Bent-Cap Confining Stress Effect on Slip of Prestressing Strands

Reverse cyclic load testing was completed involving four prestressed concrete pile specimens embedded into cast-in-place (CIP) reinforced concrete bent caps. The piles used were 18 in. (457 mm) square x 18 ft (5.5 m) long and were plainly embedded into the bent cap to depths of 18, 22, and 26 in. (457, 560, and 660 mm). The tests were conducted to determine the moment capacity and ductility of the connections along with the slipping stress of the strands. Analytical models were created in which the slipping stress of the strands was calculated using the ACI 318-08 equation for development length. In all cases, the connection achieved more moment than expected due to the confining stress produced from the bent cap, which tends to decrease the development length of the strands. A modified equation that accounts for the beneficial effect of concrete confinement due to shrinkage is introduced.