木结构设计计算理论关键问题研究（之二）

Engineering applications, scientific study and education of wood structures in our country have all gained significant development since the revival of using wood structures at the turning of this century. Due to the stagnation in the development for more than 20 years, there are many aspects of the design theory and method that can not meet the rapid development of wood structures in our country. These aspects hence need urgent investigation and improvement. Some key problems in the design theory and method once caused confusions, which include the determination of design values of wood and wood products, the stability of compression members and bending members, the calculation of load-carrying capbility of bolt connections and nail connections. Through the implementation of the last research project financially supported by the National Natural Science Foundation of China, the above mentioned problems have been resolved with success, and the results have all been adopted by the lately revised Code for Design of Timber Structures GB 50005-2017. Yet other important problems in the design theory and method are identified, which need to be addressed via further studies. They are as follows. Development of the time-dependent constitutive relationships and the time-dependent failure criteria of wood with stress relaxiation and creep, so as to reflect the nature of duration of load, and the difference of its effect, if any, on the strength and the modulus of elasticity of wood, and the difference of its effect, if any, on clear wood and structural timber; stress grading of the round and sawn timber; and the reliability analysis of the limit state of serviceability of wood structures and the related design method. This project aims to conduct in-depth invesitigations both theorectcally and experimentally, to address the above-mentioned problems, thus providing technical support in the design theory and method for the engineering practice of wood structures. And more broadly, this study is to pave the way for improving and developing the design theory and method for wood structures, which shall be advanced, independent and with Chinese characteristics.

自恢复使用木结构以来，木结构科研、教学及工程应用都取得了长足发展。但由于经历了二十余年的停滞期，木结构设计理论与方法的许多方面已不能满足木结构快速发展的需要。木材的强度设计值、受压和受弯木构件的稳定性、螺栓连接和钉连接的承载力等问题的设计计算曾一度混乱。经前一基金项目的研究，这些问题已获解决，成果已为新的《木结构设计规范》GB 50005-2017所采用。但还存在以下问题：建立考虑应力松弛和蠕变的木材时变本构关系和强度准则，以反映荷载持续作用效应的本质，荷载持续作用效应对木材强度和弹性模量的影响是否相同、对结构木材和清材的影响是否相同；对方木与原木进行应力定级使之成为现代木产品；对木结构正常使用极限状态进行可靠度分析。本项目拟对这些问题开展深入的理论和试验研究，为木结构的工程应用提供技术支持，为发展和完善先进的、独立自主的、具有中国特色的木结构设计理论与方法提供依据。

随着木结构在我国恢复应用与发展，木结构设计理论与方法需要不断改进。借助于前一个自然科学基金项目-木结构设计计算理论关键问题研究，木材与木产品的强度设计值、受压和受弯木构件的稳定性、螺栓连接和钉连接的承载力等问题得以解决，成果均已为《木结构设计标准》GB 50005-2017所采用。作为木结构设计计算理论关键问题研究之二，本项目进一步研究以下问题：建立考虑应力松弛和蠕变的木材时变本构关系和强度准则，以反映荷载持续作用效应的本质，荷载持续作用效应对木材强度和弹性模量的影响是否相同、对结构木材和清材的影响是否相同；对方木与原木进行应力定级使之成为现代木产品；对木结构正常使用极限状态进行可靠度分析。经开展深入的理论和试验研究，获得的如下主要进展和结论：荷载持续作用对木材强度和弹性模量均有影响，但对弹性模量的影响程度较轻；结构木材和清材的荷载持续作用效应并无明显差别；提出了我国方木与原木的应力定级方法，并确定了其三个等级的强度标准值；建立了木材的时变本构关系，预测木材的长期强度和蠕变；对受弯木构件进行了正常使用极限状态可靠度分析，提出了受弯木构件长期变形计算方法和挠度限值。项目研究工作为木结构的工程应用以及中国特色的木结构设计理论与方法发展和完善的提供了新的技术支持和理论依据。