LONGITUDINAL SHEAR AND THERMAL RESISTANCE OF NOVEL AND CONVENTIONAL TIES IN CAVITY WALLS

Double wythe masonry cavity walls consist of two masonry wythes, an exterior layer typically made of clay-backed or ceramic bricks (veneer) and the inner layer composed of concrete masonry units (CMU), which serves as a structural backup. In typical cavity wall design, the two wythes are assumed to share in resisting out-of-plane loads with the use of ties that resist the axial load. Cavity walls are subjected to vertical slip between the wythes producing longitudinal shear stresses in the ties. Literature shows that ties with the capacity to resist shear allow wythes to work together (i.e., composite action), leading to enhanced flexural stiffness and out-of-plane resistance of the wall (Mullins and O’Connor, 1987; Pacholok et al., 1989; Williams and Hamid, 2005). However, the Canadian standard for the design of masonry CSA A304-14 (2014a) does not have provisions for composite action in cavity walls.Considering composite action enables the possibility of designing wider cavities between the wythes to accommodate thicker insulation thickness. This is an approach to meet the growing energy efficiency standards outlined in the National Energy Code of Canada for Buildings (NECB, 2022), particularly in cold-weather regions. In such wide cavities, characterizing the strength and stiffness of such ties becomes important. Additionally, the thermal resistance of these wider cavity walls and the thermal bridging effects produced when using steel ties need to be investigated.A tie that consists of a truss geometry was developed to achieve larger shear strength and flexural stiffness compared with conventional ties. Longitudinal shear tests were conducted using 37 small-scale cavity walls to determine the shear strength and flexuraliiistiffness of three tie configurations: horizontally placed conventional (HC), vertically placed conventional (VC), and a novel tie (NC). The variables considered were the veneer brick type (clay and concrete) and the embedment length of the round bent bars attached to the ties and embedded within the veneer wythe (45, 60, and 75 mm). Results showed that the novel ties demonstrated larger flexural stiffness and peak load than conventional plate ties due a better material utilization (truss action). On average, the peak load for the HC, VC, and NC ties was 0.64, 1.74, and 4.32 kN and the flexural stiffness was 0.34, 0.47, and 0.71 kN/mm, respectively. Earlier crack propagation was observed in samples with clay bricks, while specimens with concrete bricks reached a larger peak load. Specimens with an embedment length of 75 mm reached the largest peak load. The larger strength of the novel tie enables it to be used with wider spacing in full walls. Since the novel tie is stiffer than other connectors, expansion joints should be provided to mitigate the effect of shrinkage in the inner and outer wythe.A Finite Element (FE) model was developed in the 3D FE software ANSYS to determine the thermal resistance (R-value) of wider cavity wall assemblies using both conventional and novel ties. The variables considered were the tie configuration (no tie, conventional tie, and novel tie), insulation thickness (100, 125, and 150 mm), and tie material (steel, stainless steel, and GFRP). Results showed that increasing the insulation thickness resulted in a larger R-value. The assemblies with novel and conventional ties reached similar R-values, this is explained as both ties have the same height and cross-sectional area and similar volume. The GFRP exhibited the largest R-value among the materials compared, this is due to the low thermal conductivity of the material.

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