Nonlinear Soil–Abutment–Bridge Structure Interaction for Seismic Performance-Based Design Anoosh Shamsabadi 1 ; Kyle M. Rollins 2 ; and Mike Kapuskar 3 Abstract: Current seismic design of bridges is based on a displacement performance philosophy using nonlinear static pushover analysis. This type of bridge design necessitates that the geotechnical engineer predict the resistance of the abutment backfill soils, which is inherently nonlinear with respect to the displacement between soil backfill and the bridge structure. This paper employs limit-equilibrium methods using mobilized logarithmic-spiral failure surfaces coupled with a modified hyperbolic soil stress–strain behavior LSH model to estimate abutment nonlinear force-displacement capacity as a function of wall displacement and soil backfill properties. The calculated force-displacement capacity is validated against the results from eight field experiments conducted on various typical structure backfills. Using LSH and experimental data, a simple hyperbolic force-displacement HFD equation is developed that can provide the same results using only the backfill soil stiffness and ultimate soil capacity. HFD is compatible with current CALTRANS practice in regard to the seismic design of bridge abutments. The LSH and HFD models are powerful and effective tools for practicing engineers to produce realistic bridge response for performance-based bridge design. DOI: 10.1061/ASCE1090-02412007133:6707 CE Database subject headings: Highway structures; Bridge abutments; Seismic design; Soil–structure interaction; Earth pressure; Earth pressure; Geotechnical models; Constitutive models. Introduction Bridges are one of the most crucial parts of the transportation network that have been struck by earthquakes in the past. It is generally recognized that when the bridge deck moves laterally toward the abutment during a seismic event, the bridge structure applies a lateral compressive force to the abutment that mobilizes passive resistance in the soil backfill and results in permanent soil displacement. When the bridge moves away from the abutment, a gap can form between the abutment and the soil. When bridges are subjected to small earthquake-induced lateral forces, they gen- erally remain in the elastic range. When subjected to strong earth- quake shaking, however, the dynamic response of the bridge becomes nonlinear and is largely dependent on the nonlinear soil– structure interaction effects between the abutments and the back- fill soils. The nonlinear force-displacement capacity of the bridge abutment in a seismic event is developed mainly from the mobi- lized passive pressure behind the abutment backwall. Proper mod- eling of the abutment-backfill system is therefore critical and the assumptions made for the nonlinear stiffness as well as the hys- teretic and radiation damping not addressed in this paper of the abutment have been shown to have a profound effect on the glo- bal seismic response and performance of the bridge Faraji et al. 2001; El-Gamal and Siddharthan 1998. There are many typical bridges with a seat-type abutment in which the bridge deck is supported by the abutment on bearings and the columns are supported by a pinned connection between the base of the column and the pile caps or spread footings. The performance of these bridges during seismic shaking is pro- foundly affected by the interaction between the backfill soil and the abutment structure, which involves relative displacement and soil stress-strain behavior. The performance-based design approach requires determining the mobilized resistance of the soil-foundation interaction and its individual components as a function of displacement. In current practice, these capacities are then used by bridge engineers to evaluate the structure’s response using nonlinear lateral pushover analyses. The pushover analyses produce a nonlinear load- displacement relationship normal and parallel to the abutment wall; therefore, the nonlinear load-deformation relationship of the abutment backfill should be developed for the global pushover analysis of the structural system. The monotonic pushover method is a useful and effective tool for performance-based de- sign and is intended to envelope the actual cyclic loading during earthquake shaking. The nonlinear behavior of the bridge abutment and backfill has been shown by experimental load tests Gadre 1998; Romstad et al. 1995; Maroney et al. 1990 and theoretical studies Sham- sabadi et al. 2005; Martin et al. 1996; Siddharthan et al. 1994. Experiments conducted by many researchers such as Fang et al. 1994, Rowe and Peaker 1965, and Terzaghi 1943 show that both the deformation mode and magnitude of the deformation 1 Senior Bridge Engineer, California Dept. of Transportation, Office of Earthquake Engineering, 1801 30 St., Sacramento, CA 95816. E-mail: Anoosh_Shamsabadi@dot.ca.gov 2 Professor, Civil and Environmental Engineering Dept., Brigham Young Univ., 368 Clyde Building, P.O. Box 24081, Provo, UT 84602- 4081. E-mail: rollinsk@byu.edu 3 Senior Geotechnical Engineer, Earth Mechanics, Inc., 17660 Newhope St, Suite E, Fountain Valley, CA 92708. E-mail: M.Kapuskar@ earthmech.com Note. Discussion open until November 1, 2007. Separate discussions must be submitted for individual papers. To extend the closing date by one month, a written request must be filed with the ASCE Managing Editor. The manuscript for this paper was submitted for review and pos- sible publication on September 27, 2005; approved on December 28, 2006. This paper is part of the Journal of Geotechnical and Geoenvi- ronmental Engineering, Vol. 133, No. 6, June 1, 2007. ©ASCE, ISSN 1090-0241/2007/6-707–720/$25.00. JOURNAL OF GEOTECHNICAL AND GEOENVIRONMENTAL ENGINEERING © ASCE / JUNE 2007 / 707