The Ultimate Behaviour of Composite Frames in Fire Conditions (1) (original) (raw)
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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.
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.
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.
2004
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 theoretical basis of the non-linear layered procedure used to model the reinforced concrete floor slabs, which includes both geometric and material non-linearity, is briefly outlined. Several of the full-scale 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 evaluate the influence of tensile membrane action in the concrete slabs on the structural behaviour in fire. 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 nonlinear 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 clear that the influence of tensile membrane action of concrete slabs on the behaviour of such composite structures in fire is very important, and should be accounted for in later additions and amendments to structural fire engineering design codes.
Strategies for Fire Protection of Large Composite Buildings
In this study a large generic composite flooring system with a footprint of 36m x 36m has been designed. The frame is 4 bays wide and 4 bays deep, each bay having dimensions 9m x 9m. A series of analyses has been performed, based on different patterns of fire protection to the downstand steel beams. The influence of the steel reinforcement on the structural behaviour has been investigated. It is evident that the presence or absence of tensile membrane action in the concrete slabs is a major influence on the ultimate integrity of the flooring system at high distortions. In order to optimise the mobilisation of tensile membrane action it is important to make sure that the concrete slab finally deforms in double curvature, and that it is incapable of producing folding mechanisms which do not need membrane straining. The generic composite flooring system used in the studies is based on realistic loading conditions and layouts which can test a full range of protection strategies. The behaviour is compared against the limiting temperatures for 30 and 60 minutes' fire resistance, calculated according to BS5950 Part 8, according to which all beams require fire protection for these two fire resistance periods. From the Vulcan modelling it is shown that a significant number of steel beams can be left unprotected for either fire resistance period. The analyses also demonstrate the enhancements of the fire resistance which can be achieved by increasing the size of the reinforcing mesh or using different fire protection strategies. It is clear that current structural fire design codes, which are based on standard fire tests on isolated structural members, can not properly predict the structural behaviour of real buildings in fire conditions. Future codes for the fire engineering design of composite structures should be based on an understanding of the interactions between different structural components within complete structural systems when subjected to fire. Some comparisons are made with a new design method based on a simplified model of tensile membrane action in composite slabs, generally indicating that the method is appropriately conservative while suggesting that protected members on gridlines may need some extra protection when it is used.
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.
Non-linear modelling of three full scale structural fire tests
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.
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.
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.