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The Influence of Connection Stiffness on the Behaviour of Steel Beams in Fire
Abstract
Although steel beams are often designed as simply supported, practical connection details generally afford significant rotational stiffness. When exposed to fire, this can have the effect of improving the survival of the beam. The analytical approach described provides a means of studying this. Indicative studies using this method are included to illustrate the influence of certain key parameters, including the connection type, its temperature relative to the rest of the beam, and the proportions of the beam itself. The results are compared with current simplified design approaches. The studies are based on currently available test data for a limited range of connection details, and a simple means of estimating high temperature connection characteristics for more general conditions is suggested.
... Regarding the connection behavior at elevated temperatures, El-Rimawi et al. [23] outlined the basis for studying the influence of connection stiffness on the behavior of steel beams in fire, utilizing Ramberg-Osgood formulation in representation the connection response at high temperatures. Al-Jabri et al. [24] conducted a series of elevated temperature tests on beam-to-column connections and presented moment-rotation-temperature curves for a variety of connections, using a modified Ramberg-Osgood expression. ...
... In the past few decades, several studies emphasized on the need to consider the behavior of steel connections at fire temperatures [25,23,24] as the reduced stiffness and strength of the connections undoubtedly affect the global response of the structure. Besides, the analysis of a bare steel structure exposed to fire necessitates considering numerous temperature distributions, according to the assumed fire scenarios [37]. ...
... Alternatively, previous studies revealed that the initial stiffness of a connection is directly proportional to Young's modulus of steel at both ambient and elevated temperatures and the reduction in the estimated moment capacity of connections follows directly the reduction in yield strength with temperature increase [25,23,40]; hence, the moment capacity and initial stiffness of a connection at temperature is to be calculated, using the recommended reduction factors, as: ...
... Therefore, conducting an S3 type of analysis to investigate the structural response for a given fire scenario is not only expensive but may not justify the level of accuracy afforded by full-frame analysis. To make the simplified design approach work reasonably well, it is important to incorporate realistic boundary restraints from the surrounding Nomenclature a T , b T , n T are temperature-dependent factors to El-Rimawi's M-θ model [11]; Rigid plastic load of a section at ambient temperature (N); P Internal axial force (N); P 20 Value of P at the beginning of heating (N); t Time (min); T Temperature ( • C); T cr Column critical temperature ( • C); T cr1 Column critical temperature defined at its P decreasing to P 20 ( Fig. 16) ( • C); T cr2 The temperature when snap-through phenomenon occurs in a column ( Fig. 16) [3] analytically examined the axial restraint effect on the fire resistance of steel columns. Buckling curves were plotted for steel columns of different yield strengths and subjected to different axial load ratios. ...
... (1)) depends on connection temperature, beam axial force, etc. That is to say, the moment-rotation M-θ relationship of a connection is highly nonlinear at elevated temperatures [11]. ...
... El-Rimawi et al. [11] use the Ramberg-Osgood expression to establish the nonlinear relationship between connection moment and rotation at elevated temperatures: ...
To study the structural response of a fire compartment within a steel frame, it is expensive and time-consuming to conduct full-frame finite element (FE) analyses, although they can better represent the actual behaviour. For design purposes, engineers have recourse to an isolated member or sub-frame model. Although such an approach is relatively quick, it does not simulate the effects of boundary restraints exerted on heated members. Thus, based on a simple design approach, this paper proposes a new sub-frame model and isolated member model to ascertain the fire resistance of beams and columns subjected to compartment fires. The boundary restraints are represented as a combination of linear and rotational springs, where the spring stiffnesses are derived based on the assumption of semi-rigid beam-to-column connections. The proposed models are verified using a two-dimensional (2D) full-frame analysis. The comparison shows that the member internal forces and displacements predicted by both sub-frame and isolated-member models agree well with the 2D full-frame predictions. The limitations of the proposed methods are also discussed.
... In general, there are three different numerical approaches to model connections, so called curve-fit model, finite element simulation and the component-based model, as summarised by Block [1]. The curve-fit method uses mathematical expressions to describe the moment-rotation characteristics of connections based on experimental data, which was first used by El-Rimawi [2] to represent the behaviour of connections in fire using the Ramberg-Osgood expression. This method can be easily implemented into frame analysis as rotational spring elements at beam ends. ...
... The beam span, beam section size and connection dimensions are listed in Table 1. ,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,, (2) In which the subscripts T, C and S represent the components working in tension, compression and shear, respectively. n is the number of bolt rows. ...
The component‐based model of a novel ductile connection has been incorporated into the software Vulcan in order to facilitate global frame analysis within a performance‐based structural fire engineering design process. This paper reports on the validation and verification of the model, as well as the applications of the model in order to investigate the effects of the ductile connections on the structural responses of long‐span frames at high temperature. Firstly, three single‐beam models with the novel connections at both ends, connected to rigid supports, are used to verify that the component‐based connection model has been correctly incorporated into Vulcan, via comparisons against detailed finite element modelling with Abaqus. The structural performance in fire of long‐span frames with the novel ductile connections has been compared with the performance of the same frames with idealized rigid, idealized pinned and conventional end‐plate connections, initially using a limited sub‐frame model. Results show that, compared with the above mentioned three connection types, the ductile connection provides much higher axial and rotational ductilities to accommodate the deformations generated by the connected beams as their temperatures rise. As part of this process, these connections are instrumental in greatly reducing the axial forces to which the surrounding structure is subjected. Finally, parametric studies varying several key parameters have been carried out, in order to optimize the design of the ductile connection to enhance its performance subject to catenary action at very high temperature to prevent potential connection fracture and progressive collapse.
... In order to study the structural behaviour of continuous and semicontinuous composite beams exposed to fire, finite-element models built up with beam-column elements with the incorporation of semi-rigid joints were proposed by Fakury et al. (2005), Zhao and Kruppa (2002b), El-Rimawi et al. (1997) and Moss et al. (2004). The temperature-rotation curves of composite connections at elevated temperature were adopted from physical tests. ...
... The design fire-resistant performance of composite floors can be significantly enhanced by considering the membrane and catenary actions in slabs and beams respectively with robust connections to columns (Newman et al., 2000;IStructE, 2007). The simplicity of the models (El-Rimawi et al., 1997;Fakury et al., 2005;Moss et al., 2004;Zhao and Kruppa, 2002b) means that they cannot be generalised to study other semi-continuous composite beams with different composite connections owing to insufficient connection test data or details of joint failure mechanism simulation. It is highly desirable to propose simple, yet comprehensive numerical models to predict the structural behaviour of composite beams and connections, with the reasonable incorporation of the structural behaviour of various connection structural components at elevated temperatures and under various physical actions. ...
A three-dimensional finite-element thermal model is proposed to predict the temperature histories of various components of composite connections under fire. The model was carefully calibrated against a series of standard fire tests. Temperature histories both within connections and in composite sections are compared and calibrated. A two-dimensional finite-element mechanical model is adopted and calibrated to predict the structural response of composite connections at elevated temperatures. Both of the models are efficient tools for integrated analysis and design of composite end-plate connections; they are also readily extended to study the structural behavior, under various fire scenarios, of semi-continuous composite beams and floors with different connections to columns with enhanced fire-resistant performance, considering both membrane and catenary floor actions.
... at constant temperature (isothermal analysis). Here, although case (2) corresponds to the more realistic situation of simultaneous mechanical and thermal loading (transient analysis), corresponding, for example, to the load redistribution that inevitably takes place during a fire event, the two simpler situations will be explored in greater detail. [18] Assumption: q upper beam flange =0.7 q lower beam flange ; q upper beam flange =0.7q beam web Vila Real et al. [19] Numerical modelling of HEB400 profiles. Program the furnace to follow a ISO834 fire curve: ...
... Nevertheless, because of the mass concentration within the joint area, a differential temperature distribution should be considered within the joint. Various temperature distributions have been proposed or used in experimental tests by several authors, summarised inTable 3, a detailed description being found in the literature [4,5,151617181920.Table 4 Differential relative temperature variation within the joint [20] Element Temperature, qTable 3 shows a considerable scatter of results, although, as expected, the bottom part of the joint presents higher temperatures. In the following, two alternative possibilities are considered: 1. Uniform temperature variation within the joint. ...
Recent experimental evidence has shown that steel joints exhibit a distinct change in their moment–rotation response under increasing temperature. In terms of cold design, the component method is currently the widely accepted procedure for the evaluation of the various design values. It is the purpose of the present paper to extend the component method to the prediction of the response of steel joints under fire loading. Using typical mechanical models consisting of extensional springs and rigid links, whereby the springs exhibit a non-linear force deformation response (here taken as a bi-linear approximation), an analytical procedure is proposed capable of predicting the moment–rotation response under fire conditions that incorporates the variation of yield stress and Young’s modulus of the various components as the temperature increases. An application to a cruciform flush end-plate beam-to-column steel joint is presented and compared to the experimental results obtained under various loading conditions.
... The proposed mathematical models, usually made up of physical dimension-related parameters, are then used to carry out further studies on the connections. Some research studies [48][49][50] were conducted using this method at the early stage of hightemperature connection modelling. Compared with the other methods, curve fitting is relatively simple, but the major flaw is that experimental data is required as a starting point. ...
... Moreover, as a bending component, when the beam lacks its load-bearing capacity, the rotational restraints stabilize the beam by catenary action. El-Rimawi et al. [4] have studied the effect of connection type on the behaviour of steel beams at high temperature, it was concluded that the critical temperature significantly increased in restrained steel beams comparing to simply supported beams. Dwaikat and Kodur [5,6] carried out a number of numerical analysis to study the influence of the axial and rotational restraints on the performance of the steel beam under fire conditions. ...
Most of the previous studies on restrained steel beam behaviour in fire conditions have neglected the creep effect due to the lack of applicable creep models. A finite element model (FEM) is determined in this study to investigate fire resistance and the behaviour of restrained high-strength (RHS) Q690 steel beams in fire considering the high-temperature Fields & Fields creep model. A comparison of the results obtained by the FEM with those from previous tests proved the validity of the FEM. A second FEM without a creep model is also established to study the influence of creep on the fire resistance of restrained steel beams. Results showed that creep has a serious effect on the behaviour of restrained steel beams in fire. Thus, ignoring creep will possibly lead to unsafe designs. Several parametric studies are carried out using the validated FEM with the aim to investigate the influencing factors on the fire response of RHS Q690 steel beams. Analysis shows that some of the investigated factors, such as heating rate, cross-section temperature distribution, rotational restraint stiffness and span-to-depth ratio, have been found crucial in the fire resistance of RHS Q690 steel beams. Furthermore, a simplified approach is presented for RHS Q690 steel beams based on the results of the FEM, including the creep effect, to calculate the moment capacity. This approach is also suitable for calculating the critical temperature of RHS Q690 steel beams.
... Various researchers (e.g., Burgess et al. 1991;El-Rimawi et al. 1997;Lawson 1990;Liu 1998Liu , 1999Wang and Burgess 2008) have also conducted studies to analyse the behaviour of rotationally restrained and axially unrestrained beams in fire and similar behaviour is obtained. ...
This thesis is concerned with the behaviour of composite perforated beams in fire conditions, and a new virtual hybrid simulation approach is proposed to facilitate the investigation. Composite perforated beams are an increasingly popular choice in the construction of long-span floor systems as they provide a structurally and materially efficient design solution and allow space for building services. Most of the relevant research conducted to date has been focussed on isolated beam elements, assuming simply-supported boundary conditions. These simplifying assumptions are largely due to the complexity of modelling the whole structure in high definition, as well as the significant associated computational expense. However, testing and analysing isolated components inherently ignores any load redistributions which take place in the structure and does not provide an insight into the thermomechanical interactions which develop during a fire. In this context, the two primary objectives of this work are to (i) develop a usable virtual hybrid simulation framework which assesses the response of individual structural elements subjected to fire, taking account of the surrounding structure and (ii) utilise this framework to investigate the behaviour of perforated beams exposed to fire including the effects from the surrounding structure in the form of axial and rotational restraint. In the virtual hybrid simulation method, the part of the structure which is exposed to fire is modelled in fine detail using shell and solid elements and the remaining surrounding structure is represented using simpler beam-column elements. The simulation is developed using a combination of the OpenSees, OpenFresco and Abaqus softwares and enables the user to investigate the behaviour of fire-exposed components while including the effect of the remaining structure without modelling the whole system in fine detail. The accuracy of the model is validated using available fire test data. The behaviour of composite perforated beams in fire is analysed using the developed framework and then compared with the predicted response obtained by modelling isolated simply-supported beams. The results highlight the importance of including the effects from the surrounding structure in the analysis. The virtual hybrid simulation framework is then utilised to investigate the influence of the most salient parameters including the type of fire, opening layout, restraint conditions as well as the material and geometric details. In the final part of the thesis, the current ambient temperature design standards for perforated beams are modified to account for the effects of fire. A series of analytical expressions are developed to estimate the fire resistance of composite perforated beams with different opening layouts, and these predictions are compared with the fire resistance obtained from the numerical simulations. It is shown that the proposed analytical approach provides a good estimation of the fire resistance for the majority of cases.
... Beton döşemenin, çekme kuvvetlerine karşı membran mekanizması ile yangına karşı daha dirençli olduğu bilinmesine rağmen, çelik kesme bağlantılarının bu çeşit bir sistemde dönme ve çekme kuvvetine maruz kalıp kalmayacağı tartışılan bir konudur [10,11,12]. Çelik bağlantılar, yapıların sabit kalması, özellikle kolonların yatay desteklenmesi ve burkulma yaşamaması için önemli bir fonksiyona sahiptir [1, 13,14]. Bu nedenle yangından kaynaklanan kuvvetlere karşı yetersiz bağlantı gerilimi veya yetersiz süneklik durumu, çelik yapıda çökmeye yol açabilir. ...
... Beton döşemenin, çekme kuvvetlerine karşı büyük yer değiştirmeleri (sehim) yaptığı ve yangına karşı daha dirençli olduğu bilinmesine rağmen, çelik kesme bağlantılarının bu çeşit bir sistemde dönme ve çekme kuvvetine maruz kalıp kalmayacağı tartışılan bir konudur [10,11,12]. Çelik bağlantılar, yapıların sabit kalması, özellikle kolonların yatay desteklenmesi ve burkulma yaşamaması için önemli bir fonksiyona sahiptir [1,13,14]. Bu nedenle yangından kaynaklanan kuvvetlere karşı yetersiz bağlantı gerilimi veya yetersiz süneklik durumu, çelik yapıda çökmeye yol açabilir. Kompozit döşemenin yangın altındaki yüksek sehimi ancak çelik bağlantıların dönme kapasitelerinin yükseltilmesi ve daha sünek şekilde tasarlanması ile olur [15,16,17]. ...
(Turkish) Bu çalışmada, Türkiye'de araştırma odaklı ilk çelik-beton kompozit kat döşemesi yapısal yangın deneyi gerçekleştirilmiştir. Bu çalışma, Türkiye'de gerek yangın yönetmelikleri açısından gerekse yapısal yangın konusunda teorik, nümerik ve deneysel araştırmalardaki eksiklikleri gidermeyi amaçlamaktadır. Yüksek katlı çelik bir yapı için tasarlanmış kompozit döşemenin iki kenarında simetrik bir birleşim mekanizması kullanılmıştır. Yangın fırınına monte edildikten sonra döşemenin alt yüzeyi ISO834 standart yangın eğrisiyle 105 dakika ısıtılmış ve sonrasında 90 dakika kontrollü olarak soğutulmuştur. Deney sırasında yapının sehim ve sıcaklık ölçümleri yapılmıştır. Alınan sonuçlar, beton döşemenin membran davranışı göstererek yük taşıyıcı özelliğini yangın boyunca devam ettirdiğini göstermektedir. Dolayısıyla kompozit kat döşemelerinde ikincil çelik kirişlerin yalıtımsız kalması önerilmektedir. Anahtar Kelimeler: Yangın dayanımı testi, simetrik birleşim mekanizması, yapısal yangın mühendisliği, kompozit döşeme, çelik bağlantı ABSTRACT This study conducts the first research based structural fire resistance test of a steel-concrete composite floor in Turkey. The goal of this research is to fill the knowledge gaps in the current Turkish building code for the structural fire safety and provide valuable insight for the development of new theoretical, computational and experimental research. The tested composite floor is specifically designed for a high-rise steel building. The floor is equipped with a patent pending mechanism to provide symmetric boundary conditions on two edges. The floor is subjected to ISO-834 standard fire curve from the bottom surface for 105 minutes followed by 90 minutes cooling. Displacement and temperature measurements show that at elevated temperatures, the concrete slab carries the load by the tensile membrane action without a contribution of the secondary beam. This study suggests that secondary steel beams do not need to be fire protected as the concrete slab is adequate to carry the gravity loading.
... In absence of fire protection, steel structure building was extremely vulnerable to be damaged or even collapsed in fire. To effectively prevent steel structure from damage, the major method was local spray water cooling or protective coating1256789101112. When rapidly cooled by local spray water, A large thermal stress would produced because of large temperature difference and stress concentration would happened in the region of structural break, that would produced a strong impact on the stability for the structure. ...
Locally rapid cooling of spray water had strong impact on high temperature steel structure. When temperature of beam reached 600°C and cooling rate was more than 20°C/s, the maximum axial tension could reach more than 5 times of the originally compressive force. The compressive bending moment at joint of beam-to-column changed to tensile bending moment, and the maximum bending moment could reach above 4 times as that when heated. After rapid cooling by spray water, deflection at mid-span increased slightly.
... Different end restraints are considered, viz. 25%, 50%, 75% and 100% scaling of both the stiffness and strength of the extended end-plate characteristics as proposed by El-Ramawi et al. [30]. Temperature is uniformly distributed along the beam length as well as across the section. ...
Most structural fire resistance analyses are confined to 2-D frames. To gain a more in-depth understanding of the response of a steel frame at elevated temperatures, 3-D simulations are necessary. This paper outlines the formulation of a two-noded 3-D beam-column to study the response of a steel frame in fire. The program is capable of small-strain large deformation analysis. Warping effect, a significant phenomenon in thin-walled members, is considered in the formulation. Degradation of steel mechanical properties at elevated temperatures is also considered, while thermal gradient is considered by slicing a cross-section into discrete segments. Creep can be either implicitly or explicitly taken into account. Several widely-used beam-to-column connections are approximated as zero-length semi-rigid springs. Their nonlinear moment-rotation relationships at elevated temperature are incorporated into the program. Unloading of both material stress-strain curve and moment-rotation characteristics of a connection are accounted for. The Newton-Raphson method is employed for nonlinear solving procedure. At the latter part of this paper, the program is verified against benchmark tests. All of them demonstrate the accuracy and reliability of the program.
... They pointed out that the failure mode of steel columns changed from inelastic global buckling at room temperature to local buckling at elevated temperature, due to the release of residual stress in fire. The influence of connection's stiffness on the behavior of steel beams in fire was analyzed by [8]. The studies resulted in estimating the fire temperature for some connection details. ...
In the paper the influence of boundary conditions on the thermal response of selected steel members subjected to fire loads is investigated. Simply supported and restrained steel beams subjected to uniformly distributed load and fire accidental action are analysed using the standard ISO-fire curve. The iterative procedure to investigate critical temperature is used. At each increment of temperature the ultimate limit state is checked using the EC recommendation. The special attention is focused on axial action occurring due to thermal elongation. The illustrative examples show that, proper modelling of the beam's boundary conditions plays an important role in describing and understanding the real behavior of elements in fire conditions. Moreover, beam's boundary conditions can change structural response and result in reduction of critical temperature and fire resistance time.
... The stressed documents the behaviour of the restrained composite beam during fire, see [7]. It is visible a high influence of the local buckling of the flange due to compressive forces from thermal elongation of the beam as well as from restrains of the beam by increasing of the influence of the connection stiffness due to decreasing of the bending rigidity of beam, see [8]. The beam ends yields in 17 min. ...
The paper presets a part of the results reached during the Structural integrity fire test in Cardington laboratory January 16, 2003. The work summarises the instrumentation of the experiment and focuses to the measured stresses on columns and on beams, which were instrumented to observe the tie forces in the multistorey building under natural fire. The composite action and joint behaviour play the major role in the structural behaviour under fire on the Cardington frame. The simple rules for the design of the beam may be extended into the catenary action, if the connections are designed to be adequately robust. From the measured forces in columns may be evaluated the horizontal forces in connections. The experiment confirmed all three parts of fire design by Eurocode; the fire modelling by parametric fire curve, the transfer of heat into the structure, and the structural analyses of beams, columns as well as of connections at elevated temperature.
... It may predict thermal and structural performance. In fire, the behaviour of a structure is more complex than at ambient temperatures [5]. Changes in the material properties and thermal movements cause the structural behaviour to become nonlinear and inelastic [6]. ...
This study presents the behaviour of the structural connections of multi-story buildings under fire based on the Cardington tests findings. Two types of calibration frames, braced and unbraced, simulate the typical multi-story frames. The joints and the column bases are introduced as semi-rigid. Three temperature scenarios are applied: the fire under located part of the beam, the heating of one whole beam, and the heating of the compartment as a floor. The behaviour of the joins under the heated level, on the heated level, and above the heated level was analysed. The high non-linearity of the normal forces and the rotations in the joints were observed during the simulation of the accidental actions. The required robustness of joints in the terms of the tie forces is discussed.
... Finite element models built up with beam-column elements with the incorporation of semi-rigid joints were proposed by Fakury et al (2005), Zhao & Kruppa (2002), El-Rimawi et al (1997) and Moss et al (2004). The structural behaviour of continuous and semi-continuous composite beams at elevated temperature was studied (Zhao & Kruppa, 2002). ...
An innovative type of non-uniform arrangements of f re protection materials is proposed for efficient fire protection to semi-continuous composite beams. The f re protection is arranged according to applied bending moments and shear forces. Heavy f re protection is applied in high bending and shear regions, while relative thin or even no f re protection is applied in low bending regions. Different failure modes are identified from the results of finite element analyses under different combinations of temperature profiles in both heavily and less f re-protected regions. A simple design approach is then proposed to predict load carrying capacities of semi-continuous composite beams for each of the identified failure modes. A parametric study is conducted thereafter to optimise the arrangements of fire protection materials.
... Para determinar o comportamento de ligações isoladas foram desenvolvidos processos analíticos com base em metodologias aplicadas a ligações a temperatura ambiente. El-Rimawi et al. [20,21] foram os primeiros a desenvolver uma metodologia que permitia a caracterização do comportamento momento-rotação da ligação a elevadas temperaturas. Partindo da equação de Ramberg-Osgood definiu-se uma equação modificada que permitia caracterizar a curva momento rotação da ligação a elevadas temperaturas. ...
Nesta comunicação faz-se uma breve revisão do comportamento de ligações de aço e mistas aço-betão em situação de incêndio. Aqui estão expostos os principais estudos experimentais, numéricos e analíticos desenvolvidos em ligações sujeitas a incêndios. São ainda reportados os resultados dum trabalho experimental em ligações mistas de aço e betão realizado no Laboratório de Ensaio de Materiais e Estruturas da Universidade de Coimbra. Por fim, apresentam-se os estudos numéricos neste tema que estão em desenvolvimento pelos autores.
... Commonly used approaches for investigating the large deformation and fire responses of semi-rigid joints include experimental tests [1][2][3][4], curve-fitting expressions of previous experimental results [5,6], detailed 3D finite element models [7][8][9][10][11], and the component method initially proposed by Eurocode3 [12]. Among the various available assessment methods, tests on joints are the most realistic one, but experimental data are only available for limited types of joints, and there are even discrepancies in the response between joints of the same type. ...
Joints play an important role in providing ductility for steel-composite structures subject to extreme loading conditions, such as blast, fire and impact. Due to sound energy dissipation capability and fabrication efficiency, semi-rigid joints have increasingly received attention during the last decade. This paper presents a component approach for modeling semi-rigid beam-to-column joints based on Eurocode3, where the post-elastic response, including component strain hardening and ultimate rotational capacity, is also considered. Failure criteria are defined based on the ultimate deformation capacity of components and bolt-rows. The model enables a direct integration of joint response into global frame models with the consideration of axial deformability, such that the interaction between bending moment and axial force within the joints can be realistically captured. In addition, elevated temperature can be considered in the joint model via the degradation of the component response. Through comparisons with available test data, the joint model is shown to have good accuracy, and the failure criteria are found to be reliable yet conservative. The strain hardening response of components is shown to have significant influence on the ultimate bending capacity of the joints, while neglecting it usually leads to a conservative prediction.
... However, the connections generally have a significant rotational stiffness (El-Rimawi et al., 1997), therefore the actual behaviour of bolted beam-column connections lies somewhere between a fully pinned and a fully fixed connection. Hence, this report will also look at the fixed connections in frames as comparison. ...
... Fig. 2 shows different forms of curve-fitting representations of joint's behaviour. Initial attempts to model the elevated temperature moment-rotation characteristics by mathematical expression were made by El-Rimawi et al. [23]. They proposed a simple equation with three parameters, to represent the moment-rotation-temperature data obtained from experimental fire tests, using the modified Ramberg-Osgood expression [24] of the following form: ...
This paper presents a state-of-the-art review on the behaviour of beam-to-column joints in fire and considers experimental and analytical research work on isolated joint configurations conducted with the prime objective of developing moment–rotation–temperature behaviour of joints. In addition, investigations on the effect of axial thrust on the behaviour of joints is presented because fire tests on a full-scale building, and observations from accidental fires, have demonstrated differences between the behaviour of joints when tested in isolation and considered as part of a complete building. Furthermore, joints that are routinely assumed as pinned at ambient temperature can provide considerable levels of both strength and stiffness at elevated temperature, albeit at large deformations, and this has been found to have a beneficial effect on the survival time of steel-framed buildings. It is noted that while FE analysis is capable of predicting accurately the performance of steel structures in fire, realistic models of joint performance are required. For this purpose, the use of a component approach for the prediction of joint performance is explained and appears to be a viable alternative to extensive joint testing or detailed FE analysis of joint details.
This paper presents a comprehensive numerical analysis of high-strength Q690 steel beams in fire. A FE model is developed and validated against the full-scale experiments on restrained high-strength Q690 steel beams in fire presented in a previous study by the authors. The FE model adequately replicates the deflection, axial force, and actual failure modes identified experimentally. An extensive parametric investigation was performed to determine the variables affecting the behavior of restrained Q690 steel beams in fire. The analysis indicates that several factors, including load ratio, cross-section temperature distribution, rotational restraint stiffness, and span-to-depth ratio are critical for the fire resistance of restrained high-strength Q690 steel beams. Furthermore, additional numerical analysis for the ultimate bearing capacity of high-strength Q690 steel beams under bending was carried out to estimate the flexural-torsional buckling behavior of Q690 steel beams at elevated temperatures. Additionally, based on the results of the FE models, design curves for determining the flexural-torsional buckling strength of high-strength Q690 steel beams at high temperatures were proposed. The findings were compared to the design curves specified by several design standards.
In comparison to the isolated steel beam, the restrained steel beam is able to sustain fire loads using the effect of the surrounding restraints. However, Q690 steel has lower resistance to high temperatures than other types of carbon steel. To address this gap, a series of experimental tests on restrained high-strength Q690 steel beams subjected to fire are reported in this paper. Four full-scale restrained high-strength Q690 steel beams were investigated under several fire scenarios, and one was tested at room temperature. In this study, three transient tests and one steady-state test were chosen to fully comprehend the behavior of the restrained high-strength Q690 steel beams exposed to fire. Under the two test conditions, the impact of the steady-state and transient conditions on the fire response of the restrained Q690 steel beams are compared, and the variation of axial peak force, bearing capacity, and deflection were analyzed. The results showed that the critical temperature of the steady-state test is close to the critical temperature of the transient test, and the maximum axial force of the steady-state test is comparable to the one of the transient test. Due to the combined action of load and axial forces, the restrained Q690 steel beam generally develops flexural-torsional buckling failure after reaching the critical temperature in transient tests. In comparison, the steady-state test exhibits early flexural-torsional buckling behavior before reaching the critical temperature.
Even though creep behavior in steel at elevated temperatures has been widely observed, the effect of creep on steel members is not fully understood yet due to the extensive range of steel grades. In this paper, a tension creep test on high-strength Q960 steel at high temperatures was performed. Based on the test data in this paper and available creep data on Q345, Q460, and Q690 steels, a consistent creep model for each steel according to the Fields & Fields creep model was proposed and validated by the comparison with the test data. In order to quantitatively investigate the effect of creep on fire response of restrained steel beams, a finite element model (FEM) was established to analyze the critical temperature of restrained steel beams in fire conditions, with and without considering creep strain generated in steel beams. This was also carried out to better understand the impact of high-temperature on steel beams with various steel grades under the influence of the same load. The verification of the FEM was achieved by comparing the results with experimental data on a restrained Q235 steel beam and Q690 cantilever steel beam. The study reveals that creep has a noticeable effect on the fire resistance of the restrained steel beams and that the effect varies from one type of steel to another. A comparison has been made to observe the effect of high-temperature creep on the critical temperature for each steel type.
Purpose
This paper aims to present the results of a study on the behaviour of a pre-stressed cable steel truss exposed to fire under fire conditions, basing on the results of a large programme of experimental tests.
Design/methodology/approach
The research investigated the deformation and stress change on a pre-stressed steel cable, including the deflection and displacements at different joints and fire behaviour of the pre-stressed steel cable. In other words, the structural behaviours at different loaded pre-stress, the vertical loading, steel cable height, truss dimension and the final temperature were compared in case of fire.
Findings
The results showed that the strain of longitudinal chord was far larger than those of the transverse chords, the strains of lower chords were significantly larger than those of the upper chords, strain of the chord near the longitudinal centreline were also larger than those of the outside transverse chords. During heating, the displacement and strain gradually changed from linear to nonlinear with loading, and the yielded chord had also in an order those chords which were at mid-span and near to the longitudinal centreline, yielded at first.
Originality/value
Temperatures in the furnace and at several points of the pre-stressed cable steel truss, as well as deformations, deflections and the stress changes of upper chord and the bottom steel cable and the change of displacement at different joint were measured to achieve those goals and, consequently, to assess the deformation behaviours and temperature of the pre-stressed steel cable.
This paper presents an effective, reliable and accurate method for prediction of structural behaviour of steel frames at elevated temperature. The refined plastic hinge method, which has been used successfully in the second-order elasto-plastic analysis of steel frames at ambient conditions, is adopted here to allow for time-independent fire effects. In contrast to the existing rigorous finite element programs, the present method uses the advanced analysis technique that provides a simple and reliable means for practical study of the behaviour of steel frames at elevated temperature by a limiting stress model. The present method is validated against other test and numerical results.
To understand thoroughly the effect of the column web stiffener of extended end-plate connection on the failure mode in fire and the fire resistance, a nonlinear finite element analysis is performed. For the column web without stiffener, the plastic strains in both tension and compression areas in the web increase with the elevation of temperature, and a plastic hinge is formed finally, resulting in the buckling of the column. If the column web has only compressive stiffener, the yielding of tension area in the web leads to the column buckling and the joint failure. For the column web with both tension and compression stiffeners, the plastic hinge line produced in tension area of the endplate causes the endplate to be apart from the column flange, which leads to a large rotation of the beam and local buckling of the compressive flange of beam at last, thermal axial force in the beam decrease. Smaller thermal axial force in the beam is helpful for the stability of the column and whole frame in fire.
Beam-to-column connections are one of the structural elements which were found to be of great significance in enhancing structural behaviour at ambient and elevated temperatures. In general, laboratory experiments provide acceptable results that can describe the behaviour of the beam-to-column connections. However, in many cases experiments are either not feasible or too expensive. Numerical modelling of connections at elevated temperature provides an alternative to experimental testing in investigating the connections behaviour. Therefore, in recent years, various numerical techniques have emerged to simulate the connection response ranging from simple behaviour prediction procedure, to more complicated finite element models. This paper addresses the current developments in the numerical methods that have been developed to simulate the connection behaviour at elevated temperature which could help in providing thorough understanding of the major contribution connections can play in the high performance of structures in fire.
This article provides a discussion of the mathematic modeling of connections for designing and qualifying structures, systems, and components subject to monotonie or cyclic loading. To characterize the force-deformation behavior of connections under monotonie loading, a review of the RambergOsgood, Richard-Abbott, and Menegotto-Pinto models is conducted, and it is shown that these nonlinear functions can be mathematically derived by scaling up or down a linear force-deformation function. A generalized four-parameter model for simulating connection behavior is investigated to facilitate nonlinear regression analysis. In order to perform seismic analysis of frameworks, a hysteretic model accounting for loading, unloading, and reloading is described using the established monotonie model. For preliminary analysis, a method is provided to quickly determine the model parameters that fit approximately with the observed data. To reach more accurate values of the parameters, the methods of nonlinear regression analysis are investigated and the modified Levenberg-Marquardt and separable nonlinear least-square algorithms are applied in determining the model parameters. Example case studies illustrate the procedure for the computation through the use of experimental/analytical data taken form the literature. Transformation of connection curves from the three-parameter model to the four-parameter model for structural analysis is conducted based on the modeling of connections subject to fire.
RESUMO Observações de estruturas metálicas sob incêndio e resultados de ensaios em escala real têm evidenciado a importância dos elementos de ligação na determinação do comportamento estrutural. Neste contexto, apresenta-se neste trabalho um estudo do comportamento não-linear de ligações metálicas do tipo "placa de topo estendida" em situação de incêndio. Para tal, um modelo numérico-computacional é proposto tomando-se por base o método das componentes (Eurocode 3), permitindo-se a determinação de curvas Momento-Rotação-Temperatura (M-R-T) de ligações para diferentes níveis de carregamento e altas temperaturas. Os efeitos decorrentes de gradientes térmicos na ligação, bem como, nos elementos estruturais (vigas e pilares) são estimados em função do tempo transcorrido de incêndio. A variação das propriedades térmicas e mecânicas dos materiais sob altas temperaturas é levada em consideração. As relações M-R-T obtidas são aproximadas segundo modelo exponencial dos três parâmetros (Kish-Chen): (i) rigidez inicial; (ii) momento último; (iii) fator de forma; permitindo-se uma acurada descrição do comportamento da ligação, com baixo número de variáveis. Os resultados numéricos obtidos no presente artigo são comparados com dados experimentais semelhantes propostos na literatura, indicando boa correlação. PALAVRAS-CHAVE Incêndio, ligações semi-rígidas, chapa de topo estendida, método das componentes.
A three-dimensional finite element thermal model is proposed to predict the temperature histories of various components of composite connections under fires. The model was carefully calibrated against a series of standard fire tests. Temperature histories, both within connections and in composite sections, are compared and calibrated. A two-dimensional finite element mechanical model is adopted and calibrated to predict the structural response of composite connections at elevated temperatures. Both of the proposed models are verified to be efficient tools for integrated analysis and design of composite end-plate connections under various fire scenarios.
This paper presents a state-of-the-art on the behaviour of steel joints under fire loading and some recent developments in this field, currently being carried out by the authors. Firstly, a review of the experimental research work on steel joints is presented, subdivided into isolated member tests, sub-structure tests and tests on complete building structures. Special emphasis is placed on the seventh Cardington test, carried out by the authors within a collaborative research project led by the Czech Technical University in Prague. Secondly, a brief review of various temperature distributions within a joint is presented, followed by a discussion of the behaviour of isolated joints at elevated temperature, focussing on failure modes and analytical procedures for predicting the moment-rotation behaviour of joints at elevated temperature. Finally, a description of the coupled behaviour of joints as part of complete structures is presented, describing previous work and investigations on real fire (including heating and cooling phases) currently being carried out by the authors.
Steel structures may be exposed to localized heating by a fire source nearby. Flame impingement from localized fire may lead to high temperatures in the exposed steel members, which may lead to structural failure. This paper numerically investigates the thermal and mechanical behaviors of restrained steel beams exposed to flame impingement from localized fires. Four steel beams with different dimensions and restraints were considered. Both developing and steady burning fires were investigated. The standard ISO834 fire was also used for comparison. The study finds that the temperature distributions within the steel beams subjected to flame impingement are highly non-uniform both across and along the beams. Along the beam length, the temperatures near the fire source may be hundreds of degrees higher than those far from the fire source. Due to different temperature distributions, the deformation mode for restrained steel beam subjected to flame impingement may be significantly different from that of a beam subjected to the standard ISO834 fire. The failure temperatures for restrained steel beams subjected to localized fires may be higher or lower than those for restrained beams subjected to the standard ISO834 fire. Reliance on the standard fire may lead to an unconservative design if the potential real fires are localized fires.
The structural behaviour in fire of simple sub-structured frames consisting of unprotected steel I-beams and columns, which connect through end-plate bolted connection, is studied using finite element modelling. Extended and flush end-plate connections are examined. These connections are considered as semi-rigid to rigid under ambient conditions. Various parameters which affect the behaviour of end-plate type connections were examined, such as bolt size, number of bolts and end-plate thickness. It was found that, no matter how large the moment capacity the connection can have at ambient condition, the part that may be able to enhance the load capacity of the beam at fire limit state is at most two-thirds of the moment capacity of the beam.
This paper presents an effective, reliable and accurate method for prediction of structural behaviour of steel frames at elevated temperature. The refined plastic hinge method, which has been used successfully in the second-order elasto-plastic analysis of steel frames at ambient conditions, is adopted here to allow for time-independent fire effects. In contrast to the existing rigorous finite element programs, the present method uses the advanced analysis technique that provides a simple and reliable means for practical study of the behaviour of steel frames at elevated temperature by a limiting stress model. The present method is validated against other test and numerical results.
This paper investigates the structural responses of generally restrained steel beams under fire conditions by the finite-element method. The axial restraint is represented by a linear elastic spring, whereas the rotational restraint is simulated as a semirigid rotational spring. The developments of beam internal forces, cross-sectional stresses and strains, together with displacements are examined. Moreover, the effects of some important factors such as load utilization factor, beam slenderness ratio, and axial and flexural restraint ratios, are also studied. Numerical analyses show that the critical temperature is reduced by axial restraint but increased by semirigid rotational restraint. Some stocky beams subjected to low utilization factors buckle at elevated temperature. The later part of the paper focuses on investigating the effect of cross-sectional thermal gradient; the associated thermal bowing is detrimental for a slender beam. This paper also tabulates the finite-element (FE) predictions of the lower and upper bounds of beam critical temperature for design purposes. Besides FE analyses, the simplified methods from EC3 Pt.1.2 and BS5950 Pt.8 for critical temperature are also examined.
Since the September 11th terrorist attacks, the research and engineering communities have given significant attention to building performance under combined effects of impact, explosion, and fire. This paper presents a numerical approach for inelastic transient analysis of steel frame structures subjected to explosion loading followed by fire. The approach adopts the use of beam-column element and fiber element to enable a realistic modeling of the overall framework subjected to localized explosion and fire. Detailed requirements for modeling elasto-plastic materials subjected to elevated temperature and high-strain rates are presented. Verification examples are provided. The influence of blast loads on the fire resistance of a multistory steel frame is studied.
Computer software has been developed to predict the structural response of asymmetric slim floor steel beams, used with composite concrete floor slabs consisting of deep profiled steel decking. Comparisons between predicted behaviour and that recorded in standard fire tests, showed that the software is very accurate. By including the rotational stiffness of the beam-to-column connections, the fire resistance of the beam is significantly enhanced. This is mainly due to the connections retaining most of their strength during a fire, since they are fully encased in concrete as a consequence of this type of construction. The analyses presented in this paper indicate that it may be possible to increase the fire resistance of the steel beams from 60 to 90 minutes, by including the connection behaviour. The software has also been used to aid the design of a future large-scale fire test on the asymmetric slim floor system. Predictions of the structural response have been presented. These will enable the fire load and ventilation conditions in the test to be designed. In addition the software has been used to identify the minimum amount of fire protection that is required for the supporting columns.
A series of analytical studies are presented on the behaviour of an unprotected `rugby post' frame exposed to fire. The main purpose is to assess the effects of interactive structural behaviour on predicting the critical temperature for individual members. The influence of certain key parameters is considered, including the axial load applied to the column, the type of beam–column connection, and the rate of heating of the column relative to that of the beam. Where appropriate, the results are compared with current simplified design approaches.The studies indicated that the column load has a significant effect on the development of moments in the beam. It also became evident that, at failure, the maximum moment in the column is due largely to the effects of beam expansion.
The importance of connection stiffness in influencing the behavior of steel frames is discussed. Experimental data on connection performance are cited which show that all practical forms of beam-to-column connection operate as semi-rigid joints. Against this background the assumptions of the 'simple design' method for non-sway frames are reviewed and a more rigorous behavioral study is presented. Potential benefits and disbenefits of allowing for semi-rigid action in design are discussed. For sway frames the philosophy of the 'wind connection method' is explained and conditions under which its use should result in reasonable frames are identified. Some attention is also given to the assessment of effective length factors that properly reflect the restraint available to columns in non-sway frames.
It has long been convenient to assume 'pinned' or 'fixed' connections to simplify analysis for steel frame design. Until recently, there has been little interest in research on real behaviour of joints because of mathematical complexity of solutions. However, there is a revitalised interest in this topic because of cheap computing power and new Codes highlighting a number of grey areas of design. This paper discusses the effect of joints on a number of structures and attempts to show, by use of fixity factors and a modified moment distribution method, that analysis need not be difficult and that, even when full connection data are not known, the techniques may provide qualitative data of great practical value in many cases.
The paper presents the results of fire tests on eight beam-to-column connections, typical of those used in modern framed buildings. Five tests were on non-composite beams, two on composite beams, and one on a shelf angle floor beam. Both protected and unprotected members were included. The tests demonstrated the 'robustness' these connections in fire and showed that significant moments (up to two-thirds of their design moment capacity) could be sustained in fire conditions. A method of taking account of the beneficial effect of moments transferred to connections in fire is considered. These connections are considered as 'simple' under normal load conditions. The result is to reduce the effective load ratio to which the beams are subject, leading to increased limiting temperatures and reduced amounts of fire protection. In some cases, it is demonstrated that 30 min fire resistance can be achieved in unprotected beams. This is increased to 60 min in unprotected shelf-angle floor beams.
The history of research into the behaviour of steel beam-to-column connections is traced starting from early developments in 1917. Attention is focussed on moment-rotation characteristics as this is the most important influence on the response of either individual members or complete frames. The nonlinear nature of this characteristic is identified and methods of representing moment-rotation curves for subsequent use in analytical procedures are discussed. A review is made of all available test data; this may be used as a starting point for both further studies of connection behaviour and as the basis for investigations of the effects of semi-rigid connections on structural response.
A secant stiffness method was previously developed for efficient analysis of isolated steel beams under the influence of thermal distributions caused by fire. This method has been developed considerably further, to include the thermally degrading characteristics of the connections between adjacent members and also the effects of axial force, the latter including both material and geometric nonlinearities. This enables full planar skeletal frames̀ to be analysed up to failure under local or overall fire conditions. In particular terms it allows the investigation of the real behaviour in fire of frames designed as ‘simple’ for ambient-temperature, strength and serviceability criteria. Although the normal design assumption for such frames is that beams transfer no moments to columns, the rotational stiffnesse of the connections can considerably affect fire survival, as can other restraint forces from the surrounding structure. In this paper the theoretical background to the analysis is developed, and validation studies are reported. These include comparisons with previous analytical work and with fire test results. An original example is presented of a three-bay x three-storey frame with a local compartment fire in the bottom storey.
In general the temperature profile within a steel beam subjected to fire is non-uniform. To predict the structural behaviour of such a beam the effect of the variation of the steel's characteristics with temperature across the beam must be considered. In addition stresses due to non-uniform thermal expansion need to be included. A procedure for the analysis of such cross-sections with a non-uniform temperature profile is presented. This has been developed specifically to be used on personal computers, rather than the mainframes normally employed for numerical analysis of this degree of complexity. The actual temperature profile is idealised by a finite number of constant temperature steps, and the thermal stresses induced are converted to a set of external loads. These are used in a secant stiffness approach to predict the deflection-temperature characteristics of beams. A comparison with test results for different types of beam illustrates the degree of accuracy that can be achieved using the techniques described.
Modelling the behaviour of steel frames and subframes with semi-rigid connections in fire
- J Ei-Rimawi
- I Burgess
- R Plank
EI-Rimawi, J., Burgess, I. and Plank, R., Modelling the behaviour of steel frames
and subframes with semi-rigid connections in fire. Proceedings of Third CIB/W14
Workshop on Fire Modelling, Rijswijk, 1993, ed. L. Twilt, pp. 152-168.