Experimental investigation of bottom mat reinforcement details in drilled-shaft footings

An experimental investigation on the structural behavior of drilled-shaft footings (pile caps) with different reinforcement details is presented. Four large-scale drilled-shaft footing specimens were tested subjected to a concentric vertical force up to failure. The specimens had the same geometry and amount of bottom mat reinforcement, but different reinforcement layout (uniform grid vs. banded) and anchorage details (straight vs. hooked bars). Three drilled-shaft footing specimens failed in a brittle manner upon experiencing severe diagonal and punching shear cracking, while one specimen with straight bars and uniform grid reinforcement failed by premature bar slip. Test results show that banding reinforcement over the supports does not improve the ultimate strength as compared to a uniform grid layout. Analytical strength predictions using strut-and-tie modeling provisions in AASHTO LRFD are compared to the experimental results, and modifications to current design methods are presented and discussed.