UC Berkeley

An experimental and analytical research program was conducted to characterize performance of concrete beams longitudinally reinforced with high-strength steel subjected to monotonic loading. Performance was evaluated at both service and limit states.

Experimental tests were conducted in two series of four beams each. In each series, two beams were reinforced with Grade 60 steel and two with Grade 100 steel. All beams were designed to ultimately fail in flexure. In Series 1, failure of beams was expected at high longitudinal strains. For this series, the tensile-to-yield strength ratio of the longitudinal steel (T/Y) was varied to study its effects on beam performance. In Series 2, failure was expected at low longitudinal steel strains. The strain in the steel at the crushing strain of concrete was the variable under investigation for this test series. Load-deflection relations, strain distributions, and plastic deformations for beams in each series were compared with one another to evaluate relative performance. For the Series 1 tests with low reinforcement ratio and, hence, large reinforcement strain at failure, a reduction in T/Y caused a reduction in the spread of plasticity and, consequently, a reduction in displacement capacity. For the Series 2 tests with high reinforcement ratio, deformation capacity was reduced as net tensile strain was reduced. For a given value of net tensile strain, and for a given value of the difference between net tensile strain and yield strain, the beams with Grade 100 reinforcement had equal or greater deformation capacity than the beams with Grade 60 reinforcement. Considering service-level response, typical techniques for evaluating beam performance, such as crack widths and deflections in the elastic range, were adequate for assessing behavior of beams independent of steel grade.

Moment redistribution is a concept that allows designers to shift the static moment envelope by allowing for nonlinearity at certain sections. In the present study, an analytical program examined moment distribution capacity through finite-element models. Beams in the analytical study had longitudinal reinforcement of Grade 60 and 100, with T/Y and aspect ratios spanning typical design values, which were variables identified as critical for determining moment-redistribution capability based on a theoretical derivation. The boundary condition for all beams was fixed-fixed and transverse reinforcement was provided to allow the formation of a mechanism starting with hinging at the fixed ends followed by hinging at the beam center. Input parameters for the models evaluated as part of this study were informed by the results of the experimental investigation. The theoretical moment-redistribution calculation was demonstrated to be conservative based on the output from these analyses. A relationship bounding percent allowable moment redistribution based on net tensile strain is proposed for grades of steel higher than Grade 60.