Implications of Future Seismic Bridge Design Provisions for Illinois

AASHTO and the NCHRP are currently formulating new provisions for the seismic design of highway bridges. As a member of the AASHTO Seismic Design Sub-Committee (T-3), Illinois is playing an advisory role in the process. A vote by AASHTO members on adoption of these provisions into the LRFD Bridge Design Specifications or as guide specifications, in part or in full, could come in the late spring of 2007 with initial publication as early as 2008. The proposed formulation of the new provisions is quite different than those in the current AASHTO LRFD Bridge Design Specifications as well as the Standard Specifications for Highway Bridges. There are two key aspects of differentiation between the current codified provisions and the proposed which Illinois is focusing on for implementation. The design earthquake return period is increasing from 500 to 1000 years and the newly formulated method for seismic analysis and design is primarily “displacement based” as opposed to the current “force based” or “R-factor” approach. Illinois has already begun to lay the foundation for implementation of these expected provisions. The paper presents an overview of these efforts.