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 N

2

1Post Graduate Student, Department of Civil Engineering, the Oxford College of Engineering, Bangalore

2Assistant 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.