AN INVESTIGATION OF THE STRENGTH OF BOTTLE-SHAPED STRUTS

Abstract

this investigation indicates that fis increases continuously with increase in pT. For pT contents of upto 0.01, an efficiency factor model has been proposed for bottle-shaped struts for concrete strength upto 82 MPa. In addition to the transverse reinforcement content and the concrete strength, two new geometrical parameters associated with bottle-shaped struts, namely, aspect ratio and concentration ratio, are also found to have significant influence on the strut efficiency. The effects of these two parameters have also been included in the proposed efficiency factor model. To account for the softening effect of the adjoining ties, a linear scaling factor derived from the modified compression field theory (MCFT) has been introduced into the proposed model. Limited validation using a part of the database of Brown et al. (2005) and the beam test results of this investigation indicate that the proposed efficiency factor model can conservatively predict the strength of bottle-shaped struts in prototype structures. The experimentally obtained transverse strain profiles at different stages of loading clearly suggest that unlike the assumption in current design codes, the tie locations in the strut-and-tie model of a bottle-shaped strut are not stationary. Therefore, to ensure satisfactory behaviour at all stages of loading, a uniform distribution of web reinforcement along the strut axis is preferable to lumping of reinforcement at the prospective tie locations indicated in the strut-and-tie models for bottle-shaped struts in current design codes. Although the literature suggests that steel fibrous concrete by virtue of its enhanced tensile strength and ductility has the potential to improve the strut efficiency factor, the results of exploratory tests indicate that the use of 1 % volume fraction of crimped steel fibres in concrete did not significantly improve the strut efficiency factor. However, the use of steel fibrous concrete in combination with conventional web reinforcement did improve the deformation characteristics of bottle-shaped struts when evaluated in terms of the proposed normalised toughness and ductility indices. The experimental results indicate that bottle-shaped struts need not be weaker than prismatic struts, as is implied in the ACI 318-08 provisions. Further investigations are recommended to conclusively establish the relative efficiency factors to be assigned to prismatic and bottle-shaped struts on a more objective basis. The provisions in the Indian concrete code IS 456: 2000 related to the design of members with significant extents of D-region are largely empirical in nature. It is suggested that strut-and-tie modelling procedures should be included in the next revision of IS 456: 2000. n