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.     

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.     

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。     

Response of double angle and shear endplate connections at elevated temperatures

Double angle and shear endplate connections are considered simple-beam end framing connections designed to carry gravity loads only. However, during a fire event, additional thermal induced axial forces might develop in the beam and cause failure of the connection. This paper presents the results of finite element (FE) simulations on double angle and shear endplate connections assemblies in fire. FE simulations are validated against experimental results of double angle and shear endplate connections at elevated temperature. A detailed comparison of the variation of the axial force during the heating and cooling phases of a fire is made between the two connections covering the following parameters: load ratio, beam length, and double angle and shear endplate thickness and location. The results can help future design guidelines to account for the thermal induced forces and deformations in double angle and shear endplate connections during a fire.     

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.     

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.     

Numerical modelling of timber connections under fire loading using a component model

This paper describes a component model for dowelled timber connections under fire loading. The component model of the dowelled connection is first developed and calibrated for room temperature. The constitutive relations for dowel–timber interaction are detailed and compared with experimental results. In the fire situation, a two-step approach is used: first, three-dimensional (3D) thermal analysis of the connection is performed using a conductive model with timber properties defined in Eurocode 5 in order to calculate the temperatures in the fasteners and timber; afterwards, the mechanical analysis using a component model is carried out using mechanical properties of the steel dowel and of timber adjusted to the temperatures obtained by the thermal analyses. These properties are reduced according to Eurocode 3 and Eurocode 5, respectively. Numerical simulations are presented that allow evaluation of the model behaviour and performance. Obtained results show good agreement with available experimental data, indicating that regardless of its simplicity, the component model has the capability to accurately model timber connections under fire loading.     

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.     

钢管混凝土柱-组合梁单边螺栓连接框架耐火性能研究

为明确钢管混凝土柱-组合梁单边螺栓连接框架的耐火性能,采用有限元方法分析了ISO 834 标准升温曲线火灾作用下,圆形钢管混凝土柱-组合梁单边螺栓连接框架的温度场和耐火极限。研究结果表明：提出的简化的HB 螺栓模型可以有效地对圆形钢管混凝土-组合梁框架的耐火性能进行分析。钢管混凝土柱横截面混凝土的网格划分方法对于钢管混凝土柱-组合梁框架的耐火极限影响较小。当节点连接可靠时,钢管混凝土柱-组合梁框架达到耐火极限时的破坏模式主要有柱破坏模式和组合梁破坏模式。     

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.     

火灾全过程中高层钢框架结构抗连续性倒塌性能及控制措施

建筑内部火灾的发生、发展过程较为复杂,采用标准升温曲线会造成过于保守或偏于不安全的结果。为研究火灾升降温全过程中三维部分铰接钢框架结构（周边刚接、内部铰接）的抗连续性倒塌性能,分析火灾场景、防火等级和支撑布置对结构倒塌的影响。研究表明：钢框架结构是否倒塌主要受升温段持续时间的影响,倒塌模式呈现明显的侧移式和下沉式特点,倒塌时间可偏保守地取为钢柱温度达到550 ℃时对应的时刻,结构受火的严重程度与其构件的防火等级有关,较高的防火等级可以防止结构在升温段发生倒塌,但也可能导致结构在降温段倒塌;为防止框架梁端在升温段和降温段发生断裂,梁端受拉承载力应分别不低于梁屈服荷载的30%和100%。在实际设计中,建议在顶层整层布置水平支撑体系,并在周边和内部布置竖向支撑体系,以防止结构发生倒塌;建议在确定结构的耐火等级时,应考虑可能发生的火灾场景,尤其要重视“长时-低温受火”的危险性。     

Advanced Modeling of Composite Slabs with Thin-Walled Steel Sheeting Submitted to Fire

The paper studies the behavior of composite slabs with corrugated steel sheeting at elevated temperatures. Two structural systems are considered: a simply supported composite slab and a continuous composite slab that consists of two equal spans. Both of them are designed according to the respective Eurocodes to have similar strengths at room temperature. In the sequel, sophisticated three-dimensional models of the slabs are developed. Coupled thermo-mechanical analysis is used, which takes into account the various nonlinearities that are present in the physical model (dependence of the thermal and mechanical properties of the material on temperature, nonlinear material behavior, cracking etc.). The results of the thermal analysis are compared with the temperature field that is proposed in Eurocode 4. For both the structural systems, the fire resistance, in time domain, that yields from the coupled analysis is compared with the fire resistance that results following the provisions of Eurocode 4. Another objective is to evaluate the effect of static indeterminacy on the fire resistance of composite slabs.     

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。     

The simple model for welded angle connections in fire

Due to the high sensitivity of fire affected steel behavior, fire resistance of steel structures is of great importance. Moreover, since the connections act as the main means of integration of frame members, the behavior of steel connections in fire is significantly important. Considering the importance of this matter, this paper describes a spring-stiffness model developed to predict the behavior of welded angle connections made of bare-steel at elevated temperature. The joint components are considered as springs with predefined mechanical properties i.e. stiffness and strength. The elevated temperature joint’s response can be predicted by assembling the stiffness of the components which are assumed to degrade with increasing temperature based on the recommendations presented in the design code. Comparison of the results from the model with existing experimental data shows good agreement. The proposed model can be easily modified to describe the elevated temperature behavior of other types of joints as well as joints experiencing large rotations.     

Fire design method for concrete filled tubular columns based on equivalent concrete core cross-section

In this work, a method for a realistic cross-sectional temperature prediction and a simplified fire design method for circular concrete filled tubular columns under axial load are presented. The generalized lack of simple proposals for computing the cross-sectional temperature field of CFT columns when their fire resistance is evaluated is evident. Even Eurocode 4 Part 1-2, which provides one of the most used fire design methods for composite columns, does not give any indications to the designers for computing the cross-sectional temperatures. Given the clear necessity of having an available method for that purpose, in this paper a set of equations for computing the temperature distribution of circular CFT columns filled with normal strength concrete is provided. First, a finite differences thermal model is presented and satisfactorily validated against experimental results for any type of concrete infill. This model consideres the gap at steel–concrete interface, the moisture content in concrete and the temperature dependent properties of both materials. Using this model, a thermal parametric analysis is executed and from the corresponding statistical analysis of the data generated, the practical expressions are derived. The second part of the paper deals with the development of a fire design method for axially loaded CFT columns based on the general rules stablished in Eurocode 4 Part 1-1 and employing the concept of room temperature equivalent concrete core cross-section. In order to propose simple equations, a multiple nonlinear regression analysis is made with the numerical results generated through a thermo-mechanical parametric analysis. Once more, predicted results are compared to experimental values giving a reasonable accuracy and slightly safe results.     

Modelling of I-shaped beam-to-tubular column connection subjected to post-fire conditions

This paper presents numerical results of structural post- fire bahaviour of I-shaped beam-to-chord joints in offshore platforms topside. Considering the high risk of fire events in offshore oil/gas platforms, this study focuses on the structural behaviour of these connections at post- fire condition. A highly detailed three-dimensional (3-D) finite element (FE) model of this connection has been created using the ABAQUS software. Steel members and connection components are considered to behave nonlinearly. The results of finite element and experimental tests conducted on I shape beam-to-tubular column connections in furnace fire conditions are compared, and the obtained failure modes and moment-rotation-temperature characteristics are in good agreement with those associated with experimental tests. The validated model has been used to conduct numerical parametric studies to generate theoretical data to help develop detailed understanding of the joint behaviour in post-fire condition.     

The behaviour of masonry walls subjected to fire: Modelling and parametrical studies in the case of hollow burnt-clay bricks

A thermo-mechanical model is adopted in order to investigate the fire behaviour of clay masonry walls. In this analysis, conductive, convective and radiative thermal transfers are considered together with local energy consumption due to phase changes. These latter are essentially initiated by both the vaporisation of adsorbed water and the chemical transformation of clay under rising temperatures. Therefore, experimental tests at both the material scale and the brick scale are performed in order to identify the parameters that characterise the thermo-hygral behaviour of clay. For this purpose, numerical simulations are carried out on the experimentally tested hollow bricks in order to determine by back analysis these material parameters. The thermal model being validated, the thermo-mechanical behaviour of a masonry wall subjected to fire, is thereafter investigated by adopting a full three-dimensional finite-element analysis. Numerical simulations results are compared to the experimentally measured ones in terms of both temperature and out-of plane displacement fields. In this analysis, it is shown that a non-linear elastic behaviour for bricks and mortar with temperature-dependent mechanical parameters is sufficient to retrieve the overall behaviour of thin masonry walls. Finally, a parametric study provides the influence of each material parameter on the fire behaviour of the partition walls.     

影响栓接双角钢节点抗火性能的临界因素

双角钢节点经常用于钢框架结构中,当火灾发生时,这些节点在结构构件间的荷载传递上起着至关重要的作用。对火灾下螺栓连接双角钢节点的响应还缺乏了解。采用有限元程序ANSYS,对栓接双角钢节点的抗火性能进行数值研究。分析中,考虑了材料和几何非线性,钢材的高温特性和非线性接触交互作用。通过与其他文献的试验结果进行对比,对该模型进行验证,并通过参数研究量化临界因素对螺栓双角节点的影响。结果表明:螺栓孔的大小、边距、热梯度和梁腹板的细长度对栓接双角钢节点的抗火性能影响很大。     

Degradation of Out-of-plane Initial Stiffness of Steel-concrete Walls Exposed to Fire

Steel-concrete (SC) walls, as a main lateral resisting system in nuclear power plants, have serious fire resistance problem because of their exposed steel faceplates. The out-of-plane stiffness of SC walls will degrade when exposed to fire, which has significant influence on the mechanical performance of the composite walls and even the whole structure. In this paper, a finiteelement (FE) model was developed to simulate thermo-mechanical coupling behavior of SC walls exposed to fire. One conducted ISO-834 standard fire test and two reported thermal and mechanical loading tests were assembled to verify the developed FE model. Based on the validated FE model, numerical experiments of 15 SC walls in fire exposure durations of 0~3 h were conducted to investigate the effect of steel arrangement and geometrical size on the out-of-plane initial stiffness of SC walls under elevated temperatures. Numerical results indicate that the out-of-plane initial stiffness of SC walls under ambient temperature is mainly influenced by steel faceplate thickness and section depth, while the initial stiffness degradation under elevated temperature is mainly influenced by fire exposure duration or surface temperature of exposed steel faceplate. Then, two equations were proposed to predict the out-of-plane initial stiffness of SC walls exposed to fire. The predicted results agree well with the test and numerical results, which demonstrates that the proposed equations can be used to estimate the damage of SC walls in fire.     

Effect of temperature on thermal properties of spray applied fire resistive materials

Thermal properties of fire insulation namely thermal conductivity, specific heat, thermal strain and mass loss play a critical role in determining the effectiveness of these materials to improve fire resistance of steel structural members. These properties vary with temperature and are predominantly governed by moisture content and chemical constituents. This paper presents the effect of temperature on thermal properties of different types of spray applied fire resistive materials (SFRM). High temperature property tests were carried out on three types of commercially available SFRM to measure thermal conductivity, specific heat, mass loss and thermal strain in the range of 20–1000 °C. Data from these tests show that temperature has significant influence on thermal conductivity, thermal expansion and mass loss of fire insulation. The measured test data are utilized to develop thermal property relationships for fire insulation in terms of temperature. The proposed relations can be used as input data in thermo-mechanical analysis for evaluating fire resistance of steel structures.