Performance of Ordinary Seismically Isolated Bridges Beyond Design Shaking

Gilberto Mosqueda, PhD

Professor, Department of Structural Engineering

Mervyn Lea Rudee Endowed Chair,  Jacobs School of Engineering 

Director, Caltrans Seismic Response Modification Device Facility

University of California, San Diego 

Monday, May 4, | 2 p.m. | 140 Ketter Hall

Seismic isolation is widely used to improve bridge performance under design-level earthquake demands; however, system behavior under beyond-design-basis loading remains less understood. Current Caltrans design provisions for seismically isolated bridges consider the isolation system as the primary earthquake-resisting system (ERS), supplemented by a secondary earthquake-resisting system (SERS) intended to engage for beyond-design loading before the isolation layer reaches critical limit states. The SERS can be in the form of ductile substructure columns designed to undergo controlled plastic hinging, thereby redistributing deformation demands and delaying the failure of the isolation system.

To investigate this interaction, a shake table experimental program was conducted on a quarter-scale, two-column reinforced concrete bridge bent equipped with lead rubber bearings and designed in accordance with Caltrans design requirements. Throughout beyond-design testing, the isolation system demonstrated stable hysteretic behavior, with peak shear strains in bearings limited by interaction with the substructure. 

Following SERS activation, deformation demands and energy dissipation increasingly shifted to the columns, which also maintained a stable hysteretic response. At the same time, the isolation layer continued to dissipate energy at a reduced proportion. These results demonstrate that controlled column yielding can effectively limit isolation system demands and provide a stable load path under beyond-design loading for collapse prevention. This design approach is being examined for more cost-effective and widespread implementation of seismic isolation in ordinary bridges

Gilberto Mosqueda is a Professor in the Department of Structural Engineering and the Mervyn Lea Rudee Endowed Chair in the Jacobs School of Engineering at the University of California, San Diego. Previously, he held Assistant and Associate Professor positions at the University at Buffalo. He received his PhD from the University of California at Berkeley, his MS from the Massachusetts Institute of Technology, and his BS from the University of California at Irvine, all in civil engineering.

Professor Mosqueda is the Director of the Caltrans Seismic Response Modification Device Test Facility, which tests full-scale seismic isolation and damping devices. Professor Mosqueda is the recipient of the NSF CAREER Award, the American Society of Civil Engineering Moisseiff Award, the Mexico College of Civil Engineering Jose A. Cuevas Award, and a Fellow of ASCE.