Nonlinear behaviour of unprotected composite slim floor steel beams exposed to different fire conditions

An efficient nonlinear 3D finite element model has been developed to investigate the structural performance of composite slim floor steel beams with deep profiled steel decking under fire conditions. The composite steel beams were unprotected simply supported with different cross-sectional dimensions, structural steel sections, load ratios during fire and were subjected to different fire scenarios. The nonlinear material properties of steel, composite slim concrete floor and reinforcement bars were incorporated in the model at ambient and elevated temperatures. The interface between the structural steel section and composite slim concrete floor was also considered, allowing the bond behaviour to be modelled and the different components to retain its profile during the deformation of the composite beam. Furthermore the thermal properties of the interface were included in the finite element analysis. The finite element model has been validated against published fire tests on unprotected composite slim floor steel beams. The time–temperature relationships, deformed shapes at failure, time–vertical displacement relationships, failure modes and fire resistances of the composite steel beams were evaluated by the finite element model. Comparisons between predicted behaviour and that recorded in fire tests have shown that the finite element model can accurately predict the behaviour of the composite steel beams under fire conditions. Furthermore, the variables that influence the fire resistance and behaviour of the unprotected composite slim floor steel beams, comprising different load ratios during fire, cross-section geometries, beam length and fire scenarios, were investigated in parametric studies. It is shown that the failure of the composite beams under fire conditions occurred for the standard fire curve, but did not occur for the natural fires. The use of high strength structural steel considerably limited the vertical displacements after fire exposure. It is also shown that presence of additional top reinforcement mesh is necessary for composite beams exposed to short hot natural fires. The fire resistances of the composite beams obtained from the finite element analyses were compared with the design values obtained from the Eurocode 4 for composite beams at elevated temperatures. It is shown that the EC4 predictions are generally conservative for the design of composite slim floor steel beams heated using different fire scenarios.     

高层钢管混凝土柱-钢梁平面框架抗火性能研究

利用有限元分析软件ABAQUS,对高层钢管混凝土柱-钢梁组合框架不同区域发生火灾的情况进行数值模拟,研究高温下各构件的温度场分布以及不同区域框架受火时的变形情况、耐火极限、应力分布状况。结果表明：在考虑楼板有利作用下,钢梁上翼缘的温度最低;高层建筑的底层变形最大、耐火极限最短、钢管的应力最集中、核心混凝土处于全截面受压状态。     

型钢混凝土框架结构耐火性能有限元分析

为了研究型钢混凝土框架整体结构的耐火性能,为其抗火设计提供参考,采用受火楼层建立精细化有限元计算模型、非受火楼层建立梁单元计算模型的方法,建立了型钢混凝土框架整体结构的耐火性能计算模型。考虑火灾位置、荷载分布形式、柱轴压比等参数的影响,对火灾下型钢混凝土框架整体结构的变形规律、承载机制、破坏形态以及耐火极限进行参数分析。分析结果表明：火灾下框架结构出现了整体破坏和局部破坏两种典型的破坏形态,受火构件受到的约束作用对其耐火性能有较大的影响;在局部破坏形态中,由于受热膨胀,火灾下框架梁首先出现了较大的轴压力,受火框架梁处于压弯受力状态;之后,框架梁出现了悬链线效应,轴力对框架梁的受力状态有较大影响;在整体破坏形态中,根据轴压比及荷载分布形式的不同,框架出现了中柱破坏和边柱破坏两种典型破坏形态,同时,随楼层受火位置的升高,柱的轴压比减小,框架结构的耐火极限增加。     

Structural behaviour during a vertically travelling fire

This study considers a multi-storey composite frame subject to a fire which travels vertically between three floors. Previous work has analysed the behaviour of this structure when subject to simultaneous fires on three floors. It highlighted the importance of the cooling regime adopted and the relative axial stiffness of the steel beams to the overall behaviour of the structure. This paper extends that work by investigating the more realistic case of a vertically travelling fire. Various inter-floor time delays are considered as well as two floor beam sizes. It is found that the inter-floor time delay affects the global behaviour substantially. The behaviour is also in part dependent on the stiffness of the floor beams. Axial forces caused by thermal expansion in individual floors may induce cyclic loading on the column which is not normally considered in structural fire design but may be important in determining structural behaviour. Identifying a worst-case rate of vertical fire spread is not possible due to the range of structural responses, so it is recommended that designers consider several rates of spread and ensure structural integrity for each.     

真实火灾下足尺冷成型钢复合剪力墙结构耐火性能试验研究

冷成型钢复合剪力墙结构采用冷成型钢管混凝土边柱、半刚性连接节点、组合楼盖体系和外填充式夹心组合墙体等构件,具有整体连接性强、抗震性能优越等优势.其耐火性能研究是该结构在我国推广应用的研究重点之一.为此,建造了3栋平面尺寸为7.2m×7.2m、层高3m的单层冷成型钢复合剪力墙结构模型房屋,并对其进行了不同火灾工况下的足尺...     

Fire tests on two-way concrete slabs in a full-scale multi-storey steel-framed building

This paper presents the results from two full-scale furnace tests conducted on two-way concrete slabs supported by composite beams in a three-storey steel-framed building. Each floor of the building consisted of nine panels (three by three) supported by composite beams. In two tests, a corner and an interior panel on the top of the building were heated by two specially designed furnaces respectively. Detailed experimental data in the form of describing slab cracking, the furnace temperatures, temperature distributions within the slab, vertical deflections and horizontal displacements are presented. Comparison of the results for the two tests indicates that the structural fire behaviour of two-way concrete slabs supported by composite beams in a multi-storey steel-framed building is highly dependent on the restraint provided by the adjacent structural members. Observations from the tests indicate that in addition to the extensive cracks formed on the top surface of the heated panels, regular cracks also occurred on the top of the adjacent unheated panels due to structural continuity and the interaction between the concrete slabs and the supporting beams. The test results show that both tested panels had good fire performances even under long duration fire conditions.     

Collapse analysis of regular and irregular tall steel moment frames under fire loading

This study is carried out to evaluate the progressive collapse of steel buildings under fire event. To this end, a 15‐story steel structure with moment‐resisting system and composite floors is considered. The effects of various parameters such as beam section size, gravity load ratio, vertical irregularity of resisting system, and location of fire compartments on collapse modes are investigated numerically. Different temperature‐time curves are defined across the composite floors according to the Eurocode 4. It is found that local collapse of the frames at the ground floor fire is triggered by the buckling of the interior heated columns at approximately 540°C. The redistributed loads by floors delay the global collapse at least 45 min. Increasing gravity loads accelerates the global collapse of the frames significantly. The heated columns of the middle floor buckle at higher temperature compared to the ground floor heated columns and no global collapse occur due to this scenario. In general, the potential of collapse of the regular and irregular frame due to fire in the edge bay is higher compared to the fire in the middle bay. It is also found that the local and global collapse of regular frames occur earlier than irregular frames.     

基于普通计算模型的组合结构抗火设计方法的发展

本主要研究了普通计算模型在结构抗火设计中的应用。计算机模拟为评价承载的组合构件的耐火极限提供了新的可能性。通过三个例子证明了计算机模拟比传统的用抗火试验的方法来进行结构抗火设计要好很多。第一个例子阐述了用于组合柱抗火设计的新的设计表格的形成．第二个例子是关于新型薄板梁体系的应用，最后介绍了一种组合构件抗火设计的新方法。     

Behaviour of composite cellular steel — Concrete beams at elevated temperatures

The behaviour of composite cellular floor beams is becoming important as such members are increasingly used in multistorey buildings. In the event of fire, this issue becomes increasingly critical, particularly for exposed steelwork. In a fire situation, a composite beam has a much higher perimeter area exposed to fire in its lower web-flange section than in the upper web-flange section, and so the temperature distribution across a composite beam is usually non-uniform. The reduction in fire of the strength and stiffness of the material properties of the perforated steel beam, as well as differential thermal expansion, therefore becomes an important influence on the overall behaviour of the composite beam. The objective of this research is to enhance the level of understanding of the generic behaviour of composite cellular floor beams in fire conditions. In this paper, three-dimensional nonlinear finite element models of composite cellular floor beams have been developed, taking into consideration the influence of the changes in material properties with temperature. Experimental data from furnace tests on cellular composite floor beams obtained from previous research work has been used to validate the FE models. An analytical model based on existing design guides is also presented in this paper. It is concluded that finite element analysis results are in good agreement with the experimental data, and all the failure modes have been accurately predicted. The proposed simplified analytical methods show reasonable agreement with the test and FE results, and are always conservative.     

高层钢框架在不同火灾场景下的整体响应分析

利用有限元计算软件ABAQUS模拟高层钢框架结构在火灾作用下的响应.比较4种典型火灾场景下结构的变形及构件的内力变化规律.结果表明:相同规模的火灾下,层数较低的位置发生火灾后构件破坏的危险性更高;火灾沿上部楼层的防火分区蔓延与沿同一楼层其他防火分区的蔓延相比,前者梁的膨胀变形对柱的变形和内力的影响较为突出,对结构产生的...     

Composite steel-framed structures in fire with protected and unprotected edge beams

The structural behaviour of a steel-concrete composite frame subject to a natural fire is analysed using a numerical model. The behaviour is compared when fire protection is applied to only the external beams and when no beams are fire protected. The behaviour of the structure in the two cases is significantly different. When the edge beams are protected the floor slab tends to span in 2 directions because the edge beams provide sufficient support around the perimeter of the floor for tensile membrane action to develop. When the edge beams are unprotected the slab tends to span in only one direction in a manner similar to a beam in catenary action. Catenary action is a weaker load carrying mechanism than tensile membrane action. As a consequence of the weaker mechanism, when the edge beams are unprotected, the columns displace inwards towards the end of the fire indicating the possibility of imminent runaway collapse.The pattern of mechanical strains in the floor slab reinforcement depends on the load carrying mechanism and therefore on whether edge beam protection is included. Although the average mechanical tensile strains are higher when the edge beams are protected the highest strains occur when the beams are unprotected. Conversely, an instability in the primary beam occurs at much lower temperatures when the edge beams are protected.It is concluded that fire protecting the edge beams of the structural layout studied has a number of effects on the fire resistance of the structure, some beneficial, some detrimental, however, in general, fire protecting the edge beams provides an increased level of fire resistance.     

火灾中有保护和无保护边梁的组合钢结构框架

采用数字模型分析了钢混组合框架在火灾中的结构性能。两种情况下结构的性能是显著不同的。对只带防火保护的外梁与没有任何梁受保护两种情况的结构进行了比较,当边梁被保护时,底板趋向两个方向延伸,因为边梁对板周界应力的发展提供了足够的支撑;当边梁没有被保护时,板只朝一个方向延伸,其受力方式类似于悬链线。与拉伸膜相比悬链线梁为较弱的荷载载运机制。这种弱机制的结果是当边梁无保护时,柱位移向内预示出临近坍塌的可能性。混凝土楼板的应变模式取决于其承载机理,因此与边梁是否保护有关。当边梁有保护时尽管平均拉应变较高,但是最高拉应变都发生在边梁无保护时。反之,当边梁有保护时,在很低的温度,原梁就失稳。研究结论是边梁的结构布局在结构防火等方面具有一定的效果,有些有利,有些有害,然而,一般情况下,有保护边梁可以提供较高的防火等级。     

火灾下钢筋混凝土平面框架结构受力机理分析

为研究火灾下钢筋混凝土框架结构的受力机理,采用梁单元建立了钢筋混凝土框架结构耐火性能有限元计算模型,考虑火灾位置、柱轴压比和梁配筋率等参数的影响,对火灾下钢筋混凝土框架结构的变形、内力重分布、破坏形态以及耐火极限进行了参数分析。分析结果表明,火灾下框架结构出现了整体破坏形态和局部破坏形态两种典型的破坏形态：当柱轴压比较小时,框架出现受火梁破坏导致的框架局部破坏形态;当柱轴压比和梁配筋率均较大时,框架出现受火中柱和受火边柱破坏导致的框架整体破坏形态,整体破坏形态为连续性倒塌破坏。在框架局部破坏形态条件下,三面受火梁在框架竖向分布位置不同,受约束作用不同,框架的耐火极限亦不同;而在框架整体破坏条件下,柱轴压比越大,耐火极限越小。     

Whole-building behaviour of bonded post-tensioned concrete floor plates exposed to fire

This paper discusses the whole-building behaviour of post-tensioned concrete floor plates under fire conditions. Based on the results of eight fire tests on one-way spanning bonded post-tensioned concrete slab strips, recently conducted by the authors, a nonlinear finite element model was developed to model an entire typical concrete floor plate. The considered floor plate was post-tensioned using bonded tendons and was supported on traditional reinforced concrete columns. The overall objective of the study was to provide an understanding of the structural response and modes of failure of these floor plates under fire conditions. The mechanical and thermal material nonlinearities of the floor’s components, consisting of the concrete, grout, prestressing tendon, and the anchorages, as well as the reinforced concrete columns, have been considered in the model. The interface between the tendon and grout was also considered, allowing the bond behaviour to be modelled and the tendon to retain its profile shape during the deformation of the floor. The model has been validated against published finite element results on a floor plate under normal temperature conditions. The temperature distribution throughout the floor slab and supporting columns, together with the developed displacement and stress due to heating, and the overall fire resistance of the floor were predicted by the model. Furthermore, the variables that influence the structural behaviour comprising different natural fires, applied static load during fire, use of non-tensioned reinforcement, as well the difference between unbraced and braced frames were investigated in a parametric study. The study has shown that the failure mode of the floor under fire conditions is mainly due to tensile splitting along the tendons that extended to the top surface, while at ambient conditions the mode of failure is punching shear. The restraint provided by shear walls in the considered braced frame and the use of non-tensioned flexural reinforcement affected the vertical displacement behaviour under fire conditions, but did not affect the fire resistance due to the predicted tensile splitting failure mode. From the studies presented it is concluded that the design fire resistance of the floor specified in Eurocode BSEN1992-1-2 is acceptable, while that in the UK code BS8110 is unconservative and should be modified.     

A fire test on continuous reinforced concrete slabs in a full-scale multi-story steel-framed building

To further understand the fire behavior of concrete floor slabs, this paper examines the results of a fire test on continuous concrete floor slabs in a full-scale three-story steel-framed building. The case under experimental study models the reality of fire conditions more closely than previous tests and involves the construction of a special furnace on the building's second floor to heat four panels (two by two) and steel beams on the third floor. The experimental results are investigated in detail and consider the furnace temperature, temperature distribution, vertical and horizontal deflections, and failure patterns of the structural elements during the heating and cooling phases. The testing data indicate that the number and locations of the heated panels in the floor also have a considerable effect on the continuous concrete floor's fire behavior, apart from the boundary constraint conditions provided by the adjacent structural members. In addition, the steel beams exhibit better fire-resistant performance than that observed in standard fire tests depending on their structural integrity and the interaction between structural members. In contrast to its high-strength bolt connections, the building's welded-bolted connections do not cause local buckling of the steel beams subjected to fire.     

Modeling the response of composite beam–slab assemblies exposed to fire

This paper presents the development of a three-dimensional nonlinear finite element model for evaluating the response of composite beam–slab assemblies subjected to a combination of gravity and fire loading. The behavior of typical beam–slab assemblies with different shear connection types (welded–bolted shear tab and all-bolted double-angle connection), exposed to different fire scenarios, was modeled using ANSYS. The finite element model accounts for temperature dependent thermal and mechanical properties of constituent materials, connections, and composite action. Transient time domain coupled thermal-stress analysis is performed to obtain the temperature distribution and deformation response of the composite beam–slab assembly. The finite element model is validated by comparing the predicted and measured thermal and structural response parameters of three composite beam–slab assemblies tested under fire conditions. The comparisons show that the proposed model is capable of predicting the fire response of beam–slab assemblies with good accuracy. Research from the analysis clearly shows that the composite action between the beam and slab significantly enhances the fire performance of composite beam–slab assemblies. It is concluded that the proposed finite element model could be used as a feasible tool to evaluate the fire response of composite floor systems.     

The behaviour of asymmetric slim floor steel beams in fire

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.     

足尺钢框架结构中楼板受火试验研究

采用自制火灾试验炉对足尺钢框架结构中楼板的火灾行为进行试验研究。简要介绍了试验炉的设计、温度及变形测量方案,描述了试验现象以及楼板、钢梁和节点的破坏特征,分析了楼板、钢梁的截面温度分布规律和变形规律,并与相关试验结果进行了对比。结果表明：炉内温度基本均匀,停火时平均炉温约为780℃;升温时沿板厚具有较大的升温梯度,降温时存在降温滞后;升温阶段前期受火钢梁截面存在较大的升温梯度,后期截面温度基本一致;受火板格板顶裂缝模式主要取决于其边界条件,非受火板格裂缝特征取决于受火板格的位置及数量;由于受火组合梁产生反拱效应,受火板格中心位移存在一平缓阶段;因相邻结构约束较强,钢梁具有较好的抗火性能;与高强度螺栓连接节点相比,混合连接节点可有效防止钢梁出现局部屈曲。     

Thermal and structural response of two-storey two-bay composite steel frames under furnace loading

The collapse of the World Trade Center Towers and other recent fires in tall buildings has motivated this study to understand the performance of structural frames under fire loading. Three two-storey, two-bay composite steel frames were constructed and subjected to dead loads by applying weight blocks, and to thermal load by placing the frame in a furnace. The furnace was specially designed to allow for controlled heating of the structural elements that formed the four compartments of the test frame. This paper describes the experimental results of furnace test conducted on the three full-scale composite frames. The three tests differed from each other in the number and location of compartments that were heated by the furnace. For each test, the structural elements were subjected to a heating-up phase followed by a cooling-down phase. The furnace temperatures and the steel and concrete temperatures recorded during the test are presented. The thermally induced horizontal displacements of the columns and vertical deflections of the composite beams are discussed. Observations on local buckling of the steel beam, cracking of the concrete slab and failure of the beam-to-column connections are tabulated. Experimental results of the three tests are compared with each other by studying the complete deformation process of the test frames over time. Results indicate that the deformation process of the test frames was highly dependent on the number and location of compartments that were subjected to thermal loading.