Composite beams in large buildings under fire — numerical modelling and structural behaviour (original) (raw)
Related papers
Fire Safety Journal, 2007
This paper presents a numerical investigation of the thermal and structural results from a compartment fire test, conducted in January 2003 on the full-scale multi-storey composite building constructed at Cardington, United Kingdom, in 1994 for an original series of six tests during 1995-1996. The fire compartment's overall dimensions were 11 m  7 m with one edge at the building's perimeter, using largely unprotected steel downstand beams, and including within the compartment four steel columns protected with cementitious spray. The compartment was subjected to a natural fire of fire load 40 kg/m 2 of timber, in common with the original test series, but the composite slab forming its ceiling was subjected to a uniform applied load of 3.19 kN/m 2 , which is higher than the original. Numerical modelling studies have been performed using the numerical software FPRCBC to analyse temperature distributions in slabs, manual Eurocode 3 Part 1.2 calculations for beam temperatures, and Vulcan to model the structural response to thermal and mechanical loading. These are compared with the quite comprehensive test data, and a series of cases has been analysed in order to develop a comprehensive picture of the sensitivity of the behaviour to different assumed conditions. The comparison between the modelling of basic cases and the test results shows very good correlation, indicating that such modelling is capable of being used to give a realistic picture of the structural behaviour of composite flooring systems in scenario-related performancebased design for the fire limit state. The extended sensitivity studies show the influence of extra protection to the connection zones of primary beams, and the effects of different vertical support conditions at the perimeter of the fire compartment. The effect of incomplete overlapping of the reinforcing mesh in the slab, which is believed to have occurred in one region, is also considered.
Structural Performance of Long-Span Composite Truss and Beam Systems in Fire
Composite truss and composite beam systems have recently been widely applied in longspan multi-storey floor construction. In a fire resistance design according to BS5950 Part 8, two alternative calculation approaches, a limiting temperature method and a moment capacity method, are used to assess the fire resistance of composite structures. The in-fire performance of the structures and insulation materials are required to satisfy a limiting strain level and a simple deflection criterion during the structure's specified fire resistance period. However, it is difficult to estimate the deflection of long span structures at elevated temperatures, due to the significant thermal effects on both the materials and structural mechanics. This project uses the Sheffield FE package 'Vulcan' to investigate numerically the in-fire performance of two types of long span structure which are passively protected against fire.
Structural performance of redundant structures under local fires
Most rules and investigations of the strength of structural members under fire assume that the member acts alone as an isolated structure. This matches the testing of individual members in a standard furnace test. The concept may seem appropriate where fire in a compartment effectively attacks only the individual structural members nearby. However, no account is taken of the interactions which inevitably occur with the surrounding structure. Where the complete structure is large and redundant, these interactions can completely change the structural response and effectively invalidate the design assumptions. This paper discusses the response of a structural element under fire within a highly redundant structure, such as a large building. The behaviour of the element under fire is strongly affected by the restraint provided by the surrounding parts which are not subjected to heating. A number of responses in quite simple structures are shown, to illustrate the roles of expansion, loss of material strength, the relative stiffness of adjacent parts of the structure, development of large deflections, buckling and temperature gradients. These aspects are illustrated with simple examples, and it is shown that there are several counter-intuitive phenomena in structures of this kind. The significance of these findings for the design of large buildings is explored briefly.
Thermal and structural behaviour of a full scale composite building
This paper presents a numerical investigation of the thermal and structural results from a compartment fire test, conducted in January 2003 on the full-scale multi-storey composite building constructed at Cardington, United Kingdom, in 1994 for an original series of six tests during 1995–1996. The fire compartment's overall dimensions were 11 m  7 m with one edge at the building's perimeter, using largely unprotected steel downstand beams, and including within the compartment four steel columns protected with cementitious spray. The compartment was subjected to a natural fire of fire load 40 kg/m 2 of timber, in common with the original test series, but the composite slab forming its ceiling was subjected to a uniform applied load of 3.19 kN/m 2 , which is higher than the original. Numerical modelling studies have been performed using the numerical software FPRCBC to analyse temperature distributions in slabs, manual Eurocode 3 Part 1.2 calculations for beam temperatures, and Vulcan to model the structural response to thermal and mechanical loading. These are compared with the quite comprehensive test data, and a series of cases has been analysed in order to develop a comprehensive picture of the sensitivity of the behaviour to different assumed conditions. The comparison between the modelling of basic cases and the test results shows very good correlation, indicating that such modelling is capable of being used to give a realistic picture of the structural behaviour of composite flooring systems in scenario-related performance-based design for the fire limit state. The extended sensitivity studies show the influence of extra protection to the connection zones of primary beams, and the effects of different vertical support conditions at the perimeter of the fire compartment. The effect of incomplete overlapping of the reinforcing mesh in the slab, which is believed to have occurred in one region, is also considered.
The Ultimate Behaviour of Composite Frames in Fire Conditions (1)
Large-scale fire tests in the UK appear to confirm that, in real buildings, unprotected composite beams have a significantly greater fire resistance than when furnace-tested as isolated members. This is due to interactions between the heated structure within the fire compartment and adjacent cool structure. The computer program Vulcan has been developed at the University of Sheffield to model the behaviour of composite and steel framed buildings in fire. In this paper two large-scale fire tests, with very different degrees of restraint provided by the adjacent structure, are modelled to show how this restraint affects the behaviour within the heated zone. It is evident that the influence of membrane action is important if the integrity of fire compartments is to be maintained. Where high boundary restraint is present the second-order forces caused by geometric non-linearity in the slab within the fire compartment become very significant and eventually dominate the structural behaviour. A large generic composite framed building has also been designed, and a series of analyses have been carried out on fire compartments of different extents and locations, to assist in understanding the interactions between cool and hot zones of the composite structure.
Modelling the Thermal Effects on Structural Components of Composite Slabs under Fire Conditions
Computation
This paper presents a finite-element-based computational model to evaluate the thermal behaviour of composite slabs with a steel deck submitted to standard fire exposure. This computational model is used to estimate the temperatures in the slab components that contribute to the fire resistance according to the load-bearing criterion defined in the standards. The numerical results are validated with experimental results, and a parametric study of the effect of the thickness of the concrete on the temperatures of the slab components is presented. Composite slabs with normal or lightweight concrete and different steel deck geometries (trapezoidal and re-entrant) were considered in the simulations. In addition, the numerical temperatures are compared with those obtained using the simplified method provided by the standards. The results of the simulations show that the temperatures predicted by the simplified method led, in most cases, to an unsafe design of the composite slab. Based on ...
Structural fire performance of earthquake-resistant composite steel–concrete frames
Engineering Structures, 2009
Seismic and fire design of a building structure may be two very demanding tasks, especially if included in a performance based design philosophy. For the time being, the necessary harmonization on the regulations concerning these two design fields is almost missing, thus preventing the effective possibility of an integrated design. Besides, while many countries have already moved towards the use of performancebased codes for seismic design, the application of such methodologies for the fire design of structures is still limited in scope. Within this framework, the development of suitable procedures introducing structural fire performance issues for a comprehensive design methodology is needed. In this paper, a numerical investigation for the assessment of the structural fire performance of earthquake resistant composite steel-concrete frames is presented. With reference to a case study defined in the framework of a European Research Project, a great effort was devoted to the identification of the key structural parameters allowing for a possible correlation between the predictable performances under seismic and fire loadings, when these two are considered as independent actions. At the conceptual design level, the most suitable structural solution with respect to both design actions was chosen, including composite beams and circular steel concrete-filled columns. The frame was designed in order to resist severe seismic action according to the ductile design approach provided by Eurocode 8; the parameters affecting members' sizing were outlined in this phase. Afterwards, the seismic performance of the designed frame was investigated by means of non-linear static analyses; once the seismic performance objectives were met, in order to evaluate the structural fire performance of the whole frame a set of criteria was defined. To this purpose, thermo-mechanical analyses under different boundary conditions were developed and in order to identify the possible mechanisms leading to structural failure, the state of stress at the critical cross-sections at different times of fire exposure was investigated. Another point of main concern was represented by the assessment of the influence of different restraining conditions on the achieved fire resistance rating and kind of structural failure. Moreover, the proposed methodology allowed making an estimate of the amount of axial restraint provided to the heated beams by the surrounding structure; in this view, the importance of choosing column elements in function of their flexural stiffness was revealed, in order to correlate it with the predictable performances under both seismic and fire loadings.
Analytical prediction of composite beams response in fire situations
Journal of Constructional Steel Research, 2007
In this paper an extension of the method of the Fourier series expansion to the fire analysis of composite beams is presented. In particular the extension concerns the introduction of the temperature dependent interaction of all the components: steel beams, concrete slab and steel connectors. These last are considered of finite stiffness, and a proper account is given to the combined effect of thermal degradation of the properties, and stress amplification caused by the differential thermal expansion across the interface. The proposed method compares very well with some experimental fire tests of simply supported and framed composite beams. Due to its relative simplicity and speed, it can be used for design purposes in evaluating the critical temperature in terms of critical deflection. Finally we recall that the method is capable of dealing with every type of fastening distribution, such as discontinuous or variable length.
Behaviour of reinforced concrete structures in fire
2006
In the past two decades, a significant amount of research has been conducted into the performance of composite steel-framed structures in fire. However, the same level of development has not taken place for other forms of construction. In terms of reinforced concrete construction, design is still based on simplistic methods which have been developed from standard fire tests that do not necessarily represent real building behaviour. This makes it very difficult, if not impossible, to determine the level of safety achieved in real concrete structures, or whether an appropriate level of safety could be achieved more efficiently. In this study detailed analyses of a reinforced concrete structure subject to a standard fire regime are carried out. The building is designed to Eurocode 2 and represents a commercial office building. In order to study the interactions between the cool and hot zones of the structure, a series of analyses has been carried out for different extents and positions of localised fire compartments. It is clear that adjacent cool structure provides considerable restraint and continuity, increasing the fire resistance of the structure within the fire compartment. Relatively small areas of tensile membrane force are formed within the concrete slabs, and large areas are subject to compressive membrane action during the fire. As a result the downstand concrete beams experience enhanced tension during the fire, especially in the early stages, which is mainly carried by their tensile reinforcement. It is therefore very important to keep the temperature of beam reinforcement within certain limits. Eventual structural collapse in the studies is always due to column failure, and it is clear that the performance of columns is vitally important to the survival of reinforced concrete buildings in fire.