Bridges with fixities and bearings vs isolated systems

Abstract

Seismic isolation exhibits a breakthrough in contemporary bridge engineering. The principal of isolation is to protect the bridge piers, by either reducing their seismic actions or through the increase in the damping of the structure. However, there are bridges in which the seismic loading of piers is not effectively reduced when using seismic isolation, and hence the use of expensive and expendable isolators can be avoided. The ineffectiveness of seismic isolation with typical elastomeric bearings was observed in bridges with tall piers. As such the piers can be connected with the deck through rotation-free connections, such as fixed bearings or stoppers, while their seismic loading is not significantly increased. A parametric study is conducted with alternative isolated bridge-models to identify the necessity of piers' isolation against longitudinal seismic actions. Bridge-models with bents of variable heights ranging from 5m to 30m and cross sections ranging from flexible to stiff bent-types were analyzed. All bridge-models were re-analyzed considering that shear keys placed on the piers restrict the longitudinal deck displacements. The adequacy of the piers was checked against longitudinal and transverse seismic actions. The analyses for two levels of the seismic action indicated specific bridge design cases that can utilize both rotation-free pier-to-deck fixities and bearings, while the bridge remains essentially elastic.