Based on ICT Research, IDOT Refines its Earthquake Resisting Bridge Design Methods

Thanks to a new study, researchers and engineers now have a better understanding of how bridges in the state would react to an earthquake.

Illinois does not usually come to mind when we think about earthquakes, but Cairo, Illinois, which is located in the southernmost part of the state, has a high probability of being affected by an earthquake because it is located within the New Madrid Seismic Zone. As a result, Illinoisans should be concerned because earthquake damage could have an adverse effect on roads and bridges in the region. That can, in turn, directly affect the collective transportation infrastructure.

In 2006, the Illinois Department of Transportation (IDOT) developed an Earthquake Resisting System (ERS) strategy specific to bridges built in Illinois. However, in 2009, the American Association of State Highway and Transportation Officials (AASHTO) made changes to its structural design requirements relating to bridge safety during earthquakes, which intensified the structural design standards across the country. As a result, the number of Illinois bridges that require seismic analysis and design has drastically increased due to AASHTO’s changes to the bridge design code. In response, IDOT sought to improve their ERS strategy by using experimental testing and parametric analyses to verify and calibrate some of the fundamental design assumptions in the ERS.

This led to a collaboration between IDOT and the Illinois Center for Transportation (ICT) researchers, who conducted a study that utilized a combined experimental and computational research program to investigate, validate, calibrate, and adjust high-priority components of the current Illinois ERS strategy.

The R27-070 ICT project, entitled “Calibration and Refinement of Illinois’ Earthquake Resisting System Bridge Design Methodology,” was a one-phase project that is documented in two reports. The reports on the experimental investigation and the parametric study can be found on the ICT publications page.

Diagram of a typical three-span steel integral abutment bridges (IAB).

The experimental investigation report in phase I of the research confirmed that adjustments need to be made to IDOT’s ERS. The parametric study report revealed that most bridges in southern Illinois would not experience severe damage during a 75-year design life, and that the loss of major bridge components are not likely to occur in regions with moderate seismic hazard. The parametric study also uncovered topics for future research that were needed to thoroughly update the specific provisions of the ERS. Consequently, a second phase of research was conducted to further investigate some of those topics that were not resolved during the first phase, as well as study more complex bridge configurations.

ICT project R27-133, “Calibration and Refinement of Illinois’ Earthquake Resisting System Bridge Design Methodology, Phase II,” provided further verification and refinement of the ERS design methods and was also documented in two reports, one focused on integral abutment bridges (IABs) and one on seat-type abutment bridges. The IAB report examined the seismic behavior of typical IABs in southern Illinois and developed feedback and recommendations for improving IAB seismic designs. The seat-type report assessed the seismic performance of quasi-isolated highway bridges in the southern Illinois region with seat-type abutments, validated the current IDOT design strategy, and provided recommendations for improving a bridge’s seismic behavior.

Example of a 3-D finite-element bridge model.

Dr. James LaFave served as one of the principal investigators on this project. LaFave is a structural engineering professor in the Department of Civil and Environmental Engineering at the University of Illinois at Urbana-Champaign. He indicated that by subjecting IAB and seat-type abutment bridge models representative of southern Illinois bridge design and construction practice to 1,000-year return period hazard ground motions developed specifically for that region, new insights were gained about the seismic performance of these classes of bridges.  In particular, LaFave noted that recommendations have been developed, based on the conducted nonlinear dynamic time-history analyses, to even further reduce the likelihood of unsatisfactory pier column damage and bearing unseating limit states in such bridges during strong ground shaking.

The project was guided by a Technical Review Panel chaired by Mark Shaffer, Chief of IDOT’s Policy, Standards, & Final Plan Control Unit in the Bureau of Bridges and Structures.

“This research was instrumental in obtaining a clear understanding of the behavior of IDOT bridges during a seismic event.  The results will be used to generate prescriptive details to mitigate damage, ensure public safety, and keep vital routes open to traffic in the event of an earthquake,” Shaffer said.