In an earthquake the energy resulting from the forces will be transmitted by the ground to the structure and the damage will be directed to the piers. Hence to reduce these forces a complex design becomes imperative.
The seismic effect on a structure is normally calculated separately according to three directions in space X, Y and Z. For a straight bridge, these directions are the longitudinal, transversal and vertical axis of the deck. This last component of seismic movement has to be taken into account for the verification on the bearings.
We therefore have three seismic forces acting on the bearing:
- Horizontal force that creates longitudinal distortion
- Horizontal force that creates transversal distortion
- Vertical force that causes the load path to become heavier or lighter
Imam Khomeini Bridge - Esfahan IRAN
The combination of these three forces has to be taken into account plus other loads such as permanent loads, the effects of shrinkage, concrete creep effects and displacements due to temperature variations.
For urban bridges carrying heavy traffic has to be added to the normal operating loads.
Elastomeric bearings supplemented by a blocking device taking up seismic forces
In some cases, it is useful to block the functioning of bearings in one of the two horizontal directions, for example, to preserve the integrity of the equipment (such as expansion joints or restraint systems) or else because we do not wish to design bearings for seismic design force. Obviously, the dynamic calculation model must take account of this block.
The device can be the same as the seismic coupling described above, the difference being that the clearance is reduced to a value not exceeding 15 mm. This measurement is a compromise between:
The on-site construction tolerances
- A clearance required to leave deformations free in the direction perpendicular to the blockage
- A clearance not to be exceeded so as to avoid the effects of impacts.
In this case, seismic coupling must be designed to resist the calculation effects arising from the principle of capacity design (forces resulting from the plasticisation level being exceeded in the underlying pier).
It is obvious that such bearings do not take up seismic forces. However, it must be designed to support, without damage an Elastomeric bearings in conjunction with a sliding device.