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.
Articulation
Insertion Point: Used for specifying the bearing location.
Tx [kip/in][Fixed/Free/Stiffness]: When the bearing rotation is 0 degrees, the stiffness in the longitudinal direction is represented by tx. It is typical for a continuous girder to have at least one fixed bearing (or use a real stiffness value) in the tx direction.
Ty [kip/in][Fixed/Free/Stiffness]: The stiffness in the transverse direction is represented by ty when the bearing rotation is 0 degrees.
Tz [kip/in][Fixed/Free/Stiffness]: Tz represents the stiffness in the vertical direction, and it is common to use a high stiffness value such as 1000 kip/in or a fixed.
Rx [kip-in/rad][Fixed/Free/Stiffness]: To address stability concerns, a small RX stiffness can be used in the torsional direction (Rx) under certain conditions. If the constructed girders are not connected with bracings to other girders at any stage, it can result in stability issues. Therefore, to overcome this problem, a small RX stiffness is recommended.
Ry [kip-in/rad][Fixed/Free/Stiffness]: Typically, bearings are free to rotate in Ry direction.
Rz [kip-in/rad][Fixed/Free/Stiffness]: Typically, bearings are free to rotate in Rz direction.
Bearing Rotation [deg]: Curved decks can be guided either radially from a fixed point or tangentially to the radius of curvature. When the deck is guided radially, precise geometry is crucial for the bearings that are farthest from the fixed point. For structures with a constant curvature, it is recommended to align the bearings tangentially to effectively guide the deck around the curve as it expands and contracts.
To achieve tangential alignment, as shown in the figure below, on a curved bridge, the user should set the bearing rotation to 0.
In order to achieve radial alignment, as shown in the figure below, the user needs to input non-zero bearing rotation values. Although inputting a non-zero value will not change the global behavior if the bearing is fixed (or has the same stiffness) in the x-y direction, the spring forces reported in the local axis will be different. Therefore, it is crucial to input rotation values accurately.
Transfer Forces to Substructure[Yes/No]: If the user chooses to connect to the substructure, a two-node spring is generated between the pier cap and the girder. On the other hand, if the transfer to the substructure is selected as no, a single node spring with specified fixities is generated instead.
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