Deflection Considerations in Two-Way Reinforced Concrete Slab Design

Two-way reinforced concrete flat slabs are an efficient, economical and widely used structural system. Building codes and analysis methodologies have evolved to address both slab strength and serviceability requirements. With respect to serviceability, codes permit thinner slab thicknesses when computed deflections are within prescribed limits. However, computed deflections often will not correlate with measured values. This discrepancy has led to numerous serviceability deficiencies associated with excessive two-way flat slab deflection. In many cases, this excessive deflection is related to cracking in the slab. Commercially available structural software programs will often model the slabs as uncracked and with a uniform thickness. Some of these programs are unable to account for the parameters associated with excessive deflection. Consequently, computed deflections often do not represent those of the structure in place. In this paper, we examine the effects of cracking on two-way slab deflection and serviceability.