Shear failures in pile caps: Governing mechanisms and influence of size on the resistance

Design of squat pile caps without shear reinforcement is typically governed by failures in shear, with development of inclined failure surfaces and crack sliding. Although the basic load-carrying action can be efficiently described by means of equilibrium-based models (such as strut-and-tie models or stress fields), determining the actual resistance is still subjected to scientific discussion concerning a number of topics. One of the open questions relates to the influence of size on the unitary shear resistance, which is typically extrapolated for design purposes from the size effect observed in one-way members failing in shear or from failures in punching. Such extrapolation is, however, debatable as pile caps of practical interest may have significantly larger sizes than those that can be tested in laboratories and this advises to ground design on the basis of a consistent mechanical model. In this paper, the results of an experimental program on 15 pile caps are presented in an effort to better understand the mechanics of failure and to specifically address the influence of size on the resistance. The tests were performed following perfect geometric scaling with effective depths ranging from 200 to 500 mm while keeping the material properties constant. The results of the tests show that failures are originated by shear cracks developing in the clear shear span between the edges of piles and the loading area and that they are associated with strain localisation. A notable size effect is observed, whose asymptotic slope expressed in a double-logarithmic scale results close to −1/3 (milder than the one corresponding to Linear-Elastic Fracture Mechanics). On the basis of the test results, a design approach based on the concepts of the Critical Shear Crack Theory and the upper-bound theorem of limit analysis is presented. The model allows for accurate predictions of the resistance and suitably captures the influence of size on the response of the specimens.