Experimental study of concrete breakout failure mechanism in an exposed column base with a foundation beam

Abstract

Exposed column bases are extensively used in low-to-medium rise steel structures. However, the stress transfer mechanism in the column base is extremely complicated at various failure modes. One of the failure modes mentioned in the Japanese design recommendations involves the yielding and fracture of anchor rods. To ensure this failure mode, the pull-out failure of the anchor rods—one of the reasons responsible for the concrete breakout failure—has to be prevented. However, in Japan, the current design recommendation regarding the concrete breakout failure is not clear, and the studies that have focused on this topic are limited in number. In this study, experiments were conducted to clarify: 1) the concrete breakout failure mechanism, and 2) the effects of the foundation beam reinforcement on the concrete breakout failure. A total of nine specimens, with varied configurations in anchor rods, foundation beam reinforcement, concrete strength, among others, were loaded to failure. According to the experimental results, the column rebar started to transfer strength after the tension of anchor reached the strength of concrete breakout failure. The current design recommendation overestimates the number of effective column rebars on the concrete breakout failure. As for the contribution of concrete, it decreased as the conical cracks occurred and became stable afterwards. Meanwhile, the foundation beam rebar exhibited negligible effect on the concrete breakout failure. Based on the experimental observations, a stage-wise approach is proposed to quantify the contributions of both the concrete and column reinforcement on the concrete breakout failure of an exposed column base.