Deflection control is a critical aspect of the design of concrete structures for serviceability. While there is extensive literature on detailed analysis methods for time-dependent effects of creep and shrinkage, simple multipliers applied to calculate immediate deflections remain the most common method used by engineers to calculate long-time deflections in design. Multipliers for deflection calculations are specified in design standards such as CSA A23.3 and ACI 318. This paper examines some of the limitations inherent in the simple multiplier approach. Laboratory tests were conducted on a series of oneway slabs monitored over a period of approximately 5 years to determine deflections under sustained load. Three sets of slabs were loaded at 3 days, 7 days, and 28 days respectively to determine the effect of age at loading on the time-dependent deflection. In addition the slabs were subjected to full live load at early age to simulate effect of construction loading. Deflections at various ages are compared to determine the effect of age at loading, and time varying functions are applied to the data to model the rate of increase in deflection over the time period. In addition to the test data presented, analyses at the member section level based on creep superposition are developed to study the effects of reinforcement and other parameters on long-time deflection. Suggestions are made for modifications to existing code provisions for long-time multipliers.
展开▼
