Code-based refinement and finite element validation of punching shear design for reinforced concrete footings

Punching shear failure of reinforced concrete (RC) footings is strongly influenced by soil-structure interaction; however, current design codes evaluate footing punching capacity using slab-based empirical formulations. This study presents a code-based refinement and Finite Element Model (FEM) validation of punching shear design for reinforced concrete (RC) isolated footings resting on soil. A three-dimensional nonlinear Finite Element Model (FEM) was developed in PLAXIS 3D to simulate concrete cracking and crushing, Reinforcement yielding, soil plasticity, and concrete-soil interface behavior. The model was validated against experimental results from nine square footings (750 × 750 × 120 mm) tested under monotonic vertical loading. Numerical predictions of ultimate punching load and load-settlement response agreed well with experiments, with discrepancies limited to ± 10% and coefficients of variation not exceeding 1.11%. A comprehensive parametric study comprising 81 finite element models was then conducted to investigate the effects of concrete compressive strength (34, 42, and 61 MPa), column aspect ratio (a/b = 1, 2, and 3), punching shear Reinforcement (none, 8Ø6, and 8Ø8), and soil stiffness (sand, sand-crushed limestone mixture, and crushed limestone). Results showed that increasing concrete strength enhanced punching capacity by up to 34%, while increasing column aspect ratio increased capacity by up to 74%. Soil stiffness was identified as a governing parameter, increasing punching capacity by up to 20%, subgrade reaction by up to 200%, and reducing settlement by up to 75%. ACI 318 and ECP 203 predictions exhibited significant scatter when directly applied to footings. A refined punching shear expression was therefore proposed by introducing a correction factor derived from the finite element results. The soil-related factor (η₃ = ln(5000kₛ)) was found to dominate, while other correction factors remained unity within the investigated ranges. The proposed formulation significantly improves the reliability of punching shear design for RC footings by explicitly accounting for soil stiffness effects.