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.     

Behaviour of grouted sleeve connections at elevated temperatures

Tubular steel jointing system that incorporates prestressed grout sleeve connections has superior strength under both static and dynamic loading. This paper reports an investigation into the effect of elevated temperatures on the load carrying capacity of such connections. Eleven specimens were heated in a furnace and the load was applied through an Instron machine. Three different grout lengths were chosen. The load deflection behaviour at different temperatures was compared. It was found that the ultimate load reduces almost linearly as temperature difference (TD) between outer and inner tubes increases. It is encouraging to observe the ductile behaviour of grouted connections at elevated temperature. Thermal analysis was also conducted to predict the temperature field in the connection.     

High-temperature tests on joints to steel and partially-encased H-section columns

This paper reports on a series of tests at elevated temperatures on connections between steel beams and H-section columns, both unfilled and partially-concrete-encased. Reverse-channel connections to both types of column, as well as flush endplate connections to partially-encased H-section columns, were studied. The experiments aimed to investigate the behaviour of beam-to-column connections subject to significant tying forces and large rotations in fire situations, and to provide test data for development and validation of simplified component-based connection models. It has been found that reverse-channel connections provide not only high strength, but also the high ductility which is required to reduce the possibility of connection fracture and to improve the robustness of buildings in fire.     

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.     

Behaviour of welded top-seat angle connections exposed to fire

Beam-to-column connections have been found to significantly influence the structural behaviour at ambient and elevated temperatures. When steel-framed structures are exposed to fire, the load-bearing capacity is decreased and the behaviour of the joints is of particular concern. To account for the extensive applications of welded connections, eight experimental tests were carried out in this study on two different types of beam-to-column angle connections to investigate their fire resistance capacity. Failure characteristics and fracture modes of specimens were studied, and results are presented in the form of temperature–rotation curves. In addition, the influence of different parameters such as thickness of the angles, the value of the applied moment, and other geometrical and mechanical characteristics of the connections was investigated.     

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.     

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.     

Numerical simulation of bolted steel connections in fire using explicit dynamic analysis

Behaviour of steel connections in fire is a multi-dimensional problem involving parameters such as temperature, tying forces and large deformations. Investigation of this behaviour will remain one of the main subjects for fire engineering research in the coming years. Finite element simulation plays an important role in the study of connections because fire tests are expensive to perform. Unlike normal structural analyses, finite element simulation of bolted steel connections is a challenging task, as large numbers of contacts exist in the model. This leads to convergence difficulties in static solvers. This paper explores the use of an explicit dynamic solver to analyse bolted steel connections. By comparing the results with those from static analysis and tests, it is shown that the explicit dynamic solver, with proper control, gives satisfactory predictions of the responses of steel connections up to post-failure deformations.     

Tying capacity of web cleat connections in fire,Part 2: Development of component-based model

The companion paper has reported the results from a test programme in which web cleat connections were subjected to various combinations of shear, tying and moment actions at elevated temperatures. These tests showed that web cleat connections have very good tying resistance and rotational capacity, mainly due to the large deformation of which the web cleats are capable. In this paper a mechanical model is developed to predict the behaviour of web cleats subjected to tying forces. This model considers the formation of four plastic hinges on each angle and the effect of the angles opening in enhancing their resistance. It is capable of representing the action of the angles in component-based models for web cleat connections, in which algorithms for other components, including bolts in tension, bolts in double shear and holes in bearing, are already available. Failure criteria determined from the tests have been introduced into the models for components such as web cleats and bolts in double shear. This enables the component-based assembly to predict the occurrence and the sequence of connection failure. The behaviour of the connection predicted by the component-based model shows good correlation with the test results, which indicates that the developed model can be adopted in structural frame analysis to consider connection failure.     

Behaviour of headed stud shear connectors for composite steel-concrete beams at elevated temperatures

The behaviour of composite steel-concrete beams at elevated temperatures is an important problem. A three-dimensional push test model is developed herein with a two-dimensional temperature distribution field based on the finite element method (FEM) and which may be applied to steel-concrete composite beams. The motivation for this paper is to increase the awareness of the structural engineering community to the concepts behind composite steel-concrete structural design for fire exposure. The behaviour of reinforced concrete slabs under fire conditions strongly depends on the interaction of the slabs with the surrounding elements which include the structural steel beam, steel reinforcing and shear connectors. This study was carried out to consider the effects of elevated temperatures on the behaviour of composite steel-concrete beams for both solid and profiled steel sheeting slabs. This investigation considers the load-slip relationship and ultimate load behaviour for push tests with a three-dimensional non-linear finite element program ABAQUS. As a result of elevated temperatures, the material properties change with temperature. The studies were compared with experimental tests under both ambient and elevated temperatures. Furthermore, for the elevated temperature study, the models were loaded progressively up to the ultimate load to illustrate the capability of the structure to withstand load during a fire. It is concluded that finite element analysis showed that the shear connector strength under fire exposure was very sensitive. It is also shown that profiled steel sheeting slabs exhibit greater fire resistance when compared with that of a solid slab as a function of their ambient temperature strength.     

Behaviour of column web component of steel beam-to-column joints at elevated temperatures

This paper presents an experimental investigation on stocky column web panels of semi-rigid beam-to-column joints exposed to fire conditions in order to verify an analytical prediction model. Recent experimental studies show that the degradation of material properties and high axial forces, due to restrained thermal expansion of beams at elevated temperatures, significantly affect the moment–rotation response of the joints. The component method originally established for the evaluation of the joint behaviour at ambient conditions can be adopted for elevated-temperature cases. Recently developed mechanical models for joints may not be accurate as the column web component is simultaneously subjected to bending moments and axial forces. This paper focuses on the component column web in shear in order to identify the key parameters which affect joint behaviour in shear at elevated temperatures. This experimental work was conducted on three extended end-plate connections subjected to both ambient and elevated temperature conditions. After validations by test results, detailed finite element simulations were performed for a series of parametric studies at other elevated temperatures. Both experimental and numerical results are finally compared with analytical predictions.     

Column Web Panel at Elevated Temperature

One of the most recent studied components in the connections of a steel structure exposed to elevated temperature is the column panel loaded in shear. A set of three tests were performed to confirm the analytical prediction model at the Czech Technical University in Prague. The model takes into account the distribution of the internal forces in the shear panel affected by the transfer of the forces by bolt rows in tension. Two panels were tested at ambient temperature and four panels at elevated temperature. The gas temperature in the furnace followed the temperature measured during the seventh Cardington large compartment test in 2003. This contribution is focused on the presentation of the experimental results and an analytical prediction model with a component method.     

Assessing the flexural and axial behaviour of reinforced concrete members at elevated temperatures using sectional analysis

Simplified, rational, and practical models that account for the effect of elevated temperature on concrete and steel properties are needed. These models will enable engineers to design and assess reinforced concrete (RC) structures to satisfy specific fire performance criteria. This paper introduces a simple method that predicts the flexural and axial behaviour of RC sections during exposure to elevated temperatures. The method is based on using finite difference analysis to estimate the temperature distribution within a concrete section and a modified version of the well-known sectional analysis approach to predict the axial and/or flexural behaviour. A rational approach is proposed to convert the two-dimensional temperature distribution to a one-dimensional distribution. This approach converts a complex problem to a simplified one and thus enables engineers to better understand the behaviour and have higher confidence in the results. The predictions of the proposed method are validated using experimental and analytical studies by others. Additional tests are needed to further validate and improve the proposed method.     

Tests of single shear bolted connections of thin sheet steels at elevated temperatures—Part I: Steady state tests

The current design rules on bolted connections of thin sheet steels for cold-formed steel structures are applicable for ambient temperature condition only. Investigation of single shear bolted connections at elevated temperatures is limited. In this study, 120 single shear bolted connection specimens involving three different thicknesses of thin sheet steels and 30 coupon specimens were conducted by using steady state test method in the temperature range from 22 to 900 °C. There are three main failure modes observed in the single shear bolted connection tests, namely the net section tension, bearing, and tear out. The test results were compared with the predicted values calculated from the American, Australian/New Zealand and European specifications for cold-formed steel structures. In calculating the nominal strengths of the connections, the reduced material properties were used due to the deterioration of material at elevated temperatures. It is shown that the strengths of the single shear bolted connections predicted by the specifications are generally conservative at elevated temperatures. The comparison between the deterioration of the tested connection strengths and that of the material properties at elevated temperatures showed a similar tendency of reduction.     

Experimental behaviour of a steel structure under natural fire

Current design codes for fire resistance of structures are based on isolated member tests subjected to standard fire conditions. Such tests do not reflect the behaviour of a complete building under either normal temperature or fire conditions. Many aspects of behaviour occur due to the interaction between members and cannot be predicted or observed in tests of isolated elements. Performance of real structures subject to real fires is often much better than that predicted from standard tests due to structural continuity and the provision of alternative load paths.This paper reports on the results of a collaborative research project (Tensile membrane action and robustness of structural steel joints under natural fire, European Community FP5 project HPRI—CV 5535) involving the following institutions: Czech Technical University (Czech Republic), University of Coimbra (Portugal), Slovak Technical University (Slovak Republic) and Building Research Establishment (United Kingdom). It consists of an experimental programme to investigate the global structural behaviour of a compartment on the 8-storey steel–concrete composite frame building at the Cardington laboratory during a BRE large-scale fire test, aimed at the examination of the temperature development within the various structural elements, the corresponding (dynamic) distribution of internal forces and the behaviour of the composite slab, beams, columns and connections.     

Experimental and numerical investigation on the shear behaviour of friction-welded bar-plate connections embedded in concrete

Friction-welded bar-plate connections are a basic structural component of Bi-steel steel-concrete-steel sandwich construction. In Bi-steel members, the bar-plate connections, embedded in concrete, are subject to tension, shear and bending. The static and fatigue behaviour of the embedded connections subjected to bar tension is described in another paper [Xie M, Chapman JC. Static and fatigue tensile strength of friction-welded bar-plate connections embedded in concrete. Journal of Constructional Steel Research [in press]]. This paper presents experimental and numerical studies on the static behaviour of the friction-welded connections with the bar loaded in shear. Finite element analysis is carried out to examine the effects of variations in geometric and material parameters. The experimental results are used to derive an empirical equation for predicting the shear strength of embedded connections, and compared with existing test results and code specifications. Further papers will describe fatigue behaviour of the embedded connections subjected to bar shear, and static and fatigue tests on Bi-steel beams.     

An advanced ANN model for predicting the rotational behaviour of semi-rigid composite joints in fire using the back-propagation paradigm

This paper describes an artificial neural networking (ANN) model developed to predict the behaviour of semi-rigid composite joints at elevated temperature. Three different semi-rigid composite joints were selected, two flexible end-plates and one flush end-plate. Seventeen different parameters were selected as input parameters representing the geometrical and mechanical properties of the joints as well as the joint’s temperature and the applied loading, and used to model the rotational capacity of the joints with increasing temperatures. Data from experimental fire tests were used for training and testing the ANN model. Results from nine experimental fire tests were evaluated with a total of 280 experimental cases. The results showed that the R² value for the training and testing sets were 0.998 and 0.97, respectively. This indicates that results from the ANN model compared well with the experimental results demonstrating the capability of the ANN simulation techniques in predicting the behaviour of semi-rigid composite joints in fire. The described model can be modified to study other important parameters that can have considerable effect on the behaviour of joints at elevated temperatures such as temperature gradient, axial restraints, etc.     

The study of welded semi‐rigid connections in fire

Considering the deterioration of steel properties by temperature increase and the importance of the influence of connection behavior on the behavior of steel structures, we find that the exact understanding of the behavior of a specific steel connection in fire as well as the information about the effect of fire on the principal constitutive characteristics of the connection is necessary for safe design against fire. Thus, in this paper, the behavior of welded angle connections is studied at elevated temperatures using the abaqus finite element software. Steel members and connection components are considered to behave nonlinearly; the degradation of steel properties with increasing temperature is considered according to EC3, BS5950 recommendations. The results of finite element and experimental tests conducted on welded angle connections are compared, and the obtained failure modes and moment–rotation–temperature characteristics are in good agreement with those associated with the experimental tests. In the following, since the knowledge about moment–temperature–rotation behavior of a specific connection is needed for a fire‐resistant design, these properties are accurately determined, and finally, the effect of some parameters such as the moment applied on beam, change of column axial force and change of beam shear force on the stiffness of these connections at elevated temperatures is determined. Copyright © 2011 John Wiley & Sons, Ltd.     

应变率对全焊节点影响的试验和数值分析

对2个梁柱焊接节点的足尺试件进行动力试验。为了模拟试件的非线性动力性能,用纤维梁单元模拟梁、柱,用非线性旋转弹簧模拟焊接节点。对每一纤维,均采用Perzyna准则考虑应变率对梁、柱性能的影响。通过改进Richard静力模型模拟动力性能,考虑应变率对焊接节点的影响。Krawin-kler-Zoheri模型是标准累积塑性应变的函数,可根据应变率影响量化损伤和退化性能。将有限元分析软件DRAIN-2DX升级,建立模型并进行数值分析。与试验结果比较可知,考虑应变率影响和损伤累积的数值模拟能很好预测结构动力性能,甚至能预测焊接节点的破坏模式。     

Performance of cellular composite floor beams at elevated temperatures

This paper describes an experimental and numerical study at both ambient and elevated temperatures on the behaviour of full-scale composite floor cellular steel beams. A total of four specimens, comprising two different steel geometries and loading conditions were tested under monotonic loading and at elevated temperatures. All beams were designed for a full shear connection between the steel beam and the concrete flange using headed shear studs. The beams were designed to fail by web-post buckling, which was observed in all the tests. Failure temperature observed in the fire tests indicated that failure by web-post buckling of cellular beams in fire cannot simply be estimated by applying temperature dependent reduction factors on stiffness, as given in codes. A finite element model is then established with both material and geometrical non-linearity using shell elements to compare against the experimental results. The comparison between the finite element prediction and actual test results are quite good in terms of failure modes, load deflection behaviour and ultimate loads.