Bearings [SIG]

This section will collect the boundary conditions of the superstructure that are controlled by bearings for finite element analysis. The direction, fixed/free, stiffness value for the bearings, or nonlinear stiffness of the bearings can be entered.

Bearings play a critical role in transferring forces from the superstructure to the substructure while accommodating relative movement. Various actions cause displacements and rotations at supports, including temperature changes (uniform and temperature difference), shrinkage of concrete deck slab, permanent actions (dead loads and superimposed dead loads), variable actions (primarily traffic loads), vertical loads, settlement of supports, and accidental actions such as vehicular collision. Movements can be either permanent (irreversible) or transient (reversible).

In general, the structure rotates about longitudinal and transverse axes at its supports, and these rotations must be accommodated in the bearings or designed to resist them, taking into account their effects on the structure. Horizontal displacements at supports arise from an overall change in the structure's length and bending in a vertical plane. Horizontal forces are resisted at least at one position, usually by preventing horizontal displacement at that position, while horizontal displacements at other positions are due to the expansion/contraction of the length from the fixed bearing and the vectorial sum of the movements due to bending rotation.

To minimize the maintenance liability associated with bearings and expansion joints, it is advisable to limit the number of bearings required and to minimize the movement to be accommodated by an expansion joint. Span arrangements should be designed to avoid uplift at bearing positions, particularly for skewed structures, as providing restraint against uplift in a bearing is complex and costly.

The designer should avoid locking in forces that would hinder bearing replacement. Restraint against longitudinal forces should be provided at one support, with guided restraints aligned to allow movement at the other supports. Similarly, restraint against transverse forces should be provided at only one bearing at each support. The construction sequence of the structure should also be considered to establish permanent displacements.