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Coupling effects of CLT connections under bi-axial loading Liu, Jingjing

Abstract

This dissertation presents a study on the coupling effects of tension and shear force on CLT connections in mid-rise structures. Unlike the current simplified design assumption that CLT connectors only take either pure shear or pure tension force in the structures, this research first considers the significant interactions between tension and shear loads from two orthogonal directions for CLT connections through bi-axial loading experimental tests and numerical analysis. Four sets of experimental tests were conducted: 1) AE 116 angle bracket connection tests under monotonic/cyclic tension loading with four levels of co-existent shear loads (0 kN, 20 kN, 30 kN, and 40 kN), 2) AE 116 angle bracket connection tests under monotonic/cyclic shear loading with four levels of co-existent tension loads (0 kN, 20 kN, 30 kN, and 40 kN), 3) HTT5 hold-down connection tests under monotonic/cyclic tension loading with three levels of co-existent shear loads (0 kN, 10 kN, and 20 kN), and 4) HTT5 hold-down connection tests under monotonic/cyclic shear loading with five levels of co-existent tension loads (0 kN, 20 kN, 30 kN, 40 kN and 60 kN). The specimens exhibited changes in strength and hysteresis behaviour under different configurations. Under bi-axial loading, different interactions between nails and surrounding wood embedment caused the coupling effect. Based on the mechanisms of CLT connections under bi-axial loading, a pseudo-nail model was developed using a modified protocol-independent nail connection algorithm. Two key parameters, a gap size factor β, and an unloading stiffness degradation index γ, were implemented into the original algorithm to capture the unloading stiffness degradation and coupling effect of bi-axial loading. The developed model was able to capture all the characteristics of CLT connections, including strength degradation, reloading/unloading stiffness degradation, the pinching effect, and the coupling effect. Model parameters were calibrated for all 32 configurations from the tests, and compared in terms of load carrying capacity and energy dissipation. Accurate agreements were reached. The results show that the proposed CLT connection model was able to predict the hysteresis behaviour of all nail-based connections.

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