Response of Deep Foundations to Seismic Loads in Alabama

Abstract

Deep foundations are commonly used as foundation elements for bridges in Alabama. Due to the implementation of the American Association of State Highway and Transportation Officials (AASHTO) Load and Resistance Factor Design (LRFD) Bridge Design Specifications, design of critical and essential bridges will be significantly impacted for the moderate seismic design category of Alabama. Five bridge case studies were provided by the Alabama Department of Transportation (ALDOT) to evaluate the response of typical foundations used for critical and essential bridges. FB-MultiPier, a program that couples nonlinear structural finite element analysis with nonlinear static soil models, was used to model both the soil and structure (both foundation elements and substructure components) of each case history. These models were loaded with a suite of scaled time-history events to simulate an earthquake. Displacement at the top of the pier and ground surface was recorded, as well as maximum shear force, bending moment, and demand/capacity (D/C) ratio distribution along the length of the driven pile, drilled shaft, or column. The maximum shear force, bending moment, and D/C ratio distributions indicated where the plastic hinge zones could be expected to form in the structure. FB-MultiPier was also used to develop a family of foundation response curves that were used in SAP2000 to evaluate the performance of the case histories. Pile performance under combined scour and earthquake was reviewed and two different scour depth models were developed for one case history. Buckling criteria for foundations in soft clay or liquefiable soil were also reviewed and compared to two case histories. It was found that scour depth appears to affect the dynamic response of the bridge pier. The pier modeled with 25% scour depth performed worse (structurally) than the same pier modeled with 100% scour depth. This was due to large displacements at the ground surface and large bending moments developing below the ground surface. The pier founded in soft clay over rock performed poorly due to structural failure in the foundations. This was due to flexure failure of the piles. It was also found that drilled shafts embedded in shallow bedrock tend to perform well depending on the natural frequency and structural period of the pier. Recommendations for further research address soil susceptibility to scour and liquefaction, full-scale dynamic load testing, and correlations between the natural and structural period of a bridge pier and its performance during an earthquake event.