Finite Element Modeling for Effect of Fire on Steel Frame

Mr.Madan M. Awatade.et al. Int. Journal of Engineering Research and Application www.ijera.com
ISSN : 2248-9622, Vol. 6, Issue 6, ( Part -5) June 2016, pp.15-24
www.ijera.com 15 | P a g e
Finite Element Modeling for Effect of Fire on Steel Frame
Mr.Madan M. Awatade1
, Dr.C.P.Pise2
, Prof.D.S.Jagatap3
, Prof.Y.P.Pawar3
,
Prof.S.S.Kadam3
,Prof.C.M.Deshmukh3
, Prof.D.D.Mohite3
.
1
PG Scholar, Department of Civil engineering, SKN Sinhgad College of Engineering, Pandharpur
2
Associate Professor, Department of Civil Engineering, SKN Sinhgad College of Engineering, Pandharpur.
3
Assistant Professor, Department of Civil engineering, SKN Sinhgad College of Engineering, Pandharpur.
ABSTRACT
This research is intended to be preliminary study lending to the detailed behavior of steel member i.e. Plane Frame.
This paper shows the behavior of steel structures in fire the use of steel in building construction and its behavior
when exposed to fire is presented. Fire performance of structural steel at elevated temperature includes the study of
steel frame subjected to fire. Also the effect of stress strain temperature on the fire performance of structural steel
should be observed. The behavior of a steel frame in a fire depends on many factors including the properties of the
steel and the coating material on it. Computer application based on ANSYS software used to study the various
parameters affecting the overall behavior of steel structures in fire is presented. The present paper shows the effects
of stress–strain relationships on the fire performance of steel frame exposed to uniformly increasing temperature
when steel is unprotected and protected with concrete using FEM.
Key words: Steel frame, Elevated temperature, Fire performance, Stress-Strain, Finite Element Model (FEM).
I. INTRODUCTION
Structural steel has been widely used
throughout the world. It is one of a designer’s best
options in view of its advantages over other
materials [1]. Steel is available in a range of
discrete size, and its ductile behavior allows plastic
deformation upon yielding, therefore avoiding
brittle failures. In reinforced concrete structures,
steel enhances the concrete strength by carrying the
tensile forces [2]. It is also commonly used to
reinforce timber constructions. In spite of its
advantages, steel on its own is vulnerable in fire
[3]. Elevated temperatures in the steel cause
reduction in its strength and stiffness which
eventually leads to failure due to excessive
deformations [4]. This is crucial in steel in
compared with concrete or timber members as steel
conducts heat very well and often comes in thin or
slender elements [5]. In structural design, there are
a few functional requirements such as those stated
in Clause C4 of the New Zealand Approved
Document (BIA, 1992):
“Buildings shall be constructed to maintain
structural stability during fire to:
a. Allow people adequate time to evacuate
safely,
b. Allow fire service personnel adequate time to
undertake rescue and fire fighting operations,
c. Avoid collapse and consequential damage to
adjacent household units or other property.”
There are a lot of different methods for
protecting structural steel to maintain its strength
and stability in fire, but little is known about the
True behavior of the steel members under various
support conditions and heating patterns [6].
The recommended fire resistance to be
applied to the steel structures is usually determined
based on furnace tests on single elements such as a
beam or a column [7]. Contrary to popular belief,
an unprotected steel element that is a part of a large
complex structure may have a sufficiently high
level of fire resistance to perform well in fire. This
is due to the ability of the overall structure to
redistribute loads from the heated area to the cooler
Neighboring elements [8].
The lack of understanding of the true
behavior of steel elements in fire leads to
inefficient and uneconomical design [9]. To assess
the overall performance of steel frames, it is
important to understand the detailed behavior of a
single beam with several support conditions that
represent various elements in a complex structure
[10].
II. MATERIAL USED AND THEIR
PROPERTIES
2.1. Steel
The physical properties of steel are totally
different from it component element viz. iron and
carbon one of the major property of steel is the
ability to cool down rapidly from an extremely hot
temperature after being subjected to water or oil
physical properties depends on percentage
composition of the constituent element and the
manufacturing process a particular amount of
RESEARCH ARTICLE OPEN ACCESS

carbon can be dissolved in iron at specific
temperature.
The physical properties of steel include
high strength, low weight durability, flexibility and
corrosive resistance. Steel as well all knows, offer
great strength though it is light in weight in fact the
ratio of strength to weight for steel is the lowest
than any other building material as of now .by the
term flexibility it means steel can easily molded to
form any desired shape .
Unlike the constituent element iron steel
does not corrode easily on being exposed to
moisture and water the dimensional stability of
steel is desired property as the dimension of the
steel remains unchanged even after many years or
being subjected to extreme environmental
condition steel is a good conductor of electricity
i.e. electricity can pass through steel.
Steel grade are classified by many
standard organization based on the composition
and physical properties of the metal the deciding
factor of the grade of steel is basically the hardness
of the metal which differ s depending on the
percentage of carbon content. higher the carbon
content the harder and stronger the steel metal
along with more chances of fracture high quality
steel contain less carbon yet retains the strength
and hardness.
Earlier forms of steel consisted of more
carbon, as compared to the present day steel.
Today the steel manufacturing process is such that
less carbon is added and the metal is cooled down
immediately, so as to retain the desirable physical
properties of steel. there are other types of steel
such as galvanized steel and stainless steel
(corrosion-resistance steels) .Galvanized steel is
coated with zinc to protect it from corrosion
whereas stainless steel contain about 10 percent of
chromium in its composition Structural steel has
been used throughout the world it is one of a
designers best options in view of its advantages
over other materials. Steel is available in arrange of
discrete size, and its ductile behaviour allows upon
yielding. Therefore avoiding brittle failures. In
reinforced concrete structures, steel enhances the
concrete strength by carrying the tensile forces .It
is also commonly used reinforce construction.
In spite of its advantages, steel on its own
vulnerable in fire . Elevated temperature in the
steel , cause reduction in its strength and stiffness ,
which eventually leads to failure due to excessive
deformations. This is crucial in steel compared
with concrete or timber member as steel conducts
heat very well and often comes in thin or slender
elements.
2.2. Thermal properties of steel
2.2.1 Thermal elongation
The thermal elongation of steel is the
increase in length of a member caused by heating
divided by the initial length. Figure 2.1 shows the
relationship of Elongation and the temperature of
steel according to the Euro-code 3 (EC3, 1995).
The Discontinuity in the thermal elongation
between 750o
c and 860o
c is due to a phase
Transformation in the steel. The following
equations from the Euro-code 3 (EC3, 1995)
Describe the thermal elongation relationships in
steel. The thermal elongation of steel is determined
by the Euro-code 3 formulae as a function of the
steel temperature and illustrated in Figure 2.1.
For 200
C<T<7500
C
E thermal=1.2x10-5
xT+0.4x10-8
xT2
-2.416x10-4
For 7500
c <T< 8600
C
E thermal=1.1x10-2
For 8600
C<T<12000
C
E thermal=2x10-5
xT-6.2x10 -3
Fig.2.1 Thermal elongation of steel as a function of
temperature (EC3, 1995)
In simple calculation models, assuming
the thermal elongation to have constant
relationship with the temperature of the steel, the
elongation can be taken as:
Thermal Strain=14*10-6
(TS-20)
2.2.2 Thermal conductivity
Thermal conductivity is the ability of a
material to conduct heat and is defined as the ratio
of heat flux to the temperature gradient. For steel
materials, it is dependent on steel composition as
well as the steel temperature. Figure2.2 shows that
the EC3 steel model has a linear reduction in
thermal conductivity from 20 to 800 C and is
constant. The variation of the thermal conductivity
of steel can also be determined from the following
Euro code formulae and is illustrated below:
For 200
C<TS<8000
C
Thermal conductivity=54-3.33x10-2
TS
For 8000
C<TS<12000
C
Thermal conductivity=27.3

Fig.2.2 Thermal conductivity of steel as a function
of temperature (EC3, 1995)
Thermal conductivity of steel with
temperature greater than 1200 0
C is not defined in
the Euro code 3 as most structural steel members
can hardly survive such heat. The value of 27.3
W/m K is taken for T > 1200 0
C if such case needs
to be considered as dealt with in the thermal
analysis. For simple calculation models that are
independent of the temperature, the value of 45
W/Mk can be adopted.
2.2.3. Specific heat
Specific heat is the ability of a material to
absorb heat. The specific heat of steel is
independent of steel composition and varies only
with the temperature. Figure 2.3 shows the
relationship of specific heat and temperature of
steel according to the Euro code 3 (EC3, 1995). At
730 o
c there is a metallurgical change in the steel
crystal structure that causes a peak specific heat.
The equations from the Euro-code 3 (EC3, 1995)
for the specific heat relationships are shown below.
For 200
C<TS<6000
C
Cs=425+7.73x10-2
Ts-1.69x10-3
T 2
+2.22x10-6
T3
For 6000
C<TS<7350
C
Cs=666+13002/(738-Ts)
For 7350
C<TS<9000
C
Cs=545+17820/(Ts-731)
For 9000
C<TS<12000
C
Cs=650
Fig.2.3 Specific heat of steel as a function of
temperature (EC3, 1995)
Simple calculation models take the
specific heat of steel as 600 J/kg K, independent of
the temperature of the steel. For temperature
greater than 1200 0
C, the specific heat is taken as
650 J/kg K.
2.3 Mechanical properties of steel
2.3.1 Components of strain
Strain is the measure of elongation of an element
with respect to its original length. The change in
strain with temperature is defined as:
Change in strain (dl/l) =Eth (T)+E6 (6,T)+ ECr
(6,T,t)
Where,
Eth is the thermal strain;
E6 is the stress-related strain;
ECr is the creep strain.
The thermal strain is the elongation of the
material due to heat, and is commonly referred to
as thermal expansion as described by Rotter (1999)
and is summarized in section . It is a very
important aspect especially in larger structures with
the elements restrained by the adjacent members.
2.3.2 Creep strain of steel tested in tension
Creep strain in structural steel only
becomes significant at temperatures over 400°C or
500°C. Kirby and Preston (1988) have shown that
the creep is highly dependent on Temperature and
stress level of the steel (Figure 2.4). The creep
strains increase rapidly where the curve becomes
nearly vertical at higher temperatures. Therefore,
creep deformations are important when the steel
members approach their collapse loads. The Euro
code 3 (EC3, 1995) describes that the stress-strain
relationships usedFor design implicitly include the
likely deformations due to creep during the fire
exposure.
Fig.2.4 Temperature and stress level of the steel
2.3.3Stress-strain relationship
Harmathy (1993) has obtained typical
stress-strain relationships for structural steel at
elevated temperatures (Figure 2.5). The figure
shows that yield strength and modulus of elasticity
both decrease with increasing temperature.
However, the ultimate tensile strength increases

slightly in the temperature range of 180 0
C to 370
0
C before decreasing at higher temperatures. Figure
2.5 shows the stress-strain relationships of hot-
rolled structural steels at elevated temperatures
given in the Euro code 3 (EC3, 1995) for designing
steel members subjected to elevated temperatures.
Fig.2.5 stress-strain relationships for structural
steel at elevated temperatures
2.4 Concrete
Concrete a homogeneous mixture made
up of cement, sand and aggregate. All the
constituent of concrete are bad conductor of heat
and electricity. Also it is basically used for R.C.C.
structure .but we can use the concrete as a thermal
insulating material with a suitable thickness is
provided on a steel frame so that the effect of fire
should be reduced.
III. METHODOLOGY
Analyses of steel frame with specify
reference to thermal load arising from the fire
disaster.
3.1 INTRODUCTION
For the design of plane building frame in
steel, the problem of the analysis with respect to
thermal loads arising from the fire has nowadays
acquired significant importance, large of such
activates as accidents, sabotage etc. The structural
behavior of the frame in such cases in governed by
phenomenon of material non linearity coupled with
geometric non linearity. The problem of analysis is
thus quite complex. In the present work simplistic
approach of analysis through stepwise linear
analysis is taken up Hence in this the method of
linear analysis is presented in the next chapter the
methodology presented here is applied to carry out
stepwise analysis simulating the nonlinear behavior
mentioned above.
3.2 METHODOLOGY
The linear plane frame analysis is carried
out Following steps
Step 1 Finite element idealization of the frame
structure
Step 2 Formation and solution of the equations
governing equilibrium of the idealized frame
subject to approach boundary conditions
Step 3Evaluation of the structure response of the
elements of the idealized frame
This methodology is discussed through illustrative
frame with details shown in figure3.1
Fig: 3.1 Schematic of truss
Step 4.After that calculating the thermal strain
analytically using the formula given in Euro-code
3.
Step 5.Using FEA Formulation of truss model
shown in fig. 3.2
Fig: 3.2 Truss model using FEA
Step 6.Finding the thermal strain and deformation
Step 7.After that validate the analytical result and
result using FEA.
IV. ANALYTICAL INVESTIGATION
The thermal elongation of steel is determined by
the Euro-code 3 formulae as a function of the steel
temperature.
For 200
C<T<7500
C
E thermal=1.2x10-5
xT+0.4x10-8
xT2
-2.416x10-4
For 7500
c<T<8600
C
E thermal=1.1x10-2
For 8600
C<T<12000
C
E thermal=2x10-5
xT-6.2x10 -3
The analytical investigation include the
study of behavior of steel frame subjected to fire
.the study include parameter like thermal stress,
thermal strain and temperature.
The temperature and corresponding
thermal strain is as shown in Table: 4.1 below.

Sr.No. Temperature Thermal strain
1 100 0.9984x10-3
2 110 1.1268x10-3
3 120 1.1256x10-3
4 130 1.386x10-3
5 140 1.5168x10-3
6 150 1.6484x10-3
V. FINITE ELEMENT ANALYSIS OF
SPECIMEN
5.1. Introduction:
This chapter consists of modeling and
analysis of truss members. Truss members are
made up of steel material having I section and I
section is surrounded with concrete. Effect of
temperature and self-weight of both types of
specimen on elastic strain is obtained by using
FEA package ANSYS 14.
5.2 Finite Element Modeling and Analysis:
Two dimensional finite element analysis has been
carried out in ANSYS 14.5. Schematic of truss is
as shown below in Fig.5.1
Fig. 5.1 Schematic of truss
In first step of modeling, effect of
temperature is determined for truss members
having I section which is made up of steel material.
In second step of modeling I section of steel
material is layered by concrete. CAD model of
both sections is as shown in figure.
5.2. (A) I section with steel material only.
5.2. (B) I section with geometric details.
5.2. (C) Composite I section.
5.2(D) Composite I section with geometric details.
A beam 188 element has been used to
mesh the model. It is as shown in the figure 5.3
Fig. 5.3 Beam Element
BEAM188 is based on Timoshenko beam
theory, which is a first-order shear-deformation
theory: transverse-shear strain is constant through
the cross-section (that is, cross-sections remain
plane and undistorted after deformation). The
element can be used for slender or stout beams.
Due to the limitations of first-order shear-
deformation theory, slender to moderately thick
beams can be analyzed. Use the slenderness ratio
of a beam structure. For both types of sections,
beam 188 element has been used.

Table 5.1 Details of meshing
Sr.No. Model Node Element
1 I section 853 854
2 Composite I
section
86 88
Material properties used in analysis are given table
5.2.
Table 5.2 Material properties
Sr.
No
Materials Steel Concrete
1 Modules of Elasticity in
N/mm2
200 000 22 361
2 Poisson’s ratio 0.3 0.28
3 Density Kg/mm3
7850 E-9 2400 E-9
4 Coefficient of Thermal
Expansion (aplx)
mm/mm 0
C
12 E-6 8 E -6
2D coupled field analysis is carried out. In
boundary conditions, degrees of freedom are made
zero in Y-direction at the bottom of truss member
as shown in figure 1.1. Temperature is applied on
each link of truss member and corresponding
values of strain and deflection are derived.
Temperature r is varied with range 1000
C – 1500
C
and corresponding results are plotted as shown
below.
Fig. 5.4 Response at 1000
C
Fig.5.4 shows the response at temperature
1000
C and values of strain and deformations at
temperature 1000
C.when the steel frame is
unprotected that is only I-section is used for made
up of steel frame.
C
1100
temperature 1100
up of steel frame.
C
1200
temperature 1200
up of steel frame.
C
1300
temperature 1300
up of steel frame.
C

1400
temperature 1400
up of steel frame.
C
1500
temperature 1500
up of steel frame.
Results are tabulated as shown below
Table 5.3 Results of analysis for I section with
respect to temperature when it is unprotected
Similarly, temperature is applied on each
link of truss member which has composite I
section. Temperature is varied with range 1000
C –
1500
C and corresponding results are plotted as
shown below.
C
Fig.5.10 shows the response at
temperature 1000
C and values of strain and
deformations at temperature 1000
C.when the steel
frame is protected with insulating material like
concrete that is composite I-section is used for
made up of steel frame.
C
temperature 1100
C.when the steel
C
temperature 1200
C.when the steel
C
Fig 5.13 shows the response at
temperature 1300
C.when the steel
Sr.
No.
Temperature Maximum
strain SMX
Deflection
DMX
1 100 0.254x10-3
10.4731
2 110 0.279 x10-3
11.5204
3 120 0.304 x10-3
12.5677
4 130 0.330 x10-3
13.6151
5 140 0.355 x10-3
14.6624
6 150 0.380 x10-3
15.7057

C
temperature 1400
C.when the steel
C
temperature 1500
C.when the steel
Results are tabulated as shown below
Table 5.4 Results of analysis for composite I
section with respect to temperature when it is
protected with concrete.
VI. RESULT AND DISCUSSION:
The results are as shown in below in graphical
format:
6.1 Results for analytical investigation:
Graph 6.1 Strains vs. Temperature
Above graph 6.1 shows strain vs.
Temperature relation the value of strain at different
elevated temperature having range is 1000
c-
1500
c.As temperature increases strain increases.
These values are obtained from the formula in
Euro-code 3.
6.2 Result of steel frame when it is unprotected
is as shown below:
c-1500
c
When Steel Frame is Unprotected. As temperature
increases strain increases. These values are
obtained from the ANSYS Software.
Graph 6.3 Displacements vs. Temperature
Above graph 6.3 shows Displacement vs.
Temperature relation the value of Displacement at
different elevated temperature having range is
Sr. No Temperature SMX DMX
1 100 0.00012 9.570
2 110 0.000132 10.419
3 120 0.000144 11.655
4 130 0.000156 12.709
5 140 0.000168 13.459
6 150 0.00018 14.515

1000
c-1500
c When Steel Frame is unprotected .As
temperature increases Displacement increases.
These values are obtained from the ANSYS
Software.
6.3 Result of steel frame when it is protected
with concrete is as shown below:
c-1500
c
When Steel Frame is protected with concrete. As
temperature increases strain increases. These
values are obtained from the ANSYS Software.
Graph 6.5 Displacements vs. Temperature
Above graph 6.5 shows Displacement vs.
Temperature relation the value of Displacement at
different elevated temperature having range is
1000
c-1500
c When Steel Frame is protected with
concrete. As temperature increases Displacement
increases. These values are obtained from the
ANSYS Software
VII. CONCLUSION
Based on the studies so far carried out the
following conclusions can be drawn.
1. From the above study it can be concluded that as
temperature increases strains are increases when
steel is unprotected but when steel is protected with
material like concrete then the value of strain is
reduced by 57.48% and the values are shown in
table 7.1 below.
Strain
Sr.No. Unprotected protected % reduction
1 0.254x10-3
0.000108 57.48
2 0.279 x10-3
0.000120 56.57
3 0.304 x10-3
0.000132 56.63
4 0.330 x10-3
0.000144 56.36
5 0.355 x10-3
0.000156 56.05
6 0.380 x10-3
0.000168 55.78
2. From the above study it can be also concluded
that as temperature increases displacements are
increases when steel is unprotected but when steel
is protected with material like concrete then the
value of displacement is reduced by 9.96% and the
values are shown in table 7.2 below.
Displacement
Sr.No. Unprotected protected % reduction
1 10.4731 9.4290 9.96
2 11.5204 10.4767 9.05
3 12.5677 11.5244 8.30
4 13.6151 12.5720 7.66
5 14.6624 13.6197 7.11
6 15.7057 14.6674 6.61
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Finite Element Modeling for Effect of Fire on Steel Frame

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