The role and layout principles of bridge bearings

1、 The role and requirements of bridge bearings

The support is set between the upper structure of the bridge and the pier, and its function is to:

(1) Transmitting the supporting reaction force of the upper structure, including vertical and horizontal forces caused by dead and live loads;

(2) Ensure that the structure can deform freely under factors such as live load, temperature changes, concrete shrinkage and creep, so that the actual stress situation of the upper and lower structures conforms to the static diagram of the structure.

2、 Principles for Layout of Bridge Bearings

(1) For sloping bridge span structures, it is easy to arrange fixed supports on low elevation piers and abutments

(2) For continuous beam bridges and simply supported beam bridges with continuous bridge decks, in order to disperse the longitudinal deformation of the entire beam at both ends of the beam, it is advisable to place fixed supports near the center of the bridge span; But if the bridge pier at the middle fulcrum is high or due to foundation stress or other reasons, it is very unfavorable for bearing horizontal forces, the fixed support can be arranged on other adjacent piers and abutments according to the specific situation

(3) For particularly wide beam bridges, movable supports that can move both longitudinally and transversely should also be installed. For curved bridges, the possibility of movable supports moving in the direction of the arc should be considered. For beam bridges located in earthquake prone areas, the support structure should also consider the seismic facilities of the bridge, usually ensuring that multiple piers share the horizontal force. The arrangement of fixed and movable supports should be based on the principle of facilitating the transmission of longitudinal horizontal forces between piers and abutments.

(4) For bridge span structures, it is best to compress the lower edge of the beam under horizontal forces, so as to offset some of the tensile stress generated by vertical loads on the lower edge of the beam.

(5) For bridge piers, the direction of horizontal force should be directed towards the riverbank as much as possible, so that the top of the pier is not under tension under the action of horizontal force.

(6) For bridge piers, the direction of horizontal force should be directed towards the center of the pier as much as possible to compress the top of the abutment and balance a portion of the soil pressure behind the abutment.

3、 Precautions for Bridge Bearing Layout:

The arrangement of bridge supports is mainly determined based on the structural form and width of the bridge

A simply supported beam bridge is equipped with fixed supports at one end and movable supports at the other end Due to the small width of railway bridges, the lateral displacement of bearings is minimal, and generally only unidirectional movable bearings (longitudinal movable bearings) need to be installed. For highway T-beam bridges, the possibility of lateral movement of bearings needs to be considered due to the wide bridge deck; A fixed support is set on the fixed pier, adjacent supports are set as unidirectional movable supports that are horizontally movable and vertically fixed, while a longitudinal movable support (corresponding to the fixed support) is set on the movable pier, and multi-directional movable supports are set on the rest.

Each continuous beam bridge is equipped with only one fixed support. To avoid excessive expansion joints at the active end of the beam; The fixed support should be placed on the middle pivot point of each pair. But if the pier body is relatively high, special measures should be considered to avoid or take measures to prevent the pier body from bearing excessive horizontal forces.

The arrangement of supports in curved continuous beam bridges directly affects the distribution of internal forces in the beams; At the same time, the arrangement of the supports should be able to fully adapt to the possibility of longitudinal and transverse free rotation and movement of the curved beam. Spherical supports are usually preferred and are multi-directional movable supports; In addition, a single support point is permanently installed in the middle of the curved box girder, and double supports are only set at the end of one beam (or on the bridge abutment) to withstand torque. Intentionally deviating the curved beam support point towards the outside of the curve can adjust the torque distribution of the curved beam.

When the bridge is located on a slope, the fixed support should be set at the lower end to allow the beam to be compressed under the force of the vertical load along the slope direction, so as to offset some of the tension generated by the vertical load on the lower edge of the beam. When the bridge is located on a flat slope, the fixed support should be set at the front end of the main driving direction.

The effectiveness of bridge use is closely related to whether the bearings can accurately perform their functions. Therefore, when placing the bearings, the position of the upper structure bearings should be aligned with the centerline of the lower structure bearings. However, absolute alignment is difficult to achieve. Therefore, attention should be paid to keeping possible eccentricity within the allowable range, so as not to affect the normal operation of the bearings.

Correctly determining the load borne by the support and the displacement of the movable support is related to the service life of the support. Generally speaking, in addition to bearing vertical pressure, fixed supports must also be able to withstand horizontal forces, including possible braking forces, wind forces, frictional resistance of movable supports, and the tensile force of the main beam elastic deflection on the support. These horizontal forces should always be biased towards the ground and require the support to be anchored or bolted into the upper and lower structures. For curved, inclined, and wide bridges, the stress on the bearings is relatively complex and needs to be carefully studied from three coordinate directions. Even at the same bearing position, different parts may have significant differences in stress.