【作者】 周华樟；

【导师】 祝恩淳；

【作者基本信息】 哈尔滨工业大学 ， 结构工程， 2009， 博士

【摘要】 旋切板胶合木(Laminated Veneer Lumber, LVL)是一种常见的工程木产品(Engineered Wood Product),因其良好的力学性能而广泛应用于建筑结构。蠕变是木材的显著特点之一,因木材蠕变产生的长期变形对结构工作性能的影响是极为重要的。对大跨度木结构而言,稳定承载力分析是一个关键问题,尤其是考虑木材的蠕变影响后,该问题更为复杂,使得木结构表现出不同于其它材料的结构特性。目前,对于木结构的长期荷载效应和长期刚度尚缺乏深入系统的研究,木结构因蠕变导致的屈曲研究尚属空白。因此,开展旋切板胶合木的蠕变及其对结构稳定性影响的研究便具有重要的理论意义与实用价值。本文首先对LVL开展了蠕变试验研究。在哈尔滨地区正常使用的室内环境条件下,分别进行了抗拉和抗压蠕变试验研究。拉、压蠕变试验均有三个应力水平,分别为LVL短期强度的20%、40%和60%。蠕变试验持续观测一年,获得了LVL在正常使用环境下的蠕变规律。通过将试件的变形-时间曲线转化为相对蠕变-时间曲线,得到了LVL在试验应力水平范围内蠕变与应力水平成正比的关系,并建立了LVL的蠕变本构模型,开发了用ABAQUS计算蠕变的用户材料子程序UMAT。在此基础上,针对木结构的长期变形和蠕变对木结构拱和网壳稳定性的影响进行了数值模拟研究。对基本构件的长期刚度和长期变形进行了系统研究。研究了周期荷载时间系数和其周期对基本构件长期变形的影响规律。通过对比,研究了有关国家木结构设计规范关于计算长期变形的处理方法,提出了完善我国木结构设计规范关于长期变形的计算方法。研究了木拱和单层三向网格木网壳的蠕变屈曲和蠕变对结构长期刚度的影响。将荷载水平作为无量纲化的荷载参数,通过研究不同结构参数和荷载参数的拱和网壳,得到了结构的荷载水平-蠕变屈曲临界时间的规律。结构发生蠕变屈曲的临界时间仅仅与结构承担的荷载水平有关,而与其结构参数如跨度、矢跨比和杆件截面尺寸等因素无关。木拱的蠕变屈曲荷载水平与临界时间的关系与木网壳相似,都表现表现为荷载水平随蠕变屈曲临界时间的增加先期急剧降低,而后慢慢降低,最终趋近于某一定值,即长期稳定承载力水平。50年设计基准期临界时间对应的长期稳定荷载都约为结构瞬时屈曲荷载的35%。通过数据回归分别总结了拱和网壳指数多项式形式的荷载水平-蠕变屈曲临界时间曲线方程。该方程可用以评估结构的长期稳定承载力或蠕变屈曲临界时间。研究了正常使用条件下蠕变对既有木拱和网壳的残余稳定承载力的影响。通过定义残余稳定承载力相对水平这个无量纲的参数,得到了考虑蠕变影响的结构残余稳定承载力相对水平与结构参数也无关以及在同一长期荷载水平下残余稳定承载力相对水平与结构服役时间的对数成线性关系的规律。经回归得到了不同荷载水平下的残余稳定承载力相对水平与结构服役时间的对数直线方程,利用该方程可以算得既有结构在任意服役时刻的残余稳定承载力。更多还原

【Abstract】 Laminated veneer lumber (LVL), one of the engineered wood products, is widely used in building structures due to its excellent mechanical performance. Creep is the prominent behaviour of wood, long-term deformation of wood members and wood structures due to creep is of key importance. As far as large span wood structures are concerned, buckling analysis is another key important issue for design of the structures. Creep of wood makes the structures exhibit different behaviour from those made with steel, buckling of wood structures becomes more complicated. Yet in-depth and systematic study of long-term load effect and long-term stiffness of wood structures, so far, has not been conducted, and analysis of buckling of wood structures taking creep into consideration remains untouched. It is, therefore, very useful both theoretically and in a sense of engineering practice to investigate creep of LVL and the effect on the structural stability.In this study, tension and compression tests of creep of LVL were conducted under normal in-door service conditions in Harbin. The LVL specimens were divided into three groups in both tension and compression, with each group subjected to a stress level of 0.2, 0.4 and 0.6 time the instantaneous mean strength of LVL, respectively. The test lasted for one year, and the creep behaviour of LVL under normal service conditions was obtained. It was found that creep is proportional to stress in a the range of stress levels applied. The creep constitutive model of LVL was developed, and a user-defined subroutine UMAT in ABAQUS was encoded to deal with the creep behaviour in the analysis. Then, the numerical simulations on the long-term derformation and the effect of creep on the stability of LVL structures were conducted.Long-term deflection and stiffness of elementary wood members were comprehensively studied, and the effect of the time coefficient and period of load on the long-term behaviour of wood members was obtained. By comparison, treatments of long-term deformation of wood structures in the design codes of different countries were investigated, and suggestions for China national code for design of wood structures to improve calculations of the long-term deflection of wood structures proposed.The creep buckling and the creep effect on long-term stiffness of LVL arches and single layer reticulated LVL shells were investigated. The load level, a dimensionless load parameter, was introduced to the analysis of the structures. The relationships between the load level and the critical creep buckling time of arches and single layer reticulated shells were revealed via parametric studies. It was found that the critical creep buckling time is only related with the load level of the structures, while it has nothing to do with the structural parameters, such as the span, ratio of rise to span and the cross-sectional dimensions of member. The buckling load level decreases sharply with the critical buckling time at first, then the decrease becomes slow and reaches nearly a constant when buckling time is long enough. The long-term safety load against buckling is about 35% of the instantaneous elastic buckling load in relation to the basic design period of 50 years. The equations in the form of exponential polynomials of load level-critical buckling time of the arches and the reticulated shells were established by regression of the analytical results. The equations can be used to evaluate the long-term buckling load or critical buckling time of the structures.The effect of creep on the residual buckling load of arches and the shells having underwent a certain period of normal service was also investigated. The relative residual buckling load level, also a dimensionless parameter, was introduced into the analysis. It was found that the residual buckling load level of the structures has nothing to do with the structural parameters either. A linear relationship between the relative residual buckling load level and the logarithmic service time under a certain load level was disclosed. Then the logarithmic equations were established to evaluate the relative residual buckling load level at an arbitrary service time.更多还原