BEARING

BEARINGBEARINGThe 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.Bridge bearings are of two general types,1. Expansion.2. Fixed.The expansion bearings provide for rotational movements of the girders as well as longitudinal movement for the expansion and contraction of the bridge spans. If an expansion bearing develops a large resistance to longitudinal movement due to corrosion or other causes, this frictional force opposes the natural expansion or contraction of the span creating a force within the span that could lead to a maintenance problem in the future.

FIXED BEARING The fixed bearing acts as a hinge by permitting rotational movement while at the same time preventing longitudinal movement. The function of the fixed bearing is to prevent the superstructure from moving longitudinally off of the substructure units. TYPES END BEARINGDepending 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.

1-Plate bearings (Sliding& hinged bearings). Plate bearings are simplest type of bearings. The plate bearings are used small spans upto 15 m and small end reaction of the bridge. Fig. 13-1 shows a plate bearing. The plate bearing consists of two plates.

A sole plate is attached to the bridge. The sole plate rests on bearing. The bearing plate is anchored to the concrete. The two anchor bolts fixed in concrete pass through the bearing plate and the sole plate. The size of bearing plate is found by the end reaction and the allowable bearing pressure on the concrete. The plates are made rigid to distribute the end reaction as uniformly as possibly over the required area of the concrete. When the anchor bolts pass through the circular holes in the sole plate, then, the plate bearings act as hinged bearing. One end of the bridge girder is hinged or anchored to the concrete through the hinged bearings.

The hinged bearings are designed for the end reaction (vertical load) and the lateral forces. The magnitudes of end reactions used are large. Therefore, the fixed bearings designed for end reactions (vertical loads) only strong enough to take the lateral forces.In order to allow the longitudinal movement, the slotted holes are provided in the sole plate. In order to reduce the friction, the surfaces of sole plate and bearing plate in contact are well machined and smoothly finished. The sole plate can slide upon the bearing plate. The plate bearings act as expansion bearings of sliding type. In the expansion bearing, the longitudinal movement (expansion or contraction) takes place with change of temperature and loads Fig. 13-8 shows a typical rocker bearing.

Fig. 13-82-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. Two outer vertical plates completely encircle the pin. In case, the bearing is subjected to an uplift, then, the uplift is resisted by theses plates. The middle plates provide only bearing with the cylindrical surface of the pin. The required bearing area is provided by the product of total thickness of plates and the diameter of pin. The thicknesses of all the plates are kept equal. Therefore, the end reaction is transmitted equally by these plates. The value of bending moment is found by multiplying force transmitted by outer plate of the sole to the outer plate of bearing block and center to center distance between these plates. The size of base plate is found by the allowable bearing stress in the concrete and the end reaction. The rocker bearing are also bearings are also subjected to lateral and longitudinal forces in addition to the end reaction (vertical loads). The increase of end reaction due to lateral and longitudinal forces is also taken into consideration. The lateral forces and the longitudinal forces are assumed to act at the level of cylindrical pin of the rocker bearing. The base plate is subjected to moment along both the directions. The total bearing stress in the concrete should not exceed the allowable bearing stress.The rocker bearings are designed for the end reaction and then checked for lateral forces and longitudinal forces.Figure 3.54 shows the rocker bearing for the hinged end. In the rocker bearing for free end of the bridge girder the underside of sole is curved, which rotates on the horizontal bearing plates and allows longitudinal movement. This acts as rocker type expansion.

3-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.So long as, the size of rollers is small, the complete circular rollers are provided. When the size of rollers become large, then, the sides of rollers are cut in order to reduce the length of the sole, and to make the bearings more compact. These rollers with cut sides are known as segmental rollers.

Fig 13-9In order to avoid overturning or displacement of these rollers, these are geared with upper and lower plates. The spacing between segmental rollers and the width of rollers may by found as below:It is assumed that the rollers don not slip but only roll during rolling. When, the roller rolls to the maximum position, as shown in Fig. 13-10,

Fig 13-10