加热和冷却过程中约束复合板的试验及数值研究

在曼彻斯特大学,对约束复合板进行一系列耐火试验。在不同火灾场景下,对6块不同荷载比下的复合板进行试验,观察加热和冷却过程中板内力的变化。设计试验方案,建立两种不同的非线性有限元模型,模拟复合板在加热和冷却过程中的热学和力学性能。在热分析模型中,采用平面单元模拟。在结构分析中,混凝土、钢板、锚钉分别采用实体单元、壳单元、桁架单元模拟。混凝土和钢板间的连接简化为弹簧单元。根据试验结果和有限元计算结果,详细分析了复合板的性能。最后,进行参数研究,分析了混凝土强度、钢板厚度和锚钉尺寸的影响。     

Experimental behaviour of composite slabs during the heating and cooling fire stages

Most theoretical and experimental research investigating the effect of fire on structures has previously concentrated only on the structural behaviour during the heating stages of the fire, partly due to the fact that internationally accepted standard fire tests only consider this stage of the fire. Evidence from real fires in real buildings has highlighted that the cooling phase of a fire is equally important and it is possible for structures to fail during this stage of the fire even though they have survived the heating stage up to a maximum fire temperature. This paper provides an insight into the behaviour of composite slabs under different fire scenarios considering both the heating and cooling phase of the fire. Extensive test data is presented which shows the redistribution of moments and strains in the deck and steel mesh, together with displacements during the full duration of the fire. The results show that the behaviour of composite slabs is dependent on the heating rate, the maximum temperature reached and the cooling rate. In terms of overall performance, displacements and the temperature on the non-fire side of the slab are important. For the tests presented in this paper it was shown that one fire scenario resulted in the maximum displacement but another fire scenario resulted in the maximum temperature on the unexposed face. In addition the maximum temperature of the unexposed side of the slab and the mesh reinforcement within the slab occurring during the cooling stages of the fire. This highlights the fact that the performance of structures must be checked in design under a range of possible fire scenarios, which must include both the heating and cooling stages of a fire.     

压型钢板组合楼板耐火性能的试验研究

本文通过对５块规格不同的压型钢板组合楼板的耐火试验，研究组合楼板在１５～２０小时的耐火时限中的背温和变形发展规律。试验结果表明，由于压型钢板与混凝土的共同工作，压型钢板本身不会独自升温，且混凝土具有较好的吸热和散热功能，所以能保证压型钢板组合楼板的防火安全性。     

Experimental study on semi-rigid composite joints with steel beams and precast hollowcore slabs

The concept of semi-rigid composite connection has been widely researched in the past; however, most of the researches are limited to composite joints with metal deck flooring and solid concrete slabs. Composite construction incorporating precast concrete hollowcore slabs (HCU) is a recently developed composite floor system for buildings. The research on the structural behaviour of the semi-rigid composite joints with HCU is new and without any previous experimental database. In this paper, eight full-scale tests of beam-to-column semi-rigid composite joints with steel beams and precast hollowcore slabs are reported. The variables are stud spacing, degree of the shear connections, area of the longitudinal reinforcement and slab thickness. The test set-up and instrumentation is described in detail. The experimental behaviour is analysed and based on the test data the structural behaviour of these semi-rigid composite joints is discussed. Based on the experimental data, a simplified method to predict rotation and moment capacity for this type of composite connection is proposed.     

Experimental investigation on thermal and mechanical behaviour of composite floors exposed to standard fire

This paper presents experimental investigations on the thermal and mechanical behavior of composite floors subjected to ISO standard fire. Four 5.2 m×3.7 m composite slabs are tested with different combinations of the presence of one unprotected secondary beam, direction of ribs, and location of the reinforcement. The experimental results show that the highest temperature in the reinforcements occurs during the cooling phase (30–50 °C increment after 10-min cooling). The temperature at the unexposed side of the slabs is below 100 °C up to 100-min heating, compared to the predicted fire resistance close to 90 mins from EC4. For the slabs without secondary beams, the cracks first occur around the boundaries of the slab, while for the slabs supported by one unprotected secondary beam, concrete cracks first occur on the top of the slab above the beam due to the negative bending moment, and later on develop around boundaries. Debonding is observed between the steel deck and concrete slab. The secondary beam significantly impacts the deformation shape of tested slabs. Although a large deflection, 1/20 of the span length, is reached in the tests, the composite slabs can still provide sufficient load-bearing capacity due to membrane action. The occurrence of tensile membrane action is confirmed by the measured tensile stress in the reinforcement and compressive stress in the concrete. A comparison between measured and predicted fire resistance of the slabs indicates that EC4 calculations might be used for the composite slabs beyond the specified geometry limit, and the prediction is conservative.     

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.     

Enhancing the fire resistance of composite floor assemblies through the use of steel fiber reinforced concrete

This paper presents a strategy for achieving the required fire resistance in composite floor systems through the use of steel fiber reinforced concrete (SFRC). Both experimental and numerical studies were carried out to evaluate the fire performance of floor systems comprising unprotected steel beams and concrete/SFRC deck slabs. The results from these studies show that SFRC composite deck slabs develop significant tensile forces (through tensile membrane action) that transfer load from fire-weakened steel beams to other cooler parts of the structure. Preliminary results indicate that the combined effect of composite construction, tensile membrane action, and the improved properties of SFRC under realistic fire, loading, and restraint conditions can provide sufficient fire resistance in steel beam-concrete deck slabs without the need for external fire protection on the floor assembly.     

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

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

Zur Frage der Nachnutzbarkeit verbundlos vorgespannter Stahlbetondecken nach Brandeinwirkung

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

Performance of CFST column to steel beam joints subjected to simulated fire including the cooling phase

This paper presents the experimental results of three concrete filled steel tube (CFST) column to steel beam joints with reinforced concrete (RC) slabs under combined loading and fire including the heating and cooling phases. The test parameters include heating time and thickness of the fire protection material. Temperatures and deformations of the joint specimens during the heating, cooling and post-fire phases were measured. A finite element analysis (FEA) model to simulate the action of a CFST column to steel beam joint under combined loading and fire is developed. The FEM model was verified by the experimental results. The FEA model is then used to analyse the temperature distribution in the heating and cooling phases. The moment versus relative rotation angle between the CFST column and the beam under combined loading and fire including the heating and cooling phases is also discussed.     

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.     

钢-混凝土组合楼板实用抗火设计方法

利用钢-混凝土组合楼盖抗火性能非线性有限元分析程序对不同荷载大小、不同压型钢板厚度、不同配筋的压型钢板-混凝土组合楼板进行了抗火性能分析,找出了火灾影响下组合楼板结构性能的因素,给出了计算组合楼板耐火时间的拟合公式,得到了一些适用于压型钢板-混凝土组合楼板抗火设计的结论和建议。     

The tensile membrane action of unrestrained composite slabs simulated under fire conditions

Following a major fire test programme on a full-scale, steel-framed building it was found that the composite flooring system, comprising lightweight concrete, anti-crack mesh reinforcement and steel deck, had a greater inherent fire resistance than suggested by current codified design methods. It was felt that this was due to tensile membrane action occurring in the slab at large displacements. This led to an independent test being conducted at the Building Research Establishment where a 9.5 m×6.5 m composite slab, with nominal horizontal restraint to its edges, was tested to failure. To simulate the behaviour of the slab in fire, the steel deck was removed, leaving the concrete and anti-crack reinforcement, before load was applied. Tensile membrane action was shown to occur, with the failure load being approximately double that calculated using the classic yield line theory.     

开孔薄壁钢梁-轻骨料混凝土装配式组合楼板受火试验研究

为研究由冷弯卷边H形开孔薄壁钢梁和预制轻骨料混凝土板形成的装配式组合楼板的受火性能,对6个两端简支组合楼板试件开展了高温下试验研究,得到了试件的温度及位移响应。研究结果表明：在等效均布荷载2.0 kN/m²及最高炉温为600℃的烃类火灾作用下,以轻骨料混凝土及腹板开孔薄壁钢梁形成装配式组合楼板具有良好的抗火性能,组合楼板的跨中挠度最大为L/44（L为组合楼板跨度）时,试件仍保持良好的整体性;主钢梁腹板开孔直径与分布形式对组合楼板的整体抗火性能均有显著影响,为减小组合楼板高温下的位移响应,对主钢梁腹板的开孔宜优选孔径大、数量少的分布形式;抗剪键数量对组合楼板抗火性能的响应影响远大于对预制板类型的影响。     

Non-linear structural modelling of a fire test subject to high restraint

The computer code VULCAN has been developed for the three-dimensional structural analysis of composite and steel-framed buildings in fire. In this paper, the main features of the program are outlined, with particular emphasis on the most recent development to the layered procedure for modelling of concrete floor slabs. This development has introduced geometric non-linearity into the modelling of slabs, whose layer structure already allowed temperature distributions and change of material properties through the thickness, as well as modelling the effect of the ribs at the bottom of composite decking slabs. The capabilities of the model are firstly tested at ambient temperature for a uniformly loaded ribbed reinforced concrete slab with simply supported edges, and this is followed by a very detailed modelling of the Cardington restrained beam fire test. In both cases the development of membrane action is demonstrated and the structural behaviour is compared with the geometrically linear case. A number of studies are carried out to demonstrate the influence of the major floor slab details on the behaviour of the structure in fire conditions. These studies provide evidence that when exposed steel temperatures are less than 400°C the concrete slab has little influence, other than to play a part in generating thermal curvature to composite beams. For temperatures higher than about 500°C the effect of the slab progressively becomes much greater, and it is very important to model concrete slabs correctly. The influence of membrane action cannot be ignored, particularly when the fire compartment is subject to high restraint because it is surrounded by cool, stiff structure. At very high temperatures the floor slab becomes the main load-bearing element and the floor loads above the fire compartment are carried by the membrane forces developed in the slab, with tension being carried mainly by the steel anti-cracking mesh or reinforcing bars. However, the effect of the very high in-plane restraint to thermal expansion in the particular Cardington test considered is to enhance the peripheral zone of compressive membrane force and to reduce the extent of the central area of tensile force compared with more usual cases.     

Behaviour of concrete floor slabs at ambient and elevated temperatures

This paper presents the latest developments of a simple design method used to predict the membrane action of unrestrained concrete, or concrete and steel composite, floors under fire conditions. The developments include the refinement of the assumed in-plane stress pattern and a prediction of when concrete crushing occurs in the proximity of the corners of the slab. The design method is compared against 44 small-scale tests carried out at ambient and elevated temperatures on horizontally unrestrained slabs with an aspect ratio of 1.0 or 1.55. The slabs were reinforced with either mild steel or stainless-steel welded mesh of different grades, ductility, sizes and bar spacing. Both the ambient and fire tests highlighted the occurrence of membrane action, either supporting loads higher than the theoretical yield-line load in the case of the ambient tests, or reaching higher failure temperatures compared to those calculated based on yield-line theory. Comparison between the developed simple design approach and test results showed good correlation both at ambient and elevated temperatures.     

Experimental Study on Fire Behavior of Steel–Concrete Composite Cellular Beams with Large Opening Ratio

The aim of this study is to examine the behavior of protected and unprotected steel–concrete composite I-beams with large cell diameters (D₀/H?=?0.7) and closely spaced cell configurations under the ISO 834 fire curve. Previous studies on experimental full-scale fire performances of cellular beams have been somewhat limited under vertical service loads and different insulation properties. To address this limitation, a total of four composite beams, two unprotected (one beam with a solid web, and one cellular beam) and two protected cellular beams (60 min fire resistance with implementation of water and solvent-based intumescent coatings) were tested. As outputs of the tests, the failure modes observed, such as web buckling, the Vierendeel effect, the slab behavior, including the mechanism of concrete cracking, the overall displacement behavior (i.e., deflected shapes) up to collapse at very large deflections, and temperature changes in the steel elements are discussed. It was concluded that the quality of the intumescent coating applied is crucial in achieving the desired fire resistance. Experiments showed that unprotected trapezoid deck voids did not have a decisive influence on the behavior of the beams for up to 60 min of fire testing. At high temperatures, similar crack patterns occurred in the composite slabs of the protected and unprotected steel–concrete composite cellular beams. In the protected beams, the behavior of reactive coatings was significant and resulted in a non-uniform temperature distribution in these beams’ web and flanges.

     

对组合钢梁-预制空心板中栓钉连接件的性能分析

在钢-混凝土组合梁中,剪切连接件的机械作用使得纵向剪力转移到钢翼缘/混凝土板的接合处。这种转移能力取决于剪切连接件的强度和混凝土板抵抗由剪力高度集中所导致的纵向开裂的能力。大多数对组合结构的分析都集中在传统的钢筋混凝土和金属面板结构,而对预制空心板中栓钉的剪切能力研究很少。本文对带栓钉的预制空心板组合梁结构提出标准的推力试验方法。一共进行7组测试性试验,试验结果显示新方法符合钢筋混凝土楼板规范要求。在确定这个新的标准方法后,对栓钉进行了72个足尺推力试验,确定了该类型连接件的性能,通过分析试验结果,还指出了各类参数对连接件强度和延性的作用。本文还提出了这种剪力连接件的设计公式。     

钢-混凝土组合结构抗火性能研究与应用

钢-混凝土组合结构抗火性能研究是当前的研究热点之一,国内外学者对此展开大量试验研究和理论分析。通过介绍国内外研究者在组合板、组合梁、组合柱等构件以及结构节点和结构体系抗火性能方面研究概况,分析了我国现有防火设计规范的特点以及工程应用情况,指出现有研究的不足,对组合结构抗火研究领域在高温材料热-力耦合本构关系、计算理论、数值火灾试验和设计方法等方面需进一步研究的工作进行了展望。文中指出,建立考虑升降温、多轴应力状态、不同加卸载路径的钢材和混凝土热-力耦合本构关系,建立基于整体性能、考虑升降温全过程的结构抗火分析理论,建立整体结构数值火灾试验方法,提出“三水准”结构抗火设计与灾后结构损伤评估原则以及基于时变可靠度和结构整体性能的组合结构抗火设计方法是钢-混凝土组合结构抗火性能研究的关键科学问题。表2参119     

Finite element investigation of the structural behaviour of deck slabs in composite bridges

This research studies the structural behaviour of bridge deck slabs under static patch loads in steel–concrete composite bridges and investigates compressive membrane action (CMA) in concrete bridge decks slabs, which governs the structural behaviour. A non-linear 3D finite element analysis models was developed using ABAQUS 6.5 software packages. Experimental data from one-span composite bridge structures are used to validate and calibrate the proposed FEM models. A series of parametric studies is conducted. The analysis results are discussed and conclusions on the behaviour of the bridge decks are presented.