ACI 318-14 Criteria for Computing Instantaneous Deflections

This paper investigates current procedures to compute instantaneous deflections of reinforced concrete beams. Deflections predicted using the Branson Equation with the cracking moment computed using the full modulus of rupture, in accordance with ACI 318-14, when compared to deflections of 65 simply supported and continuous beams tested by others, are increasingly unconservative and variable as the flexural reinforcement ratio reduces below 1%. Using the Branson Equation with the cracking moment computed using one-half the modulus of rupture, or the Bischoff Equation with the cracking moment computed using two-thirds the modulus of rupture give consistent and conservative deflection predictions, whether the beam is idealized as a single element or as a number of discretized elements. The Bischoff Equation can accurately compute the effective moment of inertia for members idealized using discretized elements if the exponent in its denominator is increased from 2 to 3. The cracked rigidity EcIcr of concretes that are at last 1 day old can be accurately estimated using conventional assumptions and calculation procedures. For discretized element idealizations, the use of 20 equal-length elements per span is sufficiently accurate for practical use.