Local Flexural Strength of Slabs at Interior Columns

The local flexural limit design strength of flat slabs around interior columns is analyzed by means of Yield Line Theory. The slabs are assumed to be reinforced orthotropically and to be loaded with a uniformly distributed load. Appropriate expressions are developed for the flexural collapse loads at rectangular and circular columns. Since the resulting expressions are cumbersome, solutions for a large range of practical geometries are plotted on graphs to aid designers and future researchers. Example slabs designed in accordance with ACI 318-77 are analyzed, and it is shown that they are unlikely to fail in the local flexural mode. It is also shown that some relatively small changes in those provisions of ACI 318-77 which deal with permissible bar cut off points and column strip widths could considerably enhance the local flexural strengths of most slabs. It was also found that if the reinforcement had been uniformly distributed in the example slabs as is sometimes done in test specimens and assumed in many analyses, the local flexural collapse mode would have become dominant.