IDOT Designing Bridges to Resist Earthquakes and Validate Their Capacity

Because of an increased awareness of seismic activity, the American Association of State Highway and Transportation Officials (AASHTO) expanded its requirements for bridge safety during earthquakes nationwide in 2009. The Illinois Department of Transportation (IDOT) and the Illinois Center for Transportation (ICT) responded with an innovative approach to improve bridge safety—the Earthquake Resisting System. The idea of this new design was to allow bearing anchorages to fracture during an earthquake and rely on the subsequent bearing deformation and sliding to accommodate seismic demands. Hence, the overall goal is to ensure that even in the event of a major earthquake, Illinois bridges will not collapse.

To help achieve the bridge safety goal, IDOT initiated a project, “Calibration and Refinement of Illinois’ Earthquake Resisting System Bridge Design Methodology” (R27-070) to test the efficacy of this approach. The project was led by a Technical Review Panel chaired by Dan Tobias, Engineer of Concrete and Soils with IDOT’s Bureau of Materials and Physical Research. It was successful and led to a follow-up validation project, “Calibration and Refinement of Illinois’ Earthquake Resisting System Bridge Design Methodology: Phase II” (R27-133). The second project has recently gotten under way and will expand and refine the outcome of the first project.

The initial project resulted in a two-volume final report. The first volume covered the experimental program and presented results and conclusions of laboratory testing, while the second volume focused on the analytical program and design recommendations.

Phase II will refine the results of those experiments to measure specific parameters that bridge designers will consider. This effort will require building a suite of prototype bridges that will encompass a variety of bridge styles.

The overall goal of the research is to ensure that even in the event of a major earthquake, Illinois bridges will not collapse.

The overall goal of the research is to ensure that even in the event of a major earthquake, Illinois bridges will not collapse.

As Mark Shaffer of IDOT’s Bureau of Bridges and Structures and Technical Review Panel chair for the Phase II project explains, “Phase I of the project focused on determining bridge responses to seismic events for typical IDOT bridges. In order to maintain a tight focus, the sample of bridges chosen reflected only a portion of typical bridge structures in the state. Phase II expands the size of the sample greatly and will give us results that will reflect the majority of typical bridges in the state.”

Shaffer is optimistic about the results: “This project verifies that our current bridge earthquake resisting strategy is effective and that bridges designed using the strategy will maintain some operational capacity in a the event of a 1,000-year earthquake. The project also is giving us insight as to how the design and details can be improved, resulting in a more robust statewide bridge system.”

According to Shaffer, the work done in these projects has and will continue to have an impact on the design and build requirements for bridges throughout the state, but especially in the southern part of the state, where seismic activity is more pronounced.

Many challenges still lie ahead for researchers. Principal investigator James LaFave says, “The research team looks forward to the challenge of tackling practical issues with respect to how IDOT is currently designing and constructing their bridges in southern Illinois with respect to the seismic hazard. We will be using some sophisticated laboratory experimentation plus detailed bridge system computational analyses in our investigations.” LaFave is a professor in the Department of Civil and Environmental Engineering at the University of Illinois at Urbana-Champaign.

Difficult work like this, in which researchers must understand not only the optimal structural design but also the specifications required by state and federal guidelines—and do so under the limitations of state budgets—requires a multifaceted team approach. To meet those challenges, LaFave and colleague Larry Fahnestock, associate professor in civil engineering at Illinois, consulted people from a variety of backgrounds and areas of expertise. “It’s been quite interesting to learn and use information and techniques from experts in related areas whom we’ve consulted along the way on the project,” says LaFave.