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Steel Structures Exposed To Fire - A State-Of-The-Art Report
Nwosu, D. I.; Kodur, V. K. R.
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Internal Report (National Research Council Canada. Institute for Research in
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Steel Structures Exposed ta Fire
A State-of=the=ArtReport
by D.I. Nwosu and V.K.R. Kodur
Internal Report No. 749
Date of issue: December 1997
-
STEEL STRUCTURES EXPOSED TO FIRE - A STATE-OF-THE-ART REPORT
D.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 affeciingu this behaviour
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 REPORT
D.I. Nwosu and V.K.R. Kodur
TABLE OF CONTENTS
.
1 INTRODUCTION......................................................................................................................
1
....................................................................................................................................
1
1.1 GENERAL
..........................................................................................................................
1.1.1 Advantages
2
...........................................................................................
1.1.2 Disadvantages ....................
.
2
1.2 DESIGN
CONSIDERATIONS
...........................................................................................................
2
1.3 DESIGN
FOR FIRE........................................................................................................................
3
...................................................................................................................
3
1.4 CODESOF PRACTICE
1.5 OBJECTIVES
AND SCOPE
.............................................................................................................. 4
4
2 BEHAVIOUR OF STEEL STRUCTURES IN FIRE ............................................................
2.1 GENERAL
....................................................................................................................................
4
...................................................................
4
2.2 TRADITIONAL
APPROACHES
FORFIREPROTECTION
................................................................................................................
2.2.1 Insulation Method
4
2.2.2 Capacitive Method ...............................................................................................................
5
2.2.3 Intumescent Coating Method ...............................................................................................
5
2.3 BEHAVIOUR
OF STEELSTRUCTURES
...........................................................................................
5
5
........................................................................
2.3.1 Single and Continuous Supported Elements
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 Connections .........................................................................................................................
7
.......................................................................................................................
2.4.3End Restraint
7
2.4.4 Sprinklers.............................................................................................................................
7
..........................................................................................................
2.4.5 Structural Interaction
8
8
2.4.6 Compartmentation and Localization of Fire .......................................................................
8
.................................................................................................
2.4. 7 Tensile Membrane Action
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
18
.
.
3.3.5 The Netherlands.................................................................................................................
19
3.4 SUMMARY ................................................................................................................................. 19
4 COMPUTER PROGRAMS .................................................................................................... 19
4.1 GENERAL
.................................................................................................................................. 19
4.2 DEVELOPMENT
OF COMPUTER
PROGRAMS
............................................................................... 20
4.3 SUMMARY
.................................................................................................................................25
5 CONCLUDING REMARKS ..................................................................................................25
6 REFERENCES ......................................................................................................................... 26
.
.
.
-
STEEL STRUCTURES EXPOSED TO FIRE A STATE-OF-THE-ART REPORT
by
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 fist 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,
at least as well. Moreover, current design
the whole structure should
specifications in many codes are still based on traditional prescriptive methods of fire
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 Struetures
When 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~emkntsfrom 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 midspan, 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 end
the degree of restraint may be
considerably reduced.
span,
P
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
V
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 crosssection. 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
0
When subjected to fire, an unprotected steel structure will lose its stiffness and
strength as a result of deterioration in its material properties.
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
Study Detail and Objectives
Place [Ref]
BSC and DOE
Compartment
Fire Test [II ]
The research describes a fire test on
Timber cribs:
a.
.
Full scale. loaded, 2-D mainly
steel frame consisting o f
.
.
Fire and Structural Characteristics
fire load of 25 kdmi
118 ventilation ofwalls in
companment
a) unprotected steel beam spanning
test companment at ceiling
height
b) two biock-in steel columns
I
Obtain data for the preparationof
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 a
simplified theory.
.
I
I
I
The research examines a number of
features from the collaborative test
program initiated jointly by Sweden
(BSC) and the UK (FRS).
Fire test on:
Steel columns in various locations
in a campanment
Steel frames
.
Objective:
To highlight situations where
unprotected steelwork had
adequate inherent firc resistance.
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 nleans
lhal ru;h a connru;lion cuuld
me21 the prtn ~sionsundcr lhe
IJu~ld#ng
Kcguiallons fur ground
and upper storeys in office, shop
and factorv assenihlv and storane
~~~
Objectives:
BSCIFRS Fire
Tests [ I 51
ConclusionslObse~ations
~
I
cnnncctiunr provided 30 mil! tire
rrr~sloncvuilhoul the ncud fur
addili.,nui fire proleution
Wood and woodlplastic fuels
burned undcr different ventilation
I
conditions and thermal oro~ertics.
. .
Iil~lbcr.'rihtirr. .oad Ill. I S dnd
2,. .g m' ~ ~ ( 1n2.g1,1 anJ I X
~ e n l i l a l i mdt one uall
l a w fire loads and laree
I
jmlilauun openmy .eleclcd kt
rcprt3cnl o;;upanuiz,
>u;h as
s;h~n,lj and nlulti-~h,rr.) t117ict
~
~
I
The woodlplastic fire had little
effect on the maximum steel
temperature attained.
I
i l l c lcmpcralur: rire ot',Ie~l
n
i~nctiuno f both lhr. reclion lazl%,r
(lire cxpn>r.dpcrnrnevr di, idcd h)
cross sectional area) and position. I
lernpcralure gradlent. i!crors lhc
paniall) built
,crtioll D/MIIUIIIIIS
lnlo lhc dc,uhle-lcafexlcmal uailr
reached 400°C and resulted in
substantial thermal bowing.
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.
.
I
-
BRE Fire Tests
E20.21 I
Tens were
Performed at the
Large Building
Test Facility
(LBTF) 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.
Fire Load:
Timber cribs with fire load of40
kgim' .
.
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.
BST Fire Test
Program 1231
Building tested was an eight-storey
steel framed structure.
Four tests were planned (first three
Test was
performed at the tests were completed).
Large Building
~~~t ~ ~ ~ ati l i Test
t ~ 1: One dimensional Restrained
Beam.
Cardington.
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.
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.
.
Fire Load:
Test I : Heatins was done in a gasfired furnace.
Test 2: Heating was done in a gasfire 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.
.
.
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 can
a 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
Place [Ref]
Fire Load and Structural
Characteristics
ObjectivesIStudy Details
Fire test in 4 i-storey prototype
structure.
Fire Tests of
Omce Building
at BHP
ResearchMelbourne
Laboratories
[17, 191
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 by
the
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.
Fire load:
Conciusions/Observations
The structure ofthe test building
remain undamaged during the test.
Open area: Work stations and
~h~ required load was sustained
bookcases containing large
without excessive deflection.
amounts of combustible materials ,Steel members and composite
including drawings, books,
floor slab suffered no permanent
magazines and plastic coated
deflection.
folders.
Extra light hazard sprinkler
Small Office: Fitted out as a
system was effective in
representative office with the
controlling developing and wellcontents including a desk,
developed fires in both small and
bookcases and chairs.
open pian office areas.
For refurbished building Wood Equivalent~i~ ~ ~ ~ d adequate
:
performance of
unprotected floor (slab and beams)
Test I: In small office, 52 kglm2.
in fire conditions was established.
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'.
.
.
3.2.3 Japan
ObjectiveslStudy Details
Place [Ref.]
Nippon Steel
Corporation and
Chiba
University,
Tokyo [22]
.
.
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.
.
ConclusionslObservations
Fire Load and Structural
Characteristics
Fire load:
Standard time-temperaturc Curve
according to JIS 1304 [251.
•
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
and20mminl handzh,
respectively; while those with
conventional steei were 20 mm
and 30 mm in I hand 2 h,
respectively.
.
.
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.]
Conclusions Observations
p
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 singlestorey steel frame
Obiectives:
.
.
.
~S~
~
To develop a finite element
formulation that permits full loadtemperature history of steel
frames.
To validate results from the
approach with known
experimental data from tire tests.
To investigate the effects of
various forms of partial protection
on the collapse temperature of
sway frames.
University of
Sheftield [261
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 a
three-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.
Analysis Technique:
Finite Element Method (FEM)
.
.
.
.
.
Formulation :
~~~~d~~ previous FE program
fNSTAF PI1
Element type truss and beam
elements.
-
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-D
formulation 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.
.
.
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:
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.
BRE 130,321
Structure Analyzed:
An eight storey 3 by 5 bay building
with steel columns and composite
beams was used in the experimental
study.
.
Objectives:
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.
TO
.
Loughborough
University of
Technology and
University of
shefield 1331,
Analysis Technique:
.
FEM
-
Element 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).
A series of analytical studies was
presented on the behaviour of
composite building frames in fire
Analysis Technique:
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 subassemblies in predicting the
behaviour in fire of afuil plain
composite frame.
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.
.
.
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 Of
representing 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 was
highly 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.
.
.
British Research
Establishment
1341.
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 [I61.
A nonsway ponai frame tested at
Cardington Laboratory 1371.
.
Steel
Construction
Institute and
University of
Shefield [38]
Developmento 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 Comparisonbetween 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 nonlinearity 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:
.
The average ratio o f predicted
temperatures ofthe analysis to the
test results of the portal frame of
Ref. 1 161was 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 the
cardington 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
FEM
member behaviour indicated that
the development of design
metbods for tire safety o f
structures
~
ltypes. ~
two.node~ beam ~
~
tneeds to be redirected
from its traditional emphasis on
element and four-node shell
isoiated member behaviour to
element Ihe
effect of
conQePtsbasedOn local
and
concrete slab including membrane
overall behaviour of the structure.
Validation and convergence study
Capabilities:
on composite beams indicated that
even very small numbers o f shell
~ l l o w for
s the positioning ofthe
elements
produced accurate
reference axis and location of
solutions with the program.
beam
at
Point
The tests and the computer model
on or outside the beam crossresults was that the structure
Section,
surrounding a tire zone had a
Aliows the inclusion of non-iinear
mBjor influence on the
spring elements to represent semiperformance of the directly heated
rigid connectioncharacteristics.
elements.
I ~ c ofI sheo
u elements
~ ~ ~ that
~
The effect of slabs was
the behaviour o f the floor slab
particularly important in providing
be re!Jresented (i.e.,
continuity in its own plane and
membrane action of the supported
againstdeflection.
slab be
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
Tokyo Institute
of Technology,
Yokohama 1131
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 P1).
~
(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).
.
Nagoya
University and
Sumitomo
Heavy Industry,
~~k~~ [lo]
Analysis Features
ObjectivesIStudy Details
Place [Ref.]
Analysis Technique:
.
FEM
.
.
Formulation:
-
Element type linear element
(both I- and 2-D elements)
Allows for material and geometric
nonlinearities.
.
This study presented the evaluation
ofthe strength of unprotected steel
structures during and after fire.
Analysis Technique:
.
•
Structure analyzed:
Formulation:
.
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.
.
FEM
The temperature distribution in the
section of the steel frame
members was
by 2-D
FEM
Standard temperature-timecurve
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.
.
.
Conclusions/Obse~ations
Large difference in the vetiical
midspan deflection between
elastic-plastic and elastic-plasticcreep 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 strength
was 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
Place [Ref ]
ObjectivesIStudy Details
University of
Liege. Belgium,
ERE and BST,
US [I21
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].
Analysis Techniques:
Objectives:
Formulation:
To show how the
rewmmendations presented
within the Eurocode [3,4] context
can be applied in anumedcal
model Ofthe frame behaviOur;and
how the results provided by the
numerical model compare with the
fire test results [I I].
Analysis Features
Finite difference method (FDM)
for thermal analysis.
FEM for structural analysis.
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.
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.
.
.
3.3.4 USA
American Iron
and Steel
institute [a, 141
Washington, DC
Analysis Features
ObjectivesIStudy Details
Place [Ref,]
.
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 Technique:
FEM
.
Fornulation:
element type -
elements to
represent the frame, and triangular
plate bending element to represent
the slab were
.
.
Capabilities:
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,
Conciusions/Obse~ations
The application of the model to an
actual building has demonstrated
its capability as a design tool.
19
3.3.5 The Netherlands
Place [Ref,]
ObjectivesIStudy Details
Netherlands
Organization for
Applied
Scientific
~~~~~~~hRIO
[18,281
A theoretical analysis on the
stability o f tire exposed steel frames
is presented.
Analysis technique:
Structures analyzed:
Formulation:
steel beams, braced and unbraced
steel frames
Analysis Features
FEM
- Beam elements.
Capabilities:
Can handle nonlinearity for both
geometry and material.
3.4
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.
Summary
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
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
investigated.
The overall behaviour of a structure in fire is significantlv different from that of a
single element subjected to the same fire expos;re.
-
4.
COMPUTER PROGRAMS
4.1
General
w
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.
Salient Features
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
.
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
TWOframe tests results were sirnulaled using
the program.
TASEF-2
(Temperature Analysis of
SlructuresExposed t o l r e 2-D
Version)
-
-
Wickstrom U.
Lund Institute of Technology.
Sweden [42]
Type ofAnalysis:
Only thermal analysis
Formulations:
FEM
Element type four-node rectangular element
-
Validation:
TASEF-2 used to analyze square plates
subjected to heat transfer from surrounding
gas and compared to analytical solutions.
Steel beams embedded i n concrete.
Wide flange I-beam.
Steel box-girder embedded in concrete.
Types of structures that can be analyzed:
Steel, concrete or composite (steel and
concrete) structures
..
.
Capabilities:
Allows for nonlinear boundary conditions.
Dependency o f material properties on
temperature considered.
Heat transfer by convection and radiation
hetween enclosed surfaces in structures with
voids can be considered.
Material propertics including thermal
conductivity
Has mesh generation capability.
Can he used an I'Cs and workstations
Can also accept I-and 3-D elements.
Has the capability o f mesh generation.
.
.
.
.
Limitations:
Good accuracy was not possible for concrete
temperature because the influence of mass
transfer o f water is not considered.
No graphics and post-processing capabilities.
FIREST3 @re Esponse o f
-
Structures Thermal -2
Dimensional Version)
Iding. R.. Bresler, B. and
Nizamuddin. Z.
University o f California,
Berkeley. USA [43]
Type ofAnalysis:
Only thermal analysis
Formulations:
FEM
Element types Four-node hexahedron and
six-node (degenerated hexahedron) 3-D
elements. I-and 2-D elements can also be
handled.
-
Types o f structures that can be analyzed:
Steel, concrete or composite (steel and
concrete) structures.
Capabilities:
Allows for nonlinear characteristics of
thermal properties ofmaterial.
Allows for heat transfer fmm the fire to the
environment.
Allows for any form offire exposure curve
(i.e., constant temperature, linear change,
ASTM E l 19 or natural burning)
Can be used on PCs and mainframes.
.
.
.
Limitations:
Cannot model heat transfer through cavities
i n an assembly.
Contains no post-processing and graphic
capabilities.
.
Program has been validated against a variety o f
structural problems i n lire
.
Predictio~~s
have been made of heat transfer
through steel beams with direct applied fire
protection material using the program.
SAFlR
Franssen. J.M.
University o f Liegc. Belgium
1441
Type ofAnalysis:
Heat transfer and structural analysis
Formulations:
Present application:
Protected or unprotected steel structures.
Commercial/industriaI applications:
.
FEM
Element types: triangular and quadrilateral
elements for plane sections, solid elements
with 6 or 8 nodes for 3-D structures
Types o f structures that can be analyzed:
Plane sections and 3 - 0 structures.
.
.
Capabilities:
Allows for variable materials.
Properties and evaporation o f moisture
allowed.
Allows for material and geometric
nonlinearitics.
Effect ofthermal strains can be considered.
Allows for loading reversal.
Allows for prescribed displacenient.
Allows for skewed external supports.
Residual stresses can be included.
Prestressed structures can be analyzed
Automatic adaptation o f time step is possible.
Calculation can proceed until failure.
Can be adapted to give results in a format
compatible with commercial software such as
IDEAS, etc.
Automatic mesh generation capabilities.
..
.
Fire resistance of T steel profiles protected by
intumescent insulation
Tliermal protection of sandwich steel wall.
Concrete under temperature variation
Fire resistance o f steel frames.
.
Current developments:
.
Introduction o f shell elements.
Introduction o f plane stress and 3-D plasticity
material.
European Community sponsored Research
(SAFIR used):
.
.
Buckling curves o f hot rolled H sections.
Design rules for steel structures in natural
fires in closed car parks.
Design rules for steel structures In natural
fires in large compartments.
Steel buildings through tire safety concept.
Limitations:
No post-processingand graphic capabilities.
FASBUS I 1 (Fire Analysis o f
-Steel @ildingSystems)
Chiapetta. R.. Iding. R.H. and
Bresler. B.. Illinois fnstitute o f
Technology and
American Iron and Steel
Institute. USA [45]
Type o f Analysis:
Structural analysis only
.
FEM
Element types beam and triangular plate
bending elements for frame and slab.
respectively.
-
Types ofstructures that can be analyzed:
Steel frame floor system.
Skeletal and composite steel frames.
.
.
Capabilities:
Geometty and arrangement of structural
elements.
Allows for changes in material properties.
Allows for application o f loads and
restraining conditions.
Allows for temperature profile and
distribution in the structural membea as a
result o f the fire exposure.
Can be run on PCs.
..
Limitations:
N o graphic or post-processing capabilities
Can only handle problems with 99 nodes, 180
elements (beam and slab) and 150 time steps.
FASBUS was validated against a test
conducted by the National institute of
Standards and Technology (formerly the
National Bureau o f Standards) and the
American Steel lnstitute [46] on two-storey
four-bay structural steel frame with a
concrete and steel deck floor slab.
Structural steel beams with concrete slabs
formed over steel deck (composite and noncomposite design).
STABRA (Sl'Anil voor
BRAnd: STANlL for fire)
Kerstma. J.. Biilaard. F.S.K. and
Twill. I...Institute TNO fbr
Building Materials and Building
Structures. Delft. The
Netllerlands (401
Type o f Analysis:
Thermal and structural analysis
Formulations:
.
Validation:
With the help o f STANST 1281. a comparison
was made between theoretical analysis and
the experiments presented i n Ref. [18].
FEM
Element type Beam element
-
.
Types o f structures that can he analyzed:
2-D frame structures
Capabilities:
Allows for material and geometric
nonlinearities.
Creep included in stressstrain relationship,
Allows for nonuniform distribution o f
temperatures.
Can be run on PCs.
.
-
Limitation:
3DFIRE
N a ~ aS.R
r and Burgess I.W.
University ofShefiield.. UK
1261
Influence o f thermal expansion not taken into
account
.
Type of Analysis:
The program was validated against a range of
previous analyses including:
Thermal and structural analysis
Formulations:
FEM
Element type two-node I - D element.
-
Types o f structures that can be analyzed:
2- and 3-D frames.
Capabilities:
Allows for geometric and material
nanlinearities, including changes in material
properties with change In temperature
Temperature distribution across members can
he uniform or nonuniform
Can be used on PCs.
Limitations:
No graphics or post-processing capabilities.
Cannot model effect o f composite action.
Large-deflection elastic, inelastic analyses
(ambient temperature).
Full fire analysis of2-D frame structures.
Local analysis o f mufti-storey column
subframes in various fire scenarios.
ALGOR
Type o f Analysis:
ALGOR, INC.
Pittsburgh. PA, USA [47]
Thennal and structural analysis (call also
handle nonstruclural problena).
The program was for a variety o f small and
large structural
temperatures,
problems under ambient and
Note: This is a general purpose cammercial
program.
Formulations:
-
FEM
Element types: I-.2- and 3-D elements with
linear nodes along each side.
Types of structures that can be analyzed:
I-.2- and 3-D structures, including frame
(plane and space). shell, plane stress and
plane strain structures.
Capabilities:
Allows for steady and transient heat transfer
analyses.
Geometric and material noniinearities can be
modelled.
Ambient and elevated temperature problems
(structural and nonstructural) can he handled
Allows for residual stress.
Can be run on PCs or workstations.
Can be run interactively or in a batch mode.
Has graphics and post-processing capabilities
Has automatic mesh generation capability
.
ARAQlJS
Hihbit. Karlsson & Sorensen,
Inc.. USA [48]
.
.
'Type o f Analysis:
Thermal and structural prohlems, including
nonstructural problems.
Formulations:
FEM
Element types: I-.2- and 3-D elements.
including axisymmetric elements. All the
elements are available in linear, quadratic and
cubic formuiations.
.
Types ofstructures that can be analyzed:
I-.2- and 3-D structures. including frame
(plane and space). shell. plane stress and
plane strain structures.
Capabilities:
Geometric and material nonlinearity
.
s Material can vary from one element to
another.
Prescribed displacements.
Isotropic, onhotropic or fully anisotropic
conductivity.
Latent heat for hear rransfer prohlrrns
involving phase change.
Specific heat far transient heat transfer
analysis
Can be run interactively or i n a batch mode.
Excellent post-processing and graphics
capabilities.
Contains subroutines for different
applications which can be modified by
experienced users for some specific
problems.
.
..
Program was validated on a large number of
problems which include:
.
Structures (concrete and steel) at ambient and
elevated temperatures.
.
Limitations:
No CAD capability for mesh generation.
Program requires an input file to accomplish
this.
Can only be run on mainframes.
Research and development work continues in different sectors in making better
utilization of computer generated solutions with the aim of eventually providing the
structural design engineer with solutions based on sound scientific and engineering
practice to handle the design of structures for fire.
4.3
Summary
The use of computer programs to evaluate the behaviour of structural steel in fire
is now becoming popular among structural design engineers. Information on the
computer programs that deal with this subject is presented in this section. Based on this
information, the following can be derived:
The overall structural behaviour of structures when exposed to fire is now a practical
reality with computer programs.
The influence of many parameters that affect the fire resistance of steel structures,
such as membrane or bridging action of the floor slab, continuity of support,
connection types, etc. can now be modelled with ease using computer programs.
Computer programs based on the finite element method are the most versatile
techniques to model complicated problems such as those encountered in the study of
the overall behaviour of unprotected and protected steel frames exposed to fire.
5.
CONCLUDING REMARKS
The introduction of steel as an important building material, the behaviour of steel
structures in fire and a literature survey on research activities on the behaviour of steel
structures in fire are presented in this report. The literature survey focused mainly on
overall structures rather than single element behaviour in fire. Both an experimental and
theoretical literature survey were presented.
Based on the information presented in this report, the following concluding
remarks can be made:
1.
2.
3.
4.
5.
6.
Steel compares favourably with other building materials because of the cheap
material costs, fast erection sequences and lighter foundation requirements.
Steel structures will lose their stiffnesses and strengths when exposed to fire due to
deterioration in the material property.
The problem of rapid increase of temperature in steel when exposed to fire is usually
solved by providing protection to steel structures with an insulating material, thus
delaying the temperature increase in the steel.
The fire resistance of a complete structure is entirely different from the fire resistance
of a single structural element.
The failure of some structural steel elements in a fire will not endanger the safety of
a complete steel framed building, if the rest of the structure can develop an
alternative load path to bridge the failed elements.
Membrane action from the reinforced concrete slab in steel framed composite
structures is the primary mechanism that accounts for the stability of structural
7.
8.
9.
10.
1I .
12.
elements at large deflections and for providing an alternative load path after the
failure of some structural elements.
The behaviour of three-dimensional steel frames in fire can be investigated by using
two-dimensional subframes if the frame is subjected to vertical loads. The subframes
chosen for investigation, however, must reflect the behaviour of beams and columns
in the area of interest without introducing unrealistic conditions or restraints.
Isolated elements can be used to quantify the behaviour of a whole frame in fire
when it is obvious that the condition of the beam or column in the frame dominates.
Various factors affecting the behaviour of structures in fire conditions that proved
difficult and complex to investigate using experimental techniques, can now be
investigated using computer programs.
The finite element method is the most general and versatile numerical method for
investigating various factors (e.g. non-linear material behaviour, geometric
nonlinearity, nonuniform temperature distributions and thermal strains) affecting the
behaviour of structures in fire conditions.
Structures surrounding a fire zone have a major influence on the performance of the
directly heated elements.
Factors such as continuity, element interactions, types of connections, sprinklers,
load ratio, restraint condition and temperature distribution will affect the
performance of a structure in fire.
Finally, the application of computer models to overall behaviour of steel
structures in fire have demonstrated their value as sound engineering tools. Continued
development in this direction will make it possible to use the models to identify and
evaluate different parameters. The most significant of the parameters will then be used to
develop an optimum design of structures for fire. It is, therefore, anticipated that the
design of a structure to withstand fire will eventually become a routine part of structural
design of building frames.
6.
REFERENCES
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4. Eurocode 4, "Design of Composite Structures", Part 10: Structural Fire Design
(Draft) Commission of the European Communities, 1990.
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~
-
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.. . ..
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t
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- ..
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.. . '~
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~.
'
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--
-
~
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~
~
~ 1976.
~
~
~
~
~
~
,
41. Schleich, J.B., "CEFICOSS - Computer Engineering of the Fire Resistance for
Composite and Steel Structures", Repon No. EUR10828, ABED, I.u?tembourg,
~
19x5
A,--.
42. Wickstrom, U., "TASEF-2 - A Computer Program for Temperature Analysis of
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Building Systems", Report IITRI, Project 58095, Chicago, IL, USA, 1972.
46. Jeanes, D.C, "Predicting Fire Endurance of Steel Structures", Preprint 82-033,
American Society of Civil Engineers, ASCE Convention, 1982.
47. Algor Inc., "Algor User Manual and Release Notes", Pittsburgh, PA, USA, 1993.
48. Hibbitt, Karlson and Sorensen Inc., "ABAQUS User Manual", Version 5.4,
Pawtucket, Rl,USA, 1994.
(a) Simply Supported beam [6]
(b) Continuous beam [6]
L~.fan
fln
(c) Effect of fire on moment distribution in restrained beams [7]
FIG. 1. Behaviour of Beams in Fire
FIG. 2. Moment Distribution in a Steel Frame
Exposed to Fire [7]
FIG. 3. Response of Structural Building Frame
to Fire [8]
-
FIG. 4. Plan of 2nd 3rd Floor of Eight-Storey Building at the Cardington
Large Building Test Facility (LBTF) [21]
m
0)
C
w
In
UQ171a1UB
61~h101U8
I
U)
D
i
#-O
1
'
I
R
Fixed
bases
7x3000mm
FIG.5. Section Through the Cardington Frame including Fire Compartments
Studied [33]