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Steel structures exposed to fire: a state-of-the-art report Nwosu, D. I.; Kodur, V. K. R.
Natlonal Research Conseil national
I*#
Councll Canada de rechelches CanadaSteel Structures Exposed
ta
Fire
-
A State-of=the=Art
Report
by D.I. Nwosu and V.K.R. Kodur
Internal Report No. 749
STEEL STRUCTURES EXPOSED TO FIRE
-
A STATE-OF-THE-ART REPORTD.I. Nwosu and V.K.R. Kodur
ABSTRACT
This report presents a state-of-the-art literature survey on the behaviour of steel structures in fire. The use of steel in building construction. its advantages and
disadvantages. its behaviour when exoosed
6
fire and the factors affeciing this behaviour u is presentea. Various rcscarch studies conducted using expcrimcntal and analytical techniques to determine the overall behaviour of steel structures in fire are reviewed. Conclusions derived from these studies are given. Computer programs developed to study the various parameters affecting the overall behaviour of steel structures in fire are presented.STEEL STRUCTURES EXPOSED TO FIRE
.
A STATE-OF-THE-ART REPORTD.I. Nwosu and V.K.R. Kodur
TABLE OF CONTENTS 1
.
INTRODUCTION...
1 1.1 GENERAL...
1...
1.1.1 Advantages 2...
1.1.2 Disadvantages...
.
.
2 1.2 DESIGN CONSIDERATIONS...
2...
1.3 DESIGN FOR FIRE 3
1.4 CODES OF PRACTICE
...
3 1.5 OBJECTIVES AND SCOPE...
4...
2
.
BEHAVIOUR OF STEEL STRUCTURES IN FIRE 42.1 GENERAL
...
4 2.2 TRADITIONAL APPROACHES FOR FIRE PROTECTION...
4...
2.2.1 Insulation Method 4
...
2.2.2 Capacitive Method 5
...
2.2.3 Intumescent Coating Method 5
2.3 BEHAVIOUR OF STEEL STRUCTURES
...
5...
2.3.1 Single and Continuous Supported Elements 5
...
2.3.2 Overall Structural Behaviour 6
2.4 FACTORS AFFECTING BEHAVIOUR OF STEEL STRUCTURES IN FIRE
...
7...
...
2.4.1 Loading.
.
7...
2.4.2 Connections7
...
2.4.3 End Restraint7
...
2.4.4 Sprinklers 7...
2.4.5 Structural Interaction 8...
2.4.6 Compartmentation and Localization of Fire 8
...
2.4. 7 Tensile Membrane Action 8
...
2.4.8 Temperature Distribution 8...
2.5 SUMMARY 8 3.
LITERATURE REVIEW...
9 3.1 GENERAL...
9 3.2 EXPERIMENTAL STUDIES...
9...
3.2.1 United Kingdom 10...
3.2.2 Australia 12 3.2.3 Japan...
12 3.3 ANALYTICAL/NUMERICAL STUDIES...
13...
3.3.1 United Kingdom 14 . -...
3.3.2 Japan 17...
3.3.3 Belgium 18...
3.3.4 USA 183.3.5
The Netherlands
...
193.4 SUMMARY
...
194
.
COMPUTER PROGRAMS...
194.1 GENERAL
...
194.2 DEVELOPMENT OF COMPUTER PROGRAMS
...
204.3 SUMMARY
...
255
.
CONCLUDING REMARKS...
25STEEL STRUCTURES EXPOSED TO FIRE
-
A STATE-OF-THE-ART REPORTby
D.I. Nwosu and V.K.R. Kodur
1. INTRODUCTION 1.1 General
Steel is one of the construction materials which has been in use for a long time. It is a versatile and economical building material whose use is on the increase. The
continuing improvement in the quality of fabrication, erection techniques and
manufacturing processes in conjunction with the advancements in analytical techniques, made possible by computers, have permitted the use of steel in just about any rational structural system for buildings of any size.
Steel frames for building were f i s t introduced in buildings approximately one hundred years ago, and since then, have made it possible to build different kinds of buildings from those previously in common use. No doubt the kind of buildings that emerged were in response to market requirements of the day. Early buildings with steel frames were generally heavy in dead weight, contained much masonry, were lightly serviced and were generally not of a large span. In some instances, the steel structure was used as a substitute for masonry and timber, and was simply a skeleton around which the building fabric was wrapped.
In the early use of steel as a substitute for a building framework, compatibility between the steel frame and the building as a whole was obviously relatively easy to achieve. The requirement was for short spans, heavy cladding and partitioning which substantially stiffened the framework and, in many situations, provided the entire lateral load carrying resistance of the building. Fire protection requirements were not onerous or non-existent. This may be as a direct consequence of the widespread practice of using encasement to protect the steel structure. As a result, the steel frames used in the early buildings were very simple, mainly pin-jointed in design, with simple non-welded connections, which proved quick and simple to erect.
Modem steel-framed buildings, by comparison, are of much lighter construction, are often required to have longer spans giving more flexible use, have lightweight partitions incapable of carrying lateral load, are heavily serviced and are liable to alterations in layout and use. With modem construction practice, there are more
extensive requirements for fire protection. However, the relatively cheap material costs, fast erection sequences and lighter foundations, achieved by using steel in modem buildings, compare very favourably with those of other building materials.
One of the main costs in the use of steel, for the main framework of a building, is the additional protection required to provide adequate safety in the event of a fire. The problem arises because steel softens at high temperatures with significant degradation of strength and stiffness. Current structural fire protective measures concentrate on limiting the rate of temperature rise of the steel framework by a combination of measures which usually include some form of protection to some or all of the exposed surfaces of the members.
Despite its problems for use in fire resistant construction, there are advantages in the use of steel for structural members that constitute the primary supporting system of a building.
1.1.1 Advantages
Some of the advantages of steel as a structural material are:
The high strength of steel per unit weight, results in small cross-sectional sizes and thus minimizes the dead loads of structural members in buildings.
The behaviour of structural members made of steel is predictable because of the uniformity of the material (isotropy), unlike other commonly used structural materials.
The addition of structural members made of steel to existing buildings can be achieved easily.
Since most steel members that make up the primary supporting system of buildings are shop fabricated, high quality control can be easily achieved.
The stress-strain characteristic of steel makes it easy to model the response of a structure.
Erection of buildings using steel is much faster when compared to other building materials. This can lead to a considerable reduction in construction costs.
The ductility property of steel makes it an attractive material for buildings, especially those subjected to seismic loads.
I . I . 2 Disadvantages
Some of the disadvantages of steel as a structural material are:
Steel has a low resistance to corrosion. Consequently, when exposed to a corrosive environment, high maintenance costs may be involved.
Steel loses its integrity at high temperatures. Therefore, where a high fire resistance rating is needed, there are additional costs involved in providing fireproofing. Although the low dead load of steel-framed buildings is one of the advantages of using steel, the light weight of a steel frame system can, in certain cases, result in problems with vibration.
1.2 Design Considerations
Two design philosophies are currently in use for the design of steel structures: the working stress design and the limit states design. For the working stress design, the structures are designed so that unit stresses computed under the action of working, or service loads, do not exceed predesignated allowable values. In the limit state design philosophy, the structures are designed to satisfy the condition of serviceability limit states (those states concerned with occupancy of the building, such as deflection, vibration, and permanent deformation) and ultimate limit states, which are those concerned with strength, accidental failure due to a fire (fire limit state), buckling, fatigue, fracture, overturning and sliding.
The working stress design philosophy has been around for quite some time; however, limit state design is becoming a widely accepted approach. While the limit state design approach has been developed for structural steel design under room
temperature, its application for fire resistance design is not fully developed at the present time. Although codes of practice for fire resistant design have recently emerged
[I, 21,
they are still not fully implemented in Building Regulation Documents.1.3 Design for Fire
Structural design for fire is still in the developing stage in various parts of the world. Traditionally, steel structure designers have not played a part in assessing the structural fire endurance requirements. Instead, the structural design of a steel frame for a building has been independent of any consideration of the thermal effects of a fire. Fire protection is then added on to the completed assembly in accordance with the
requirement for fire resistance ratings. With the cost of fireproofing typically
representing a significant proportion of the cost of the structural frame for a building, structural engineers are becoming increasingly interested in the proper design of steel structures for fire endurance.
Some designs for steel structures to withstand fire have emerged in the recent past. These approaches are based on the fire resistance for individual members. The computation of fire resistance in these design approaches, is based on predicting the behaviour of a structural member were it to be exposed to the heating regime used in fire resistance tests. It does not necessarily imply the capability to predict the performance of that structural member when involved in a fire in a building.
Methods for calculating limiting temperature and design temperature are proposed in the British and European Standards for the fire resistance design of steel structures [l,
3,4]. These codes set out a methodology for determining fire resistance, based on fire tests and analytical methods. Methods of fire safety whereby designers can select an appropriate thickness of fire protection or, in many cases demonstrate that no protection is needed, are presented in the British code [I].
In this code, the structural effects of a fire in a building, or part of a building, are considered as a fire limit state. To check the strength and stability of the structure at the fire limit state, the applied loads are multiplied by relevant load factors. The performance criteria used for the final design are: (i) the members should maintain their strength under the factored loads for the required period of fire resistance, and (ii) any specified requirements for the insulation and integrity of compartment walls and floors should be satisfied.
1.4 Codes of Practice
Buildings are subjected to regulatory control for health and safety purposes. The safety requirements include provisions for fire protection. In Canada, fire protection requirements in the National Building Code of Canada
[5]
are given according to the building use and occupancy.In most codes, fire protection is concerned with safeguarding the occupants in the building where the fire may occur, minimizing risk to the adjacent buildings, and thereby avoiding a large and destructive fire. To achieve these aims consideration is given to the planning, layout and construction of the building to control the growth of a fire, prevent it from spreading, safeguard means of escape and prevent collapse of the building. In the codes, provisions are made for subdivision of a building into compartments in order to keep the size of the fire under control.
The control of the fire growth and size is concerned with the complete building. However, the code requirements for structural fire resistance at present applies to - individual members. The assumption is that if the individual members are satisfactow, the whole structure should at least as well. Moreover, current design
protection, where thc structure is designed for the ambient temperature situation and a certain thickness of fire protecrion is specified to achieve the fire resistance requirements.
1.5 Objectives and Scope
The main objective of this report is to conduct a comprehensive literature review on the behaviour of steel structures in fire conditions. Attention is focused mainly on structural frames. Both experimental and analyticallnumerical studies relating to the subject are reviewed.
In Section 2, the behaviour of steel in fire and the methods currently used for their protection against fire are given. Reviews of previous studies are given in Section 3.
Section 4 presents a review of computer programs for analysis of structures exposed to fire. The report is summarized in Section 5.
2. BEHAVIOUR OF STEEL STRUCTURES IN FIRE
2.1 General
Steel structures, when exposed to fire, will lose their strength and stiffness. This may cause excessive or permanent deformation that, in some situations, will lead to structural collapse. This situation certainly will violate the serviceabilitv and ultimate strength criteria. Thus, it is common praciice to provide protection to steel framework so that the integrity of the structure can be preserved for a sufficient period to enable safe evacuation of the occupants and to limit property damage.
The failure of a structural element under fire would mean that the element is no longer capable of sustaining the applied load on it. However, the failure of some elements in a fire would not necessarily cause the total collapse of the building. Therefore, the behaviour of an isolated structural element in a fire can be significantly different from the behaviour of the same element within a complete structure.
2.2 Traditional Approaches For Fire Protection
To achieve a required fire resistance rating, steel structural elements are protected so
as to control the rise in temperature when exposed to fire. The most common method of protecting a structural steel member is to encase it in a material which will act as a thermal insulator. Other, less common methods are: the capacitive method, the installation of automatic sprinklers, or the use of intumescent coating. Some of these methods are briefly discussed in the following sections.
2.2. I Insulation Method
In this method, materials such as gypsum, perlite, and vermiculite board are used to protect a steel fiame from fire. Other materials which are used include: mineral fibre, ceiling tiles, Portland cement concrete, Portland cement plaster, masonry materials and spray coatings. The insulation may be used as a membrane fire protection, in which a fire resistive barrier is placed between a potential fire source and the steel member to be
protected. On the other hand, the insulation may be sprayed directly onto the member to be protected. The latter method is widely used for structural steel frames.
2.2.2 Capacitive Method
This method of fire protection is based on using the heat capacity of a material to absorb heat. Using this method, the temperature rise in the element of a building exposed to fire is delayed and its fire resistance increased. Two examples in which the heat capacity of a material is used to gain fire resistance are: (i) water filling in Hollow Structural Section (HSS) columns, where part of the heat is used for heating and evaporation of the water, and (ii) concrete filling in HSS columns, in which the concrete absorbs some of the heat.
2.2.3 Intumescent Coating Method
For this method, intumescent coatings are used which foam and form a stable thickness of insulating cover for the steel on application of heat. The method avvears attractive, particularly as the material is appliedto steel in the form of a coat ofpaint. However, the method has its limitations. Materials available, at present, give only a relatively short period of fire resistance and require several on-site applied coats. Moreover, there is a danger of mechanical damage to the coating during the normal use of the structure in which case there would not be a complete cover of insulation available should a fire occur.
2.3
Behaviour of Steel StrueturesWhen a steel structure is exposed to fire, its load carrying capacity is reduced due to the loss in strength and stiffness of the material with increase in temperature. There is a critical temperature at which steel loses a considerable strength such that it can no longer carry the load imposed on it. The time for a structural element to reach this critical temperature depends on many factors such as the applied loads, stress level and the ratio of its surface area exposed to fire to the mass of material per unit length.
In a building, various elements are normally linked together and, consequently, the structure responds as a combined or total system to any external loading condition, transmitting stresses and strains to adjoining members. Such interactions also occur under fire conditions and, in general, have a beneficial influence on the behaviour of structures in fire. This is one-of the reasons why, with few exceptions, total structural collapse is not a frequent occurrence in actual buildings when a fire occurs.
2.3.1 Single and Continuous Supported Elements
The mode of failure in steel elements exposed to fire depends on boundary conditions and the rate of temperature increase. Steel elements can be of flexural or compression type. Those that resist loads primarily through bending, such as beams, are termed flexural elements, while compression elements are those subjected to loads tending to decrease their lengths (e.g., columns).
The behaviour of flexural structural elements when exposed to fire is influenced by the continuity of the structural system. The effect of continuity is illustrated in Fig. 1
[6] for a simply supported and a continuous beam subjected to a uniformly distributed load (udl). The beam (Fig. l(a)) is placcd on two simple supports, and the structure is therefore statically determinate, which means that the distribution of forces can be determined from equilibrium. The maximum stresses will occur at the centre of the beam, and as such, the section will yield at this location first when heating is applied. A 'plastic hinge' is formed, which means that, at a certain temperature, the middle section reaches its ultimate strength and will no longer offer resistance to rotation. At this stage,
the beam has become unstable and collapses (Fig. l(a)). One hinge is therefore sufficient for a statically determinate flexural member to collapse.
In Fig. I(b), a two-span continuous beam subjected to a udl is shown. In this case, the beam is statically indeterminate; the distribution of forces cannot be determined from equilibrium alone, compatibility must be satisfied as well. When exposed to fire, the centre support section reaches its ultimate capacity and the hinge will be formed, so that this section can rotate freely (Hinge 1 in Fig. l(b)). If the span and the loading of the beams in Figs. ](a) and (b) are equal, this hinge will be formed at the same temperature as that in the beam in Fig. l(a). The formation of the hinge at the centre support, however, does not lead to collapse in the beam and the beam can cany further load by transferring the load to other critical sections, namely the span section. Further heating will cause another hinge to be formed in the span region (Hinge 2 in Fig. l(b)). At this instant, the structure will become unstable and collapse occurs.
The continuous beam case illustrates the beneficial effect of continuity in structural elements when they are exposed to fire. When one portion of the member reaches its ultimate strength, the moment is then redistributed to other sections of the member that are yet to attain their ultimate capacity, and in so doing, the fire resistance of the element is improved.
2.3.2 Overall Structural Behaviour
There is another phenomena which enhances the fire resistance of a continuous beam. This arises in a situation where the beams are either restrained by some means or
are
active components of a whole structural system where they are restrained by adjacent elements. In these beams, before heating, the mid-span region would be under positive moment, and supports would be under negative moment as shown in Fig. l(c). When the beam is uniformlv heated from the bottom in both svans. the central soan is restrained bv the adjacent e~emknts from deflecting, thereby yen&ating an additionh negative mome; throughout the beam. This reduces the load-induced positive moment at the mid-span and increases the load-induced moment over the support. This has a beneficial effict as the moment capacity (strength) over the support decreases at a lower rate than at mid- span, thereby increasing fire resistance. Further heating can lead to formation of a plastic hinge at mid-span. If the restraint provided by the adjacent elements is adequate, the beam can be transformed into two cantilevers, thereby allowing it to resist collapse as long as the moment capacity over the support remains sufficient for the loads. The foregoing situation will usually occur in buildings with a continuous beam system where the internal spans are heated and the surrounding spans are unheated. However, if all the spans are heated or if the heated span is an endspan,
the degree of restraint may be considerably reduced.P
V
In practice, there is always some interaction between various elements. The simplest form is a beam-column assembly. Frictional resistance at supports and
closeness of adjacent elements can develop restraining forces. Fig. 2 [7] shows the effect of heating on the moment distribution in a steel portal frame. This is a typical example of the influence of interaction between a beam and a column element on the behaviour of a
steel frame in fire. The figure also illustrates the effect of restraint provided by a column on the moment distribution in the beam and bending moment induced in the column. The restraint provided by the column during heating reduces the moment distribution in the beam thereby enhancing the fire resistance of the frame.
In a multi-storey building, a fire is likely to occur in only a part of the structure, particularly where good compartmentation is provided as illustrated in Fig. 3 [S]. This
figure shows the response of a structural building frame subjected to a localized fire. Such a fire has two effects on the exposed niembers of the building. Firstly, the restraint to thermal expansion provided by the surrounding cold members increaseithe axial forces in the heated members which, in the case of columns, can cause instability at lower temperatures than would occur in isolated members. Secondly, additional support is provided by the cooler members around and above the heated area which can divert load paths from the weakened members to the stronger members.
2.4 Factors Affecting Behaviour of Steel Structures in Fire
Several factors influence the behaviour of steel structures exposed to fire. Some of these factors are discussed in the following sections.
2.4.1 Loading
One of the major factors that influences the behaviour of a structural steel element exposed to fire is the applied load. The limiting temperature and the fire resistance of the element increases if the applied load decreases. Failure will occur when the applied loading is equal to the ultimate strength of the element. Therefore, lowering the applied load on the element will increase its fire resistance.
2.4.2 Connections
The beam-to-column connections in modern steel framed buildings are generally designed with shear-resisting connections. The forces are also resisted by vertical bracing or shear walls. When deformations occur in the frame due to fire, moments are transferred to the connections and to the adjacent members, reducing the mid-span moments in the beams. The moment resisted by connections will reduce the effective load ratio to which the beams are subjected, thereby enhancing the fire resistance of the beams. This beneficial effect is more pronounced in large multi-bay steel frames with simple connections, but is smaller in frames designed with moment connections because the effect of continuity is already utilized under normal conditions.
2.4.3 End Restraint
The structural response of a steel element under fire conditions can be significantly enhanced by end restraints. For the same loading and fire conditions, a beam with a rotational restraint at its ends would deflect less and survive longer than its simply supported counterpart. The addition of axial restraint to the beam will result in an initial increase in the deflections, due to the lack of axial expansion relief. With further heating, however, the rate of increase in deflection will decrease.
2.4.4 Sprinklers
Sprinkler systems are also very important in protecting steel structures in fire. Automatic sprinkler systems are considered to be the most effective and economical way to apply water to suppress a fire [9]. In the event of fire in a building, the temperature rise in the structural elements located in the vicinity of sprinklers is controlled.
Therefore, the fire resistance of such elements is enhanced. In addition, fire protection is rarely used for the elements in the area where sprinklers are used.
2.4.5 Structural Interaction
In contrast to an isolated element exposed to fire, the way in which a complete structural building frame performs during a fire is influenced to varying degrees by the interaction of the connected structural elements in both the exposed and unexposed portions of the building. This is beneficial to the overall behaviour of the complete frame, because the failure of some of the structural elements may not necessarily
endanger its structural stability due to the ability of the remaining interacting elements to develop an alternative load path to bridge over these failed elements.
2.4.6 Compartmentation and Localization of Fire
Buildings are usually subjected to fires which remain localized due to
compartmentation. This has a two-fold effect on the heated elements. First, the restraint to thermal expansion provided by the surrounding cooler elements increases the axial force in the heated elements which, in the case of columns, can cause more instability at lower temperatures than would occur in isolated elements. However, the cooler area also provides support which can divert load paths from the weakening members increasing the stability of the structure.
2.4.7 Tensile Membrane Action
A tensile membrane action is developed by reinforced concrete floor slabs in a steel framed comoosite building. This action occurs when the applied load on the slab is taken by the steefreinforcemes, due to cracking of the entire ddI;th of concrete cross- section. It enhances the fire resistance of a complete framed building by providing an alternative load path after failure of some structural elements has occurred. It also has the potential of providing a means to eliminate fire protection for some steel elements in framed buildings, thereby reducing the cost of fire protection.
2.4.8 Temperature Distribution
Depending on the protective insulation and general arrangements of members in a structure. steel members will be subiected to temperature distributions that vary along the length or'over the cross-section.
embers
subjected to temperature variation across their sections may perform better in fire than those with uniform temperature distribution. This is due to the fact that sections with uniformly distributed temperatures, will attain their load capacity at the same time. However, for sections subjected to non-uniform temperature distribution, some parts will attain the load limit before the others. This will decrease the effect of the fire on the members.2.5 Summary
The following points summaize the information given in this Section:
0 When subjected to fire, an unprotected steel structure will lose its stiffness and
strength as a result of deterioration in its material properties.
0 The traditional approach to the above problem is to provide insulation to protect the
structure's load bearing elements to meet the requirements for fire resistance.
The most common and effective method of protecting structural steel is encasing the steel in a material that will act as an insulator.
The behaviour of an isolated structural element in fire differs significantly from that of an element within a complex, highly redundant structure.
The continuity, elements interaction and restraint to thermal expansion present in a complete structure enhances the fire resistance of the structure.
In general, indeterminate structures perform better in fire than determinate structures. The fire resistance of a structure increases with increase in the degree of static
indeterminacy.
The fire resistance of a complete building may be independent of the failure of some structural elements in the building due to the ability of the remaining elements of the building to develop an alternative load path.
The membrane tensile action produced by a reinforced concrete slab can increase the stability of a structural element at large deflection and provide an alternative load path to bridge over failed elements in a framed structure.
3. LITERATURE REVIEW
3.1 General
A large amount of research into the behaviour of structures exposed to a fire has been concerned with the structural performance of single structural elements, and the development of analytical techniques. In recent years, various studies have been undertaken on the overall behaviour of structures when subjected to fire.
A number of experimental and numerical studies for protected and unprotected steel framed buildings have been conducted or are in progress [lo-181. The following section presents a review of these studies. The review, presented below, is divided into the separate countries where they were conducted.
3.2 Experimental Studies
Due to the very high cost of providing facilities for fire tests, coupled with the difficulty of preparing the tests to investigate the true response of a complete structure, experimental studies on the overall behaviour of steel structures in fire are progressing at a very slow rate. There is a limited amount of research being carried out at this time [l 1,
15, 16, 17, 191.
Objectives of the experimental studies included: (i) observing and monitoring structural and non-structural elements within a real compartment subjected to a real fire, (ii) assessing the overall behaviour of the structure under actions that correspond, as closely as possible, to the impact of real fires, and (iii) providing data over a range of fire scenarios so that the computer models can be verified. Other studies have used scale models to study the overall behaviour under fire conditions.
In the United Kingdom 120,211, fire tests have been conducted on a full-scale eight-storey composite frame at the British Research Establishment Large Building Test Facility (LBTF) at its Cardington Laboratory (see the results obtained from some of the tests in the table below).
In Australia [17, 191 a fire test on a 41 -storey prototype structure was conducted to investigate the likely effect of fire in the building: (i) if the existing automatic sprinkler system complying with the requirements of extra light hazard is left in the building, and (ii) if the previously protected parts of the building are left unprotected. The experimental study was also aimed at providing data for a risk assessment on fire safety in the prototype building.
In Japan [22], fire tests on the behaviour of multi-storey steel frame buildings were conducted to investigate the impact of using fire-resistance (FR) steel for structural elements for buildings.
The features of the above experimental studies and the observations and conclusions derived from them are highlighted in the following tables.
3.2.1 United Kingdom
I
I
Fire test on: Place [Ref] BSC and DOE Compartment Fire Test [I I ] BSCIFRS Fire Tests [ I 51Steel columns in various locations in a campanment
Steel frames
Study Detail and Objectives The research describes a fire test on
a.
Full scale. loaded, 2-D mainly steel frame consisting o f a) unprotected steel beam spanning
test companment at ceiling height
b) two biock-in steel columns Objectives:
Obtain data for the preparation of design guidelines and
developmest o f analytical techniques to simulate the structural stability o f steelwork in natural fires.
.
To relate the observed deflection behaviour o f the frame with asimplified theory.
The research examines a number of features from the collaborative test program initiated jointly by Sweden (BSC) and the UK (FRS).
I
Objective:~~~ ~
lhal ru;h a connru;lion cuuld me21 the prtn ~sions undcr lhe IJu~ld#ng Kcguiallons fur ground Fire and Structural Characteristics
Timber cribs:
fire load of 25 kdmi
.
118 ventilation ofwalls in companmentand upper storeys in office, shop
I
and factorv assenihlv and storaneI
ConclusionslObse~ations.
The observed survival time was greater than that expected from the individual beam, which indicates the potential benefit from continuity..
Attainment o f a 30 min fire resistance for the frame nleanscnnncctiunr provided 30 mil! tire rrr~sloncv uilhoul the ncud fur
addili.,nui fire proleution
Wood and woodlplastic fuels The woodlplastic fire had little burned undcr different ventilation effect on the maximum steel conditions and thermal oro~ertics. ~ ~ . . I temperature attained. I
I il~lbcr.'rih tirr. .oad Ill. I S dnd i l l c lcmpcralur: rire ot',Ie~l n 2,. .g m' ~ ~ ( n g 1 2. 1,1 anJ I X i~nctiun o f both lhr. reclion lazl%,r ~ e n l i l a l i m dt one uall (lire cxpn>r.d pcrnrnevr di, idcd h) l a w fire loads and laree I cross sectional area) and position. I jmlilauun openmy .eleclcd kt lernpcralure gradlent. i!crors lhc rcprt3cnl o;;upanuiz, >u;h as ,crtioll D/MIIUIIIIIS paniall) built s;h~n,lj and nlulti-~h,rr.) t117ict lnlo lhc dc,uhle-lcafexlcmal uailr
.
To highlight situations where unprotected steelwork had adequate inherent firc resistance.I
reached 400°C and resulted in substantial thermal bowing.I
.
Changing compartment lining affected the combustion eas-
vmpcralure. inrli;aling thc ncud tu a;ioun~ t j r lhcrmdl pn,pcnies o i enclosure in any analytical approach.BRE Fire Tests E20.21 I Tens were Performed at the Large Building Test Facility (LBTF) at Cardington BST Fire Test Program 1231 Test was performed at the Large Building ~~~t ~ ~ ~ i l i t ~ at Cardington.
Experimental Building: Steel-frame structure:
.
Eight storeys with 5-bay long by 3-bay wide (approx. 945 mi rectangular area) with composite (Concrete-Steel) floors. Two wmer tests representative of a comer office and one Large Compartment test which is representative of a large open plan office..
In the comer tests, unprotected steel beams within compartment were tested to investigate shedding and bridging mechanisms between beams and slabs.Objectives:
To examine the behaviour of multi-storey, steel-framed buildings subjected to real fires. To use data from tests to validate computer models for structural analysis at elevated temperatures. To assess the tire pans ofthe forthcoming Eurocodes.
Building tested was an eight-storey steel framed structure.
Four tests were planned (first three tests were completed).
Test 1: One dimensional Restrained Beam.
Test 2: Two-dimensional plane frame.
Test 3: Three-dimensional comer tests.
Objectives:
Test I: To check the effect of expansion of heated members on the movement in the frame.
Test 2: To examine the behaviour of the frame around the connections. Test 3: To determine the extent of membrane action provided by the composite metal deck floor system.
Fire Load:
Timber cribs with fire load of40 kgim' .
Fire Load:
Test I : Heatins was done in a gas- fired furnace.
Test 2: Heating was done in a gas- fire furnace nearly 5 times longer than the typical floor fumace BS 467
(IS0 834) [241 fire resistance tests on floor beams.
Test 3: Heating was provided by wood cribs with a fire load of 45 kg/m1.
Preliminary results obtained fmm the wmer fire test indicated that: The maximum temperature of 903°C was recorded after 114 min at the middle section on the bottom flange of the unprotected beam 8 2 of Fig. 4.
A maximum temperature of 690°C was obtained after 114 min for the edge beam which was totally covered by the fire. The slab maximum deflection at its centre which was 266.9 mm after 130 min was found to have reduced to 159.7 mm once the cooling occurred.
The primaly beam on the west end boundmy ofthe compartment remained straight throughout the test. This was attributed to the restraint provided by the secondary beam framing into its web and partly due to the position of the partitions relative to the underside of the lower flange of the beams.
.
The integrity of the structures was demonstrated by restriction of all damages to the areas within the compartment.Contributing to the enhancement of the structure's performance was the membrane action provided by the wmposite floor slab.
.
The beneficial effect of continuity provided by surrounding structure was shown in Test I , where a steel temperature of up to 900°C was achieved,.
Test showed [hat, in real structures, steel members cana very large deformation without structural collapse.
.
The derived from membrane action provided by the floor slab when one or more members lose stiffness and strength was also shown when the steel temperature attained in Test 3 was approximately 1000°C. There was a wnservative correlation behveen the measured time temperalure response in Test 3 and the parametric equation of Eurowde 1.Thermal expansion of members can be constrained by the surrounding structure resulting in a negligible movement ofthe building's external facade.
12 3.2.2 Australia 3.2.3 Japan Place [Ref] Fire Tests of Omce Building at BHP Research- Melbourne Laboratories [17, 191 Conciusions/Observations
The structure ofthe test building remain undamaged during the test.
.
~h~ required load was sustained without excessive deflection.,
Steel members and composite floor slab suffered no permanent deflection.Extra light hazard sprinkler system was effective in controlling developing and well- developed fires in both small and open pian office areas.
For refurbished building - adequate performance of unprotected floor (slab and beams) in fire conditions was established. ObjectivesIStudy Details
Fire test in 4 i-storey prototype structure.
Building tested
Represents a segment ofone typical storey of prototype building with two parts: a) purpose part; and b) existing part
Objectives:
.
To observe the nature, duration and severity of fire generated bythe and Of
offices typical of those in the prototype building.
.
To investigate and evaluate the influence of fire on the unprotected composite slab and steel framing.To investigate the effectiveness of an extra light hazard sprinkler system.
To generate data to be used for risk assessment study.
ConclusionslObservations
• The results From the test showed that FR steel columns will not fail until the steel temperature exceeds 600'C.
.
The deformation behaviour of FR steel agreed with other forms of steei, although the tire resistance time differs with protection thickness..
The protection thicknesses that satisfied the required fire resistance (in hours) were 10 mm a n d 2 0 m m i n l h a n d z h , respectively; while those with conventional steei were 20 mm and 30 mm in I hand 2 h, respectively.Fire Load and Structural Characteristics Fire load:
Open area: Work stations and bookcases containing large amounts of combustible materials including drawings, books, magazines and plastic coated folders.
.
Small Office: Fitted out as a representative office with the contents including a desk, bookcases and chairs.Wood Equivalent ~i~ ~ ~ ~ d :
Test I: In small office, 52 kglm2. Test 2: In open plan area, 53.5 kglm'.
Test 3: In open plan area, 53.9 kg/m2, and in smali office, 52.1 kdm2.
Test 4: In small ofice, 67.5 kglm', and in open plan area, 64.3 kglm'.
Fire Load and Structural Characteristics Fire load:
Standard time-temperaturc Curve according to JIS 1304 [251. Place [Ref.] Nippon Steel Corporation and Chiba University, Tokyo [22] ObjectiveslStudy Details Stage I:
.
Test on steel columns made of fire-resistant (FR) steel.Stage 2 (Anaiyticai-design):
.
Fire-safe design of Yokohama Sogo warehouse where FR steel was used for its interior columns and beams.Fire-safe design of Tobihata building where FR steel was used for its external frames
Objectives:
To confirm experimentally that protection thickness could be reduced.
.
To show that the strength of building elements at high temperatures could be retained.3.3 AnalyticaVNumerical Studies
A number of analytical/numerical studies have been conducted on the overall behaviour of steel structures in fire [3,4, 14, 18,26,27]. The main objectives of these studies were to demonstrate the inherent fire resistance of unprotected steel structures or to show that the amount of fire protection required can be reduced by investigating the overall response of a complete frame exposed to fire.
Most of these studies concentrated on the development of models, which were generally based on the finite element method (FEM) with a few models developed using the finite difference method (FDM) to determine the temperature distribution across a section (thermal analysis).
In the U.K., at the University of Sheffield, University of Nottingham,
Loughborough University of Technology and British Research Establishment (BRE), considerable research on the development of numerical models to study the overall behaviour of steel structures in fire is being carried out. Similar studies have been conducted in Japan, Belgium, the United States and the Netherlands [8, 12, 13,281.
Some of the models deal with two-dimensional steel frames (composite and noncomposite) [27,29] and others with three-dimensional steel frames (composite and noncomposite) [26,30]. These models have been used to study the effects of parameters such as continuity, end restraint and continuous floor slabs, which influence the
behaviour of the heated steel frame.
With the exception of a few recent studies [6, 18,291, most of the models that deal with the concept of composite action have included the effect of the floor slab by representing the beams as composite elements based on the effective width concept. The models, however, did not include any membrane or bridging action of the slab, which may be a major influence on the structural response. Also, most of the models have only been used to assess the effectiveness of using various types of sub-assembly (subframes) in predicting the structural behaviour in fire. This approach has been employed in order to reduce the amount of computation and computer memory required.
The research studies carried out on the overall structural response of steel
structures when exposed to fire are outlined in the following tables with the observations - and conclusions reached by various studies.
3.3.1 United Kingdom Place [Ref.] University of Sheftieid and University of Nottingham 1271
The study presented the
development of a numerical program on the behaviour of a steel frame under fire conditions
Structures studied:
2-Dsingle- and two-bay single- storey steel frame
Obiectives:
a) A critical temperature of 450PC was attained when all members In the frame were unprotected. b) Protection of the beam only
raised the critical temperature to 436°C.
c) Insulation of one beam and one column provided a21% increase in critical temperature. d, Ofeither three
columns or one complete ring frame increased critical temperatures to 64S°C and 634°C. respectively.
.
Comparison with full fire frame analysis of previous studies exhibits an excellent agreement..
Comparison with large-deflection elastic solutions showed an excellent correlation through the linear and nonlinear ranges.at a
lower load using the formulation. NO explanation was given for this observation.
University of Sheftield [261
p
Analysis Technique:Finite Element Method (FEM)
Formulation :
.
~~~~d~~ previous FE program fNSTAF P I 1.
Element type-
truss and beam elements.~S~ ~
.
To develop a finite element formulation that permits full load- temperature history of steel frames..
To validate results from the approach with knownexperimental data from tire tests.
.
To investigate the effects of various forms of partial protection on the collapse temperature of sway frames.The study formulated and developed a computer program for nonlinear analysis of 3-D steel frames in fire (3DFIRE)
Structure analyzed: Multi-storey frame.
,
simple column subframe in athree-storey framed structure with fire in a ground-floor
compartment. Objectives:
.
To analyze multi-storey column subfmmes in various tire conditions..
To provide analytical and numerical formulations for conducting parametric on 3-D column subframes from multi-storey frame construction.Conclusions Observations
.
Comparisons between analysis and test data for columns and frame indicated that in all cases satisfactoly agreement was obtained.Results from the analysls of2-D two-bay single-storey steel frame to demonstrate the potentials of the formulation show that:
Capabilities:
.
Allows for materiai and geometric nonlinearities.Variation oftemperature across and along each member is possible with the program.
.
Allows for different material models..
Thermal strains, residual stresses and thermal bowing can be considered.Analysis Technique:
FEM
Formulation:
.
~~~~d~~ the program ~NSTAF PI]..
All basic principles of INSTAF were retained. but 3-Dformulation of element stifiess and elevated temperature characteristics were included.
.
Element type -twonode I-D beam elements.Capabilities:
.
Allows for material and geometric nonlinearities..
Allows for materiai variation as temperature increases..
Capable of analysis to high deformation levels at ambient and elevated temperatures..
For steel frames subjected to vertical loads, subframes may be used for elevated temperature analysis..
Connected beam in a frame may be designed as pinned support from coiumns for fire safety if the wlumn is designed to reach its limiting temperature in fire (bending moment in the column is neglected)..
Built-in support conditions may be assumed when calculating the load-bearing capacity of columns in a rigidly connected steei frame under fire conditions..
The influence of thermal restraint is much more important in columns than in beams..
Extrapolation of values from the code to constructions where stability is the governing factor may result in overestimation of limiting temperatures..
The study suggested that steel frames in a fire can be adequately represented by appropriate subframes similar to those used in conventional structural design. Subframes Ohosen were capab1e Ofrepresenting the behaviour of the beams and columns in the area of interest without introducing unrealistic conditions or restraint. Only when it is obvious that the beam or the wiumn behaviour dominates, were individual elements considered in isolation to quantify the behaviour ofthe whole frame.
.
Overall results ofthis study showed that failure in most cases was precipitated by the columns. Indicative study for frames with protected columns demonstrated a contrast behaviour to the above observation, therefore, it washighly recommended to cover all possibilities in the choice of subframes.
Boundary conditions that caused artificial restraint to axial thermal expansion were avoided.
.
Subframe models were consistent and reliable in predicting failure temperature. However, they were less reliable in predicting internal forces in beams.Analysis Technique:
.
FEMElement type
-
two-node beam element.Fuii composite action between beams and exists,
.
Uniform and non-uniform temperature distributions for columns beams, respectjveiy, The fire is confined to a compartment of one bay in one storey only.Conditions of Analysis
.
Frame was analyzed as a whole with fire in each C o m p ~ e n t . Subframes with different boundary condition combinations were analyzed.Subframe was formed by extending heated compartment by one bay (let? and right) and one storey (top and bottom).
Analysis Technique: FEM
Formulation:
.
All analysis based on previous finite element (FE) program NARR2 [I (1.Element type - Beam and plate elements.
Composite action:
The beams were treated as fully composite, with an effective concrete flange width equal to a quarter of the span.
,
~h~ method developed is oftaking into account strain reversal. It is therefore, possible to assess the residual stress effects on the frame elements after a iocal fire was extinguished, and the frame was returned to ambient temperature. BRE 130,321 - Loughborough University of Technology and University of shefield 1331, Structure Analyzed:An eight storey 3 by 5 bay building with steel columns and composite beams was used in the experimental study.
Objectives:
.
TO carry out a comprehensive parametric study of steei frame behaviour in fire conditions. To examine the applicability of using subframe to predict behaviour of structures in fire..
To compare the behaviour of complete multi-storey framed building with associated subframes.A series of analytical studies was presented on the behaviour of composite building frames in fire Structure analyzed:
,
unprotected plain composite steel-concrete frame (see frame and fire location in Fig. 5 -this is a section through the Cardington frame). Both rigid and semi-rigid beam-column connections were investigated.Both full frame and subframe analysis were performed. Objectives:
.
To assess the effectiveness of using different 2-D sub- assemblies in predicting the behaviour in fire of afuil plain composite frame.British Research Establishment 1341. Steel Construction Institute and University of Shefield [38]
This study described a finite element computer program for studying the structural behaviour o f steel frames at elevated temperatures. Structures Analyzed:
2-D one-bay sway at ambient temperature [35].
.
3-D column assemblage with beam-column connections tests at ambient temperature [36]. Three portal frames tested at elevated temperature [I 61. A nonsway ponai frame tested at Cardington Laboratory 1371.Development o f a computer program for both 2-D and 3-D analysis o f framed buildings under tire conditions is presented in this study. Structures analyzed:
.
Simply supported composite beams from hvo tests conducted in 1982 1391.8-storey composite frame building from the Cardington tests on unprotected secondary beam at the seventh floor level ofthe building. Objectives:
.
To study the ways in which different boundary conditions to the 3-D subframe affect structural response predictions..
To make Comparison between the overall behaviour ofbuildings and single elements in tire..
To develop the model to ensure that the software can be run on personal computer hardware, even for fairly large structures. Validation and Convergence Studies:.
Convergence studies against classical theoretical solutions were carried out to test the bending and membrane characteristics o f shell elements included in the program (see next column)..
Validation ofthe program and convergence studies [29] on composite beams were conducted.Analysis Technique: FEM
Formulation:
Element type - 2-node beam element with six degrees of freedom per node. Capabilities:
Material and geometric non- linearity were considered. Allowed for uniform and nonuniform temperature distributions across and dong elements cmsssection and length, respectively.
• The inclusion o f the semi-rigid, beam-column connections were possible.
Analysis Technique: FEM
.
~ ltypes. two.node beam ~ ~ ~ ~ t element and four-node shellelement Ihe effect of concrete slab including membrane
Capabilities:
.
~ l l o w s for the positioning ofthe reference axis and location of beam at Pointon or outside the beam cross- Section,
Aliows the inclusion of non-iinear spring elements to represent semi- rigid connection characteristics. I ~ c I u ~ ~ ~ ~ of sheo elements that the behaviour o f the floor slab
be re!Jresented (i.e., membrane action of the supported
slab be
.
The average ratio o f predicted temperatures ofthe analysis to the test results of the portal frame of Ref. 1 161 was 0.94 and the coeficient o f variation was 0.07. This, suggested by the authors, can be accepted as satisfactoty agreement..
Comparison with the Cardington frame test, shows that the fire resistance ofthe rigid frame was about 12% lower than the test result. No explanation was given for the difference..
The differences in the steel temperatures between thecardington test and the analysis were much smaller, because the steel temperatures in the later stage ofthe fire test increased slowly with time.
.
Comparison behveen overall structure behaviour and isolated member behaviour indicated that the development of design metbods for tire safety o f structures needs to be redirected from its traditional emphasis on isoiated member behaviour to conQePts based On local and overall behaviour of the structure..
Validation and convergence study on composite beams indicated that even very small numbers o f shell elements produced accuratesolutions with the program. The tests and the computer model results was that the structure surrounding a tire zone had a mBjor influence on the
performance of the directly heated elements.
.
The effect of slabs wasparticularly important in providing continuity in its own plane and
against deflection.
.
The continuity effect offered by the slabs not only provided support and reduced deflections but also caused considerable restraint to axial thermal expansion, which increased deflections.17 3.3.2 Japan Place [Ref.] Tokyo Institute of Technology, Yokohama 1131 Nagoya University and Sumitomo Heavy Industry, ~~k~~ [lo] Analysis Features Analysis Technique: FEM Formulation:
.
Element type-
linear element (both I - and 2-D elements).
Allows for material and geometric nonlinearities.Analysis Technique:
.
FEM Formulation:The temperature distribution in the section of the steel frame members was by 2-D
FEM
.
Standard temperature-time curve specification was according to fire resistance test [241..
Heat transfer source to members were given by radiation and con~ection.Three-node seven degrees of freedom beam elements was adopted for the structural analysis potiion.
ObjectivesIStudy Details Developed a general computer program for the analysis of large displacement elastoplastic thermal creep deformation in a fire compartment
Structures analyzed:
H cross-section steel beams with simple, hinged and built-in ends.
.
Two steel frames:(i) Single-bay portal frame, fixed at base with loaded beam ( T Y P ~ 1).
(ii) Multt-storey rectangular frames -computation time reduced by assuming column ends hinged and free to displace
longitudinally at the points 0.5 and 0.7 column length from the heated beam (Type 2).
This study presented the evaluation ofthe strength of unprotected steel structures during and after fire. Structure analyzed:
2-D plane frame. Objectives:
.
To study the progress of deformation and reduction of strength of steel framed structure during fire.To study how the strength ofthe frame members are recovered after fire.
.
To determine the length of strengthening plates, necessary for recovery ofthe initial strength aRer fire.Conclusions/Obse~ations
.
Large difference in the vetiical midspan deflection between elastic-plastic and elastic-plastic- creep analyses of beams at 80 to 90 min offire exposure (approximately 450°C). For frame Type I, deformation of beam with small flexural rigidity follows that of the columns. because the columns which have large flexural rigidity deflect towards the inside of the fire wmparlment,.
inclusion of creep at fire temperatures exceeding 4509C is crucial especially for the beam cases.• The strength of the frame structure during the fire decreases according to the increase in fire duration. This reaches only 43% ofthe pre-fire strength in the case of fire duration of 30 min.
.
The structure can recover full initial strength (pie-fire) only when the duration of fire is less than 10 min.,
For a 30 fire duration, the reduction of post-fire strength compared to the pie-fire strengthwas up to 79%,
.
By increasing the cross-sectional area of the centre part of the horizontal element (beam) of the frame structure the strength of the post-fire structure can be enhanced.I8 3.3.3 Belgium 3.3.4 USA Place [Ref ] University of Liege. Belgium, ERE and BST, US [I21 Conciusions/Obse~ations The numerical model simulated with reasonable agreement the thermal and structural behaviour of the steel frame tested at Cardington.
.
With the exception of the local buckling of the beam that occurred at the moment of failure which could not be modelled using a beam element, the frame behaviour was accurately predicted up to the failure point. The yield strength at ambient temperature has a great influence on the fire resistance of the structure.
The behaviour of an isolated column and bean1 during tire is different from the behaviour of the frame as a whole.ObjectivesIStudy Details The study presented the results of a number of numerical simulations of the Cardington fire test on a full-size loaded mainly unprotected steel frame [i I].
Objectives: To show how the
rewmmendations presented within the Eurocode [3,4] context can be applied in anumedcal model O f t h e frame behaviOur;and
how the results provided by the numerical model compare with the fire test results [I I].
Conciusions/Obse~ations The application of the model to an actual building has demonstrated its capability as a design tool. Place [Ref,] American Iron and Steel institute [a, 141 Washington, DC Analysis Features Analysis Techniques:
Finite difference method (FDM) for thermal analysis.
FEM for structural analysis. Formulation:
Element type for structural two.node beam element.
.
Effect oflarge displacements were taken into account,stress.strain relationships and thermal strains of materials can be wnsidered.
ObjectivesIStudy Details Structures analyzed:
.
Composite (steel and concrete) floor beams..
Two-storey structural steel frame with concrete and steel deck floor slab.Portion of 42storey offlce building located on the West coast of the United States.
Objectives:
.
To predict fhe performance of structural steel framed floor systems underexposure to high temperature.* To develop a design concept for structural fire endurance using wmputer models FIRES-T3 and FASBUS iI for thermal and structural analyses, respectiveiy.
Analysis Features Analysis Technique:
FEM Fornulation:
.
element type - elements to represent the frame, and triangular plate bending element to represent the slab wereCapabilities:
.
Geometric and material nonlinearities can be modelled. Temperature distribution across elements can be either uniform or nonuniform.
Effect ofthermal restraint by the slab on the elements can be modelled,19 3.3.5 The Netherlands 3.4 Summary Place [Ref,] Netherlands Organization for Applied Scientific ~~~~~~~h RIO [18,281
Based on the literature survey presented, this section can be summarized as follows:
The traditional approach of using standard fire tests on isolated members in rating buildings vrovides limited information on the actual verformance of a building durine
ObjectivesIStudy Details A theoretical analysis on the stability o f tire exposed steel frames is presented.
Structures analyzed:
steel beams, braced and unbraced steel frames
-
w exposure io fires.Full-scale fire testing is expensive and time consuming, however, it provides data over a range of fire scenarios in a real structure so that analytical/numerical models
can - - - . he tested. - - - .
Numerical modelling of steel behaviour in fire allows the effect of parameters such as continuity, restraint, and the membrane characteristics of a floor slab to be easily
Analysis Features Analysis technique: FEM Formulation: - Beam elements. Capabilities:
Can handle nonlinearity for both geometry and material.
investigated.
The overall behaviour of a structure in fire is significantlv different from that of a
ConclusionsIObservations
Theoretically determined critical temperatures in all cases were lower than the critical temperatures resulting fmm the experiments. The maximum deviation was IOO°C and the minimum was 25°C. In the case ofthe unbraced frames, the extent o f initial inclination ofthe frame had a significant effect on the critical temperature.
I t was suggested by the authors that both theoretical and experimental analysis should be performed on the stability ofan unbraced frame loaded by additional horizontal forces.
single element subjected to the same fire expos;re.
4. COMPUTER PROGRAMS
4.1 General
Evaluation of structural fire resistance based on the overall structural behaviour of steel framed buildings has become a practical reality with advancements in computer technology. Initially, the fire resistance of a structure was determined based on a standard fire test of a single element in a furnace. These test procedures have several shortcomings including the size of the element, available loading capacities, and the restraint characteristics. These problems can be solved by conducting full-scale tests on complete frames. However, such tests are expensive, time consuming and difficult to perform, especially on complete structures.
Considerable progress has been made in the development of simple analytical models for the evaluation of fire resistance of elements, and fire resistance can now be estimated using these simplified calculation models. However, these models do not apply to all cases and have severe limitations when more parameters have to be taken into consideration in order to simulate real situations.
During the last decade or so, there has been considerable progress in the development of computer programs to predict the overall response of structures in fire [8, 14, 18,28, 33,401. These new models are now making it possible to analyze different kinds of structures under realistic fire exposure conditions. Unlike the traditional fire test methods and empirically derived calculation solutions, the numerical models are now providing solutions considerably closer to reality.
4.2 Development of Computer Programs
There have been a number of computer programs developed in recent years aimed at modelling the behaviour of frame structures in fire. The most ceneral and vcrsatilc approach isbased on the finite element method (thermal and stru&ral analysis). Using this method, various factors affecting the behaviour of structures in fire have been modelled (for example, nonlinear material behaviour, geometrical nonlinearity,
nonuniform temperature distributions and thermal strains). The main advantage of the method is that it enables the behaviour of complicated structures to be studied. There are a few computer programs with thermal analysis formulations based on the finite
difference method. The majority of these programs model two-dimensional behaviour. Recently, computer programs capable of full three-dimensional analysis of frames, which can take into account composite action, the effects of slabs and other panels and rotational stiffness of connections, have been developed. Some of these computer programs and their salient features are presented in the table below.
Name and Reference CEFlCOSS
(computergngineering of the ~i~ resistance f o r ~ m p o s i t e -
and Steel Structures) Schleich, J.B.
ABED, Luxembourg 1411
Salient Features Type of Analysis:
.
Thermal and Structural Analysis Formulations:Thermal analysis by finite difference method (FDM).
Stmctural analysis by FEM
Element type - two-node beam elements. Types of structures that can be analyzed:
.
Single element - columns and beams (steel or concrete) with or without protection. 2- and 3-D composite or nonwmposite frames.Capabilities:
.
Allows for geometric and material nonlinearities..
Allows for strain hardening effect to be included.Evaporation of moisture in wncrete can be accounted for.
Cooling effect alongside structural elements as well as real end conditions can be simulated.
.
Can be used on personal computers (PC) and workstations.Limitations:
Creep cannot be considered implicitly. No graphics and post processing capabilities.
Applications
For columns, beams and frames of any of the following:
a) Bare steel profiles
b) Steel sections protected by any type of insulation; and
C) Any type ofcomposite stee,.concrete cross. section
TWO frame tests results were sirnulaled using the program.
![FIG. 2. Moment Distribution in a Steel Frame Exposed to Fire [7]](https://thumb-eu.123doks.com/thumbv2/123doknet/14215504.482709/35.915.150.777.162.993/fig-moment-distribution-steel-frame-exposed.webp)
![FIG. 4. Plan of 2nd - 3rd Floor of Eight-Storey Building at the Cardington Large Building Test Facility (LBTF) [21]](https://thumb-eu.123doks.com/thumbv2/123doknet/14215504.482709/36.915.135.791.238.921/plan-floor-storey-building-cardington-large-building-facility.webp)
![FIG. 5. Section Through the Cardington Frame including Fire Compartments Studied [33]](https://thumb-eu.123doks.com/thumbv2/123doknet/14215504.482709/37.915.120.719.119.960/fig-section-cardington-frame-including-compartments-studied.webp)
