Beam Strength Enhancement at Design Ductility Factor Demands

Abstract

A refined bending‐analysis model is presented that accurately predicts available experimental results. By means of this model, the relations of the ultimate‐to‐yielding moment ratio versus the longitudinal reinforcement ratio are deduced for a beam that is rectangular in cross section with two reinforcement layers. Two values of the curvature‐ductility factor were assumed to be boundary values of the demand during a severe earthquake. From each of the two values a moment value is then derived, which is considered ultimate. The results show that the strength enhancement due to strain‐hardening of the reinforcing steel can vary over a wide range of values (12‐65%) depending on the longitudinal (in tension and compression) and transverse reinforcement ratios. Hence the ACI Committee 352 recommendation of a constant (40%) increase in the sum of beam's nominal moment strength at beam‐column joints is found to be inadequate. To provide an approximate evaluation of the strength enhancement, thus avoiding the iterative analytical solution, a simple numerical model that gives reasonable agreement with the more exact values is proposed.