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.     

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.     

Behavior of bolted angle connection under catenary condition in fire

The behavior of bolted angle connections under the combination of shear and tensile forces is studied in this paper to simulate the force applied on a connection in a real fire. First, ansys is used to develop a 3D model of these connections. These models are analyzed in a similar condition to experimental tests, ignoring the tensile force, and the results are compared with those of the experimental tests. Having assured of the accuracy, we studied the connection models in several conditions under the combination of shear and tensile forces. The results show that the strength of connection is rapidly decreased when the temperature is increased, and the decrease pattern of connection strength is similar to decrease pattern of bolts used in the connection. Moreover, investigation of strength reduction value of these connections by the increase of temperature under shear and tensile forces obviously shows that it is possible that the failure of steel frames at elevated temperatures occurs at the connections, and thus utilization of catenary action to enhance the fire resistance of structural steel beams requires investigation of the capacity of steel connections to resist the tying forces generated at the ends of the beams. Copyright © 2012 John Wiley & Sons, Ltd.     

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.     

Modeling of end-plate bolted composite connection in fire considering axial force effects

This paper proposes the development and validation of a simplified model, which is based on spring-component-based model, in an attempt to calculate the initial stiffness and ultimate moment capacity for flush end-plate composite joints at elevated temperatures. This method may be adopted as the basis of the flush end-plate joint design in high temperature. In this paper, the resulting axial force due to the expansion of the beam at elevated temperatures is considered as a significant influence on the moment bearing capacity of joints. A 3-D finite element model using ANSYS is presented to simulate the real response of flush end-plate composite joints with axial force at elevated temperatures. This model is verified by comparing the simulated temperature distribution and the behavior of the joint at ambient temperature with experimental results. Then the finite element model is used to validate the accuracy of the simplified component model. Close agreement is found between the results of the simplified component-based method and that of the finite-element method. Furthermore, a simplified model based on the three-parameter exponential model proposed by Kishi-Chen [15] to predict the moment–rotation relationship of the composite joint at elevated temperatures considering the effect of the axial force is proposed, the parameters in which is obtained based on the parametric analysis with the FE model. The proposed component model and moment–rotation relationship provide a convenient path for designers to assess the connection behavior in the semi-rigid frame analysis.     

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.     

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.     

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.     

The development of a component-based connection element for endplate connections in fire

This paper describes the development of a component-based element for endplate connections in fire. The reported research is part of an ongoing project aimed at understanding joint behaviour in fire. The paper summarises the derivation of the stiffness matrix of this new element, based on a spring model, and the incorporation of the element into the non-linear finite element program Vulcan. It also states the component characteristics that have been used for the individual zones of deformation in an endplate connection. Furthermore, the additional features of the element, necessary for correct response at elevated temperatures, such as the consideration of the temperature distributions across the connection as well as cooling and unloading, are summarised. The proposed element is then used to predict the moment–rotation curves of connection experiments at ambient and elevated temperatures. Finally, the advantages and limitations of the new high-temperature connection element are listed.     

正弦腹板网格结构中复合钢和混凝土连接节点的受弯承载力

多年来,钢梁已经被成功地运用于复合钢及混凝土结构领域中。制造技术的进一步发展和改进导致了新型结构型材的产生。其中复合正弦腹板网格结构在近些年被运用于巴西建筑结构中。尽管这种复合结构有很多优点,仍然缺少技术规范来计算这种梁结构的性能。因此,仍需要更多的设计建议妥善处理这些构件及它们与相邻构件连接处的弯扭承载力和初始刚度问题。所以:1)试验测试中所用构件按十字形排列;2)使用有限元分析软件AN-SYS构建构件和组合梁的有限元模型;3)有限元模型通过试验数据的验证;4)对其进行参数研究;5)提出一个理论模型。所提出的理论模型虽然包含了很多复杂因素,仍与试验和数值分析结果相吻合。     

Moment resistance of composite steel and concrete connection in sinusoidal-web girders

Steel girders have been successfully used for many years in the composite steel and concrete construction market. Further developments and advances in manufacturing technology have led to a new generation of structural shapes. One of these developments, the composite sinusoidal-web girders, has been recently introduced in the construction of Brazilian buildings. In spite of the advantages this type of composite construction may offer, there are no design standards dealing with the behavior of such girders. As a result, there is a need to develop design recommendations that properly address the flexural and rotation capacities and initial stiffness of these elements and their connections to the adjacent members. To this end: (i) experimental tests of connections on cruciform arrangements were performed, (ii) finite-element models for the connections and the composite girders have been developed, using the commercial finite element software ANSYS, (iii) finite element models were validated by experimental data, (iv) a parametric study was carried out and (v) a theoretical model is proposed. It is shown that the proposed theoretical model, despite all the complexities involved, is in compliance with both experimental and numerical analyses.     

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.     

Behavior of semi‐rigid connections and semi‐rigid frames

Semi‐rigid connections are widely used in different countries. These connections are usually used in semi‐rigid frames with bracing system. Considering the frequent use of these connections, studying their behavior as an individual connection or as a frame with a semi‐rigid connection is of great importance. In this paper, moment–rotation behavior of bolted angle connection as a usual semi‐rigid connection is studied, and the affecting factors on there are investigated. Finite element connection models are developed. These models are verified by comparing the results of finite element analyses with the results of experimental test, and the verified models are used to investigate the behavior of the connections. The behavior of semi‐rigid frame with bolted angle connection is then studied conducting time‐history analyses. The results show that the increase in shear stiffness of angle bolted connections significantly decreases the lateral drift and increase the frame stability against lateral loading. Copyright © 2012 John Wiley & Sons, Ltd.     

Experimental Investigation of T-Stub Link-to-Column Connections in Eccentrically Braced Frames

Link-to-column connections in eccentrically braced frame should transfer large shear and moment developed in a fully plastic and strain hardened link. Moreover, the connection should allow the link to undergo large plastic rotations. It was observed in previous studies that link-to-column connections are failed before achieving the plastic link rotation capacity. Design of link-to-column connections in seismic-resistant eccentrically braced frames is still an unresolved problem. In this paper, experimental tests are carried out on long link-to-column connections. T-stub connections are used for connecting the link to column. The test parameter is the strength of connection by changing the connection details. The results of the tests indicate that the proposed connections can successfully achieve the desired rotations without any strength loss.     

Finite element analysis of flush end-plate connections connected to column web

Eurocode 3: Part 1.8 (2005b) does not cover the empirical design for flush end-plate connections connected to column web. Thus, experimental works need to be performed to investigate the behaviour of the minor axis end-plate bolted connection. However, the experimental tests are expensive, tedious and time consuming to be conducted. Alternatively, finite element modelling and analysis can be adopted to predict the behaviour of the connection accurately. Five specimens of flush end-plate connections connected to column web with variable parameters have been tested and the results are compared with finite element analysis. Finite element models with enhanced strain solid and contact joint elements are used to stimulate the connection behaviour. The objective of this paper is to compare and validate the accuracy and reliability of the finite element model by correlating moment rotation (M-Φ) curves between the finite element model and experimental tests. The comparison shows a reasonable agreement between the predicted results from FEA and the experimental tests.     

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.     

Finite element simulation of new RHS column-to-I beam connections for avoiding tensile fracture

Many beam-to-column connections, consisting of rectangular hollow section (RHS) columns and wide flange I-beam connections, sustain brittle fracture of welded connections at the beam ends during a large earthquake. These fractures most frequently occur in regions around the beam bottom flange groove welds. A series of tests was conducted on an improved RHS column-to-I beam connection. The aim of the tests was to find possible solutions for avoiding premature occurrences of brittle fracture in RHS column-to-I beam connections. Research results show that the improved connection does not fail by fracture as observed in the conventional connections and has a larger energy dissipation capacity than the conventional types. This paper describes the finite element modeling method employed to analyze the new RHS column-to-I beam connection. The ABAQUS finite element package is used to simulate the experimental behavior, and three highly detailed 3D finite element models are created. These are complex models accounting for material nonlinearity, large deformation and contact behavior. The connection models have been analyzed through the elastic and plastic ranges up to failure. Comparisons with experimental data show that the models have high levels of accuracy.     

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.     

Design of screwed steel sheeting connection at ambient and elevated temperatures

Cold-formed profiled steel roof sheeting can be directly connected to the top chord of a steel roof truss through self-tapping screws. At ambient temperatures, neither EN 1993-1-8 nor EN 1993-1-3 can be used directly for this type of connection. Besides, no design rules are available in EN 1993-1-2 for designing screwed connections in fire. A 3D Finite Element (FE) model for a single-lap shear screwed connection is developed using ABAQUS software. After the validation by tests, the model is used to predict the ultimate resistance of connections at both ambient and elevated temperatures. Further, the effects of edge and end distances on the connection resistance are investigated. Based on the analyses results, revised design equations for predicting the connection resistance are proposed. The design resistance is calibrated by testing and FE analyses results according to the procedure given in EN 1990 and the partial safety factor is derived.     

常温和高温下钢板螺栓连接的设计

通过自攻螺钉可以将冷弯压型钢板与钢屋架的上弦杆直接连接。常温下,无论是EN1993-1-8规范还是EN1993-1-3规范都可以直接作为此类连接的设计规范。此外,在EN1993-1-2规范中,没有针对火灾中螺栓连接的设计条文。利用ABAQUS软件,建立了一个单剪螺栓连接的三维有限元模型。通过试验证实此模型的有效性之后,采用该模型计算常温和高温下螺栓连接的极限承载力。并且,分析了边缘效应和末端距离对连接承载力的影响。根据EN1990规范,考虑了分项系数,在试验和有限元分析结果的基础上,确定承载力设计值。