Behavior of bolted top-seat angle connections in fire

Beam-to-column connections have been found to be of great significance in influencing structural behavior at ambient and elevated temperatures. When steel-framed structures are subjected to fire, the load bearing capacity is decreased and the behavior of the joints is of particular concern. Observations from full-scale fire tests and damaged structures confirm that connections have a considerable effect on the stability time of structural components in fire. The cost of high temperature tests on the broad range of connections used in practice means that their influence is not well detailed in current design codes. The paucity of data also limits the effective use of numerical models developed to simulate the behavior of complete structures at elevated temperatures. In this study, 12 full-scale tests were conducted at elevated temperatures on two types of bolted angle beam-to-column connections in order to investigate their resistance to fire. The failure modes and deformation patterns of these specimens were studied and the results are shown as rotation-temperature curves. In addition, the influence of different parameters such as thickness of the angles, the grade of bolts, and other geometrical and mechanical characteristics of the connections were studied.     

Tying capacity of web cleat connections in fire,Part 1: Test and finite element simulation

The tying capacity of connections between beams and columns is very important in maintaining structural integrity when beam deflections are high due to accidental loads such as fire, but has not so far been thoroughly studied. The project which is the subject of this paper has investigated the robustness of common types of steel connection when subjected to fire. The results reported here concern the performance of web cleat connections in fire, and are drawn largely from experimental investigations. During the testing programme, short cantilever stub beams were subjected to different combinations of shear and tying force. The rotational capacities and resistance to tying forces of their connections at high temperatures were investigated in the presence of other concurrent actions.Test results show that web cleat connections have excellent rotational ductility, and that their resistance reduces rapidly with increase of temperature. Web cleat connections can fail in a number of modes, the selection of which is highly dependent on the connection temperature. Finite element simulations of the test results have been shown to be able to reproduce the behaviour accurately up to the stage at which material failure happens. However, as the ultimate behaviour of connections is often controlled by material fracture, finite element analysis is limited in predicting the ultimate resistance of connections. Investigation of the behaviour of the connection, with some proposed modifications to the general finite element model, showed that finite element analysis can help to interpret the test results and expand the test observations to other similar applications.     

Zur Frage der Nachnutzbarkeit verbundlos vorgespannter Stahlbetondecken nach Brandeinwirkung

The connection between fire exposure and the structural behavior of prestressed slabs after cooling This article shows the influence of fire exposure to the structural behaviour of slabs and prestressed slabs after cooling. The connections between the different fire exposures, the behaviour under fire and the structural behaviour after cooling are carried out. Concluding this, the text gives some information for constructing slabs with a good structural behaviour after fire exposure. The article “Verbundlos vorgespannte Decken – Nachnutzung nach Brand”, published in “Beton‐ und Stahlbetonbau” in August 2004, deals with the material properties of concrete, reinforcing steel and prestressing steel during and after fire exposure. In addition to this the results of some experiments on single‐span slabs and two‐span slabs are described. This new article has to be considered as a continuation and update.     

Effect of preloading in high-strength bolts on bolted-connections exposed to fire

Preloading is an important process for bolted connections. Previous studies over the last few decades investigated the fire behaviour of typical bolted connections. However, there is a paucity of studies that examine the effects of preloading in high-strength bolts with respect to the responses of bolted connections that are exposed to fire. This study includes a series of numerical analyses to investigate the fire behaviour of two types of bolted connections, namely extended endplate connections and fin plate connections with and without preloading. Various parameters including preloading and thickness of the connecting plate were considered. The study also demonstrates numerical methodology with respect to preloading and parameters of the damage index by using an explicit dynamic solver. The failure modes, mid-span deflections, displacements of beam ends, and fire-induced axial forces in beams are also discussed in detail. The results indicate that preloading in the bolts has little or no effect on the response of endplate connections exposed to fire because the preloading in endplate connections is offset at the thermal expansion stage. In contrast, the fire behaviour of the fin plate connections is influenced by preloading and fin plate thickness. The use of a thinner fin plate results in the connections receiving a limited effect from the preloading in the connections in fire. The use of a stronger connecting plate in fin plate connections indicates that it is not possible to ignore the effect of preloading on the fire response of connections, and this can improve safety.     

The effects of protected beams and their connections on the fire resistance of composite buildings

According to full-scale fire tests, it is noticed that tensile membrane action within the concrete floor slabs plays an important role in affecting the fire resistance of composite buildings. It is well known that the development of tensile membrane actions relies on the vertical support along the edges of the slab panel. However, there is at present a lack of research into the influence of vertical supports on the tensile membrane actions of the floor slabs. In this paper, the performances of a generic three dimensional 45 m×45 m composite floor subjected to ISO834 Fire and Natural Fire are investigated. Different vertical support conditions and three steel meshes are applied in order to assess the impact of vertical supports on tensile membrane action of floor slabs. Unlike other existing large scale modelling which assumes the connections behave as pinned or rigid for simplicity, two robust 2-node connection element models developed by the authors are used to model the behaviour of end-plate and partial end-plate connections of composite structures under fire conditions. The impact of connections on the 3D behaviour of composite floor is taken into consideration. The load-transfer mechanisms of composite floor when connections fail due to axial tension, vertical shear and bending are investigated. Based on the results obtained, some design recommendations are proposed to enhance the fire resistance of composite buildings.     

Moment-rotation-temperature curves for semi-rigid joints

Although connections are known to have a very significant effect on the behaviour of steel and composite framed buildings in the event of fire, the cost of high temperature tests on the broad range of connections used in practice means that their influence is not well detailed in current design codes. The paucity of data also limits the effective use of numerical models developed to simulate the behaviour of complete structures at elevated temperature. This research describes a series of elevated temperature tests conducted on beam-to-column connections. This paper presents moment-rotation-temperature curves for a variety of connections.     

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.     

Finite element modelling of steel fin plate connections in fire

Recent structural collapses caused by fire have focused attention on research concerning fire safety in building design. Steel connections are an important component of any structural steel building, as they provide links between the principal structural members. The evaluation of the performance of steel connections at elevated temperatures has been a topic of several research programmes in the last few years. Determining the behaviour, available strength and stiffness of moment connections in fire conditions has been a dominant theme in these research works; however very little information on the behaviour of simple shear connections in fire conditions has been disseminated. Fin plate shear connections are easy to fabricate and install; as a result, they have gained popularity with fabricators because of their economy. In this research, the robustness of simple fin plate beam-to-column connections is being investigated under catenary tension from highly deflected beams in fire. A highly detailed three-dimensional (3-D) finite element (FE) model has been created using the ABAQUS software. This is a complex model accounting for material and geometric non-linearity, large deformation and contact behaviour. Contact is critical to model the shear behaviour of the joint, and contact elements have been used both at the bolt–hole interface and also at the surface between the web of the beam and the fin plate, taking into consideration friction between the surfaces. The connection model has been analysed through the elastic and plastic ranges up to failure. Bolt shear and bending, and plate and web bearing have been observed as failure modes. A comparison between available experimental data at ambient and elevated temperatures and other analytical results shows that the model has a high level of accuracy. When the connection model was extended to include an attached beam, it was found that it eventually experiences large tensile force when exposed to fire.     

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.     

Development of a yield-line model for endplate connections in fire

Connection designs at ambient temperature generally consider only the initial stiffness and the plastic resistance, whereas ductility and failure modes are more important for connections in fire, as large deformations are generally experienced. This paper describes the development of a T-stub model to capture the behaviour of endplate connections at large deformations, including failure wherever relevant. The model is based on the virtual work principle, which allows it to be easily applied to endplate connections, and considers material hardening after yielding for both the T-stub flange and the bolts. Compatibility between these two components is also maintained. Validation against T-stub tests at both ambient and elevated temperatures shows that the proposed model can predict the behaviour of T-stubs of various characteristics. Comparison with finite element analysis results shows that this model represents the behaviour of endplate connections very well.     

Experimental investigation on flush end-plate bolted composite connection in fire

Three tests at elevated temperatures were carried out to invest the fire-resistant capacity of flush end-plate composite joints. The axial force was considered in two of the three tests simulating fire conditions to investigate its effect on the connection's load carrying ability (one with fire protection and the other without fire protection). Summaries have been given on the failure mechanisms, temperature distribution, loading carrying capacity of the composite joint and the mutual influence between joints and beams at elevated temperatures. Date shows that the failure mode of the flush end-plate joint without stiffener is usually dominated with the yielding of the bottom flange of steel beam near the supports, elements of the composite connection have different rate of temperature increase, and the axial force from restrained composite beam influences the rotational stiffness and moment capacity of joints. This paper also theoretically presents how the interaction of joints and beam developed in fire conditions. A practical simplified method is proposed to calculate the non-linear variable characteristics of composite connection in the analysis of catenary action of beams at elevated temperatures. The proposed method is verified by the experimental investigations. This study offers a feasibility of fire-resisting design and evaluation for the composite beam with semi-rigid connections in the complete temperature range.     

A connection element for modelling end-plate connections in fire

In this paper a robust 2-noded connection element has been developed for modelling the bolted end-plate connection between a steel beam and column at elevated temperatures. The connection element allows the element nodes to be placed at the reference plane with offset and the non-uniform temperature distributions within the connection. In this model the connection failure due to bending, axial tension, compression and vertical shear are considered. The influence of the axial tensile force of the connected beam on the connection is also taken into account. This model has the advantages of both the previous simple and component-based models. A total of 23 fire tests were used to extensively validate the model. It can be seen that the current model is robust and has a capability to predict the behaviour of a bolted end-plate connection under fire attack with reasonable accuracy. Compared to the tested results the predictions of the current model were mainly on the conservative side. Hence, the model can be used for structural fire engineering design on steel-framed composite buildings. The idea described in this paper can also easily be applied to develop other kinds of connections, such as simple connections, column based connections or hollow section connections, and so on.     

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。     

一般大气环境下锈蚀H型钢梁柱焊接节点抗震性能试验

为研究一般大气环境锈蚀对梁柱焊接节点抗震性能的影响,对6根H型钢焊接梁柱节点试件进行了户外周期喷淋加速腐蚀试验与低周往复荷载试验,研究不同锈蚀率对焊接梁柱节点损伤过程、破坏模式、滞回曲线、骨架曲线、承载力、变形能力、刚度及耗能能力的影响,得到了其承载力、延性及耗能能力随锈蚀率增大的退化规律。研究结果表明：一般大气环境下节点呈现出全面锈蚀特征,且不同位置的锈蚀量存在差别,梁下翼缘的锈蚀量最大,梁腹板、柱翼缘和柱腹板的锈蚀量次之,梁上翼缘的锈蚀量最小;锈蚀对梁柱焊接节点失效模式具有重要影响,锈蚀率为5.03%~8.85%的节点梁根部发生严重局部屈曲,产生了明显的承载力退化现象,破坏特征由突然破坏向混合破坏转变,且锈蚀导致断裂临界裂缝萌生位置上移及扩展路径改变;锈蚀节点屈服荷载、峰值荷载、峰值位移和位移延性系数均呈线性退化规律,并产生初始刚度损伤,破坏时塑性循环加载次数较未锈蚀节点降低2~4次,累积耗能呈幂函数降低趋势,锈蚀率为5.03%~8.85%节点梁端总转角不能满足总转角5%的抗震设计要求。     

Methodik zur Beurteilung des Brandverhaltens von Metallkonstruktionen

The paper presents a unified theory of metal structures at elevated temperature. The theory deals with time effects, which means that the influence of velocity of heating metal structures on the decrease of their load bearing resistance is considered. Defining time effects is indispensable when the analysis of structures with different temperature‐time curves of fire is recommended. Such requirements were formulated in Standard Eurocode1. Due to the lack of implemented methodology of analysis of load bearing resistance, they are not respected while analysing metal structures in fire. Such analyses must be connected with justification of the choice of a temperature‐time curve (describing a more or less intensive fire) and calculations carried out consistently according to a theory of load bearing capacity which enables to consider the influence of heating velocity on the structure behaviour.     

Predicting the maximum compressive beam axial force during fire considering local buckling

Local buckling in floor beams has been one of the important observations in several fire events in steel buildings such as World Trade Center Tower 7 and large-scale fire experiments such as Cardington building test in U.K. Utilizing three dimensional finite element methods for complex geometry and nonlinear behavior of such connections, local buckling of the web followed by the buckling of the lower flange is observed to occur in early stages of fire, which causes instability to the floor system, and a significant reduction in the connection strength. The observations also suggest that the maximum compression in the floor beam is limited to the buckling capacity of the web and flanges near the connection. This paper contributes to such knowledge by investigating the local buckling of floor beams for different connection types at elevated temperatures using nonlinear finite element models. Moment connections are found to be more resistant to local buckling when compared to the shear connections. The results are also compared to the AISC design equation for plate buckling under ambient and elevated temperatures. Compared to the finite element analyses of this study, it is observed that at ambient temperature the AISC curve conservatively captures the buckling capacity of webs and flanges; at higher temperatures, AISC overestimates the capacity.     

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.     

Comparative experimental investigation on steel staggered-truss constructed with different joints in fire

A total of three fire tests have been conducted to compare the mechanical performance of the steel staggered-truss (SST) exposed to pool fire constructed with welded joints and gusset plate connections. The data of temperatures and deflections of typical members were recorded during the three tests. The experimental results showed that the performance of the connections had a significant influence on the mechanical behavior of the structure under elevated temperatures. The truss constructed with gusset plate connections (TRG) possesses better fire performance than the truss adapted with welded joints (TRW) because the gusset plate connection has better resistance capacity in fire than the welded joint does. Particularly, a chain of destruction might be aroused in the TRW due to local buckling and joint fracture, and then the progressive collapse of the whole structure might be induced. However, better coordination of the truss members is shown in the TRG because the gusset plate connections possess better hinged performance in fire, and the instantaneous damage of the structure was not induced; though the failure of some specific members was observed. The difference in the manifestation of the local buckling between the TRW and TRG was also found through the tests. That is, in-plane local buckling was observed in the TRW, while either in-plane or out-plane local buckling might be induced in the TRG.     

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.     

Flush Endplate Connections in Fire: Time-Dependent Behavior of Tension Bolts

Steel connections play a crucial role in maintaining the integrity and stability of steel building frames especially when exposed to fire temperatures. The behavior of flush endplate connections in fire is shown to be governed by tension bolt failure as bolts lose their strength and stiffness more rapidly at higher temperatures. As a result, the ability to predict the development of stresses in tension bolts of flush endplate connections at different stages of fire is of special importance. One of the factors influencing bolt stresses in fire is the thermal creep or time-dependent inelastic response of steel to elevated temperatures. Therefore, time- and temperature-dependent behavior of tension bolts of flush endplate connections in fire is the focus of this study. Stress-time histories in tension bolts are obtained by explicit consideration of thermal creep of steel in FE models of flush endplate connections at elevated temperatures. To better understand the effect of thermal creep on tension bolt behavior, the correlation between time-dependent rotational deformation of flush endplate connections and bolt stresses is also investigated. Further, the isochronous representation is utilized to study the rotational deformation and the tension bolt stresses under various applied moments ranging from 50% to 95% of the moment capacity and fire temperatures ranging from 450°C to 600°C with 25°C increment. Through such representation, it is indicated that the connection behavior is not only dependent on bolt strength degradation and applied moment, but also affected by the time duration of applied moments and temperatures. Also, with the inclusion of thermal creep of steel, the connection experiences higher rotation and excessive endplate deformation with stress relaxation leading to top tension bolt failure at earlier stages of fire. More specifically, for time exposure greater than or equal to 60 min, the failure temperature of the connection decreases from 600°C to around 550°C. Therefore, neglecting thermal creep of structural steel may result in an unsafe prediction of the overall response of flush endplate connections in fire.