Displacement-Based Seismic Design Integrating AASHTO and IBC Codes

As energy costs soar, public demand for massive transportation systems has increased. Massive transportation systems often include structures such as station buildings that are linked by elevated bridges. It is essential for these aerial structures to withstand large earthquakes. Structural engineers typically design station buildings according to the IBC which references ASCE 7, while bridge design is based on the AASHTO LRFD bridge design code and the Guide Specification for seismic design. IBC and AASHTO LRFD both use force-based design, but with different spectral loads, analysis parameters, and levels of acceptance. The newly published AASHTO Seismic Guide Specification applies displacement-based seismic design as its framework. However, there is no unified code for the seismic design of this kind of mixed structural systems in the United States. How will these codes be implemented in the design of mixed structural systems without violating either code? Based on displacement-based seismic design concepts, this paper uses a case study to show how the seismic design of mixed structural systems can satisfy the performance objectives of both the IBC and AASHTO codes without producing a design that is overly conservative.