Title: Rational Approach for Computing Long-Term Deflection of Reinforced Concrete
Author(s): Peter H. Bischoff and Lluis Torres
Publication: Structural Journal
Appears on pages(s): 215-223
Keywords: beams; creep; deflection; effective moment of inertia; longterm; shrinkage; slabs
The ACI 318 Building Code computes time-dependent deflections from creep and shrinkage as the product of immediate deflection from sustained loads and a deflection multiplier that depends on the duration of loading and amount of compression reinforcement. An alternative methodology is proposed that extends the approach developed for an effective moment of inertia Ie, recently adopted by ACI 318-19 for computing immediate deflection to include time-dependent deflections from creep under sustained load and shrinkage. Separate components are developed for creep and shrinkage deformation based on a rational tension stiffening model that accounts for the reduced member stiffness from creep, shrinkage warping of the uncracked and cracked sections, and loss of stiffness from a lower cracking moment that results as tensile stresses develop in the concrete from restraint to shrinkage and cause additional cracking. Comparison is also made with the Eurocode 2 approach. Validation with long-term test data shows close agreement within a few percent for the proposed approach and Eurocode 2 approach. ACI 318-14 underestimates deflection by 27% and ACI 318-19 overestimates deflection by 23% for the test results evaluated in this study.