Deflection Control for Serviceability of Reinforced Concrete Floor Systems

Design of concrete floor systems for deflection control has for many years relied on empirical rules of thumb for selection of span to depth ratios or simplified deflection calculation procedures and empirical deflection limits based on previous experience. With increasing use of high strength materials and advanced analytical techniques there is a need to develop a more rational approach to design for deflection control. This paper reviews current practice for deflection control and suggests an alternative to the ACI Code minimum thickness factors to account for a wider range of design variables. The paper also proposes the use of utility theory as a basis for proportioning members to provide acceptable deflection control. The procedure reduces to the minimization of total cost consisting of initial construction cost and the cost of serviceability failure. The cost of serviceability failure includes consideration of uncertainties in loads and response to load (including time-dependent effects) through Monte Carlo simulation and non-discrete thresholds for serviceability limit states. Construction issues related to deflection control also discussed.