国产结构用规格材的抗弯强度可靠度分析和设计值

Reliability analysis and design value on bending strength of domestic dimension lumber for structural purpose

我国现行木结构设计规范中缺乏国产规格材的强度设计值取值方法,其强度设计值仅参照方木与原木的设计方法确定。强度设计值取决于木材树种,而与规格材的材质等级、尺寸规格无关。为了合理确定国产规格材的强度设计值,开展了不同等级（Ⅰc、Ⅱc、Ⅲc、Ⅳc）的国产东北落叶松（兴安）规格材的足尺抗弯强度试验研究,并基于一次二阶矩验算点法对其进行了可靠度分析。建议采用前25%抗弯强度数据拟合结果确定的平均值和标准差准确拟合抗弯强度的尾部概率分布,规格材几何尺寸的统计平均值和变异系数设置为1.00和0.05。基于可靠度分析,计算得到了东北落叶松（兴安）规格材在不同数据拟合、短期抗弯强度概率分布、荷载组合类型、荷载比值等条件下的可靠度指标与抗力分项系数之间的变化规律。结合GB 50068—2001《建筑结构设计可靠度统一标准》规定的目标可靠度要求,给出了适于工程应用的规格材受弯构件的抗力分项系数及其对应抗弯强度设计值。

In current Chinese timber structure design codes, due to lack of a determination method, design strength value of domestic dimension lumber can only be determined with reference to the design method of sawn and log timber structure, which is dependent only upon wood species and is not related to visual grade and size of dimension lumber. For rational determination of the design strength value of domestic dimension lumber, full scale tests for bending strength were conducted on different visual grades （ grade I c, II c, III c, IVc） of Chinese larch dimension lumber, and then reliability analysis on bending strength of domestic dimension lumber was carried out by the first order second moment method. In order to predict accurately the tail of probability distribution of bending strength, this paper has suggested that the mean value and standard deviation could be determined with the truncated 25% tested data of bending strength. The statistical mean vaule and coefficient of variation of geometry size of Chinese larch dimension lumber should be modified and be equal to 1.00 and 0. 05, respectively. Based on reliability analysis results, relationships between reliability index and partial factor for different visual grades dimension lumber under different fitting data, probability distribution models of bending strength, load combinations, and load ratios were investigated. Finally, according to the target reliability requirements specified in the GB 50068--2001 ＇Unified standard for reliability design of building structures＇ , the partial factors for resistance and the corresponding design values of bending strength for Chinese larch dimension lumber were ziven.