PUNCHING SHEAR RESISTANCE OF FLAT SLABS - INFLUENCE OF REINFORCEMENT TYPE AND RATIO

As a part of a research program to study the performance and design of flat-slab systems, this investigation aimed to determine the effect of shear reinforcement type and ratio on punching shear strength of monolithic slab-to-column connections. The first type of shear reinforcement consisted of hat-shaped units, very advantageous from the points of view of prefabrication and field installation. The second type consisted of double-hooked shear bars, more difficult to install but with very efficient anchorage. A total of 13 specimens were fabricated and tested. The dimensions of the slab were 2 x 2 x 0.2 m (79 x 79 x 7.9 in.), with a centrally located column 0.3 x 0.3 m (11.8 x 11.8 in.) in crass section. The column extended above and below the slab for a length of 0.3 m (11.8 in.). Monotonic static load was applied downwards at eight points symmetrically distributed around the column center at a distance of 0.75 m (29.5 in.). The specimen was supported by the column stud. Shear reinforcement varied from 0 to 1.53 percent in the first series, and from 0 to 1.98 percent in the second series. Longitudinal bars were placed on both compression and tension sides. The tensile flexural reinforcement was 1.23 and 1.53 percent in the first and second series, respectively. Load, deflection, and strain were monitored (at various locations along the shear and flexural reinforcement) during the tests. Experimental results showed that the hat-shaped shear reinforcement was not effective because of lack of proper anchorage and large spacing. Double-hooked reinforcement showed high effectiveness, which resulted in a considerable increment of the punching shear resistance of the connection. Comparisons with the provisions of ACI 318-89 were made.