500 mm may therefore be suitable for the example cited above. It is also a practice sometimes to provide an additional plate called the sole plate. Sole and bed plates shall be planed smooth and straight.
ADVERTISEMENTS:
At expansion ends the sliding surfaces shall be planed. The bottom of webs and connecting angles of pedestal should also be planed before the base plates are connected. Steel castings shall also be planed so that they are smooth and straight. All steel casting shall be true to the required sizes after annealing.
Spans over 25 m in length should be provided with pin bearings and pedestals at both the ends. A pedestal is usually built-up of web plates and base plates, not less than 20 mm in thickness. The webs of pedestals must be connected to the base plates by angles not less than 150 mm x 100 mm x 12 mm size with the 150 mm leg placed vertically.
ADVERTISEMENTS:
In order to distribute the load over the bearings, the pedestal should be of sufficient height. The bottom of the pedestals and the length of the bed plate will be made equal. It is advisable to provide the top of the bridge seat above high water. Usually the height is about one-half the length. Fig. 13.49 (a) shows the elevation of the end joint of a plate girder bridge. The end view of the same joint is shown in Fig. 13.49 (b). The plan of the pedestal is shown in Fig. 13.49 (c) while Fig. 13.49 (d) shows the top view of the rollers.
The bottom plate of the pedestal is usually 35 mm to 40 mm thick and the angles connecting the vertical webs to the base plate are usually 150 mm × 150 mm × 20 mm angles. These angles and the base plate are connected by rivets counter sunk on the underside. The pedestal is stiffened by riveting the diaphragm to the vertical webs.
A diaphragm is inserted in the chair so that the load from the floor beam may be more evenly distributed between the two sides. One end of the bridge is fixed and the other is placed on rollers. At the fixed end the base plate will rest directly on masonry. The plate is anchored to it by means of anchor bolts. At the roller end rollers are placed under the base plate so as to allow the end to move forwards and backwards due to temperature changes.
Rollers:
Spans over 25 m in length should be provided with rollers at one end. A roller shall not be less than 75 mm in diameter and shall be turned down to a groove 6 mm deep to fit guiding strips of this thickness on the bearing plates above and below the rollers.
Rollers are spaced above 6 to 8 mm apart. These are provided for the full length of the pedestal. These are made at least as long as the distance between the outer edges of the angles g-g. The rollers are connected to each other so as to form a roller nest short struts m-m about 20 mm in diameter are set into the ends of each cylinder for 40 mm and the ends will be left to project 12 mm at each end.
A flat bar is placed at each end of the rollers and connected by short bolts. The bars n are held in position by bolts O and extra bars p at each end of the nest. A plate 50 mm in thickness is provided under the rollers to afford a smooth surface to the rollers and also to distribute the reaction evenly on the masonry.
In order to prevent the pedestal and rollers from moving side wise the bearing surface above and below rollers are provided with projections which fit into grooves placed in the rollers.
Rocker Bearing for Plate Girders:
When the span of a girder exceeds 25 m the ends are supported on rocker and provided at one end on rollers. When it is felt necessary to keep the bridge seat close to the bottom of the girders, an arrangement shown in Fig. 13.50 can be adopted.
In this arrangement the outspread legs of the lower flange angles are cut for a short distance at each end so as to allow the lower flange to enter between the vertical plates of the pedestal. The web of the girder is reinforced at the end and a pin is passed through the pedestal and the web. The pin shall be atleast 150 mm in diameter.
A bearing or bed plate shall be so designed that with the eccentricity of the loads due to the combined effect of dead load, live load, impact centrifugal force, longitudinal and racking forces etc. the maximum pressure on the bearing or bed plate shall not exceed the limits given below.
(The diameter of pin shall not be less than 10 mm)
Safe stresses in turned and fitted knuckle pins shall be taken as follows:
i. Bearing – 120 N/mm2 of projected area
ii. Bending – 210 N/mm2
iii. Shear – 100 N/mm2
For steel sliding on steel, hard copper alloys or cast iron the bearing stress may not exceed 30 N/mm2.
Roller bearings shall be arranged such that the loading is uniformly spread on the rollers longitudinally as well as laterally. The rollers should have free movement and they shall be provided in sufficient number so that the stresses in the rollers are within safe limits.
The rollers shall be so arranged that they are suitably accessible to clean them periodically.
Safe Loads on Cylindrical Rollers:
Safe load P on a roller is usually expressed per unit length of the roller.
(a) Cylindrical Rollers Supported on Curved Surfaces:
(b) Cylindrical Rollers Supported on Flat Surfaces (Diameter not less than 100mm):
Segmental Rollers:
Let b be the allowable clearance between the rollers after the horizontal displacement.
Let a be the clear distance between the rollers in the normal position.
We have, (b + d) = (d + a) cos θ
a = b sec θ + d (sec θ – 1)