Loss of tendon force due to long-term creep in unbonded post-tensioned CLT panels

Post-tensioned (PT) Cross-Laminated Timber (CLT) panels are innovative wood structures utilized as load- resisting components in tall wood buildings, offering self-centering capabilities that enhance building stability during seismic events. The self-centering capability of PT CLT panels primarily relies on maintaining PT forces in the tendon. As a hygroscopic material, CLT's modulus of elasticity (MOE) is directly influenced by variations in ambient relative humidity (RH). These variations can alter the strains in both CLT panels and tendons, potentially leading to a loss of tendon forces in PT CLT panels. Controlling post-tensioning force losses is crucial, as these losses may diminish the self-centering capability of CLT panels, thereby reducing the stability of tall building structures during seismic events. This paper presents a numerical approach to estimate the loss of tendon forces in CLT panels under changing RH conditions. A simplified equation is proposed to calculate the tendon force loss over time. Numerical solutions from various CLT configurations and stress levels were compared with results obtained using the simplified equation. Under an RH step between 50 % and 70%, minimal differences in tendon forces were observed at different periods (63 days, 179 days, and 271 days). The total loss of tendon forces over 271 days ranged from 0.6% to 6.5% across all tested specimens. The proposed simplified equation showed good agreement with numerical model results, with differences of less than 2 %. Additionally, a reliability analysis for the design equation is presented, allowing for the estimation of long-term tendon force loss with a reliability index of 3.5.