Research on stress relaxation behavior of confined Chinese fir subjected to compression

The anchorage system can enhance the bending resistance and initial stiffness of timber structure joints. The system applies pressure through squeezing plates and a surrounding steel tube, compressing the enclosed wood significantly. However, if the wood within the anchoring steel tube experiences stress relaxation, it will gradually diminish the force-transfer capacity of the anchorage system over time. In order to quantify the stress relaxation occurred in the confined wood, specimens of 54 were fabricated and compressed under lateral constraints. During the testing process, six fixed temperatures and three distinct compression ratios were taken into account. Thereafter, the evolution of relaxation modulus was discussed according to various temperatures and compression ratios. A linearized Arrhenius equation was proposed and used to determine the parameters of the Arrhenius equation based on the time–temperature superposition principle and experimental data obtained. Besides, the relationship between the compression ratio and the parameters of Arrhenius equation was formulized and the relaxation modulus and relaxation times were presented for five-element general Maxwell model. The results indicate that the stress relaxation behavior of fir wood is closely related to temperature, time, and compression ratio. The relationship between the horizontal shift factor and temperature follows the Arrhenius equation. Additionally, the five-element Maxwell model obtained can be used to predict stress relaxation behavior of confined Chinese fir.