What are fixed bearings on bridges? Aren't plate rubber bearings supposed to be the same, without distinction between fixed and movable bearings?

This world isn't just about plate rubber bearings!

Bearings function to transfer various loads from the superstructure and accommodate displacements caused by temperature, shrinkage, creep, and other factors.

They are generally divided into fixed bearings and movable bearings. Fixed bearings primarily constrain vertical and horizontal displacements, while movable bearings constrain vertical displacement and may also constrain some horizontal displacement.

For simply supported beams, a fixed bearing at one end and a movable bearing at the other are generally used. For continuous beams, a fixed bearing is typically installed on one pier within each span. The bearing placement should facilitate the transfer of horizontal forces from the pier.

Main types of bearings: simple bearings, curved steel plate bearings, and rubber bearings. Rubber bearings are further divided into plate rubber bearings and pot rubber bearings. Medium- and small-span highway bridges generally use plate rubber bearings.

Large-span continuous beam bridges generally use pot rubber bearings.

Railway bridges generally use steel bearings.

1. Simple Bearing: Made of several layers of tar paper or asbestos, compacted to a thickness of not less than 1cm, used for slab girder bridges with spans less than 10m.

2. Curved Steel Bearing: Applicable to spans of 10~20m.

The upper and lower contact surfaces of the bearing are curved, allowing for concentrated force transmission and free rotation of the beam ends. However, it requires overcoming significant frictional resistance during expansion and contraction, thus it is only suitable for beams with smaller spans.

3. Plate Rubber Bearings

These are avail
able in rectangular and circular shapes.

1 indicates a reinforcing steel plate, 2 indicates a rubber layer.

Due to the different elastic moduli and stiffness of steel plates and rubber, the rotation of the bearing is achieved through different deformation magnitudes when there is uneven elastic compression; horizontal displacement is achieved through the shear deformation of the rubber. Plate rubber bearings do not distinguish between fixed and movable bearings!

PTFE sliding plate rubber bearings are formed by bonding a PTFE sheet of a certain thickness to the top surface of a regular plate rubber bearing. Other variations include spherical crown circular plate rubber bearings, sloped plate rubber bearings, and lead-core rubber bearings.

4. Pot Rubber Bearings

This is where the distinction between fixed and movable bearings begins! Pot rubber bearings feature large horizontal displacement, flexible rotation, light weight, compact structure, simple construction, low building height, convenient processing and manufacturing, steel saving, and reduced costs, making them ideal bearings for large-span bridges.

Pot rubber bearings include two main categories: fixed bearings and movable bearings. Movable bearings are further divided into unidirectional and bidirectional movable bearings. Generally, fixed bearings are used at the fixed end of a bridge, and movable bearings are used at the movable end. For example, in a simply supported beam bridge, a fixed bearing should be installed at one end of each span, and a movable bearing at the other end; in a continuous beam bridge, a fixed bearing should be installed on one pier in each span, and movable bearings should be installed on the remaining piers. However, if the bridge deck is wide, the distance between the two bearings at the fixed end is large, and the transverse expansion and contraction cannot be ignored, then a unidirectional movable bearing cannot be used at the fixed end. This bearing is rotated 90 degrees and placed under the beam, thus ensuring both longitudinal fixation and transverse movement. Furthermore, to reduce the stress on the piers, for simply supported beam bridges, fixed bearings should be placed on the lower piers; for continuous beam bridges, to distribute the longitudinal deformation of the entire beam at both ends, fixed bearings should be placed at the middle support point.

Bidirectional movable bearings can move in any direction within the horizontal plane. Therefore, this type of bearing should be selected for movable piers on curved bridges. As for unidirectional movable bearings, they can be used on straight bridges. However, it should be noted that they are only suitable when there is only one bearing on the movable pier, or when the lateral thermal expansion and contraction between the bearings is very small.

The movable pot bearing consists of an upper bearing plate, a polytetrafluoroethylene (PTFE) plate, a pressure-bearing rubber block, a rubber sealing ring, an intermediate bearing plate, a steel clamping ring, a lower bearing plate, and an upper and lower bearing connecting plate. A fixed bearing can be formed by combining the upper and middle bearing plates or by using the upper and lower bearing connecting plates.

5. Spherical steel support

a. Fixed support

Fixed supports constrain the structure in both the X and Y directions, while spherical supports exhibit no displacement in the Z direction. Therefore, the superstructure cannot translate in any of the X, Y, or Z directions. The internal spherical sliding plate allows for a degree of rotation within the structure.

b. Unidirectional movable supports

A unidirectional sliding support restricts sliding in one direction while allowing free sliding in the other. As shown in the diagram, the support is vertically movable but horizontally constrained in the plan view.

c. Bidirectional sliding support

There are no constraints in the X and Y directions, only in the Z direction. Rotation is also unconstrained, similar to a vertical hinged support in structural mechanics.

The main structure of the support moves synchronously with the beam, and the force distribution on the beam does not change with displacement, ensuring that no additional moment is generated during beam displacement. The support transmits force through a spherical surface, resulting in relatively uniform reaction forces acting on the supporting concrete pad. The support has a small rotational torque and consistent rotational performance in all directions.