Abstract:

Objective: In order to provide data for the refined design on fire resistance of timber structures and the evaluation on residual bearing capacity of structural members exposed to fire, the parallel-to-grain tangential shear strength of heated wood within structural member with a large cross section under the protection of the outermost charred layer was tested, and its degradation model was built. Method: Imported Larix gmelinii and Pseudotsuga menziesii, and domestic fast-growing Populus spp. with a high strength grade and potential application in timber structure, were selected as the research objects. A test chamber filled with nitrogen was used in this study to simulate the oxygen-free conditions. The temperatures were set to nine levels, such as 20, 50, 70, 110, 150, 200, 220, 250 and 280 ℃. As many as 216 specimens were tested to determine the parallel-to-grain shear strength of wood specimens at temperatures between 20 and 280 ℃ under oxygen-free conditions. The main chemical compositions of wood specimens at 150, 180 and 220 ℃ was also determined. Result: At room temperature, the shear strengths of Larix gmelinii, Pseudotsuga menziesii, and Populus spp. wood specimens were 9.65, 8.94 and 9.48 MPa, respectively. When the temperature reached 150 ℃, the shear strength of Larix gmelinii, Pseudotsuga menziesii, and Populus spp. of wood specimens decreased to 60.7%, 68.0% and 65.6% of the initial values, respectively. The shear strength of wood specimens decreased faster, as the temperature was 150 ℃ and higher. The shear strengths of Larix gmelinii, Pseudotsuga menziesii, and Populus spp. of wood specimens were determined as 1.05, 0.91 and 0.61 MPa at 280 ℃, respectively, which were only 9.0%, 10.2% and 6.4% of the initial values. The thermal stability of cellulose was the highest and that of hemicellulose was the lowest among the main chemical compositions within wood. The hemicellulose contents of Larix gmelinii, Pseudotsuga menziesii, and Populus spp. of wood specimens were 25.3%, 25.7% and 16.3% at room temperature, respectively. At 150 ℃, hemicellulose started to be decomposed and its content decreased by 1.3%-9.1%. Hemicellulose was decomposed rapidly at 200 ℃, and its content decreased by 29.4%, 18.6% and 25.9%, respectively. The degradation on parallel-to-grain shear strength of wood specimens was attributed to the serious pyrolysis of hemicellulose at high temperature. With the increase of temperature, the effects of wood density on parallel-to-grain shear strength gradually decreased. The ratio of shear strength to wood density was determined as 14.2 MPa·(g·cm^-3)^-1 at room temperature, and decreased to 1.1- 3.1 MPa·(g·cm^-3)^-1 at 200 ℃ and above. The degradation on relative shear strength of three wood species was similar with each other at elevated temperatures. Conclusion: High temperature might be negatively related with parallel-to-grain shear strength, causing by the reduction in hemicellulose content. The reduction factor of wood shear strength at a high temperature is less than that in European standard due to the oxygen-free conditions used in this study. Based on the test data, a bilinear model of wood parallel-to-grain shear strength deterioration at a high temperature was proposed.

Key words: structural wood, chemical composition, parallel-to-grain tangential shear strength, high temperature, oxygen-free condition