正常使用极限状态受弯木构件变形计算及可靠度分析

Calculations of deflection and reliability analysis of bending wood members in serviceability limit state

北美锯材、欧洲锯材、目测定级层板胶合木及机械定级层板胶合木等标准化、工业化的木产品已被纳入GB 50005—2017《木结构设计标准》,而其中仅对原有的方木与原木受弯构件开展了正常使用极限状态下的可靠度分析。设计标准中所采用的各类结构木材的力学性能和木结构设计中的荷载情况均发生了很大变化,受弯木构件变形计算及验算方法需随之改进。为此,开展规格材长期受弯试验,研究其蠕变性能,并建立结构木材幂律函数蠕变模型。考虑木材蠕变和荷载历程的随机性,提出受弯木构件长期变形计算方法。将长期变形和极限挠度均视为随机变量,分别建立感知损坏和局部损坏极限状态对应的功能函数,并开展可靠度分析,获得以短期变形表示的挠度限值。研究结果表明:挠度限值随结构木材的种类、荷载组合及活荷载与恒荷载之比ρ的变化而变化,进而给出基于方木与原木的受弯构件挠度限值计算式;对于层板胶合木、欧洲锯材等,当荷载比ρ≤0.5时,挠度限值取方木与原木构件挠度限值乘以系数0.85;当ρ>0.5时,乘以系数0.90;对于目测应力定级木材,挠度限值则取方木与原木构件挠度限值乘以系数0.80。经验算,采用所建议的受弯木构件的挠度限值及改进的变形验算方法,可满足正常使用极限状态可靠度要求,且更为合理。

Standardised and industrialised wood products like North American sawn lumber,European sawn timber and glulam made of visually graded and machine graded laminations are already incorporated into GB 50005—2017 ‘Standard for design of timber structures',yet only the reliability of bending members made of the original sawn and round timber in the serviceability limit state had ever been analysed.With quite different mechanical properties of the newly incorporated structural timber and different load situations in design of timber structures,calculation and verification of deformation of the bending members thus need to be improved.In this study,long-term bending test of dimension lumber was conducted,the creep behaviour was investigated and the creep model in the form of power law function for structural timber developed.The calculation method of deformation of wood members in bending was suggested,in consideration of the creep model and the randomness of load history.The performance functions with respect to the conceptive damage limit state and the local damage limit state were developed,in which both the deflection and the ultimate deflection were treated as random variables.Reliability analyses were conducted and the deflection limit values for bending members made of various types of structural timber in terms of short-term deformation obtained.It is revealed that the deflection limit value varies with type of structural timber,load combination and the live to dead load ratio ρ.The formulae for the deflection limit value for the bending members made of sawn and round timber were thus presented.For members made of glulam,European sawn timber and so on,the deflection limit value is suggested to take the value for sawn and round timber members timing a factor of 0.85 if ρ≤0.5 and a factor of 0.9 if ρ>0.5;for members made of visually stress rated lumber,the deflection limit value is suggested to take the value for sawn and round timber members timing a factor of 0.80.It is verified that the suggested deflection limit value and the improved deformation verification method all meet the reliability requirements more reasonably.