Study on the fire resistant design of reinforced concrete flexural members

IJRET: International Journal of Research in Engineering and Technology eISSN: 2319-1163 | pISSN: 2321-7308
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Volume: 09 Issue: 09 | September-2015, Available @ http://www.ijret.org 52
STUDY ON THE FIRE RESISTANT DESIGN OF REINFORCED
CONCRETE FLEXURAL MEMBERS
Jothis K Mathew1
, Arunkumar B N2
1
Post Graduate Student, Department of Civil Engineering, the Oxford College of Engineering, Bangalore
2
Assistant Professor, Department of Civil Engineering, the Oxford College of Engineering, Bangalore
Abstract
The inherent fire resistance property of concrete is one of its benefits. This thesis mainly focuses on fire resistant design of
RC flexural member’s viz. beams and slabs using finite element software ANSYS 13. Both thermal and thermo-structural
analysis was carried out for various parameters. Thermal analysis is done by taking into concern several parameters viz.
aggregate type, cover to the reinforcement, concrete thickness, strength of concrete etc. The results are compared with IS
456:2000 provisions. Thermo-structural analysis is conducted for various support conditions and load ratio. Elements were
modeled using SOLID 70 element and LINK 33 element for thermal analysis. For thermo-structural analysis instead of SOLID 70
element, SOLID 65 element was used. The parameters having a paramount influence on fire resistance are support
conditions, dimensions of members, action of members under load, cover to the reinforcement, type of aggregates etc. Effect of
the parameters on fire resistance is found out. Techniques to develop fire resistance are then found out. Moreover, it is found
out that by changing some parameters, better fire resistant design for structural elements can be achieved.
Key Words: Thermal analysis, Thermo-structural analysis, SOLID 70, SOLID 65, LINK 33
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1. INTRODUCTION
Fire resistant building components and systems have
specific fire resistance ratings on the basis of fire resistance
tests. These ratings are expressed in terms of minutes and
hours. It describes the time duration a given building
component or system is capable of maintaining specific
functions whilst being exposed to a specific simulated fire
event. To most buildings and structures, fire is a severe
potential risk. As a structural material, the use of concrete is
widespread. Upon baring RC member to fire, temperatures
in steel reinforcement as well as concrete escalates. This
leads to a declination of strength and stiffness, and a
potential damage to the structure. The existing technique of
valuing fire resistance of reinforced concrete columns and
beams is based on prescriptive approaches, and is generally
a function of concrete cover thickness, size of the member
and aggregate type.
1.1 Process of Fire Development
The process of fire development is shown in figure 1. For
the fire to reach flashover, ample amounts of fuel and
oxygen is needed. The object initially ignited do not contain
enough energy and do not release it swiftly enough (heat
release rate), flashover will not take place (e.g., small trash
can burn in the middle of a large room). Similarly, if the
fire tends to deplete the oxygen that is available, drop the
heat release rate then fire in the compartment should not
attain flashover. In the post flash over stage, the rate of
energy release is large amount but it is usually restricted by
ventilation.
Fig.1 Time - temperature curve
1.2 Objectives
In order to develop an overall different and safe structure, it
is important to understand the response of components
during loading. Experimental based testing is widely used to
analyze each individual element and the effects of concrete
strength under loading. Though this method is real life
response it is very time consuming and costly. Here, first of
all study the behavior of reinforced concrete flexural
members namely beams and slabs subjected to fire load.
And the study extended to get the result of effect of support
conditions, cover to the reinforcement, aggregate type, and
strength of concrete as different parameters on the behavior
of these members. The structure is analyzed in ANSYS 13.

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2. FIRE RESISTANT ANALYSIS OF BEAMS
FEA is an alternative to this, which is numerical method.
This is performed using a commercial software ANSYS
which is suite powerful engineering simulation programs.
More efficient and better analysis can be made with the use
of finite element packages. Development in the field of
computer aided engineering resulted in several benefits for
the engineering industries area last two decades. However in
the building industry advance finite element tools are used
for the development of accurate design methods.
ANSYS deals with 3 different stages for the structural
analysis:-
a) Pre-processing – Environmental factors and FE model
defining stage and to be applied to it.
b) Analysis solver - solution of finite element model.
c) Post-processing of results like deformation contours for
displacement, etc., using visualization tools.
2.1 Description of SOLID 70 element
SOLID 70 have a 3-D thermal conduction capability. The
element has eight nodes with temperature, a single degree of
freedom at each node. The element is applicable for a 3-D,
steady-state or transient thermal analysis.
F
ig.2 Geometry of SOLID 70 element
2.2 Description of LINK 33 Element
Basically LINK 33 comes under uniaxial element. And
which have the ability to transfer heat between its elements.
The element has eight nodes with temperature, a single
degree of freedom at each node. And the conducting bar is
applicable to steady-state or transient thermal analysis.
Fig.3 Geometry of link 33 element
2.3 Modeling and Meshing
Using “volume” option concrete block is created and
meshed with solid 70 using “mesh tool”, “volume sweep”
command. The command between steel reinforcement and
concrete is assumed as perfect and no loss of bond between
them is considered. The nodes for main steel, stirrups and
concrete are made common thus ensured the connectivity of
nodes. The meshed finite element beam with the lines
showing main beam, stirrups and concrete is shown in the
figure 4.
Fig.4 Finite element mesh for 200 mm Χ 350 mm beam
2.4 Parametric Study
Case 1: Aggregate Type
Fig.5 Thermal conductivity of different aggregate types

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Fig.6 Temperature profile for 200 mm Χ 350 mm beam for
various time exposure
Fig.7 Time –temperature curve for aggregate types
Case 2: Strength of Concrete
Fig.8 Thermal conductivity for normal strength and high
strength concrete
Fig.9 Time –temperature curve for NSC and HSC at the
position of reinforcement
Case 3: Cover to the Reinforcement
Fig.10 Thermal analysis of 200 mm Χ 350 mm beam for
different cover
Fig.11 Effect of cover on fire resistance of reinforced
concrete beams

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Beam
Parameter
Rebar temperature
Dimension
(mm)
(o
C )
Aggregate
Siliceous
aggregate 948
type
Carbonate
942
aggregate
200 Χ 350
25 671
Cover 30 647
(mm) 35 623
Strength of NSC 623
concrete HSC 660
Table-1: Effect of various parameters on the fire resistance
of RC beams
2.5 Thermo Structural Analysis Using Ansys 13
In finite element method the most commonly used
application in all probability is analysis of structure. The
particular term structure is not limited to civil engineering
structure like buildings and bridges but also connected to
most other engineering fields like mechanical structure.
2.5.1 Parametric Study
Case 1: Support Conditions
Fig.12 Deflection of simply supported beams for various
time exposures
Fig.13 Deflection of fixed beams for various time exposures

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Fig.14 Deflection of continuous beams for various time
exposures
Fig.15 Increase in deflection for various times of exposure
3. FIRE RESISTANT ANALYSIS OF ONE WAY
SLAB
Reinforced concrete one way slab is modeled using the
finite element software ANSYS-13. The slab was modeled
as 3-D block element in numerical structural model.
Restraint conditions on the supports are varied. Thermal and
thermo-structural analysis was carried out and deflection of
slab was found out for different time of thermal exposures.
3.1 Thermal Analysis of One-Way Slab
Thermal analysis was carried out for a slab of 6000
mmΧ3000 mmΧ100 mm. ISO fire curve was given as
thermal load. Analysis was done for 1/2, 1, 2, and 3 hour
thermal exposure and reinforcement bar temperature was
found out.
Fig.16 Temperature profile for 100 mm thick slab
Fig.17 Reinforcement bar temperature for different time of
exposure
3.2 Thermo Structural Analysis of One-Way Slab
Slab of 6000 mm Χ 3000 mm Χ 100 mm was analyzed with
a structural load of 5.4 kN/m2. Properties were given as
indicated in the previous chapter. 8 mm diameter
reinforcement was provided at a spacing of 200 mm center
to center. Deflection of slab was found out for various times
of exposures along with the application of structural load.
Fig.18 Modeling of 6000 mm Χ 3000 mm Χ 100 mm with
simply supported ends
Fig.19 Deflection of simply supported slab for 3 hour fire
exposure

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Fig.20 Deflection of simply supported slab for various times
of exposures
Fig.21 Deflection of fixed supported slab for various times
of exposures
4. FIRE RESISTANT ANALYSIS OF TWO WAY
SLABS
Slab is a major structural component. In this chapter fire
resistant analysis of two way slabs is studied. It consists of
both thermal and thermo structural analysis. Fire ratings are
determined for parameters like aggregate type, cover to the
reinforcement, thickness of slab, load ratio, support
conditions etc. using finite element software ANSYS13.
4.1 Modeling
Model of two -way slab used for the analysis is shown in the
figure 5.2. Slab of dimensions 4000 mm Χ 3000 mm Χ100
mm was used for the analysis. Model after meshing is
shown in figure 5.3. Meshing is done as per the requirement
in thermal analysis.
Fig.22 Model for slab 4000 mm Χ 3000 mm Χ 100 mm
Fig.23 Slab model after meshing
Slab Properties
Description Tested by Linus Lim
Cross Section 4000 mm Χ 3000 mm
Reinforcement 12mm diameter @200mm grid
Applied load 5.4kN/m2
Concrete cover 20mm
Thickness 100mm
Support condition Simply supported
Aggregate type Siliceous aggregate
Table-2: Description of the test slab
Fig 24 Comparison of test result with thermal
criteriaobtained from ANSYS for test slab

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4.2 Thermal Analysis Using Finite Element
Software
Case 1: Aggregate Type
Fig.25 Effect of aggregate on the fire resistance of slab
Case 2: Concrete thickness
F
ig.26 Effect of concrete thickness on fire resistance of slab
Case 3: Cover to the reinforcement
F
ig.27 Effect of cover on the fire resistance of slab
4.3 Thermo Structural Analyses Using Finite
Element Software
Case 1: Support Condition
Fig.28 Deflection of fixed slab for various time exposures
Fig.29 Deflection of simply supported slab for various time
exposures
Fig.30 Effect of support condition on deflection of slab
5. SUMMARY AND CONCLUSIONS
Fire resistant design of reinforced concrete flexural element
is dealt with in this project. Finite element software ANSYS
13 is used for analysis. Elements were modeled using
SOLID 70 element and LINK 33 element for thermal
analysis. For thermo-structural analysis instead of SOLID
70 element, SOLID 65 element was used.

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Beam is analyzed for various parameters like aggregate
type, concrete cover, and strength of concrete, support
conditions and load ratio. Effect of these parameters on the
fire resistance of beams was discussed.
Thermal and thermo-structural analysis was carried out In
order to find out the behavior of slabs under fire exposure.
To understand the effect of support conditions on fire
resistance one way slab is analyzed. Fire resistant analysis
of two way slab was done by considering various parameters
for different time of exposures.
Both thermal and thermo-structural analysis was carried out
for various parameters like aggregate type, concrete
thickness, concrete cover, type of exposure, hydrocarbon
fire, support conditions and load ratio and the results were
compared.
Some of the specific conclusions derived from the analysis
are listed below:-
[1]. As per IS code provisions, beam of 200mm width has
a fire rating of 120 minutes even with 20mm cover to
the reinforcement. However, as per ANSYS fire
rating; in order to get 120 minutes fire rating minimum
of 40mm cover is required.
[2]. Rebar temperature was reduced from 673°C to 623°C
upon increasing the cover from 25 to 35 mm.
[3]. For zero hour exposure, stress in concrete is only
8.96Χ106 N/m2. But it increases to 18.5Χ106N/m2
for a fire exposure of 30 minutes, i.e., with even 30
minutes of fire exposure, the stress in concrete is
increased by 51 %.
[4]. Moment capacity is reduced by 80% when exposed to
fire for 180 minutes.
[5]. In case of two way slab, for 30mm cover fire resistance
is 236 minutes. But for 20mm cover it is 216 minutes
only, i.e., cover thickness is directly proportional to
fire resistance.
[6]. In 180 minutes, for standard fire, temperature in
reinforcement is recorded as 556°C whereas due to
hydrocarbon fire it is 780°C. Hence, special care
should be taken in case of hydrocarbon fire.
[7]. Increase in load ratio from 0.2 to 0.5, deflection is
increased by 16.6 mm, for 60 minutes fire exposure.
General Conclusions obtained from fire resistant analysis of
beams and slabs include:-
[1]. The thermal response of concrete beams modeled in
ANSYS shows good agreement with available
experimental results.
[2]. For the same cross-section of the beam, theoretical fire
rating is less than that of IS code provisions.
[3]. During fire exposure, a thermal failure criterion is
more critical compared to deflection criteria and rate of
deflection criteria.
[4]. For structures exposed to fire, carbonate aggregate
concrete is favorable.
[5]. Large increase in rebar temperature resulting from
reduced concrete cover is associated with large plastic
and creep strains, leading to increased deflections in
the beam.
[6]. For structures subjected to fire loading, use normal
strength concrete instead of high strength concrete,
wherever possible.
[7]. Fixed beam behaves in better way compared to simply
supported beam under fire load, i.e., rotational restraint
at both end supports have much better behavior and
significantly higher fire resistance than pin-supported
ends.
[8]. For the three-bay beam, the continuity over the
supports does not greatly enhance the fire resistance
compared to a simply supported beam.
[9]. Increased load ratio causes early yielding of the steel
reinforcement and therefore accelerates the plastic and
creep strains. This in turn leads to lower stiffness in
the beam and results in substantial increase in
deflection and rate of deflection. Hence, in order to
improve fire resistance, keep the load ratio to a lower
value.
[10]. Temperature profiles generated for slabs exposed to
one side fire exposure shows good agreement with the
Eurocode2 temperature profile in shape.
[11]. In case of two way slab fire resistance is not much
affected by aggregate type as in case of a beam.
Increase in concrete thickness and concrete cover
reduces temperature in the reinforcement and thus
increases the fire resistance.
REFERENCES
[1]. Abu A, Ramanitrarivo V and Burgess, “Collapse
Mechanisms of Composite Slab Panels in fire” Sixth
international conference on structures in fire, June
2010, pp 382-389.
[2]. Allam.M.Said and Hazem M.F, “Behaviour Of One-
Way Reinforced Concrete Slabs Subjected To Fire”,
Alexandria engineering journal, December 2013,pp
749-761.
[3]. Amer M. Ibrahim, Mohammed Sh. Mahmood, “Finite
Element Modelling of Reinforced Concrete Beams
Strengthened with FRP Laminates”, European Journal
of Scientific Research, Volume.30, 2009, pp.526-541.
[4]. Antonio F. Barbosa and Gabriel O. Ribeiro, “Analysis
of Reinforced Concrete Structures Using Ansys
Nonlinear Concrete Model”, Computational
mechanics, 1998, pp 1-7.
[5]. Aqeel Ahmed, Venkatesh Kodur, “The Experimental
Behaviour Of FRPStrengthened RC Beams Subjected
to Design Fire Exposure”, Engineering Structures,
March 2011, Volume33, pp2201–2211.
[6]. Cashell K.A, Elghazouli And Izzuddin, “Influence Of
Reinforcement Properties On the failure Of Composite
Slabs In Fire”, Sixth international conference on
structures in fire ,June 2010 ,pp 373-381
[7]. Chang.J, Buchanan A.H, Dhakal R.P & Moss P.J,
“Simple Method For Modelling Hollow Core Concrete
Slabs Under Fire”, University of Canterbury, February
2005, pp 1-6.

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