24-01-2013, 02:15 PM
Bearing
Bearing.ppt (Size: 736 KB / Downloads: 188)
END BEARING
The bearings are provided at both the ends of a bridge girder. One end of the bridge girder is fixed in position while, the other end is kept free for the horizontal movement. The bearings are provided for the following functions:
1- The bearings are provided to transmit the end reaction to the abutments and/ or piers and to distribute it uniformly, so that the bearing stress does not exceed the allowable bearing stress of the material.
2- The bearings are provided to allow the movement in the longitudinal direction (expansion and contraction) due to change in temperature and stresses.
3-The bearings are provided to allow rotation at the ends, when the bridge girders are loaded and deflections take place.
For all spans in excess of 9 m, the provisions are made for change in length due to temperature and stress variation. The provisions for expansion and contraction should be such as to permit movement of the free bearings to the extent of 10 mm for every 10 m of length.
For spans greater than 15m, on rigid pier or abutment, the bearings, which permit angular rotation at the girder ends, are provided, and at one end, there shall be a roller or other effective type of expansion bearing.
TYPES END BEARING
Depending upon the magnitude of end reaction, and the span of bridge, the different types of bearings used for the bridges are as follows:
1-Plate bearings (Sliding& hinged bearings).
2-Rocker bearings
3-Roller bearings.
4-Bearing adopted by Railway Board.
Hinged (Rocker) bearings
The cast steel sole and cast steel bearing block are used in these types of bearings. A cylindrical pin is inserted in between the cast steel sole and the cast steel bearing block. This pin allows rotations at the ends of bridge girder. The rocker bearing acts as hinged bearing. The end reaction of a bridge girder is transmitted to the pin
by direct bearing through the sole attached with the girder. The vertical plates are used to transmit the end reaction. The number of plates (two or three) depends upon the magnitude of end reaction. The end reaction is further transmitted to the cast steel bearing block and then to the supporting structure.
Roller bearings.
The roller bearings as shown in Fig. 13-9 are also used for the long span bridges. Fig. 3.55 (A) shows a single roller used in the bearing.
The rollers provide the rotation as well as the longitudinal movement. Fig. 3.55 (B) shows number of rollers used in the bearing. The bearings act as roller type expansion bearings. The rollers are kept in position by means of dowels, lugs or keys as shown in Fig. 3.55 (A).
The roller bearings for spans above span 35 m should preferably be protected from dirt by oil or grease box.