Evaluation of ACI 440.11 Shear Strength Provisions for Members without Stirrups

Presented By: Stephanie Walkup, Villanova University
Recently codified language in ACI CODE-440.11-22 provides an equation for concrete shear capacity and imposes a lower bound on this calculation. An experimental study consisting of 39 flexural members without shear reinforcement and tested to failure in shear was used to evaluate the current code provisions, including, most specifically, the lower bound. Comparison of experimental and analytical shear capacities demonstrates that the current code provisions are conservative. More lightly reinforced specimens have a higher variability in experimental-to-nominal concrete shear strength than more heavily reinforced specimens, and this variability appears to be dominated by the depth between the elastic cracked section neutral axis and the depth of the tensile reinforcement, which is the area where aggregate interlock occurs. Based on a comparative reliability study, the lower bound, kcr = 0.16 (5kcr = 0.8), in the code, causes more lightly reinforced specimens (kcr less than 0.16) to have lower factors of safety against shear failure than more heavily reinforced specimens (kcr greater than 0.16). Rather than imposing a lower bound of 5kcr on the current shear strength equation, it would be more prudent to resolve the overprediction of the equation for all specimens.

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Unconservative approach