AISI announces advancements in steel bridge seismic design

The Steel Bridge Task Force of the Steel Market Development Institute (SMDI), a business unit of the American Iron and Steel Institute (AISI), announced Wednesday that new research has yielded design examples for steel plate girder bridges that meet the requirements for the Type 2 design strategy outlined in the “AASHTO Guide Specifications for LRFD Seismic Bridge Design, 2nd Edition, with 2012 and 2014 Interim Revisions.” The research results could significantly limit the damage sustained in a large earthquake, allowing for quicker repairs and reopening to traffic.

The groundbreaking research was conducted by Ahmad M. Itani, Ph.D., P.E., S.E., F.ASCE, professor in the Department of Civil and Environmental Engineering at the University of Nevada, Reno (UNR). Dr. Itani and his team addressed the lack of a design procedure for steel plate girder bridges in the Type 2 response strategy in the “AASHTO Guide Specifications for LRFD Seismic Bridge Design.” Currently, there are three Global Seismic Design Strategies provided by AASHTO (American Association of State and Highway Transportation Officials) based on the expected behavior characteristics of bridge systems. The Type 2 design strategy is dedicated to an essentially elastic substructure with a ductile superstructure. The current guide specifications do not provide bridge engineers with a complete design procedure for achieving the desired performance when utilizing this strategy. Dr. Itani’s research proposes a Force-Base design procedure that will achieve an essentially elastic substructure and ductile superstructure. 

Six design examples were developed that provide a viable alternative to the conventional design procedure for steel plate girder bridges. They will achieve a better response than the conventional design by allowing inelasticity to occur in the support cross-frames of straight bridges. In the event of a large earthquake, these members would buckle and yield without premature fracture since they are detailed to withstand large cyclical deformations. The cross-frames would bear the brunt of the earthquake’s energy while keeping the superstructure intact. Damage occurring to the support cross-frames could be easily replaced.

The next step is to use these guidelines in the design of actual bridges at two separate locations in the United States. One location will be Los Angeles, California, representing a high-level seismic site; and the other location will be selected in the New Madrid area, representing a mid-level seismic site.