Performance and design of steel pipe pile to concrete cap connections subject to seismic or high transverse loading

Abstract

This research investigated the seismic behavior of the connection between concrete-filled steel tube (CFT) piles and concrete pile caps. This connection is an important component of an accelerated bridge construction technique, which involves driving steel piles to a finished elevation just below the design deck level, forming a pile cap around the ends of the driven piles, reinforcing this cap, and then filling the piles and the cap formwork with concrete. This cap then serves as the support for the superstructure of the bridge. Conventional configurations of the CFT to pile cap connection often involve congested and complex reinforcing schemes, which can limit the use of this bridge support system. The research discussed herein evaluated the performance of a new detailing scheme that significantly reduces congestion and construction issues, and developed a mechanics-based analysis tool to predict load carrying capacity of this type of connection. This new reinforcing scheme uses U-shaped reinforcing bars to encircle the tip of the embedded CFT pile within the cap, which act to confine the concrete immediately around the pile and transfer the loads throughout the cap. In this research, six connection specimens with various details were tested under lateral loads until failure while monitoring applied loads and lateral displacements. As intended, five specimens experienced failure in the concrete cap, with the remaining specimen failing through plastic hinging in the CFT at the face of the cap. The focus of this test program was on characterizing the failure behavior of the concrete cap; typically, this behavior was observed to consist of crushing of the concrete adjacent to the pile near the face of the cap and at the tip of embedment, followed by yielding of the longitudinal reinforcement, and concluding with yielding of the transverse reinforcement and the formation of diagonal cracks extending from the embedded pile to the edge of the cap. Based on the results of this and a previous investigation, an analysis methodology (similar to moment-curvature analysis) was developed and evaluated. This methodology proved to be an effective means for predicting the ultimate capacity of CFT to concrete pile cap connections.