Effects of Reinforcement Details on Behavior of Drilled Shaft Footings

This paper presents an experimental study on the structural behavior of reinforced concrete drilled shaft footings, also commonly referred to as pile caps, supported on four drilled shafts. The test program investigated the effects of three design parameters related to reinforcement detailing: layouts of bottom mat reinforcement (grid versus banded), anchorage details for bottom mat reinforcement (straight versus hooked bar), and ratios of side-face reinforcement to control shrinkage and temperature cracking. Seven large-scale drilled shaft footing specimens, each constructed with a center-located square column stub, were tested under uniform axial compression column loading. The specimens represent typical drilled shaft footings, which can generally be considered as three-dimensional (3-D) deep (D-region) elements. The test results indicate that the ultimate strengths of the footings were comparable when the bottom mat reinforcement was properly developed, regardless of the type of layouts and anchorage details. Nevertheless, different crack patterns were obtained for different bottom mat reinforcement layouts. The absence of sideface reinforcement led to lower capacity and more brittle failure as compared to footings having a minimum amount of side-face reinforcement. Finally, some refinements are proposed to existing 3-D strut-and-tie modeling guidelines to more accurately reflect the ultimate strengths of the test specimens.