Punching shear behaviour of reinforced concrete slabs

Abstract

Punching shear is an important consideration in the design of reinforced concrete flat plates, flat slabs, and column footings. Present design rules for punching shear failure of reinforced concrete slabs, given in various Codes of practice, are largely based on studies of the behaviour and strength of simply-supported, conventional specimens extending to the nominal line of contraflexure. As punching shear provisions incorporated in various Codes of practice are a direct result of the empirical procedures, they do not usually provide an accurate estimate of the ultimate punching load capacity of a slab with its edges restrained against rotation. This is because no direct account is taken of the significant enhancement of punching capacity due to the in-plane restraint in many types of reinforced concrete slab systems. The present study describes punching tests conducted on reinforced concrete slabs with their edges restrained as well as unrestrained. Here, edge restraint has been provided, by means of edge beams of various dimensions, to mimic the behaviour of continuous slabs. A total of 16 model slabs have been tested in an effort to ascertain the influence of the degree of boundary restraint, percentage of steel reinforcement, and slab thickness of the slab models on their structural behaviour and punching load-earrying capacity. The cracking pattern and loaddeflection behaviour of the slabs tested have also been monitored closely. The test program was carried out to provide basic information on the real punching behaviour of restrained slabs subjected to concentrated loading and may also be usefully applied in the assessment of existing structures with laterally restrained slab construction. The significant positive effect of edge restraint on the punching failure load, resulting in enhancing the ultimate punching strength, has been noticed. The code-specified strength of the specimens was calculated in accordance with the American, British, Canadian and European codes. It became apparent that no code-specified method predicts an enhancement in the punching shear strength of a restrained reinforced concrete slab with an increase in the degree of such restraints. Present Codes do not recognize the role of percentage oflongitudinal steel on the punching strength effectively either. It has been understood that inclusion of the findings of the study in the design Codes will result in a rational design of structural sy~ems where punching phenomenon plays a vital role.