Punching behaviour of footings

Abstract

Punching shear is an impOltant consideration in the design of flat plates, bridge decks, column footings and mat foundations. Present design rules for punching shear failure of reinforced concrete slabs and footings, 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 punching load capacity of a slab with lateral restraint. Again, present codes indiscriminately use equations, base on punching tests conducted on slabs, for the design of footings and mat foundations. More impoltantly, a majority of the present codes usually represent punching shear strength merely as a function of concrete strength and ignores the possible effects of percentage of steel, span to depth ratio, etc. The present thesis describes punching tests conducted on eighteen footings, seventeen of which resting on sand. During tests, percentage of steel reinforcement, depth, size and concrete strength of model footings, as well as punching area were varied. Again, the rate of loading as well as the degree of saturation of sand have been varied. The test program was carried out to provide basic information on real punching behaviour of footings subjected to concentrated loading. The punching strength of footings resting on sand have been found to be significantly higher than the predictions of present design provisions. The code-specified strengths of the specimens were calculated in accordance with ACI 318-95, BS 8110-85, CAN3- A23.3-M84, CEB-FIB and BNBC codes. It appears that no code specified method adequately caters for the effect of the upward soil pressure on the punching strength of footings. Most of the codes do not recognise the role of percentage of longitudinal steel on the punching strength either. The overall footing size has been found to have very little effect on the punching strength of footings. Since pressure distribution under a footing is expected to vary depending upon soil type, the code provisions for footings should address this issue faithfully to arrive at a realistic prognosis. During the course of the investigations it became apparent that the concrete strength has a very positive influence on punching strength of footings, as has already been recognized in the codes. The effect of concrete strength, however, becomes almost insignificant as the compressive strength of concrete reaches high values. The present series of tests confirmed, albeit tentatively, that the presently adopted code methodology, where thickness has been considered to have a linear contribution towards the punching strength, may be considered to be valid. Rapid load tests conducted on footings resting on saturated sand demonstrated that the difference between experimental failure load and code predictions narrows down as the punching loads are applied at a very fast rate, in comparison to its normal rate counterpart. It has been understood that inclusion of the tenets of the thesis in the design Codes will result in an economic and rational design of footings where punching phenomenon plays a vital role.