Ultimate restraint considerations in base-isolated bridges

ABSTRACT

Whether specifically carried on seismic isolation bearings or, as is more usual, supported on normal bridge bearings, bridge decks require displacement control to prevent the deck falling off the supporting structure in strong seismic shaking. Transverse restraint is frequently provided by some form of slack "key" whereas longitudinal movement is often controlled by using the abutments as buffers. Such restraints have to be designed to allow normal differential movements such as are prompted by thermal changes, traffic vibration and braking, or small earthquake shaking, but they must be capable of surviving strong earthquake motion generated displacements. It is a matter of concern lest the protection conferred by the flexible bearings om piers, columns or abutments may be depleted significantly by the effects of shock throughout the bridge as the displacement control is activated. This paper describes progress with an ongoing project involving the mathematical modeling of a simple bridge structure supported on bearings such that the longitudinalnatural period is substantially lengthened from the non-isolated condition. Provision is made for the superstructure to be subjected to longitudinal displacement control as the deck impacts the abutments, or stops provided at the tops of the pier walls or columns, under the action of strong ground motion.(AU)