On the Effective Stiffness of Slender Concrete Masonry Walls

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The slender masonry wall design procedure in the Canadian Standard for Design of Masonry Structures, CSA S304-14, (2014) has been found to be overly conservative. The calculated effective stiffness term, EIeff, which is used to determine the secondary moment due to wall deflection, was noted as a potentially significant cause of this conservatism. In this thesis, the degree to which the effective stiffness term contributes as a source of error is assessed by analyzing the results from a concrete block wall testing program (Hatzinikolas et al., 1978a) and comparing the recorded lateral wall deflections to the deflections calculated as per CSA S304-14 (2014). The effective stiffness is shown to be particularly underestimated by the procedure in the Standard (leading to overdesign of walls) when a wall is designed to be loaded at a low axial load eccentricity. A more accurate equation for calculating the effective stiffness of a wall as it is being loaded is developed and recommended in this thesis, whereby the effective stiffness depends on the wall’s thickness, critical buckling load and the applied axial load and its eccentricity. A discrepancy between the expected and observed failure modes of slender walls loaded at low axial load eccentricities is noted in the analysis, whereby slender masonry walls commonly experience compressive material failure when loaded at a low axial load eccentricity, even though slenderness effects are expected to contribute to out-of-plane failure. A potential alternative design procedure template is presented, whereby a wall is first categorized based on its expected failure mode and only then designed either as a wall which is expected to experience slenderness effects (a wall expected to deflect significantly and experience out-of-plane failure) or a wall for which slenderness effects need not be considered because it will likely experience material failure and not deflect much laterally (despite the fact that it would be currently categorized as slender by CSA S304-14 (2014) based on its slenderness ratio).
Bogoslavov, M. (2022). On the effective stiffness of slender concrete masonry walls (Master's thesis, University of Calgary, Calgary, Canada). Retrieved from https://prism.ucalgary.ca.