Proposed Framework for the Performance-Based Seismic Design of Highway Bridges

The current AASHTO Guide Specifications for LRFD Seismic Bridge Design and the Seismic Provisions contained in AASHTO LRFD Bridge Design Specifications are mainly based on prescriptive design methodologies that only marginally relate important design parameters to the seismic performance of bridges. With the current specifications, a designer does not have direct control over the seismic performance of bridges, other than to ensure that their capacity exceeds the demand. In recent years, there is an emerging need for bridge designers to be able to incorporate multiple levels of seismic performance into bridge design based on owner's input and budgetary considerations. The new performance-based seismic design (PBSD) methodology enables engineers to make informed decisions about the seismic performance of bridges through incorporation of different seismic hazard levels and a multitude of operational expectations. Once incorporated, this design methodology allows designers and owners to better understand and quantify the seismic performance of bridges while considering its post-earthquake designated functions such as instant access by emergency responders or immediate availability for traffic. Although PBSD is relatively well developed for buildings, its application to bridges is still at a nascent stage. There is therefore a need for a clear and consistent approach to relate typical engineering demand parameters (EDPs) like displacements or strains, to potential damage (e.g., concrete spalling and cracks, steel yielding and fractures, instability or excessive elastic or permanent deformations) to the risks associated with such damage (e.g., loss of life, substantial repair costs, and downtime or indirect risks such as productivity slowdown and economic loss). This paper examines some existing knowledge gaps in implementing PBSD to highway bridges and proposes a framework to close some of these gaps. The proposed framework will provide more clarity toward a more meaningful assessment of bridge performance and evaluation of its potential use following an earthquake.