杉木目测等级规格材足尺抗压和拉伸强度的设计值研究

Design values of compressive and tensile strength for visually graded Chinese fir dimension lumber

杉木具有生长周期快、力学性能好、耐腐性强等优点,被广泛应用于工程木产品的制备,如规格材、胶合木和木基复合材料。但是由于缺少设计值,其在建筑结构领域的使用存在安全隐患。先根据加拿大NLGA分等规则,将杉木规格材分为SS(structural select)、No.1和No.2三个等级,再根据ASTM D4761-02和ASTM D198-02分别进行足尺的抗压和拉伸测试,选取正态分布和对数分布对测试数据进行拟合,利用K-S检验对正态分布和对数分布进行拟合优度检验。参照木材容许应力法和ASTM D2915-03计算杉木规格材设计值。结果表明:No.1等级规格材在15%含水率条件下的抗压和拉伸强度最小,其主要缺陷为节子,对力学性能影响较大;No.2等级规格材主要缺陷为钝棱,对强度影响不大。SS等级规格材的抗压和拉伸测试数据与对数分布拟合效果更优;No.2等级规格材的抗压和拉伸测试数据与正态分布拟合效果更佳。SS、No.1和No.2等级规格材抗压强度参考设计值分别为11.50,11.33和11.05 MPa,拉伸强度参考设计值分别为8.95,8.15和7.18 MPa。

Being widely used for dimension lumber, glued lumber and wood-based composites, Chinese fir has the advantages of fast-growing, good mechanical performance and decay resistance. However, the application in the building structures was limited due to the lack of design values of mechanical properties. To utilize the green building material, this study aimed at obtaining the design values of Chinese fir for the building structures. The dimension lumbers for Chinese fir were divided into structural select （ SS）, No. 1 and No. 2 grades according to the NLGA Standard of Canada. The compressive strength and tensile strength of specimens were full-size tested according to ASTM D4761-02 and ASTM D198-02, respectively. The normal and lognormal distributions were selected to fit the experimental data. The K-S testing was used to estimate goodness of fit for normal and lognormal distributions. The design value was calculated according to Chinese allowable stress design method and ASTM D2915-03. The results showed that the UCS15 （compressive strength under 15% moisture content） and UTS15 （tensile strength under 15% moisture content） of No. 1 grade lumber were lower than that of other lumber grades due to the great influence of knots. Besides, No. 1 grade lumber contained more knots. In addition, the wane and skips in No. 2 grade lumber had no significant effect on the wood strength. The experimental data of compressive and tensile strength for SS grade lumber fitted better by using the lognormal distribution. Conversely, the normal distribution fitted No. 2 grade lumber better than the lognormal distribution did. The calculated design values of Ucs were 11.50, 11.33 and 11.05 MPa for SS, No. 1 and No. 2 grade lumber, respectively. The calculated design values of UTS were 8.95, 8.15 and 7. 18 MPa for SS, No. 1 and No. 2 grade lumber, respectively.