An appraisal of design provisions for bottle-shaped struts

Ever since their inception, strut-and-tie modelling (STM) provisions in most of the current design codes have remained virtually unchanged, in spite of significant research-driven advances in the recent past. An appraisal of the ACI 318 STM provisions related to bottle-shaped struts has been carried out in the context of research findings reported in the literature. This appraisal has been used to propose a simple and robust analytical model for the efficiency factor which is used to estimate the strength of bottle-shaped struts. The proposed strut efficiency factor model, besides taking into account the strength of concrete, also considers the effect of inclination of the strut with an adjoining tie. For struts with inclination angles less than a certain threshold value that depends on the concrete strength, the efficiency factors recommended by current design codes are shown to be unconservative when compared with the predictions of the proposed model and other similar models in the literature. An alternative to the ACI minimum reinforcement formulation for bottle-shaped struts, which takes into account material strength characteristics, is discussed and the corrected form of the minimum reinforcement formulation has been used to identify a conflict in the deep beam web reinforcement provisions of ACI 318-08.