PUNCHING CAPACITY OF SPREAD FOOTINGS USING ACI 318-19 AND THE STRIP MODEL

In design, the sectional depth of reinforced concrete spread footings is usually governed by design code provisions for punching shear, which are derived primarily from experiments on slab-column connections. Previous experiments have shown that the punching behavior of concentrically loaded spread footings differs from that of slab-column connections. This paper describes punching of a concentrically loaded spread footing by combining conventional strut and tie modeling with the concept of an arch strip, part of the Strip Model. By itself, the Strip Model describes the behavior of slab-column connections under a variety of loading conditions. For spread footings, Strip Model concepts need to be combined with conventional strut and tie modeling to adequately describe load transfer in a concentrically loaded spread footing. Two methods are explored, each producing closed-form expressions for the footing capacity that agree well with experimental results (112 tests from the literature). The analyses make it possible to estimate the fraction of footing load that is carried by conventional strut and tie behavior. The experimental results are also compared to punching shear capacities in accordance with ACI 318-19. The Strip Model produces results with roughly the same average test-to-predicted ratio (in the order of 1.3) as ACI 318-19 but with a lower coefficient of variation (10.3% compared to 15.8%). This work shows how a lower-bound plasticity-based model can be used for the practical case of determining the capacity of reinforced concrete spread footings failing in punching shear.