Numerical investigations on the thermo-mechanical behavior of steel-to-timber joints exposed to fire

A three-dimensional finite element model is developed to predict the thermo-mechanical behavior of steel-to-timber doweled joints in tension parallel to grain exposed to fire. To manage the plastic yielding of the materials, the mechanical model is based on the von Mises criterion for steel and the Hill criterion for timber. In fire, the material characteristics depend on the temperature. Two different meshes are used for the thermal and the thermo-mechanical models. The thermal model is continuous, to take account of the thermal continuity between the joint components. The thermo-mechanical model is discontinuous, to consider the contact evolution between the joint components. The thermal model is used to predict the evolution of the temperature field inside the joint which depend on the gas temperature. It is validated on the basis of measured temperatures during fire tests. The complex transformations in wood during fire are represented by apparent values of thermo-physical characteristics proposed in the bibliography and calibrated on the basis of the experimental measurements. The mechanical model is validated by comparison with the experimental results of joints in normal conditions. The thermo-mechanical model is validated by considering the experimental failure times of some joints. The numerical models showed a good capacity to simulate the behavior of the timber joints in cold and in fire situations. These developed and tested models can be used as a general tool to analyze the behavior of a large variety of joint configurations to constitute a data base that can be used in safe and economic practice of fire engineering of wood joints.     

Ductility aspects of reinforced and non-reinforced timber joints

Even though brittle failure modes in timber joints may be avoided by the proper design of the connection, the use of minimum timber dimensions and minimum spacing and distances of fasteners often leads to timber splitting in the connection area. Due to the highly nonlinear behaviour of timber loaded in compression as well as the steel used for mechanical fasteners, timber joints can behave in a rather ductile manner. Ductile behaviour is preferable in timber structures.Technical innovations regarding engineered wood products as well as fastener and steel technology led to the development of high-performance timber connections. In these high-performance connections, brittle failure modes are prevented by reinforcing the timber in the connection area perpendicular to the grain or using cross-laminated timber members. The improvement of the ductility levels is shown based on several experimental studies comparing non-reinforced to reinforced connections.     

Numerical 3D finite element modelling and experimental tests for dowel-type timber joints

This study is a contribution to various scientific researches on dowel-type timber joints. In this paper, a three-dimensional (3D) finite element model is developed to predict the mechanical behaviour of steel-to-timber joints loaded in parallel-to-grain tension. The adopted material model is based on the isotropic behaviour of the steel and the transverse isotropy behaviour of the timber. In order to take into account the asymmetric mechanical behaviour of timber material in tension and compression, Hoffman failure criterion is associated with the Hill criterion to control the plastic yielding combined with the damage evolution of timber. Besides, the interaction among materials is modelled using contact elements associated with friction. The results obtained in numerical simulations are evaluated and compared with experimental results. The model showed the capacity to simulate the behaviour of the timber joints. Moreover, an analytical model based on stress interactions is presented to calculate the failure loads of the timber joints.     

Experimental and numerical analysis of a halved and tabled traditional timber scarf joint

Traditionally constructed heavy timber trusses, found in timber framed buildings and bridges, employ various traditional joints, among them the lower chord scarf joint. This paper examines the behavior of a halved and tabled scarf joint, which was studied as an isolated structural component using experimental tests and finite element analyses. Experimental tests identified two different limit states for these particular scarf joints: shear failure parallel to grain and tension failure perpendicular to grain. The possibility of failure due to tension perpendicular to grain results from variations in grain angle and means that the limit state of shear failure parallel to grain, typically assumed in analysis and design, is unconservative. For the purposes of design and rehabilitation, the authors propose that the scarf joint be treated as a member subject to combined bending and axial tension forces. The results of the finite element analysis, performed using solid continuum elements in ABAQUS, are in good agreement with the experimental test results. In addition to finite element models, the authors use analytical spring models to demonstrate that when developing a two-dimensional model of a truss with lower chord scarf joints, serviceability limit states be checked with a model that reduces the lower chord section properties in the vicinity of the scarf joints.     

木销半榫节点转动性能试验研究与有限元分析

为研究木销半榫节点平面内的转动性能和节点区域木材损伤演化,通过木销半榫节点试件的单调加载试验和有限元分析研究了此类节点平面内的转动性能.试验研究表明,加载初期预钻孔附近木材会发生横纹劈裂破坏,随着荷载不断增加,榫头变截面处木材会出现顺纹受拉破坏.节点有限元分析结果表明,采用弹塑性损伤模型建立的有限元模型对于节点刚度和受...     

Experimental and numerical analysis of a stop-splayed traditional timber scarf joint with key

Traditionally constructed heavy timber trusses employ scarf joints to transfer tensile forces between lower chord members. Scarf joints significantly impact the global stiffness of heavy timber trusses, yet little is known about their structural behavior. This paper describes a study recently completed on the commonly used stop-splayed scarf joint with a key. Experimental tests were performed on scarf joints replicated from a covered wooden bridge in Pennsylvania and compared to the numerical results of full, three-dimensional finite element models created using ABAQUS. The authors determined that the key has the most influence on scarf joint behavior as its orientation causes it to be loaded in compression perpendicular to grain. Experimental tests also revealed the importance of clamping bolts, without which a limit state of “key rolling” was observed. With two clamping bolts in place, the limit state was shear failure parallel to grain, which provides a more stable failure and a higher ultimate strength. The authors recommend to engineers that the scarf joint not be analyzed as a pure tension member; rather, it should be considered a member subject to combined tension and bending forces.     

木结构螺栓连接横纹受力性能试验研究

木结构螺栓连接横纹受力在现代木结构应用中难以避免,其横纹劈裂承载力计算目前尚缺乏相关规范作指导.设计制作了185根锯材木梁和54根胶合木梁螺栓连接横纹受力试件,进行了横纹劈裂承载力试验研究.结果表明:劈裂承载力与梁截面宽度成正比,与连接节点相对加载高度呈非线性关系,而与螺栓直径无关,螺栓布置方式对劈裂承载力无明显影响;...     

基于三维弹塑性损伤模型的木材非线性分析

为预测木材复杂的受力行为,在弹塑性理论和连续介质损伤力学的框架内建立了木材三维弹塑性损伤本构模型。采用Hill屈服准则和Voce强化模型描述木材受压硬化行为;通过修正后的Hashin破坏准则和指数型损伤演化模型控制木材受拉、受剪的损伤演化过程。基于应变增量法求解本构模型的数值解,并采用Newton-Raphson迭代法求解塑性应变。通过编写用户自定义子程序（UMAT）将本构模型嵌入商业有限元软件ABAQUS,并根据试验结果对本构模型进行了验证。针对木材顺纹和横纹受压试验的数值模拟结果表明,该模型可以有效地描述木材的受压非线性硬化行为。针对木材斜纹受拉试验的数值模拟结果表明破坏准则可以较为准确地识别木材在横纹拉应力和顺纹剪应力作用下的破坏模式,损伤演化模型可以合理地控制木材的损伤演化过程。     

低周反复荷载作用下钉节点性能试验研究

介绍了轻型木结构框架剪力墙结构中的面板与木框架钉节点在低周反复荷载作用下的试验研究;对采用不同OSB面板厚度、不同钉边距、顺纹或横纹受力的10组共100个钉节点试件的承载力、破坏模式、延性、刚度退化、强度退化和耗能性能作了比较分析。得到以下结论:钉边距影响钉节点的破坏模式;钉边距和面板厚度的增大有利于提高节点承载力、耗能和变形能力;板厚对顺纹受力试件承载力及横纹受力试件承载力相对关系影响较大,顺纹受力试件的延性与耗能能力相对横纹受力试件较差;钉边距增大使强度退化的速率逐渐降低;各因素对刚度退化的影响可忽略。     

共面闭合非贯通节理岩体贯通机制和破坏强度准则研究

简述共面闭合非贯通节理岩体的破坏机制及其贯通强度依赖于节理和岩桥的特性，阐述现有的共面闭合非贯通节理岩体的强度准则及其不足。研究直剪应力状态下共面闭合非贯通节理的受力特点，提出共面闭合非贯通节理岩体破坏机制，引入起裂角，在此基础上建立含起裂角的共面闭合非贯通节理岩体贯通破坏强度准则。通过与前人的试验进行对比，结果表明提出的破坏机制能较好地解释试验现象，理论计算的峰值强度与试验实测值吻合较好。     

考虑局压效应的榫卯节点力学性能研究

木结构榫卯节点在受力过程中处于局部受压状态,通过木材横纹、顺纹方向的中部局压、端部局压、全截面受压性能的试验研究,得到了相应的材性参数;设计了钢榫头-木卯口、木榫头-钢卯口、木榫头-木卯口三组不同形式的分离式直榫节点试件,通过单调加载试验,对局部受压区域的应变分布特征进行了研究;基于试验研究得出的榫卯节点受力机理,建立了考虑局压效应的直榫节点力学模型。研究结果表明：木材横纹局压的抗压强度和弹性模量均高于全截面受压,顺纹局压与全截面受压的材性参数基本一致;榫卯节点受力区域的榫头应变远大于卯口应变,榫卯节点的受力挤压变形主要集中在榫头,而卯口变形可忽略;考虑局压效应的榫卯节点力学模型计算结果比未考虑局压效应所得的结果更符合试验结果。     

有空洞地基破坏过程的试验和数值模拟

首先对该地基的破坏过程进行了室内模型试验,用数码照相记录下了地基的渐进破坏,然后运用所建立的各向异性损伤数值模型,对位移和载荷控制条件下该地基的渐进破坏过程进行了数值模拟试验,重点分析了地基的破坏模式和应力应变关系,并与不考虑拉伸损伤的理想弹塑性模型的计算结果进行了对比。研究发现：有空洞粘土地基首先发生竖向拉伸破坏,然后沿对角连线发生剪切塑性屈服并不断向连线中间扩展,最后在对角连线中间发生拉伸破坏,导致地基发生失稳。数值结果与模型试验结果基本一致,从而为进一步揭示复杂条件下有空洞粘土地基的渐进破坏全过程提供了一种新的数值模拟研究手段。     

木材非线性受力行为的表征方法研究进展

木材非线性受力行为的表征方法对木结构构件和节点受力行为的有限元分析具有至关重要的影响。为此,从木材失效行为的判别依据和材料本构模型两方面对该领域的研究进展进行了梳理总结。结果表明,已有的强度准则各有其局限,难以全面地反映木材复杂的失效行为。在数值模拟中常需要结合不同强度准则以提高预测模型的计算精度。弹塑性本构模型能够合理反映木材在压应力作用下的硬化行为,并且能够描述木材出现的残余变形,但该模型难以描述木材在拉应力和剪应力作用下的刚度退化和强度软化行为。弹性损伤理论可在一定程度上反映材料的软化行为和反复加载过程中的刚度退化行为,但该理论不能考虑材料的塑性变形。基于上述理论建立的黏聚区单元的初始刚度和相邻单元长度对计算结果具有显著的影响。三维弹塑性损伤模型已经被应用于木结构构件和螺栓连接节点的有限元分析中,此类模型能有效反映构件和节点区域木材的损伤演化规律。随机损伤本构模型和非局部化损伤模型是未来的发展趋势。     

轻型木结构中覆面板钉连接承载性能试验研究

通过对222个采用中国产圆钢钉制作的轻型木结构覆面板钉连接试件进行单调加载试验,研究了它们的承载性能。试验结果表明,钉连接发生模式M-c1屈服、覆面板断裂、覆面板剪坏、规格材撕裂和钉子穿透等5种破坏;直钉连接的承载力比斜钉高,而两者刚度基本相同;覆面板主方向与规格材木纹夹角对钉连接承载力和刚度的影响很小;荷载方向与规格材木纹夹角影响钉连接的刚度,但不影响其承载力。通过与欧洲木结构设计规范钉连接计算公式的分析对比,获得了符合中国轻型木结构覆面板钉连接的刚度计算公式,为计算分析剪力墙和横隔(楼盖和屋盖)的抗侧力性能提供参考。     

火灾下采用榫钉和螺栓连接的钢木连接件的性能

重点研究火灾时平行于晶粒的拉伸荷载作用下钢木连接件扣件间的荷载分布。给出各种几何布置的不同类型连接件的试验结果。研究扣件类型(螺栓、榫钉)对连接件热力学性质的影响。采用经过试验验证的3D有限元模型,温度和失效时间的有限元分析结果与试验结果相一致。金属扣件的类型对连接件的热力学性质起着决定性的作用。研究中采用的连接件,每行扣件均使用1个螺栓,以确保其能够与安装构件分隔开来。螺栓的存在极大地影响了火灾下连接件的性能。研究了仅使用榫钉或改变螺栓位置的各种几何布置的连接件。通过改变特定连接件的一些几何特性,提出新的分析方法。数值试验设计能够用于计算连接件的破坏时间。     

Stochastic spatial modelling of material properties and structural strength of unreinforced masonry in two-way bending

The paper describes the development of a computational method to predict the strength for unreinforced masonry walls subject to two-way out-of-plane bending considering unit-to-unit spatial variability of the material properties of mortar joints and bricks. The study involves conducting a numerical simulation of full-sized walls subject to two-way bending using stochastic analysis in the form of Monte Carlo simulations and comparing the results with experimental work. A 3-D non-linear Finite Element Analysis is used to study how the spatial variability of material properties affect non-load bearing wall failure progression. The numerical results are compared to the experimental results in terms of the wall failure progression and wall capacity. It is shown that the model which considers the spatial variability of brickwork can best capture the failure patterns and predict the cracking and ultimate loads for walls subjected to two-way bending.     

焊接空心球节点的拉压极限承载力分析

采用三维退化曲壳有限元对焊接空心球节点的极限承载力进行了计算。推导了其切线刚度矩阵和弹塑性矩阵,采用多线性等向强化模型和Von-Mises屈服准则,在对6组受拉节点和6组受压节点的试验结果进行了数值模拟的基础上,提出了适于有限元分析的强度破坏准则和极限准则;采用柱面弧长子增量技术,跟踪了节点在轴向荷载作用下的荷载-位移曲线。通过计算发现,焊接空心球节点在轴向受拉荷载作用下为强度破坏,而在轴向受压荷载作用下为弹塑性压曲破坏,两者均与钢材的设计强度有关。最后,在对32组受拉节点和32组受压节点承载力回归分析的基础上,提出了球外径适用范围为160～900mm的焊接空心球节点承载力计算公式。     

冻融循环后轻骨料混凝土双轴拉(劈拉)压强度试验和破坏准则研究

进行海水中不同冻融循环次数后轻骨料混凝土双轴拉(劈拉)压状态下的强度试验,考察试件的破坏形态和表面裂缝的走向特征。根据试验结果分析双轴拉压状态下的极限抗压、劈拉强度随冻融循环次数和应力比的变化规律。并在此基础上,建立不同冻融循环次数后主应力空间的破坏准则和考虑冻融循环影响的八面体应力空间的统一破坏准则,为寒冷海洋环境条件下轻骨料混凝土结构在经受拉压组合荷载作用时的强度分析提供试验和理论依据。     

Elasto-plastic constitutive law for wood behaviour under compressive loadings

In this paper, a constitutive law describing the wood behaviour, as elasto-plastic orthotropic material with 3D finite element model, is presented. Anisotropic elasto-plastic constitutive law with hardening associated with material densification, without distinction between radial and tangential properties, was used for the compressive behaviour. The main novelty of the developed 3D F.E. model, among the models accounting for hardening is the coupling between the anisotropic plasticity and the ductile densification. The model was successfully implemented in the commercial ABAQUS software and validated in the context of uniaxial compressive loading and on one-bolt wood connection tests. The results for the uniaxial compressive loading were compared with the published experimental ones, showing good agreements and demonstrate clearly the effect of the densification on the plastic behaviour. Further, the results obtained on one-dowel wood connection were compared with experiments showing the capability of the model to simulate accurately the non-linear behaviour of bolted joints until failure.     

胶合木曲梁横纹应力及开裂研究

研究了胶合木曲梁纯弯曲区横纹应力与各向同性材料曲梁纯弯曲区径向应力.结果表明：胶合木曲梁纯弯曲区横纹应力与各向同性材料曲梁纯弯曲区径向应力分布规律大致相同,都是中性轴附近大,随距中性轴距离的增大而减小;胶合木曲梁纯弯曲区横纹应力以胶缝为分界线,呈锯齿状分布,层板髓心位置对应力峰值影响较大,髓心位于层板大面的胶合木其横纹应力最大,应力峰值是简化计算式的1.5倍.根据胶合木的应力特点和脆性开裂特点,建立了胶合木开裂损伤本构模型,并编制了相应的有限元软件ABAQUS用户材料子程序UMAT,成功预测了曲梁的横纹开裂及其极限承载力,与曲梁承载力试验相比,其刚度和极限承载力吻合良好.