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.     

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.     

A connection element for modelling end-plate connections in fire

In this paper a robust 2-noded connection element has been developed for modelling the bolted end-plate connection between a steel beam and column at elevated temperatures. The connection element allows the element nodes to be placed at the reference plane with offset and the non-uniform temperature distributions within the connection. In this model the connection failure due to bending, axial tension, compression and vertical shear are considered. The influence of the axial tensile force of the connected beam on the connection is also taken into account. This model has the advantages of both the previous simple and component-based models. A total of 23 fire tests were used to extensively validate the model. It can be seen that the current model is robust and has a capability to predict the behaviour of a bolted end-plate connection under fire attack with reasonable accuracy. Compared to the tested results the predictions of the current model were mainly on the conservative side. Hence, the model can be used for structural fire engineering design on steel-framed composite buildings. The idea described in this paper can also easily be applied to develop other kinds of connections, such as simple connections, column based connections or hollow section connections, and so on.     

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.     

Analytical and experimental studies on composite slabs utilising palm oil clinker concrete

The utilisation of waste materials in the construction industry is an effective way to sanitise the environment and reduces the cost of construction. In this research, palm oil clinker (POC) aggregates was used to fully replace normal aggregates to produce structural lightweight concrete. This concrete was used in the construction of composite slabs with profiled steel sheet. A total of eight full scale composite slabs, six palm oil clinker concrete (POCC) slabs and two conventional concrete slabs were constructed and tested in accordance to Eurocode 4: Part 1.1 and BS 5950: Part 4: 1994. Two shear spans were used, 450 mm for short shear span and 900 mm for long shear span. The structural behaviour of the slabs was investigated and compared. The horizontal shear-bond strength between the concrete and the steel was determined according to two methods; m–k and partial shear connection methods. Test results show that the structural behaviour and the horizontal shear-bond strength of the POCC slabs are nearly similar to the conventional concrete slabs. The mechanical interlock (m) and the friction (k) between the steel and the concrete are 117.67 N/mm² and 0.0973 N/mm², respectively and the design horizontal shear-bond strength using m-k and PSC methods is 0.248 N/mm² and 0.215 N/mm², respectively. The difference between the two methods is 13.3%. POCC is therefore suitable to be used for structural applications with a reduction in weight of 18.3% compared to conventional concrete composite slabs.     

Behaviour of precast concrete floor slabs exposed to standardised fires

This paper quantifies the thermal movements of 14 simply supported precast reinforced concrete floor slabs of 4.5 m span and 900 mm width exposed to two standardised heating regimes used in fire resistance furnace tests. The tests were designed to show the effect of varying the slab thickness, type of concrete, imposed load, soffit protection and nature of fire exposure on the mid-span flexural deflection and axial movements of the slab ends. Measured deflections showed that during the 90 min design period of fire resistance thermal bowing was dominant and the effect of the 1.5 kN/m² design imposed load was small. The NPD hydrocarbon fire exposure caused a doubling of the flexural deflections achieved using the standard BS 476: Part 8 (now Part 20) fire exposure in the first 20 min of exposure.     

Rehabilitation of splice connections of wood trusses with FRP composites

Glass fibre reinforced polymer (GFRP) composite laminates were investigated for improving the tensile capacity of joint splice connections in the chords of the arch wood trusses of the roof of a historic structure, as part of an overall seismic rehabilitation. Specimens consisting of two 76 mm × 305 mm butt ended heavy dimension lumber members with GFRP laminate connections were tested in tension to simulate the chords of the wood trusses of the historic structure. The GFRP-reinforced splice connections were of two types: Type I had the composite applied to only one 305 mm wood face due to accessibility restrictions; Type II had the composite applied to the two 76 mm wood faces. Three different GFRP composite materials were used with either clamped or mechanically fastened splice connections. Splice connections with intentional offsets were also tested. The GFRP-reinforced splice connections provided a significant improvement of the tensile capacity for resisting the combined gravity and seismic loads. The rehabilitation method was considered to be successful and it was determined that implementation would reduce the overall seismic vulnerability of the historic structure. As a result, the method was implemented to connect the chord joint splices of the roof arch wood trusses of the structure.     

Punching shear tests on flat concrete slabs exposed to fire

Underground parking structures often consist of flat slabs connected by columns, for which punching shear is often the most critical design criterion. In fire conditions, the punching load can increase due to restraint of the thermal curvature of the slab or due to the expansion of the columns. This increase of the punching load is discussed in the paper by means of a literature review. On the other hand, during fire the punching resistance of the slab decreases due to a gradual reduction of the material properties. This reduction in bearing capacity is studied by means of real scale fire tests, consisting of 6 slabs measuring 3.2×3.5×0.25 m with a connected column stub and tested for punching shear with a specially designed loading frame. Two reference tests are executed at ambient temperature conditions and four slabs are submitted to ISO 834 curve for 120 min. Comparison of the test data with the expected increased axial load due to thermal restraint found in the literature, shows a potential danger for premature punching failure of flat slab-column connections exposed to fire.     

Effect of Aspect Ratio on Fire Resistance of Hollow Core Concrete Floors

Several factors can affect the behaviour of the hollowcore floor slabs in fire. This paper analytically investigates the relationship between the fire resistance of the overall floor system and the floor length to width ratio as well as the type of side supports. The study uses beam grillage and shell elements to model the hollowcore slabs and the topping concrete under the platform of the non-linear finite element program, SAFIR. Different methods to model precast, prestressed concrete hollowcore floor slabs subjected to fire are also investigated. The results show that side supports can enhance the fire performance of hollowcore floor slabs provided that the spacing of the side supports does not greatly exceed the span length.     

Numerical modelling of light gauge cold-formed steel compression members subjected to distortional buckling at elevated temperatures

Fire safety design of building structures has received greater attention in recent times due to continuing loss of properties and lives during fires. However, fire performance of light gauge cold-formed steel structures is not well understood despite its increased usage in buildings. Cold-formed steel compression members are susceptible to various buckling modes such as local and distortional buckling and their ultimate strength behaviour is governed by these buckling modes. Therefore a research project based on experimental and numerical studies was undertaken to investigate the distortional buckling behaviour of light gauge cold-formed steel compression members under simulated fire conditions. Lipped channel sections with and without additional lips were selected with three thicknesses of 0.6, 0.8, and 0.95 mm and both low and high strength steels (G250 and G550 steels). More than 150 compression tests were undertaken first at ambient and elevated temperatures. Finite element models of the tested compression members were then developed by including the degradation of mechanical properties with increasing temperatures. Comparison of finite element analysis and experimental results showed that the developed finite element models were capable of simulating the distortional buckling and strength behaviour at ambient and elevated temperatures up to 800 °C. The validated model was used to determine the effects of mechanical properties, geometric imperfections and residual stresses on the distortional buckling behaviour and strength of cold-formed steel columns. This paper presents the details of the numerical study and the results. It demonstrated the importance of using accurate mechanical properties at elevated temperatures in order to obtain reliable strength characteristics of cold-formed steel columns under fire conditions.     

Interactive fire spread simulations with extinguishment support for Virtual Reality training tools

Virtual Reality training for fire fighters and managers has two main advantages. On one hand, it supports the simulation of complex scenarios like big cities, where a fire cannot be simulated in the real world. On the other hand, fire fighting VR simulators allow trainees to experience situations as similar as possible to real fire, reducing the probability of accidents when they are practising exercises with real fire.The success of the Virtual Reality training tools also depends on how close to reality the simulation process is. This work provides fire spread algorithms for forest and urban environments, which can be used at interactive rates. Due to the interactive nature of the algorithms, the users are able to fight the fire by throwing extinguishing agents.Although the algorithms assume many simplifications of the problem, their behaviour is satisfactory. This is due to the efficient management of the cell states in a 3 m×3 m cell grid. Also the variables that have more influence on fire propagation constitute the core of the algorithms. The overall system deals with user extinguishment actions, natural and artificial firebreaks, variable wind conditions (even at a cell level) and non-contiguous fire propagation (embers and spotting fires). The unified forest/urban model leads to an object oriented architecture which supports the fire propagation algorithms. This also allows the system to compute efficiently mixed forest–urban environments.     

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.     

有单（双）向面内约束力混凝土板火灾行为机理分析#br#

为考查面内约束作用对混凝土双向板火灾行为影响,对八块面内约束混凝土试验板进行数值模拟和对比分析,研究了几何(非)线性和混凝土膨胀应变对约束试验板变形、弯矩分布和薄膜机理影响规律。同时,开展参数分析,研究约束类型、约束水平、长宽比、配筋率和板厚等对面内约束混凝土板温度场、变形、破坏模式和耐火极限等影响。结果表明,对任一约束工况,较大混凝土膨胀应变导致约束板较大变形,且几何非线性影响不可忽略;面内约束作用倾向于降低跨中弯矩和增大板边负弯矩,且不利于板大变形阶段受拉薄膜效应发展;约束板火灾行为取决于约束类型、约束水平和长宽比等相互作用;增加板厚和配筋率有助于提高约束板抗火性能。     

Effectiveness of vertical barriers in preventing lateral flame spread over exposed EPS insulation wall

Insulation panels made of organic, combustible materials are frequently used in the exterior thermal insulation systems (ETIS) for buildings. Such combustible insulation panels have been involved in several catastrophic building fires in recent years in China. One potential strategy to mitigate this fire hazard is to limit fire spread over the ETIS. The present work evaluates the effectiveness of vertical fire barriers in inhibiting fire spread over exposed insulation walls made of expanded polystyrene (EPS) panels. Reduced-scale experiments were carried out indoors using EPS panels with or without two vertical barriers made of non-combustible mineral wool, the fire started at the bottom center of the middle panel. The interval and width of the barriers were varied systematically, while the temperature distribution on the wall, the radiation heat flux from the fire, and the infra-red (IR) images were recorded. To demonstrate the validity of the concept, an outdoor, full-scale experiment was carried out using a 7-floor building. Our reduced-scale experiments showed that the installation of two vertical fire barriers successfully stopped the lateral flame spread, decreasing the peak temperatures of the two side panels by about 300 °C for all barrier configurations tested. When barrier width was fixed at 5 cm, an increase of the barrier interval from 30 to 90 cm led to increases in the peak temperatures, radiation heat flux, and the maximum rate of upward flame spread. By contrast, when barrier interval was fixed at 90 cm, an increase of the barrier width from 2 to 5 cm had little influence on the combustion dynamics of the middle panel but the peak temperature on the side panels dropped, consistent with the smaller heat transferred with wider fire barriers. In the regions of the side panels next to the barriers, pyrolysis and deformation could be observed with barrier widths of 2 and 3 cm, but not 5 cm. Finally, our outdoor, full-scale experiment demonstrated that a 30 cm wide vertical barrier made of air-filled cement successfully stopped the lateral flame spread over exposed EPS wall. The study highlights the effectiveness of vertical fire barriers in preventing the lateral flame spread over the exposed EPS insulation wall and provides another option for enhancing the fire safety of the combustible insulation systems.     

Behaviour of composite beam-column flush end-plate connections subjected to low-probability, high-consequence loading

This paper aims to investigate the performance of beam-column flush end-plate connections when using blind bolts. The paper studies the strength on medium rise buildings in regions of medium to high seismicity. This paper presents the result of both experiments and finite element analysis on composite beam-column flush end-plate connections subjected to low-probability, high-consequence loading. The earthquake actions were determined using the earthquake code, AS1170.4, Standard Australia (2004) [44]. Considerable savings were apparent when moving from a pinned connection to a semi-rigid connection. Not only in terms of weight savings, but more importantly the ability to reduce the floor-to-floor heights, which can considerably reduce the cost of services and facades and subsequently mass and inertia of the structure. This can also result in more lettable floor space due to reduced column sizes and the possibility of adding another floor for the same total height compared with the pinned connection. The experimental studies and finite element analysis enables improvement to the design of the connections to be used in engineering practice.     

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.     

Brandverhalten von Hohlkastendecken aus Holz

Fire behaviour of timber slabs made of hollow core elements. This paper presents an overview of experimental tests on the fire behaviour of timber slabs made of hollow core elements. The fire tests aim at supplying basic data and information for an extended safe use of timber structures, particularly in multi‐storey buildings. A series of large scale fire tests on timber slabs was performed by the Institute of Structural Engineering of the Swiss Federal Institute of Technology Zurich in order to enlarge the experimental background of the design methods for a fire resistance of 60 and more minutes. Further a series of small scale fire tests permitted to investigate the fire performance of different types of joints between the timber elements, the influence of acoustic perforations as well as the fire behaviour of hollow core elements filled with different insulation materials.     

火灾下钢结构楼板的薄膜作用

通过对真实火灾中的足尺火灾试验和观察显示，合组合楼板和承载钢梁的建筑物的结构承载力比现行杭大设计方法的建议值高出许多。因此规范中规定所有承载钢梁都要添加被动防火保护是不必要的。现行设计方法和实际结构性能之间产生这种差异是由于设计方法中忽略了楼板的薄膜作用。本根据国外有关资料给出了几种简单计算方法，允许在钢结构杭大设计中考虑楼板的薄膜作用。从而可以更精确地评估火灾下建筑物的真实承载能力，在给定的耐火时间内能减少相当数量钢梁的防火保护。     

A numerical and experimental analysis of unbonded brickwork panels laterally loaded

The recent earthquakes occurred in Italy (Umbria and Marche 1997 and Molise 2002) have shown that the collapse of masonry buildings is mostly due to out-of-plane actions, caused, for instance, by an inadequate interlocking between perpendicular walls. In this paper, a combined numerical–experimental analysis on in-scale unbonded masonry panels out-of-plane loaded is presented. Three different Series (A, B and C) of mortarless walls subjected to in-plane compressive loads and concentrated out-of-plane actions are considered. Series A is restrained at two perpendicular edges and subjected to an eccentric out-of-plane load, whereas Series B and C are constrained at the base and on both vertical edges and are subjected to an eccentric (Series B) or centered (Series C) out-of-plane load. Experimental tests are repeated in presence of three different vertical membrane compressive loads equal to, respectively, 7, 10 and 13 kN. Two different numerical models, relying in an upper bound heterogeneous FE limit analysis and in a FE homogenized limit analysis approach, are employed to have a better insight into the experimentation conducted. The combined numerical–experimental information obtained shows that anisotropic masonry behaviour and vertical compressive loads are key aspects to consider to have a reliable prediction of both collapse loads and failure mechanisms.     

火灾下钢结构建筑楼板的薄膜效应机理及理论模型

通过试验及火灾实例发现,建筑结构中的钢筋混凝土楼板或压型钢板组合楼板在高温下能产生很大变形,进而产生薄膜效应,能大大提高楼板的抗火性能。因此,在结构防火设计中正确考虑薄膜效应的影响,对发挥楼板的抗火潜能,降低结构抗火成本,有重要意义。本文论述了火灾下楼板的薄膜效应产生的机理,建立了分析模型,将楼板划分为5部分,包括周边4个刚性板块和中间一呈椭球面的板块。分别对板块的内力和弯矩进行了分析,然后根据板块内力及弯矩的平衡方程及位移协调方程,推导出了火灾下考虑薄膜效应后楼板极限承载力的计算公式,为考虑薄膜效应时的楼板抗火设计提供了参考。