Title: Designing T-Beams for Less Ductility than Required by ACI

Author(s): German Gurfinkel and Fernando Fonseca

Publication: Structural Journal

Volume: 93

Issue: 1

Appears on pages(s): 116-127

Keywords: beams (support); bending; bridges (structures); buildings and prefabrication; continuous beams; ductility; parking facilities; precast con-crete;

DOI: 10.14359/9682

Date: 1/1/1996

Abstract:
ACI Building Code 318-89 imposes a certain amount of minimum ductility in reinforced concrete beams. Ductility is measured indirectly by ultimate strain in tensile reinforcement. By requiring that the latter not exceed a certain percentage of the balanced steel, the code guarantees that tensile reinforcement would be strained substantially beyond initial yielding if the section were subjected to ultimate. It is shown herein that for rectangular sections this amount is a quantity that depends only on the yield strength of the reinforcement. However for T-sections, the code imposes a greater demand for minimum ductility that depends also on the geometry of the section. This penalizes T-sections by reducing the maximum amount of tensile reinforcement that could otherwise be allowed. This discrepancy is fully discussed using various tables, graphs, and a design example. For the latter a T-section is designed to: 1) comply with code requirements for ductility, and 2) to provide only the same minimum ductility allowed for rectangular sections. The second design requires less steel reinforcement while providing acceptable ductility even after the effects of loss of concrete strength are calculated. There is little justification for code-imposed additional demand on minimum ductility in T-beam sections.