System Level Interactions and Floor Diaphragm Out-of-Plane Stiffness Effects on the Lateral Response of Light Frame Wood Shear Walls

Abstract

The lateral performance of light-frame wood shear walls is significantly influenced by system-level interactions that are not fully captured by conventional design approaches. In particular, the impact of floor diaphragm out-of-plane stiffness on the lateral behaviour of wood shear walls, and its role in mitigating cumulative deformations along the height of multi-storey systems, introduces complexities in the interpretation of global response parameters such as inter-storey drift, structural stiffness, and force distribution among structural elements. This study investigates the impact of floor diaphragm out-of-plane stiffness on the behaviour of wood shear walls through a combined experimental and numerical approach. An experimental program was conducted on single-storey, single- and multi-segment shear walls subjected to monotonic loading to characterize local and global deformation mechanisms under realistic system-level boundary conditions. A total of 14 test configurations were investigated. The key experimental parameters include shear wall configuration (single- and multi-segment), floor assembly configuration (continuous joists versus blocking), openings of varying lengths, types of anchorage systems (e.g., discrete hold-downs and continuous rods), shear wall aspect ratio, and wall-to-floor boundary conditions. A numerical modelling framework incorporating system-level effects was developed and validated using the results from the single-storey shear wall tests. The validated model was subsequently extended to multi-storey shear wall systems to evaluate the influence of system-level interactions, including diaphragm out-of-plane stiffness, on the global response of buildings. Particular attention was given to the cumulative effects of hold-down elongations along the height, global structural stiffness, and force distribution among shear walls at both the storey level and along the building height. In addition, the four-term deflection equation in Canadian Standard Association for Engineering Design in Wood standard (CSA O86) was examined through comparisons with experimental results from single-storey, single- and multi-segment shear wall tests and numerical predictions. Its applicability was further evaluated for multi-storey systems by comparing its predictions with results from parametric studies considering various system-level characteristics and modelling assumptions related to wood shear wall behaviour. The experimental results demonstrated that the out-of-plane stiffness of the floor assembly and system-level effects significantly reduce hold-down force and deformation demands, with average and maximum reductions of approximately 50% and 77%, respectively, while increasing nail deformations by about 50% on average. This behaviour indicates that a floor assembly promotes a racking-dominated response by restraining rocking deformations associated with hold-down elongations. Shear walls with higher aspect ratios exhibited greater reductions in hold-down demands and larger increases in nail deformations. While shear walls equipped with Anchor Tie-down System (ATS) rods showed similar trends to those with discrete hold-downs, the uplift stiffness of the ATS system was approximately 50% lower than the theoretical design value. Both experimental and numerical results indicated limited bending deformation in chord members, which is not consistent with the assumptions made in the four-term deflection equation in CSA O86, which is based on cantilever beam theory. Sheathing panel shear deformations were found to be significantly lower than those predicted by the CSA O86 equation and remained relatively constant as the nail connections entered the nonlinear range. The CSA O86 equation also underestimates nail deformations and does not capture their increase due to system-level effects. Similarly, it overestimates the hold-down force and deformation demands. Part of this overestimation is attributed to the contribution of anchor bolts, which are neglected in CSA O86 but were observed to resist up to 30% of the hold-down force. In contrast, perpendicular-to-grain deformation of the bottom plate was significantly higher than predicted by the CSA O86. However, this was a localized behaviour that primarily increased the nail deformations near the base rather than contributing to rigid-body rocking. In multi-storey systems, the CSA O86 deflection equation, which includes cumulative effects, produced results similar to analyses that neglect floor diaphragm stiffness. However, excluding cumulative effects more closely represents buildings with realistic floor systems. Incorporating floor diaphragm out-of-plane stiffness significantly influenced inter-storey drift, global structural stiffness, and force distribution among shear walls both along the height and at the storey level. Neglecting floor out-of-plane stiffness can lead to overestimation of hold-down forces and deformations, which in turn results in underestimation of global structural stiffness and, consequently, design base shear. Furthermore, inclusion of floor effects altered force distribution among shear walls, with some walls experiencing increases in force demands of up to 100–200%, potentially leading to perceived premature failure if not properly accounted for in design. The findings of this study provide insight into the deformation mechanisms of wood shear wall systems and emphasize the need for refined modelling and interpretation methods, particularly for multi-storey applications.