Pulse-like near-fault ground motion effects on the ductility requirement of bridge bents

This paper discusses the ductility requirement for a typical bridge pier or bent subjected to extreme loads such as those from pulse-like near-fault ground motions. The ductility requirement as stated by prevailing design codes may not be valid for such structures in near-fault (less than 10 miles) zones. Due to dominance of the pulses, medium to long period structures are highly affected; often resulting in high residual or permanent deformations after the cessation of seismic ground motions. As per most design codes, the magnitude of displacements associated with P-Delta effects is required to be captured using non-linear time history analysis. The higher the value of the target displacement ductility demand or the P-Delta ratio, the larger is the magnitude of the residual displacements. Because permanent deformations in a bridge bent would lead to a higher eccentric loading from the superstructure, the bent would be subjected to higher secondary moments from it own design load. A suitable balance should therefore be made between ductility and residual displacements in the design of such structures using Performance-Based Seismic Design procedures.