Simplified Seismic Design of Steel Tube Pile Bents for Bridges

Pile bent substructures are a popular choice for many ordinary bridges with deep soil subsurface profiles. Two methods of construction that make use of steel tube pipe piles in pile bent substructures are (1) hollow tubes (HT), and (2) concrete-filled steel tubes (CFST). The design of pile bent substructures subjected to seismic loading has yet to be standardized in design specifications. This paper proposes simplified methods for plastic shear and displacement capacity estimation for CFST and HT bridge substructures. Through pushover analyses using software and curve-fitting procedures, the paper presents a methodology that will enable engineers to estimate key parameters without resorting to detailed computer modeling. Limitations on diameter-to-thickness (D/t) ratios for both HT and CFST construction are summarized. One primary advantage of CFST construction is that much larger D/t ratios are permitted because the concrete fill delays buckling of the steel tube, and the steel tube confines the concrete fill. The proposed methodology is limited to tubes with D/t ratio in the range of 28 to 72 and clear height (HC from bottom of the bent cap to the point of in-ground fixity) to diameter ratios in the range of 5 to 20. Geometric nonlinearity was not considered in this study. Such effects will be significant for tall, slender bridge bents and are beyond the scope of this research. The paper also includes a brief discussion of depth-to-fixity estimation, using results for various soils analyzed through software. This information has been added to the paper to alert engineers to situations where typical rule-of-thumb estimates for depth to fixity may be erroneous.