I.G. Buckle ⁽¹⁾, T.P. Murphy ⁽²⁾, M.L. Marsh ⁽³⁾, M. Kowalsky ⁽⁴⁾

This paper reviews two developments in the seismic design of bridges in the United States, both of which have recently been adopted by the AASHTO Committee on Bridges and Structures. In addition, progress is reviewed on a new initiative to better estimate inelastic displacements in bridges during earthquakes.

The first of the new developments is a set of guidelines for the performance-based seismic design (PBSD) of bridges. In these guidelines, a methodology is developed to implement PBSD using strain-based engineering design parameters (rather than ductility-based parameters) to control the performance and expected damage under a two-level seismic hazard approach. The methodology includes determination of analysis and capacity requirements based on the desired performance, seismic hazard, and attributes of the bridge.

The second development is a major change in the way seismic hazard is characterized for bridge design in the U.S. Previously it was based on set of uniform hazard maps in which the return period of the hazard was the same throughout the country. But the consequence of this approach is a non-uniform likelihood of collapse in the life of the bridge. To address this concern, risk-targeted ground motions have been adopted along with their corresponding design spectra. These involve the integration of the hazard curve for each site with bridge fragility to achieve a target collapse probability of 1.5% in the life of a bridge, taken to be 75 years. Uniform risk in the life of a bridge is therefore the focus of these motions rather than uniform occurrence. 

Finally, a fundamental step in displacement-based seismic design is the estimation of displacement demand. Current approaches are based on elastic methods. Often called the ‘equal-displacement’ rule, it has recently been shown to significantly underestimate displacements for some common bridge configurations. The reason is believed to be the way damping is modeled in the benchmark structures. This recently funded project will investigate the limitations of the ‘equal-displacement’ rule and perhaps propose an alternative for use in design.