Master of Engineering Science
Civil and Environmental Engineering
Dr. F. M. Bartlett
Excessive deflection of concrete beams is a recurring serviceability problem. Provisions in current building codes, CSA A23.3-14 and ACI 318-14, account for some but not all of the contributing factors. The effect of loading concrete members at very young ages on the associated deflections remains uncertain.
Concrete stress-strain data reported by others are used to investigate if conventional stress-strain relationships, and empirical equations in A23.3-14 for tensile strength and elastic modulus are accurate for young concretes. Moment-curvature analyses based on conventional simplifying approximations used for flexural analysis are performed. For concretes less than one day old, the conventional relationships and empirical equations yield unconservative results. For older concretes, however, the conventional stress-strain relationships, empirical equations and conventional simplifying assumptions yield accurate results.
Current practice is to compute deflections using either a whole-member analysis with an average effective moment of inertia, or a discretized analysis with unique effective moments of inertia for each discrete element. Branson proposed equations for the effective moment of inertia for use in either analysis. Bischoff proposed an improved equation, for use in whole-member analysis only. The current research quantifies suitable modifications to the Bischoff Equation for use in discretized analysis: the exponent applied to the cracking-to-applied moment ratio term should be increased from 2 to 3.
Test-to-predicted ratios for 65 beams investigated by others are used to quantify the accuracy of the various alternative deflection calculation procedures and to identify key parameters for accurate computation of deflections. The Bischoff Equation with the cracking moment computed using full modulus of rupture yields the best results. When the recommended reduced modulus is used, the difference between results using the Branson and Bischoff Equations is indistinguishable.
Mancuso, Caitlin, "Short-term Deflections of Reinforced Concrete Beams" (2016). Electronic Thesis and Dissertation Repository. 3862.