Creep and long-term strength of CLT in shear-governed flat-wise bending

Mass-timber construction is a cornerstone of sustainable development, combining reduced embodied carbon, renewable resources, and efficient prefabrication. Cross-laminated timber (CLT) panels are widely used for floor systems, where serviceability (SLS) and ultimate limit state (ULS) design are influenced by long-term load duration. The Canadian Standard for Engineering design in Wood, CSA O86 accounts for load duration through a modification factor to reduce the resistance, and a creep factor ( K creep,CLT = 2.0) to amplify deflection. However, limited data exist to validate this creep value for shear-governed spans. In this study, the creep of full scale 24 CLT panels in major and minor strength axes under flat-wise four-point bending (shear-governed members) was recorded over one year in a semi-controlled dry service condition. Two load levels were considered: 55% of short-term resistance; and factored shear resistance per CSA O86. At the first load level, creep values were below 1.8 (major axis) and 2.0 (minor axis). At the design load level, both values averaged 1.6, suggesting that the CSA O86 creep factor of 2.0 was conservative for the studied specimens. The experimental results served to evaluate the predictive performance of several analytical models; among which, the modified General Kelvin model was best suited to capture the long‑term flatwise bending behaviour of CLT. Finally, after long-term loading of the panels, the short-term resistance was within 83–96% of and the effective short-term bending stiffness was not significantly different.