AbstractThe existence of recesses (hollow pots) in hollow ribbed (waffle) reinforced concrete slabs makes the slab more vulnerable when subjected to concentrated loads. The punching shear failure inside waffle slabs has not been thoroughly investigated and this is also reflected in the current code of practice, which, in the main, relates the design of punching shear in waffle slabs to that in solid slabs. In this study, both experimental and theoretical studies have been conducted to investigate the failure pattern and ultimate failure load of punching shear failure of waffle slabs.
Twelve model waffle slabs, 1/3 scale, were tested to failure to study the failure pattern, the influence of the local configuration of the ribs and presence of local solid area, and the effectiveness of the stirrups supplied inside the ribs. The experimental results were compared with the calculated results by following the provisions of BS8110.
The theoretical study was carried out in parallel; the Upper Bound Analysis and Nonlinear Finite Element Analysis were employed to predict the ultimate punching load and also to serve the purpose of investigating the failure mechanism. Alternative methods, modified from the methods recommended by the code, are proposed to predict the punching load.
From the comparison of punching loads obtained by tests and calculation using BS8110, it was found that the punching capacity of waffle slabs without stirrups was under-estimated and the shear resistance of stirrups was over-estimated.
By the experimental study and theoretical study, it was found that the punching capacity of waffle slabs were not sensitive to the loading directions nor to the local configuration of the ribs in the vicinity of the loading pad; the punching load was dissipated to a larger area than the loading pad and resisted by both the orthogonal ribs and deck of the slab. The use of local solid area in the waffle slabs can increase the punching shear capacity more efficiently than the use of shear reinforcements; the upper bound analysis revealed that the dimension of the punching perimeter would be reduced by the supplement of the stirrups and resulted in less strength of the stirrups being mobilised than expected.
By comparing the results obtained from tests with those obtained employing various analytical methods two methods are recommended for the prediction of the punching shear capacity of R.C. waffle slabs with a degree of confidence because of their inherent simplicity and accuracy, which are the Upper Bound Analysis and the Alternative Method using the Affine Solid Slabs.
|Date of Award||Nov 1994|