New Research Results Advance the Seismic Design of Steel Bridges
The Steel Bridge Task Force of SMDI announced 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 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.
“These design examples will significantly alter the way that steel bridges are built in the future,” said Alex Wilson, chairman of SMDI’s Steel Bridge Task Force and manager of customer technical services for ArcelorMittal USA. “They will provide significant cost savings on bridge repairs and time savings in returning the bridge to serviceability. Instead of having to repair the superstructure, which can be extensive, engineers can now focus their repair efforts on the support cross-frames. We are appreciative of the work that Dr. Itani and his team have accomplished in developing these design examples and are anticipating the next phase of research, which will put the design examples into actual practice.”
Wilson said that SMDI plans to present these results to the AASHTO Subcommittee on Bridges and Structures – T-3 Seismic Design. SMDI will also advocate for adoption of the proposed design language for ductile superstructure.
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, Calif., representing a high-level seismic site; and the other location will be selected in the New Madrid area, representing a mid-level seismic site.
The results of Dr. Itani’s research are published in a technical report titled “Seismic Design and Nonlinear Evaluation of Steel I-Girder Bridges Using Conventional and Ductile Support Cross-Frames,” which is available for free download here
SMDI’s Steel Bridge Task Force
was formed more than 40 years ago to coordinate research that establishes safe, cost-effective steel bridges and to implement these developments into steel bridge design codes, specifically the AASHTO design codes. Its members include steel producers, steel fabricators, the AASHTO Technical Committee for Structural Steel Design, university faculty, consultants and representatives from the Federal Highway Administration and National Steel Bridge Alliance.
serves as the voice of the North American steel industry in the public policy arena and advances the case for steel in the marketplace as the preferred material of choice. AISI also plays a lead role in the development and application of new steels and steelmaking technology. AISI is comprised of 24 member companies, including integrated and electric furnace steelmakers, and 124 associate members who are suppliers to or customers of the steel industry. AISI’s member companies represent over three-quarters of both U.S. and North American steel capacity
The Steel Market Development Institute
grows and maintains the use of steel through strategies that promote cost-effective solutions in the automotive, construction and container markets, as well as for new-growth opportunities in non-traditional steel markets. SMDI’s Steel Bridge program is conducted under the Construction Market Council, whose investor companies include 12 North American steel producers.