In strut-and-tie modeling the efficiency factor is used to take into account the lower strength of struts relative to the uniaxial strength of the concrete. The reduction in strength is justified on the grounds of the brittle nature of concrete and on the basis of the effects of transverse tension fields on the strength of the struts. In this paper, the available models for the efficiency factor are reviewed and new models proposed, which are compared with the test results of 135 nonflexural structural elements such as deep beams, corbels, and nibs. The results show that the efficiency factor has only a week relationship with the concrete strength and that the more important parameter is the angle of the strut relative to the longitudinal axis of the member. Efficiency models based solely, or primarily, on the concrete strength are found to have a poor correlation with the data. Efficiency models that account for the angle of the strut and models based on the modified compression field theory are found to give the best correlation with the test results.
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