Fundamental principles of structural behaviour under thermal effects (original) (raw)

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.

Model Studies Of Composite Building Frame Behaviour In Fire

Fire Safety Science, 1994

A series of analytical studies is presented on the behaviour of an unprotected plane composite steel-concrete frame under fire scenarios which occur across single storeys of the structure. The main purpose is to assess the effectiveness of using various types of sub-assembly in predicting the structural behaviour in fire. The studies are based on modelling a full-scale experimental multi-storey frame in which some fire testing is to take place shortly, and in part originate from a programme of analyses in which the authors have participated whose aim has been to determine the test parameters required. For the basic modelling studies the frames and subframes are assumed to be rigidly connected, but the effect of the semi-rigidity of real connections is also investigated. The analyses are all performed using a program NARR2, whose most recent development has been the capability to take into account strain reversal whenever it happens. This allows an assessment of the residual effects on the frame members after a local fire has been extinguished and the frame has returned to ambient temperature.

Behaviour of Light Weight Composite Trusses in Fire-a Case Study

2007

On September 11 th 2001, the twin towers of the World Trade Center in New York City were struck by two hijacked airplanes. Despite severe local damage induced by the impact, the towers were able to sustain 102 and 56 minutes of the subsequent multi-storey fires before collapsing. The purpose of this study is to contribute to the understanding of the in-fire performance of composite trusses by examining the behaviour of the longer-span type used in the towers. It makes no attempt to be a forensic study of the actual events. Using the finite element package Vulcan, the structural mechanics of typical long-span composite floor trusses are explained, under a variety of scenarios, as the fire temperatures rise. Different boundary conditions, degrees of protection and loading are all covered, the results being presented mainly in the form of graphs of deflection and internal force of members against time.

Composite beams in large buildings under fire — numerical modelling and structural behaviour

Fire Safety Journal, 2000

A good engineering assessment of the "re safety of a building structure should be based on a sound understanding of the mechanics of its behaviour under "re. Existing standards and methods of design for "re assume that the structural behaviour is e!ectively the same as that at ambient temperature, allowing for the reduced material properties. This simple assumption is valid for statically determinate structures, but is in serious error for highly redundant structures, and may be unconservative in certain cases. In particular, the e!ect of thermal expansion is generally ignored, even though it may swamp the e!ects of all other phenomena in a large highly redundant building under a local "re. This paper presents some of the results of an extensive investigation (Usmani et al., DETR-PIT project, "nal report (draft), March 2000) in which the structural action in a two-way slab and composite beam structure subjected to a compartment "re has been explored. These results show that thermal expansion dominates the response of highly redundant structures under local "res, and that local yielding and large de#ections can be bene"cial in reducing damage to the complete structure. However, it is now clear that explicit cognisance should be taken of thermal expansions in design calculations, but this can only be done when a thorough understanding of the behaviour, appropriately generalised, is in place. This is the main motivation behind the results presented in this paper.

Non-Linear Modelling of Steel and Composite Structures in Fire

A computer program Vulcan has been progressively developed for some years at the University of Sheffield, with the objective of enabling three-dimensional modelling of the behaviour of composite buildings in fire. In this paper the current theoretical basis of the program is very briefly outlined. Three of the fire tests carried out in 1995-96 on the composite frame at Cardington, representing cases in which different degrees of in-plane restraint are provided by the adjacent structure, are modelled to show how this restraint affects the structural behaviour within the heated zone. In order to illustrate the influence of membrane action and its relationship with boundary restraint, all cases have been analysed using both geometrically linear and non-linear slab elements. A series of parametric studies has been carried out as an initial investigation into the characteristics of steel reinforcement which allow this action to take place. It is evident that the influence of membrane action in slabs can be very important to the ultimate integrity of compartments, and should be taken into account in the modelling of this type of structure in fire conditions.

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.

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.

Experimental behaviour of a steel structure under natural fire

Fire Safety Journal, 2006

Current design codes for fire resistance of structures are based on isolated member tests subjected to standard fire conditions. Such tests do not reflect the behaviour of a complete building under either normal temperature or fire conditions. Many aspects of behaviour occur due to the interaction between members and cannot be predicted or observed in tests of isolated elements. Performance of real structures subject to real fires is often much better than that predicted from standard tests due to structural continuity and the provision of alternative load paths. This paper reports on the results of a collaborative research project (Tensile membrane action and robustness of structural steel joints under natural fire, European Community FP5 project HPRI-CV 5535) involving the following institutions: Czech Technical University (Czech Republic), University of Coimbra (Portugal), Slovak Technical University (Slovak Republic) and Building Research Establishment (United Kingdom). It consists of an experimental programme to investigate the global structural behaviour of a compartment on the 8-storey steel-concrete composite frame building at the Cardington laboratory during a BRE large-scale fire test, aimed at the examination of the temperature development within the various structural elements, the corresponding (dynamic) distribution of internal forces and the behaviour of the composite slab, beams, columns and connections. r

Experimental Study on the Behavior of Full-Scale Composite Steel Frames under Furnace Loading

Journal of Structural Engineering, 2009

In current design codes, structural fire behavior is evaluated by conducting experiments on isolated structural elements in a standard fire condition. In such tests involving single isolated elements, many aspects of structural behavior that occur due to the interaction between adjacent members as well as the role of connections cannot be studied. Performance of real structures subject to fires is often much better than that predicted from standard tests due to structural continuity and the interaction between members. In addition, tests performed under the current design codes subject the structural element to a heat up phase only and do not consider the cool-down phase during which local/global failure of the structure may occur. This paper describes the results of a furnace test conducted on two full-scale composite steel frames. In one frame the beam-to-column connections were protected while in the second frame the columns as well as the beam-to-column connections were protected. A special test furnace was built in which full-scale frames could be tested under load. The structural frame was subjected to a heat up phase followed by a cool-down phase. During the test, the furnace temperature, steel and concrete temperatures as well as the horizontal and vertical deflections were recorded. The complete deformation process of the test frame observed during the heating phase and the cooling phase, including failure of the frame is described in this paper. A comparison of the data obtained from the two tests indicates that the fire resistance of a composite beam is significantly better than that of a steel column. Finally, experimental data on fire resistance of composite frames with conventional floor slab construction was compared with data from slim floor slab construction. Results indicate that the fire resistance rating of frames constructed with slim floor slabs is at least as good as that of frames with conventional floor slab construction. It is proposed that in certain engineering design scenarios, the composite beam may not have to be protected due to its higher fire resistance rating, while the steel column should be protected from fire heating.