Punching shear failure in three-pile caps: Influence of the shear span-depth ratio and secondary reinforcement

Abstract

The strut-and-tie model (STM) is currently established as the best approach for pile cap design. This model leads to efficient estimations of the main reinforcement placed in strips between piles. However, good practices and some international Concrete Design Standards recommend some secondary distributed reinforcement, and even vertical stirrups that are not considered by the STM. An experimental campaign with nine three-pile caps tested by a centered load is presented to show the influence of both secondary reinforcement and the shear span-depth ratio on pile cap strength.The experimental results show a potential redistribution of internal forces in pile caps after yielding of main reinforcement, finally collapsed due to punching. Secondary reinforcement proves efficient to enhance pile cap strength since it takes part in complementary resistance mechanisms. As expected, the failure load increases with shear span-depth ratio reduction. The STM neither captures the effect of this ratio nor considers punching failure. Checking this failure mode is also required for pile caps.The punching formulation of Eurocode 2 allows considering the influence of this ratio, but some interpretation is required whether one deals with pile caps, regarding the effective width of the shear enhancement factor and the definition of the basic control perimeter. A proper definition would prevent unsafe or very conservative results. Therefore, some recommendations for the verification of deep pile caps following the Eurocode 2 are presented. The contribution of vertical stirrups as punching reinforcement is also investigated. The proposed approach is applied to the existing experimental database of three- and four-pile caps to check formulation validity, and conservative predictions with low coefficient of variation are reached.