Creep behaviour of thin cross-laminated timber under short-term loading

Abstract

Cross-laminated timber (CLT) has been developed and used in building applications, such as walls, roofs, and floors, which are subjected to long-term loading in the perpendicular-to-grain direction. Thus, the creep performance of CLT is critical in the design and construction of a building using CLT panels. The creep behaviour of a material, component, connection, or system is defined as the deformation increases with time under constant load. This thesis report examined the creep of CLT using the thin specimens. Two loading levels (60% and 30% of proportional limit (PL), or 93kg and 25kg), two relative humidities (30% and 65%), and two panel directions (major and minor). A total of four groups of thin CLT specimens were fabricated, and each group had six specimens (three for major and three for minor), producing a total of 24 specimens. The third-point static bending tests were performed to determine the load levels for creep tests. A constant load was applied on each specimen, and the flexural deflection was recorded in the course of loading. It was found that 1) In third-point bending test, the PLs of thin CLT specimens in the major and minor directions were 3,000N and 800N, respectively. The average MOE[subscript app] and MOR[subscript app] of the specimens in the major direction were 8,510 MPa and 46.64 MPa, respectively, and the average MOE[subscript app] and MOR[subscript app] in the minor direction were 809 MPa and 9.48 MPa, respectively; 2) With increasing load level up from 30% to 60% PL, the minor strength direction specimens deflected 81.04% than that of 30% PL. And for major strength direction specimen, the deflection increased 32.07%; 3) With decreasing the RH, the deflection of CLT specimens showed a slower increment. At the load level of 60% PL, the deflection of creep specimens in the minor direction showed a difference of about 2 mm after 7 days. However, at the load level of 30% PL, RH showed little impact on creep. 4) CLT in the major direction exhibited a superior load-bearing capacity than those in the minor direction. At the load level of 30% PL, the maximum deflection in the minor direction was about 1.8 times larger than that in the major one, and at the load level of 60% PL, the deflection in the minor direction was approximately 2.5 times larger than that in the major one. 5) The Burger model could well describe the 7-day creep of thin CLT specimens with a high r-square value around 0.9135 tested in this study. It could be recommended to investigate the long-term creep behaviour of full-size CLT members by taking cyclic humidity and edge-bonding into account for at least one to three months. Keywords: Creep, cross-laminated timber, modelling, panel direction, relative creep, relative humidity, load level