Balloon-type CLT Shearwalls: Lateral Deformation and Kinematic Behaviours

Abstract

Balloon-type Cross-Laminated Timber (CLT) shearwall systems have been increasingly adopted as lateral load-resisting systems in mid-rise timber buildings. Compared with platform-type configurations, balloon-type shearwalls mitigate bearing failure associated with perpendicular-to-grain compression at floor levels, while also reducing cumulative vertical shrinkage and the number of structural connectors and panels. Despite these advantages, research on balloon-type CLT shearwalls remains limited, and their behaviour is not explicitly addressed in many current timber design codes. This thesis presents a comprehensive investigation of the lateral deformation behaviour and kinematic mechanisms of balloon-type CLT shearwall systems through a combined analytical, numerical, and experimental approach. A parametric sensitivity analysis was first conducted using finite-element (FE) modelling to examine the influence of hold-down stiffness, vertical joint stiffness, panel aspect ratio, number of panels and vertical loading on lateral stiffness, kinematic modes, and deformation components of multi-panel shearwalls. The results demonstrate that bending deformation is a fundamental component of the lateral response of balloon-type shearwalls, and that kinematic mode transitions significantly affect the contribution of rocking deformation. Based on these observations, an analytical model was developed to estimate the total lateral displacement of balloon-type CLT shearwalls. The model accounts for bending deformation using the γ-method, as well as shear and rocking deformation components, initially formulated for a two-panel configuration. Validation against FE simulations showed good agreement, with the analytical predictions of top lateral displacement exhibiting an average error of less than 5%. A full-scale experimental programme was subsequently carried out on eleven two-storey multi-panel balloon-type CLT shearwall specimens. The test matrix covered a wide range of parameters, including the number of hold-downs (one to four), number of vertical joints (eight to sixty-five), panel aspect ratio (4.17 to 8.33), and the number of panels (one to four). The experimental results provided detailed insight into deformation sequences, failure mechanisms, kinematic modes and deformation compatibility among panels. A systematic comparison between experimental results and corresponding analytical and numerical predictions was performed, highlighting both consistencies and sources of discrepancy. The findings confirm the validity of the proposed analytical framework while also identifying its limitations. Overall, this research advances the understanding of deformation and kinematic mechanisms in balloon-type CLT shearwalls and provides a foundation for further development of analytical models and future design provisions. The results demonstrate that bending deformation represents a significant component of the lateral response of balloon-type CLT shearwalls and should not be neglected in the deformation prediction. The study also shows that the kinematic behaviour of multi-panel shearwalls is strongly influenced by panel aspect ratio and connector stiffness, which govern the contribution of rocking deformation and the transition of kinematic modes. Experimental observations further indicate that hold-down failure is generally the governing failure mechanism. Moreover, for a given total wall width, the use of slender panels can significantly enhance deformation capacity without reducing lateral resistance, providing favourable implications for the seismic design of balloon-type CLT shearwall systems.