Improving one-way slab design: A new span-to-depth ratio formula based on ACI-318 deflection limits

This paper examines the ACI-318 guidelines for span-to-depth ratios in solid non-prestressed one-way slabs, focusing on deflection control when nonstructural element damage is not a concern. ACI-318's current guidelines offer conservative slab thickness estimates by presenting slab thickness as a portion of the span, with modifications for steel yield strength and concrete density. However, these guidelines do not account for key design factors like live load and concrete compressive strength, often leading to conservative slab thickness with minimum steel reinforcement ratio. The introduction of the Bischoff formula for the effective moment of inertia in the ACI-318-25 code, which predicts larger deflections compared to the older Branson formula, prompts a reassessment of these span-to-depth ratios. This paper aims to evaluate the compatibility of ACI-318's provided span-to-depth limits and the deflection predicted using the Bischoff formula and to develop a more efficient span-to-depth ratio formula that incorporates design factors influencing deflection. A parametric study was conducted to evaluate the impact of concrete compressive strength, live load, and slab span on deflection. Based on this study, a new span-todepth ratio formula was derived from deflection calculations and ACI deflection limits. The proposed formula was verified against selected slabs designed using different values of design parameters, and the predicted deflections remained within ACI-318 limits, demonstrating that the new formula offers a more efficient design approach.