Thermo-mechanical behaviour of timber-to-timber connections exposed to fire

A numerical model is developed to analyse the thermo-mechanical fire behaviour of timber-to-timber dowelled and bolted connections in tension parallel to wood grain. The experimental results of two series of tests are used to validate the model based on three-dimensional (3D) finite element modelling. The thermo-mechanical performances of the connections are determined using two different meshing and calculation procedures. The thermal model meshing is continuous to take the thermal continuity between the connection components into account. Regarding mechanical and thermo-mechanical calculations, the meshing is discontinuous to consider the gap and the contact changes between the connection components. The thermal model takes into account the evolution of physical properties of materials as a function of the temperature and is calibrated according to the experimental measurements. The thermo-mechanical model considers the mechanical properties – temperature evolutions of the materials provided by the corresponding Eurocodes. The thermal model can predict the temperature fields inside the connection components. Its validation is conducted through comparison with the experimental temperatures measured in different places inside the connections. The mechanical model is validated using the experimental load–slip curves of the connections in standard conditions. Finally, the thermo-mechanical model is validated by considering the global time–slip curves and by determining the fire resistance rating of the connections. A good agreement is obtained between simulated fire resistance times and experimental ones.     

Predicting the Fire Resistance of Wood–Steel–Wood Timber Connections

This paper presents an investigation on the fire performance of wood–steel–wood timber connections with slotted-in steel plates. In the first part, a three-dimensional thermal model was employed that uses the finite element method to analyze heat transfer within timber connections exposed to the standard fire. The temperature-related properties were obtained from the literature and imported into the thermal model. A validation of the proposed thermal model was achieved by comparing predicted temperatures with experimental results. In the next phase, a reduction in the embedding strength method was adopted to estimate the load-carrying capacity of connections in fire. Based on the temperature profiles within the connection calculated by the thermal model, the reduction of the embedding strength was determined and used to calculate the load capacity at elevated temperatures. Furthermore, a formula was proposed to evaluate the fire resistance rating of timber connections and compared with the results of fire resistance tests. The parameters considered included the load level, fastener diameter and wood member thickness.     

Using artificial neural networks for calculation of temperatures in timber under fire loading

Artificial neural networks have been used in recent years as a tool to model properties and behavior of materials in many areas of civil engineering applications. Because of their ability to learn and adapt they can be used to find complex relations between different properties. In the present paper artificial neural networks are used for predicting the temperatures in timber under fire loading. The artificial neural network model has been trained and tested using available numerical results obtained using design methods of Eurocode 5 for the calculation of temperatures in timber under fire loading. A multilayer feed forward network has been used with input data arranged in a format of three input parameters that cover the density of timber, the time of fire exposure and the distance from exposed side and the output parameter being the temperature in timber. The training and testing results in the neural network model have shown that neural networks can accurately calculate the temperature in timber members subjected to fire.     

Experimental Testing and Analytical Prediction of the Behaviour of Timber Bolted Connections Subjected to Fire

The ultimate strength of bolted and dowelled connections in timber members at ambient temperatures have been assessed using Johansen’s yield equations in Europe and USA. More recently, several researchers have begun to investigate the strength of bolted and dowelled connections at elevated temperatures. Research has been carried out at the University of Canterbury to investigate the application of Johansen’s yield equations to the prediction of the failure strength of bolted connections in fire conditions. A series of single bolted connections using steel side plates was heated at constant temperature for several hours, then loaded to failure and used to determine the embedment strength of the wood over a range of temperatures from ambient to 300°C. The temperature-dependent embedment strengths are employed in Johansen’s equations for connections using a central steel plate as well as connections using steel and wood side members. Comparisons are also being made with the results of several similar connections tested in fire conditions and show considerable promise for predicting failure of such joints. A proposal for implementation of an easy-to-use approach for the prediction of the fire resistance of bolted joints is discussed in the paper, based on an extension of the Johansen’s yield equations to fire conditions, including a model for the variation of the embedment strength with temperature.     

Component modelling of flexible end-plate connections in fire

This paper describes a component-based model for simulating the behaviour of flexible end-plate connections between beams and columns in steel framed structures in fire conditions. In this method, a simple steel connection was split into a number of active components for which mechanical properties are represented by non-linear springs. The behaviour of a steel connection is then determined by assembling the individual behaviour for each active component into a spring model. The component model presented in this paper is capable of predicting the behaviour of steel connections under varied loading conditions. It is also capable of predicting the tying resistance and critical components of failure for steel connections in fire. Compared with experimental test data, a good correlation with the simplified model has been achieved and this method, combined with finite element modelling, may be used to examine the performance of simple steel connections in fire conditions.     

Fire analysis of timber composite beams with interlayer slip

The purpose of this paper is to model the behaviour of timber composite beams with interlayer slip, when simultaneously exposed to static loading and fire. A transient moisture-thermal state of a timber beam is analysed by the Luikov equations, and mechanical behaviour of timber composite beam is modelled by Reissner's kinematic equations. The model can handle layers of different materials. Material properties are functions of temperature. The thermal model is validated against the experimental data presented in the literature. Generally, the model provides excellent agreement with the experimental data. It is shown that the material properties of timber play an important role in the fire resistance analysis of timber structures when exposed to fire.     

Experimental analysis and modelling of bolted T-stubs under cyclic loads

Bolted connections have recently attracted new research efforts after the unexpected failures of welded connections during Northridge and Kobe earthquakes. The criteria and the formulations for predicting the rotational stiffness and the flexural resistance, under static loading conditions, of the most common connection typologies have been codified by Eurocode 3 [CEN. EN 1993-1-8 Eurocode 3: Design of steel structures — Part 1.8 Design of joints. 2005] which is based on the so-called component approach. In order to extend the component approach to the prediction of the seismic response of partial-strength connections, the modelling of the cyclic response of the joint components is necessary. Starting from the observation that the main sources of deformability and plastic deformation capacity of bolted connections can be modelled by means of an equivalent T-stub, an experimental program devoted to the cyclic response of the most important component of bolted connections has been carried out aiming at the modelling of the cyclic force-displacement curve of bolted T-stubs. In this paper, starting from the analysis of the results of the experimental tests performed, stiffness and strength degradation rules are derived as a function of the displacement amplitude required at any cycle and of the energy dissipated in the previous loading history. The combination of these rules with the theoretical prediction of the monotonic envelope leads to a proposal for predicting the cyclic behaviour of bolted T-stubs starting from the knowledge of their geometrical and mechanical properties.     

火灾中组合梁-板组件的响应模型

介绍一种立体的有限元模型,可用来评估受重力和火灾荷载的组合梁-板组件的响应。使用有限元分析软件模拟典型梁-板组件在不同剪切条件(栓焊混合连接和全部螺栓连接的双角钢节点)和不同火灾场景下的性能。有限元模型中包含随温度变化的组成材料的热力学性能、连接件和混合作用。使用瞬态时域热应力耦合分析来获得组合梁-板组件的温度分布和变形响应。通过对所预测的和所测量的火灾情况下3个组合梁-板组件热学参数和结构响应参数进行对比,验证了这个有限元模型。通过对比显示,所提出的有限元模型可以很准确地预测梁-板组件的火灾响应。分析研究可知,梁和板的混合作用很大程度上提升了组合梁-板组件的耐火性能。因此,可以推论出所提出的有限元模型可用来评估组合楼板结构火灾响应。     

Modeling the response of composite beam–slab assemblies exposed to fire

This paper presents the development of a three-dimensional nonlinear finite element model for evaluating the response of composite beam–slab assemblies subjected to a combination of gravity and fire loading. The behavior of typical beam–slab assemblies with different shear connection types (welded–bolted shear tab and all-bolted double-angle connection), exposed to different fire scenarios, was modeled using ANSYS. The finite element model accounts for temperature dependent thermal and mechanical properties of constituent materials, connections, and composite action. Transient time domain coupled thermal-stress analysis is performed to obtain the temperature distribution and deformation response of the composite beam–slab assembly. The finite element model is validated by comparing the predicted and measured thermal and structural response parameters of three composite beam–slab assemblies tested under fire conditions. The comparisons show that the proposed model is capable of predicting the fire response of beam–slab assemblies with good accuracy. Research from the analysis clearly shows that the composite action between the beam and slab significantly enhances the fire performance of composite beam–slab assemblies. It is concluded that the proposed finite element model could be used as a feasible tool to evaluate the fire response of composite floor systems.     

火灾下椭圆中空截面钢柱有限元数值分析

为对已有的椭圆中空截面(EHS)钢柱性能试验进行补充,该文根据烃类火灾曲线对其进行了数值分析。基于12根EHS钢柱试验建立3D有限元分析(FEM)模型,其中6根钢柱无约束,6根钢柱受到轴向约束,钢柱包含两种长细比λz=40.1和λz=50.8。在校准过程中使用三种不同的荷载水平(αL=0.3,0.45,0.6),测得EHS的温度曲线和轴向位移。将EC3第1～2部分详细描述的高温下碳钢的材料和热力性质,应用于钢结构设计中。轴向位移曲线表明,在校准过程中强调如何判断膨胀系数和几何缺陷时,有限元分析结果和试验结果十分吻合。最后,该文将详细介绍椭圆中空截面钢柱(钢号S355J2H)的热学分析,为综合性参数性火灾研究提供平台。     

Determination of fire resistance ratings for glulam connectors within US high rise timber buildings

Multi-storey mass timber buildings constructed with cross laminated timber and glulam are being developed globally. Where engineered timber such as glulam is utilized, the column to beam connections need to be constructed with a fire resistance rating equal to that of the connecting members. The preferred glulam connectors are either a concealed steel plate with bolts and dowels; or a concealed proprietary screw-in sleeve type connector. The fire resistance of connectors for glulam members is an unresolved design issue, as there is no clear methodology to assess their capacity under fire, when the timber is exposed and not clad behind fire protective plasterboard. There is limited fire test data on concealed connectors under shear forces, which is the normal loading condition within a constructed building. Fire test data is also limited on full-size specimens. Correlations developed to date to calculate concealed connector fire resistance have only limited application.A methodology for the design of glulam beam to column connections has been developed based on an extensive literature review, examining the key issues for connection failure. It has been determined that char rate for the timber at the connection needs to be increased above the normally accepted design values, due to the influence of the steel connectors. Secondly, the reduction in timber strength behind the char layer needs to be accounted for, by including a greater depth of reduced strength and stiffness timber, such that the connection can effectively transfer the applied forces through the timber to the steel connector. The methodology detailed within this paper provides a simple approach to evaluate the timber cover to the concealed steel connector, where the timber strength and stiffness are effective.     

胶合木梁-柱摩擦型节点滞回性能研究

将摩擦型连接和形状记忆合金（SMA）板引入重型胶合木结构,提出了胶合木梁-柱摩擦型节点并研究了其滞回性能。分别以钢材和木材作为摩擦板材料,设计并制作了3个1∶2缩尺节点,开展了水平低周反复加载试验,获取了节点的典型破损模式和弯矩-转角滞回曲线,对比了节点的弹性转动刚度、峰值弯矩、耗能能力和残余变形。基于胶合木梁-柱摩擦型节点的工作机理建立了节点弯矩-转角滞回模型。结果表明：摩擦型节点主要发生SMA板受拉断裂,节点连接区域木材基本保持完好。相比普通螺栓钢填板节点,摩擦型节点的弹性转动刚度基本保持不变,峰值弯矩下降2%~17%,但各级位移加载幅值下节点耗能增加64%~278%,等效黏滞阻尼系数基本大于0.2,残余变形减小。采用钢制摩擦板的摩擦型节点残余变形更小,采用木质摩擦板的摩擦型节点的等效黏滞阻尼系数较大。胶合木梁-柱摩擦型节点的弯矩-转角滞回模型与试验滞回曲线吻合良好,表明其可用于工程结构分析。     

Hysteretic performance of frictional glulam beam-to-column connections

将摩擦型连接和形状记忆合金（SMA）板引入重型胶合木结构，提出了胶合木梁-柱摩擦型节点并研究了其滞回性能。分别以钢材和木材作为摩擦板材料，设计并制作了3个1∶2缩尺节点，开展了水平低周反复加载试验，获取了节点的典型破损模式和弯矩-转角滞回曲线，对比了节点的弹性转动刚度、峰值弯矩、耗能能力和残余变形。基于胶合木梁-柱摩擦型节点的工作机理建立了节点弯矩-转角滞回模型。结果表明：摩擦型节点主要发生SMA板受拉断裂，节点连接区域木材基本保持完好。相比普通螺栓钢填板节点，摩擦型节点的弹性转动刚度基本保持不变，峰值弯矩下降2%~17%，但各级位移加载幅值下节点耗能增加64%~278%，等效黏滞阻尼系数基本大于0.2，残余变形减小。采用钢制摩擦板的摩擦型节点残余变形更小，采用木质摩擦板的摩擦型节点的等效黏滞阻尼系数较大。胶合木梁-柱摩擦型节点的弯矩-转角滞回模型与试验滞回曲线吻合良好，表明其可用于工程结构分析。     

Study on nominal values of mechanical properties of high strength steel at elevated temperature and after fire exposure

高强钢高温下和高温后的力学性能是进行高强钢结构抗火设计和火灾后评估的重要基础。我国GB 51249—2017《建筑钢结构防火技术规范》和欧洲规范EC3中针对普通低碳钢提出了高温下屈服强度和弹性模量计算公式，但其不适用于高强钢。国内外学者对高温下和高温后高强钢力学性能已开展了一系列试验研究，但由于钢材强度等级、试验设备、加热速率和加载制度等影响，导致试验结果离散性较大，不能应用于实际工程中。同时不同学者提出的力学性能指标计算式各不相同，均不具有普遍适用性。采用数理统计中t分布与置信区间的方法对高强钢高温下和高温后力学性能试验数据进行统计分析，得到不同温度下力学性能指标具有95%保证率的标准值，拟合出高强钢高温下和高温后力学性能指标的计算式，并与GB 51249—2017和欧洲规范EC3预测结果进行对比。结果表明：自然冷却和浸水冷却条件下，高强钢高温后屈服强度发生明显下降的转折点分别是600℃和 500℃；高温下高强钢的屈服强度折减系数低于普通结构钢；高强钢弹性模量折减系数在作用温度小于600℃时低于普通结构钢的，而在温度大于600℃时高于普通结构钢的。     

高强钢高温下和高温后力学性能指标的标准值研究

高强钢高温下和高温后的力学性能是进行高强钢结构抗火设计和火灾后评估的重要基础。我国GB 51249—2017《建筑钢结构防火技术规范》和欧洲规范EC3中针对普通低碳钢提出了高温下屈服强度和弹性模量计算公式,但其不适用于高强钢。国内外学者对高温下和高温后高强钢力学性能已开展了一系列试验研究,但由于钢材强度等级、试验设备、加热速率和加载制度等影响,导致试验结果离散性较大,不能应用于实际工程中。同时不同学者提出的力学性能指标计算式各不相同,均不具有普遍适用性。采用数理统计中t分布与置信区间的方法对高强钢高温下和高温后力学性能试验数据进行统计分析,得到不同温度下力学性能指标具有95%保证率的标准值,拟合出高强钢高温下和高温后力学性能指标的计算式,并与GB 51249—2017和欧洲规范EC3预测结果进行对比。结果表明：自然冷却和浸水冷却条件下,高强钢高温后屈服强度发生明显下降的转折点分别是600℃和 500℃;高温下高强钢的屈服强度折减系数低于普通结构钢;高强钢弹性模量折减系数在作用温度小于600℃时低于普通结构钢的,而在温度大于600℃时高于普通结构钢的。     

Assessment of Eurocode 5 Charring Rate Calculation Methods

The base hypothesis for the assessment of fire resistance of timber structures by simple calculation models is that for temperatures above 300°C, timber is no longer able to sustain any load. Consequently, the determination of the location of the 300°C isotherm, the char depth, is decisive for the result of fire resistance calculation methods. The charring rate (CR) of timber is dependent of numerous factors, such as wood species (density, permeability and composition), moisture content and direction of heat transfer (parallel or perpendicular to the grain). Eurocode 5, Part 1–2, presents several methods for the calculation of fire resistance of timber structures that are divided into simplified and advanced. In this paper simplified and advanced methods are compared regarding the calculation of the char depth and residual cross section strength. Finite element simulations have been performed, employing the proposed timber properties of Eurocode 5 using the finite element code SAFIR. The influence of parameters such as timber density and moisture has been investigated. The results obtained with finite element calculations were then compared with Eurocode 5 simplified models. Some inconsistencies between the two methods have been observed. Proposals are presented to overcome some of the inconsistencies as well as to extend the applicability of the models.     

Effect of Transient Creep Strain Model on the Behavior of Concrete Columns Subjected to Heating and Cooling

In the numerical analysis of structures in fire, the material models that are used have important implications on the global behavior of the structure. In concrete, a particular phenomenon appears when subjected to high temperatures: the transient creep strain. Models integrating explicitly a term for transient creep strain have been proposed in the literature but, in the current Eurocode 2 model, there is no explicit term for transient creep strain. This phenomenon is included in the Eurocode 2 model, but it is implicitly considered in the mechanical strain term. A series of experimental fire tests on axially restrained concrete columns subjected to heating and cooling has been recently performed at South China University of Technology and described by Wu et al. (Fire Technol 46:231–249). In the original paper, it was shown that using the implicit model of Eurocode 2, the behavior of the columns cannot be simulated properly, especially during the cooling phase. The objective of the present paper is to perform again the fire tests simulations using a new formulation of the Eurocode 2 model that contains an explicit term for transient creep. In the first part of the paper, the explicit formulation of the Eurocode 2 model is presented. In the second part, the fire tests are modeled with the software SAFIR using, on the one hand, the implicit Eurocode model and, on the other hand, the new explicit model. It is shown that the transient creep model has significant implications on the global behavior of structural concrete members, as the residual axial load sustained by the columns at the end of the fire can differ by up to 25% of the initial applied load depending on the transient creep strain model that is used for the calculation. The experimental behavior is better matched with the new explicit model than with the current Eurocode model. Particularly, the results given by the Eurocode model during the cooling phase are unconservative as the residual axial load is overestimated. Finally, it is explained why, on the basis of an example, in a performance-based approach, these results can have important implications on the global fire resistance of a structure.     

Review of concrete flat plate-column assemblies under fire conditions

The flat plate-column connection is a critical region in concrete structures because of the possible punching shear failure due to brittleness, which is aggravated in the presence of fire. Many studies have been carried out on flat plate-column connections in ambient conditions. However, only a handful of experimental works have examined this region's behaviour under high temperature conditions. This paper aims to present, discuss, and compare the available experimental tests on the mechanical behaviour of flat plate-column connections under high temperatures. The effects of decay on concrete material properties, as well as test configurations, support types, load conditions, and other parameters, are discussed. Moreover, this paper presents the available thermo-mechanical models that evaluate the behaviour of this region in fire conditions.     

Experimental analysis of laterally loaded nailed timber-to-concrete connections

This work intends to assess the accuracy of the methodology proposed in the Eurocode 5 for laterally loaded nailed timber-to-concrete connections. First, the adequacy of the methodology proposed by Eurocode 5 for the dowel-type fasteners is analyzed and discussed, using timber elements from Pinus pinaster Ait. Square and round smooth nails have been used, initially on push-out tests of single and double shear dowel-type connections. Later, a similar investigation is carried out on timber–concrete connections, using the same kind of fasteners (nails), in single shear, wood species and experimental procedure. The use and the influence of permanent formwork are also investigated. As a result, comparisons between the values obtained for the load-carrying capacity and the slip modulus using the analysis provided by the Eurocode 5, when existing, and the experimental results are presented.     

Nonlinear analysis of reinforced concrete cross-sections exposed to fire

A nonlinear structural analysis of cross-sections of three-dimensional reinforced concrete frames exposed to fire is presented. The analysis includes two steps: the first step is the calculation of the transient temperature field in cross-sections exposed to fire and the second step is the determination of the mechanical response due to the effect of thermal and mechanical load. A nonlinear finite-element procedure is proposed to predict the temperature field history. In this thermal analysis, the effect of moisture has been taken into account by introducing a water vapor fraction function to define the variation of enthalpy. A mechanical nonlinear analysis of the cross-sections is performed for each temperature distribution and for the applied exterior load using an algorithm of arc-length control. The mechanical and thermal properties of concrete and steel are taken according to the European Standard ENV 1991-1-2 [ENV. Eurocode 2, design of concrete structures, part 1–2: general rules—structural fire design. ENV 1992-1-2, 1995]. In order to validate the proposed thermal and mechanical models, comparisons between numerical and experimental results have been performed. The agreement found is in both cases, fairly good. In addition, a numerical example of the structural analysis of several cross-sections of a reinforced concrete waffle slab under external load and fire is shown.