Müller, Anna Louisa1; Isfeld, Andrea2; Hagel, Mark3 and Shrive, Nigel4

1 Research Associate, Department of Civil Engineering, Schulich School of Engineering, 2500 University Dr NW, Calgary, AB, Canada, anna_l_mueller@gmx.de
2 Postdoctoral Scholar, Department of Civil Engineering, Schulich School of Engineering, 2500 University Drive  NW, AB, Canada, acisfeld@ucalgary.ca
3 Executive Director, Alberta Masonry Council, P.O.Box 44023, RPO Garside, Edmonton, AB, Canada, markhagel@albertamasonrycouncil.ca
4 Professor, Department of Civil Engineering, Schulich School of Engineering, 2500 University Dr NW, Calgary, AB, Canada, ngshrive@ucalgary.ca

ABSTRACT
In Canada, masonry walls appear to be constrained more by slenderness than wood frame walls. The Canadian masonry design standard appears to underestimate the capacity of slender masonry walls, reducing efficiency in the use of the material. The capacity of a slender concrete masonry wall subjected to axial loads is affected mainly by its slenderness ratio, the eccentricity of the applied load, the deflected shape of the wall resulting from the ratio of end eccentricities and its flexural rigidity. To take account of slenderness and second order effects, the current Canadian design standard allows use of the moment magnifier method, or calculation of the P-Δ effect. Several investigations indicate that these approaches are generally appropriate for considering the effects of secondary moments. The main reason for the underestimation of the capacity is the effective flexural rigidity used in the code. Due to material nonlinearity and a reduction of the cross-sectional depth caused by tensile cracking, the effective flexural rigidity is limited to 0.4 and 0.25 times the initial flexural rigidity for unreinforced and reinforced masonry, respectively. Examination of experimental test results reported by different researchers shows that the limits lead to overestimations of the capacity reducing effects of slenderness for most of the allowable slenderness ratios. The effective rigidity is particularly conservative for small load eccentricities and thus for walls undergoing compression dominant failure. For increasing eccentricities, however, the reduction of the capacity due to slenderness becomes more important. We review the reports on experimental programs and demonstrate that further testing is required with loading conditions similar to those found on site in order to produce recommendations for less conservative and more consistent design of slender masonry walls.

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