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.

The strip model describes a load path prior to failure that can be tailored to a wide range of slabs under concentrated loads, both concentric and eccentric. Eccentric punching shear can occur in concrete slab-column connections subjected to shear and unbalanced moment, such as edge and corner columns. This paper shows how the strip model is applied to experiments of edge column-slab connections tested with outward eccentricities, and compares this approach to the traditional and simplified ACI 318-19 approaches for dealing with eccentric punching shear. This work shows how a lower-bound plasticity-based model can be used for the practical case of the slab-column capacity for edge and corner columns, how it may explain the mechanics behind the ACI code provisions, and can help researchers identify adequate test setups for future experiments.