Experimental and Numerical Investigation of Shear Behaviour of Reinforced Concrete Deep Beams with Web Openings

Deep beams with web openings are commonly encountered in transfer structures, pile caps, and service coordination details of multi-storey buildings, yet the interaction between web openings and the strut-and-tie load transfer mechanism introduces complexities that empirical design rules in current standards do not fully capture. This study presents an experimental and numerical investigation of the shear behaviour of reinforced concrete deep beams with circular and rectangular web openings, motivated by design scenarios relevant to commercial buildings in the South West geopolitical zone. Twelve deep beam specimens were fabricated with shear span-to-depth ratios of 0.5 to 1.5, incorporating openings at three vertical positions and two sizes, and tested under monotonic two-point loading to failure. Crack patterns, load-deflection responses, and failure modes were recorded. Finite element models were developed using ABAQUS with concrete damaged plasticity constitutive parameters calibrated against control specimens without openings, and the calibrated models were used to extend the parametric range beyond the laboratory programme. Results indicate that openings positioned within the natural strut trajectory, extending from the load point to the support, caused the most significant strength reductions, averaging 34 percent below the equivalent solid beam capacity. Rectangular openings were more detrimental than circular openings of equivalent area due to stress concentration at corners. The finite element models predicted failure loads within 8 percent of experimental values across all specimens. Modified strut-and-tie model provisions are proposed that account for opening geometry and position, offering an improvement over the conservative and uniform strength penalty factors in ACI 318. Keywords: deep beams, web openings, shear behaviour, strut-and-tie, finite element analysis.