Title: Flexural Behavior of Beams with Confinement
Author(s): Mohamad M. Ziara, David Haldane, and Atallah S. Kuttab
Publication: Structural Journal
Appears on pages(s): 103-114
Keywords: beams (supports); compressive strength; confined concrete; ductility; flexural strength; structural design; tests; Structural Research
The principal aim of this paper is to examine, both theoretically and experimentally, the flexural behavior of structural concrete beams in which confinement stirrups have been introduced into the compression regions. The laboratory-based part of the investigation included 12 under-reinforced beams, eight of which had the concrete compression regions, as defined by the compression force path (CFP) concept, confined with rectangular closed stirrups. The remaining beams were traditionally detailed. In a comparison with traditionally detailed beams, the presence of confinement was shown to increase the ductility of the beams; however, no comparable increase was found in their respective flexural capacities. A method for the evaluation of the flexural capacity of beams in which confinement of the compression regions is present has been proposed. A method for the design of over-reinforced beams utilizing the ductility resulting from confinement has also been outlined and investigated experimentally using four types of over-reinforced beam selected from another extensive test program. The results obtained have shown that although the beams with confinement were able to achieve a flexural capacity of up to 246 percent of the value corresponding to the maximum longitudinal reinforcement ratio rho max allowed by the ACI code, they still failed in a ductile manner. A unique relationship between rho b' (enhanced longitudinal reinforcement ratio under the new balanced-ductile failure conditions) and rho b (as defined by the ACI code) was found to be a function of the confinement characteristics. All the results predicted using the proposed approaches for both under- and over-reinforced beams were found to be in close agreement with the corresponding measured values obtained from the beams included in the test programs.