Revisiting Unreinforced Strut Efficiency Factor

A comprehensive experimental study investigating the strut compression strength of deep beams without stirrups by array of varying strut angles and concrete strengths is presented. The specimens were designed to strut failure, eliminating unwarranted coupled failure modes. The discrepancies of dependent parameters in strut efficiency factors were reconciled with provisions in design standards, along with various researchers’ proposals. It was affirmed that a single constant value is best to represent the strut efficiency, echoing the design recommendation in ACI 318-11. Nonetheless, a less-conservative recommendation was put forward based on the experiment and a suite of supplementary filtered database results. A compatible lower-bound shear limit to prevent strut failure was set forth to unify the strut strength and sectional shear stress limit through a generic linear shear enhancement factor. This model was found to be robust and conservative for a wide range of concrete strengths and strut angles