seminar original

8/11/2019 Seminar Original

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CONTENT

CANDIDATES DECLARATION.................................................................................... i

ABSTRACT..................................................................... Error! Bookmark not defined.

ACKNOWLEDGEMENT.............................................. Error! Bookmark not defined.

LIST OF FIGURES .........................................................Error! Bookmark not defined.

LIST OF TABLES ...........................................................Error! Bookmark not defined.

CHAPTER - 1

INTRODUCTION ............................................................................................................. 1

1.1 GENERAL ......................................................................................................... 1

1.2 FIRE LIMIT STATE .......................................Error! Bookmark not defined.

1.3 IS CODES FOR FIRE RESISTANCE ............................................................ 16

1.4 NEED OF STUDY ............................................................................................ 2

1.5 AIMS AND GOALS ....................................... Error! Bookmark not defined.

1.6 METHODOLOGY ..........................................Error! Bookmark not defined.

1.7 ORGANISATION OF DISSERTATION .......Error! Bookmark not defined.

CHAPTER - 2

LITERATURE REVIEW............................................... Error! Bookmark not defined.

2.1 GENERAL ...................................................................Error! Bookmark not defined.

2.2 IS CODE PROVISIONS FOR FIRE RESISTANCE .....Error! Bookmark not

defined.

2.3 EFFECT OF HIGH TEMPERATURE ON CONCRETE .... Error! Bookmark

not defined.

2.3.1 Spalling of Concrete .........................Error! Bookmark not defined.

2.3.2 Modulus of Elasticity. ....................... Error! Bookmark not defined.

2.3.3 Concrete Compressive Strength ....... Error! Bookmark not defined.

2.4 STRESS-STRAIN RELATIONSHIP FOR CONCRETE AT ELEVATED

TEMPERATURE ..................................................Error! Bookmark not defined.

2.4.1 Stress-Strain Relationship as per Eurocode2 ...Error! Bookmark not

defined.

2.4.2 Stress-Strain Relationship for Confined Concrete (Youssef et al,

2007) ..........................................................Error! Bookmark not defined.

2.5 CONSTITUTIVE MODEL COMPARISON .. Error! Bookmark not defined.

CHAPTER3


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THERMAL ANALYSIS.................................................Error! Bookmark not defined.

3.1 GENERAL ....................................................... Error! Bookmark not defined.

3.2 THERMAL PROPERTIES ............................. Error! Bookmark not defined.

3.2.1 Thermal Properties of Concrete ........Error! Bookmark not defined.

3.2.2 Thermal Properties of Steel ..............Error! Bookmark not defined.

3.3 ANALYTICAL STUDIES .............................. Error! Bookmark not defined.

3.3.1 Sectional Details: Beam and ColumnError! Bookmark not defined.

3.3.2 Fire Scenario and Exposure Condition ............Error! Bookmark not

defined.

3.4 ANALYSIS: PROCEDURE AND THEORY . Error! Bookmark not defined.

3.4.1 Uncoupled Heat Transfer Analysis ...Error! Bookmark not defined.

3.4.2 Effect of Boundary Condition ..........Error! Bookmark not defined.

3.5 HEAT TRANSFER ANALYSIS RESULTS ..Error! Bookmark not defined.

3.5.1 TNC1: Results and Comparison .......Error! Bookmark not defined.

3.5.2 THC4: Results and Comparison .......Error! Bookmark not defined.

3.5.3 THC8: Results and Comparison .......Error! Bookmark not defined.

3.5.4 B1: Results and Comparison ............Error! Bookmark not defined.



CHAPTER4

STRESS ANALYSIS .......................................................Error! Bookmark not defined.

4.1 GENERAL ....................................................... Error! Bookmark not defined.

4.2 ANALYTICAL STUDIES .............................. Error! Bookmark not defined.

4.2.1 Test Conditions and Procedure .........Error! Bookmark not defined.

4.3 ANALYSIS: PROCEDURE AND THEORY . Error! Bookmark not defined.

4.3.1 Element Description ......................... Error! Bookmark not defined.

4.3.2 Material Model for Concrete ............Error! Bookmark not defined.

4.3.2.1 Linear Elasticity .................Error! Bookmark not defined.

4.3.2.2 Concrete Damaged Plasticity Model Error! Bookmark not

defined.

4.3.3 Material Model for Reinforcement ...Error! Bookmark not defined.

4.3.3.1 Classical Metal Plasticity ...Error! Bookmark not defined.

4.4 STRESS ANALYSIS OF BEAMS .................Error! Bookmark not defined.

4.4.1 Beam B1 ...........................................Error! Bookmark not defined.


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5.3 RECOMMENDATIONS FOR FURTHER WORK ......Error! Bookmark not

defined.

REFERENCES................................................................Error! Bookmark not defined.


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1INTRODUCTIONObjective:- Briefly explain the problem statement, its significance andscope of the work carried out.


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CHAPTER1

INTRODUCTION

1.1 GENERAL

Major advances in fire engineering of building structures have occurred in the past decade with

considerable effort made in the understanding of how real structures behave in fire. The

knowledge base is by no means complete but the advances that have occurred to date allow a

more realistic assessment of the fire performance of structural components. This study

summarises the current knowledge with respect to both reinforced and pre stressed concrete

structural elements although some of the discussion can be readily applied to other materials.

Concrete elements can generally be divided into two types: flexural members such as beams and

floors, and compression members such as columns and walls. The ability of loadbearing

elements to remain loadbearing is usually a major concern in a fire, while walls, ceilings and

floors may also be required to contain fire by preventing its spread directly through the elementor by restricting excessive heat transmission from the side exposed to the fire to the side not

exposed.

Existing simplified practices used to confirm adequate member sizes or to design new members

rely on a number of assumptions, which are necessarily conservative. In a large number of cases

these practices will still be adequate as fire resistance requirements may not govern the design.

However, there may be instances where member design is over conservative where factors such

as the beneficial effects of continuity and restraint are not currently adequately assessed, or

unsafe where the effects of restraint have been incorrectly assessed.

Much of the early work and effort put into investigating the fire performance of concretemembers was conducted in the United States on behalf of the pre-stressed concrete industry. Pre-

stressed concrete beams were known to be more susceptible to fire damage due to the rapid

deterioration in strength of cold-worked pre-stressing tendons and wires at high temperature.

Member sizes for pre-stressed elements also tend to be more slender than for reinforced

construction. Fortunately, most of the work on pre-stressed concrete can easily be extended to

include reinforced concrete construction.

Reinforced concrete (RC) beams function as critical load bearing structural members in a

building, and hence the provision of appropriate fire resistance is one of the major design

requirements in buildings. The basis for this requirement can be attributed to the fact that, when

other measures for containing the fire fail, structural integrity is the last line of defence. Fireresistance is the duration a structural member (system) exhibits resistance with respect to

structural integrity, stability and temperature transmission under fire conditions. Typical fire

resistance requirements for specific building members are specified in building codes. Fire

resistance can play a crucial role in the performance of buildings and infrastructure in the event

of fire, as seen in the collapse of the World Trade Centre twin towers.


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1.2 NEED OF STUDY

First, the methods now used for assigning fire resistance classifications are being seriously

challenged. These methods are based on results of standard fire tests of building materials and

constructions, i.e., ASTM E-119. Nevertheless, testing agencies, as well as architects and

engineers appears to be facing an insurmountable task because no single standard fire test can

accurately reflect the behavior of an assembly under the many different ways the assembly can

be used in buildings.

Existing simplified practices used to confirm adequate member sizes or to design new members

rely on a number of assumptions, which are necessarily conservative. In a large number of cases

these practices will still be adequate as fire resistance requirements may not govern the design.

However, there may be instances where member design is over conservative where factors such

as the beneficial effects of continuity and restraint are not currently adequately assessed, or

unsafe where the effects of restraint have been incorrectly assessed.

Generally, concrete structural members (made from NSC) exhibit good performance under fire

situations. The fire resistance of RC structural members is evaluated using a prescriptive-based

approach. While the Eurocode specifications provide some options for performance-based fire

resistance design, the specifications in American as well as Indian standards are highly

prescriptive. There are many drawbacks to the prescriptive-based approach of evaluating the fire

resistance, since this approach is based on standard conditions and does not account for

realistic scenarios. Therefore, the current design approaches may not be fully applicable for use

under the recently introduced performance-based codes that facilitate innovative, cost-effective

and rational designs

Firstly, the development of rational design approaches for fire safety evaluation requires an

insight into the extent of the influence of various factors governing the fire resistance of46

(Kodur

and Dwaikat) RC beams, namely fire scenario, load level, concrete cover thickness, failure

criteria, aggregate type and span length.

Secondly, it is often possible to provide added fire resistance more economically through

structural design than by other more conventional methods. Considerable research has been

carried out into the effect of fire on the behaviour of steel and composite beams and reinforced

concrete slabs, while very less similar research undertaken into the behaviour of concrete beams.

Reinforced concrete beams within real structures may be subjected to varying support conditions

because neither total horizontal restraint nor total rotational restraint is likely to be realised.

Thus, the behaviour of the beams with varying support conditions has to be analysed.

The previous research on steel and composite beams has shown that the effect of a decay phase

of the fire can have detrimental consequences on the structure. Therefore there is great interest in

the behaviour of a concrete beam exposed to the full process of fire development.

In view of all the factors outlined above, this literature review is set out to investigate the

behaviour of reinforced concrete beams having a range of support conditions and subjected to

ISO and parametric fires.


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Basic Definitions

Fire Endurance. A measure of elapsed time during which a material or

assembly continues to exhibit fire resistance under specified conditions of testand performance.

Fire Resistance. This is the property of a material or assembly to withstand fire

or to give protection from it. As applied to elements of a building, it is

characterized by the ability to confine a fire or continue to perform a given

structural function or both.

Notwithstanding this definition given above fire resistance and fire endurance

have been used interchangeably by various authors.

Fire resistance can also be defined as the measure of the ability of a building

element to resist a fire. Fire resistance is most often quantified as the time for

which the element can meet certain criteria during exposure to a standard fire-

resistance test. It is a property assigned to building elements; individual materials

do not possess fire resistance.

During a fire test, a structural element must perform its load-bearing function and

carry the load for the duration of the test without collapse. It then meets the

stability criteria. Barriers like walls and slabs additionally have to meet the

integrity and insulation criteria to prevent fire spreading from the room of origin.

Therefore, the test specimen must not develop any cracks or fissures which allow

smoke or hot gases to pass through and the temperature of the cold side must

not exceed a specified limit (Buc-01).

It is important to realise that the fire-resistance time does not express the time a

structure might resist in a real fire, as the duration of an actual fire cannot be

precisely specified.The construction in a building may perform satisfactorily for a

shorter or a longer period depending upon the characteristic of the fire (Mal-82).

Failure Criteria2(purkiss)

1. Insulation (denoted as I): The average temperature on an unexposed face

achieves a temperature of 140C or a local value exceeds 180C.

2. Integrity (denoted as E): Cracks or openings occur in a separating element such

that ignition can occur on the unexposed face.


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3. Load-bearing capacity (denoted as R): The element being tested loses load-

bearing capacity when the element is no longer able to carry the applied loading.

In practice, however, deflection limits are imposed, partly in recognition of the

fact that at collapse, large deflections occur due to the formation of plastic hinges

in beams or slabs, or due to incipient buckling in walls or columns and partly toavoid the specimen collapsing into the furnace with possible consequential

damage to the furnace and loading system. For any members such limits should

not be applied until the deflection reaches L/30. Then for

Flexural members:

Limiting deflection is L2/400d (mm) or rate of deflection L2/9000d (mm/min)

Where d is the depth of the member and L the span, both in mm.

Vertically loaded members:Limiting vertical contraction is h/100 (mm) or rate of contraction 3 h/1000

(mm/min)

Where h is the initial height of the member (mm).

Isotherm

A Line drawn on the cross section of a member connecting points of same

temperature.


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GENERAL

The fire resistance of an RC beam depends on a number of factors including fire

scenario, sectional characteristics, load level, geometric properties and support

conditions. The current fire resistance provisions in codes and standards are

prescriptive and do not account for many of these factors. The purpose of this

study is to quantify the influence of these parameters on the fire resistance of RC

beams and to develop a simplified approach for fire resistance design of RC

beams under a performance-based code environment.

Fire Models

Most fire resistance tests follow time-temperature curves that serve as

standard fires which are idealized simulations of room fires. Since the tests

follow established time-temperature curves, the heat load imposed on a test

specimen is calculable at any point during testing. Standard fire test time-temperature curves for various countries can be seen in Figure 2.1 (Lie 1992). The

most widely used standard test conditions are the ASTM E119 (United States and

Canada) and ISO 834 (Australia, New Zealand, and England) (Buchanan 2001).

Several models of time-temperature relationships are available for the simulation

of fires for design purposes:

A simplified equation that approximates the ASTM E119 curve is given by (Lie

1992):

where th (hours) is the time. The conditions for failure for reinforced concrete

components exposed to the ASTM E119 protocol are (Ellingwood & Shaver 1979):

o Collapse of the component or failure to inhibit passage of flame or

hot gases

o Attainment of the limiting average temperature of 593C in

reinforcemento Rise of 139C in the average temperature of the unexposed surface

of the test component.


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The ISO 834 fireis the basis of most fire resistance tests and is defined according

to the following equation:

T = 345 log10(8 t +1) + T0

where t is the time (minutes) and To is the ambient temperature (C).

Where a structural member is engulfed in flames from a large pool fire, the

hydrocarbon fire curve according to EC1 (EC1-03) should be used. The

hydrocarbon fire curve is defined as follows:

T = 1080 (10.325 e0.167

t0.675

e2.5t) + 20

Where t is the time (minutes).

Structural members located outside a burning compartment will be exposed to

lower temperatures than the members inside a compartment unless they are

engulfed in flames.They can be designed according to:

T = 660 (10.687 e0.32

t0.313

e3.8t) + 20

Where t is the time (minutes).

The fires mentioned above are shown in Figure

The Eurocode (EC1-03) also provides an equation for parametric fires, which

allows a time-temperature relationship considering any combination of fuel loads,

ventilation openings and wall lining materials. Thus the fire curve can be adjusted

to any existing fuel load to produce a realistic temperature development to allow

for a performance-based design.


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The temperature during the heating period is defined as:

Where

Time Equivalent Methodology

The methodology for establishing time equivalency between standard and design

fire scenarios is based on equivalent energy concept. The energy based concept is

better suited, than equal area or maximum temperature approach, for

establishing equivalency since equal energy concept relates the fire severity, and

thus resulting fire resistance, to the amount of energy transferred to the beam.

Accordingly, two fires will have the same fire severity if they transfer same

amount of energy to an RC beam. The amount of energy transferred to an RC

beam exposed to fire is related to the heat flux on the fire exposed boundaries ofthe beam, which involves heat transfer through convection and radiation. The

convection and radiation heat flux on the boundary of an RC beam exposed to fire

can be given by the following two formulae respectively: [5]


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or

The term E represents the energy bound by the time- temperature curve of

given fire exposure.Both A and are assumed to be constant

where A = Area of boundary exposed to fire and E = total energy

where t e(FE) =time equivalent computed from maximum deflection method (or

FE analysis) and

t e(energy) =time equivalent computed from equivalent energy method and

Tmax = maximum temperature of design fire.


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This energy based approach can only be applied if the temperature of thecompartment can be assumed as a single temperature and the convective heat

transfer coefficient and emissivity have to be homogeneous along the structural

element. In the current study, an emissivity (e) value of 0.5 and convective heat

transfer coefficient (hc)of 25 W/m2K are used. Thus using the equivalent energy

principle, a design fire will have the same severity as that of the standard fire if Es

= Ed

where Es = total energy under the heat flux (q/) curve of the standard fire, and

Ed = total energy under the heat flux (q/) curve of the design fire.

Consequently, the equivalent time can be computed by equating the total area

under the heat flux (q/) curve for the design fire with the area under the heat

flux (q/a) curve for the standard fire as shown in Fig. 5 (for the standard and

design fires). To arrive at equivalency, first the total area under the heat flux

curve for the design fire (area B in Fig. 5) is computed. The area under the heat

flux of a standard fire (area A in Fig. 5) is computed at various time steps. The

time at which area A (which varies as a function of time) equals area B is the time

equivalent of the design fire(kodour)2.

2.1.2 Numerical and Analytical Methods

Due to the costs involved in performing fire tests, numerical and analytical

methods have been developed as an economic alternative for determining fire

resistance. These methods have proven to be successful in predicting the fire

resistance of structural elements (Lie 1972; Lie 1992), and the application and


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limitations of each are explained. The main advantage of analytical methods is

that simple graphs and formulae can be used to estimate the fire resistance

(Bushev, Pchelintsev, Fedorenko, & Yakovlev 1972; Lie 1972; Malhotra 1982;

Wade 1991). These techniques eliminate the need for computers and special

testing devices, and estimations can be done quickly without much effort byapplying simple algebra.

However, analytical procedures are less accurate in determining temperatures in

structural elements than numerical and testing procedures because their

application is limited to specific conditions and assumptions. Numerical methods,

albeit more complicated, have several advantages over their analytical

counterparts (Harmathy 1979; Hertz 1981; Munukutla 1989; Lie 1992). For

instance, they enable the solution of complex heat transfer problems for which

analytical solutions have not yet been developed. Additionally, solving the

governing heat transfer equations numerically allows for the implementation and

investigation of temperature- dependent material properties. On the other hand,

use of numerical methods is more complicated and time consuming than the use

of analytical methods. Time is needed to develop and input the model as well as

to review and interpret the body of results. Computers have reduced calculation

time significantly but the preparation phase before execution is still cumbersome

and involves programming equations into software applications as well as

determining material properties as a function of time.

Application Of Structural Engineering Principles To Design For Fire Safety

The rational design procedure described in this document applies structural

engineering principles and material properties at elevated temperature to the

calculation of fire resistance of reinforced or pre-stressed concrete beams and

floor slabs.

Current methods used for reinforced or pre- stressed concrete structural

elements are based on tabular data. This tabular data specifies fire resistance

according to the minimum width or thickness of the element and the amount of

concrete cover provided to tensile steel. In many cases, the use of this tabulated

data will be sufficient, particularly when fire resistance requirements do not


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govern the design of the element. When fire resistance governs the dimensions of

the element then a rational design approach may well be appropriate particularly

for members requiring fire resistance of more than about 90 to 120 minutes for

reinforced concrete and about 60 minutes for pre-stressed concrete.

When fire resistance requirements do govern cover requirements for flexuralelements, the rational fire design procedure can be used to most advantage by

optimising design to take into account a range of properties (such as restraint,

amount of steel and degree of loading) other than just element width or thickness

or concrete cover to tensile steel. All of these parameters can have an influence

on the amount of fire resistance achieved.

The failure criterion used by this design procedure is that of 'Loadbearing

Capacity' (or collapse) for slabs and beams, and in addition 'Insulation' for slabs.

These criteria are consistent with internationally accepted criteria for fire

resistance testing of beams and floors, except that vertical deflection of theelement is not considered. This factor is not regarded as significant as an

increasing rate of deflection will normally precede the collapse of the member.

The mode of failure is assumed to be in flexure and the time at failure is taken as

the time at which the moment capacity of the element, which reduces as the

temperature within the section increases, becomes less than the moment applied

to the element. Shear failure of flexural members in fire rarely occurs, but can be

checked for in the usual way using material properties at elevated temperatures

and factored loads.

As safety factors are already incorporated into the fire resistance period and thenormal design loads, in fire design it is usual to allow the load factors and design

live loads to be reduced.

STRUCTURAL RESPONSE AND DESIGN PROCEDURES

Concrete structures have a very good record for their performance in fire. There

are likely to be a number of reasons for this:

(a) Standard fire resistance testing has traditionally treated structural elements in

isolation, treating failure of an individual element as unacceptable, even if thestructure as a whole remains satisfactory due to the presence of structural

redundancy. In a real building, the failure of one structural element is not

necessarily indicative of structural collapse of the whole building.

(b) At higher temperatures concrete elements become more flexible, due to a

reduction in elastic modulus and are therefore capable of greater structural

deformations.


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(d) The imposed load may be much less than the design load assumed in the

determination of fire resistance.

(d) The imposed load may be much less than the design load assumed in the

determination of fire resistance.Structural Design Loads for Fire

In standard fire resistance testing, loadbearing structures are usually loaded to

produce their maximum permissible stresses, thus assuming that the full design

load is present at the time of the fire. The Institution. good performance of many

concrete structures to the fact that the imposed load on the structure in a fire is

often much less than the full design load.

As fire can be considered an accidental load, then its simultaneous occurrence

with other accidental loads can generally be ignored. Buchanan*


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Design Process

2.2 Code recommendations for fire design of concrete beams

This section gives an overview of the regulations concerning the design of fireresistance of different codes, with emphasis on the design of concrete beams.

2.2.1 Eurocode

Eurocode 1 Part 1-2: Actions on structures exposed to fire (EC1-03) regulates

calculation models for the determination of temperature and load effects. The


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fire scenario is treated as exceptional action and does not need to be

superimposed with other, independent exceptional actions. For the fire design,

different time-temperature curves for the determination of the hot-gas-

temperature are provided. These are stated in section 2.4.

Eurocode 2 Part 1-2: Design of Concrete Structures Structural Fire Design

(EC2-02) (EC2-95) deals with the design of concrete structures for the case of fire.

It states the design values for material properties and combination factors for

actions and treats methods of the passive and structural fire precautions; active

fire protection methods are not included.

For the determination of a sufficient fire rating, Eurocode 2 (EC2-02) (EC2-95)

gives three alternatives of design methods:

Tabulated data

In dependence on the fire resistance rating, the tabulated data gives minimum

values for cross-sectional dimensions and the axial distances of the longitudinal

reinforcement to the concrete surface. For beams there is a distinction between

simply supported and continuous beams.

For statically determinate structures the axial distance of the reinforcement is

determined such, that for the fire resistance time, the critical temperature in the

steel bars is 500C. Reaching this critical temperature, the reinforcement reachesits yield stress if the loads under fire conditions equal 0.7 times the load under

cold conditions. If the applied load in the event of fire is less, the tabulated axial

distances may be reduced. a reduced cross-section consisting of cooler parts of

the member. Therefore, the temperature profiles within the member and data of

the temperature dependant changes of the material properties are needed. More

details on simplified calculation methods are given in section 2.3.1.

Advanced calculation methods

Advanced calculation methods can be used for the simulation of the structural

behaviour of single members, parts of the structure, or the entire structure.

The advanced calculation methods provide a realistic analysis of the structures

exposed to fire. They are based on fundamental physical behaviour leading to a

reliable approximation of the expected behaviour under fire conditions.


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Thermal analysis shall be based on acknowledged principles and assumptions of

the theory of heat transfer and include the temperature dependant thermal

properties of the materials. Mechanical analysis shall be based on the

acknowledged principles and assumptions of the theory of structural mechanics,

taking into account the changes of mechanical properties with temperature. Non-linear geometrical effects and the effects of thermally induced strains and

stresses shall be considered, as well as all strains due to the temperature,

mechanical loads, creep and transient creep (EC2-02).

The code provides details on the thermal and mechanical properties of concrete

and reinforcing steel when they are subjected to elevated temperatures. 2.2.2

German Standard

The German regulations on determining the fire resistance of concrete structuresare regulated according to DIN 4102-4 (DIN-4102-4). DIN 4102-4 gives tabulated

data considering the size of the member, the axial distance of the longitudinal

reinforcement to the concrete surface and the minimum number of reinforcing

bars within the beam, related to the fire resistance rating.

At present, DIN 4102-4 is not applicable to the current German concrete standard

DIN 1045-1, as it is based on stress analysis and not on the partial safety

coefficient concept used in DIN 1045-1. This discrepancy is going to be closed with

the introduction of DIN 4102-22, which is a direction for use of DIN 4102-4. At themoment, the first draft of DIN 4102-22 (DIN-4102-22) has been published. For the

design of concrete beams the application of DIN 4102-22 will not result in great

changes in the use of the tabulated data.

At the moment, the determination of the fire resistance has to be performed

using the tabulated data of Eurocode 2 (EC2-95), considering additional

regulations stated in DIBt-Richtlinie zur Anwendung von DIN V ENV 1992-1-2 in

Verbindung mit DIN 1045-1" (DIBt-02).

1.3 IS CODES FOR FIRE RESISTANCE

Most of the countries have their own fire protection codes and consider fire

resistant design in their normal construction practices. However in India fire

resistance design has not yet gained attention. This is clear from the fact that all

codes for fire safety of buildings were introduced in latter half of 1980s and since


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then have not been revised. These codes were based on then research done on

fire protection in countries like USA, UK and Canada. Even the code for fire

resistance test of structures is based on, then ISO-834.

Above points show the need for introduction of new fire resistant design code for

structures in Indian context. Indian codes are prescriptive codes and they lay

down guidelines to be followed to achieve fire resistance of required duration.

From design point of view, provisions in code are limited to minimum dimension

and minimum cover to reinforcing steel to achieve fire resistance ranging from 0.5

hr to 4 hr.

IS: 1641-1988 defines fire load as heat in kilocalories which is liberated per sq. m. of floor area.

This amount of heat is used as the basis for classification of occupancies. It also defines 3 types

of fire zones, which are basically groups of different types of buildings (classified on based on

occupancy). IS: 1642-1989 classifies type of construction according to fire resistance into 4

categories namely type 1, type 2, type 3 and type 4.The code also enlists fire resistance ratings

for various type of construction and building elements (walls, beams, column etc). It also enlists

minimum dimensions of building elements for different fire resistance. NBC:2005 Part- 4

provides the criterion for limit state of insulation. It states that on an unexposed face maximum

temperature should not exceed 180

0

C at any location on the face and average temperature of theface should not exceed 150

0C. IS: 3809-1979 describes the procedures for conducting standard

fire resistance tests. Indian standard for RC design of buildings (general) IS: 456-2000 ensures

fire safety of structure by laying guidelines regarding the minimum dimension and cover

requirements for various fire rating (duration) of columns, beams and floors.


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Table 2.2:- List of various IS codes related to fire safety of buildings (General)

IS 1641:1988Code of practice for fire safety of buildings (General): General

principles of fire grading and classification

IS 1642:1989

Fire safety of buildings (General): Details of construction-code of

practice

IS 1643:1988Code of practice for fire safety of buildings (General): Exposure

hazards

IS 1644:1988Code of practice for fire safety of buildings (General): Exit

requirements and personal hazard

IS 1646:1997Code of practice for fire safety of buildings (General): Electrical

installation

IS 3808:1979 Method of test for non-combustibility of building materials

IS 3809:1979 Fire resistance test of structures

Code of practice for fire safety of special/particular buildings and institutions like industrial

buildings, temporary structures, libraries, hotels, educational institutions had also been

introduced in 1990s [32, 33, 36-39, 49]

2.3 Design methods

2.3.1 Simplified calculation methods

The design of beams under fire conditions is normally performed in the strength

domain. Thus it has to be demonstrated, that the design resistance under fire

conditions is greater than the design action at a particular duration of the fire. For

the determination of the design resistance, the reduced strength of the concrete

and the reinforcement has to be determined. Eurocode 2 (EC2-02) gives twomethods to determine the residual strength of a structural member using reduced

cross-sections:

The 500C isotherm method comprises a general reduction of the cross-

section size with respect to a heat-damaged zone at the concrete surfaces. The

thickness of the damaged zone is made equal to the average depth of the 500C


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isotherm in the compression zone of the cross-section. Thus concrete with

temperatures in excess of 500C is assumed not to contribute to the load bearing

capacity of the member, whilst the residual cross-section retains its initial values

of strength and modulus of elasticity. Taking the reduced strength of the

reinforcement bars into account, the ultimate load bearing capacity can bedetermined using conventional calculation methods, such as those stated in the

Eurocode (EC2-02), by the CRSI (Gut-80), Harmathy (Har-93), or the ACI

Committee (ACI-81).

The Zone method subdivides the cross-section into several zones of

equivalent thickness and evaluates the reduced strength of each zone. Out of the

single value of each zone, a damaged zone and the residual strength of the

reduced cross- section can be determined. For the standard fire curve, the

Eurocode provides several diagrams which allow for a quicker determination of

the reduced cross- section. This method is more accurate, but also more

laborious, compared to the 500C isotherm method.

For the determination of the temperature profiles needed, published generic

temperature contours for structural members can be used. Temperature contours

have been published among others by FIP (FIP-78), ACI (ACI-81), Wade (Wad-91)

and the Eurocode (EC2-95) (EC2-02) for standard fires. For fire conditions other

than standard fires, the temperature profiles should be determined using suitable

computer programs.


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PROPERTIES OF STEEL AND CONCRETE AT ELEVATED TEMP

Effect of Fire on Building Materials

A relatively new method for determining fire exposure used by fire protection

engineers is to first calculate the fire load density in a compartment. Then, based

on the ventilation conditions and an assumed source of combustion determinethe compartment temperature at various times. Another factor considered in the

analysis is the effect of active fire protection systems e.g. sprinklers or fire

brigades on the growth of the fire. The size and timing of the fire growth

determined by fire analysis is sensitive to changes in the fuel load over time and

changing ventilation conditions during the fire. This method of fire analysis

requires special software and extensive training and is used only in very large or

unusual buildings. Once the temperature time relationship is determined using a

standard curve or from the method described above, the effect of the rise intemperature on the structure can be determined. The rise in temperature causes

the free water in concrete to change from a liquid state to a gaseous state. This

change in state causes changes in the rate with which heat is transmitted from

the surface into the interior of the concrete component. The rise in temperature

causes a decrease in the strength and modulus of elasticity for both concrete and

steel reinforcement. However, the rate at which the strength and modulus

decrease depends on the rate of increase in the temperature of the fire and the

insulating properties of concrete. Note that concrete does not burn.

Concrete

The change in concrete properties due to high temperature depends on the type

of coarse aggregate used. Aggregate used in concrete can be classified into three

types:


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(a)Carbonate,

(b)Siliceous and

(c)Lightweight.

Carbonate aggregates include limestone and dolomite. Siliceous aggregate

include materials consisting of silica and include granite and sandstone.

Lightweight aggregates are usually manufactured by heating shale, slate, or clay.

Temp

Figure 1 Effect of high temperature on the compressive strength of concrete.

Figure 1 shows the effect of high temperature on the compressive strength of

concrete. The specimens represented in the figure were stressed to 40% of their

compressive strength during the heating period. After the designated test

temperature was reached, the load was increased gradually until the specimen

failed. The figure shows that the strength of concrete containing siliceous

aggregate begins to drop off at about 800 F and is reduced to about 55% at

1200F. Concrete containing lightweight aggregates and carbonate aggregatesretain most of their compressive strength up to about 1200 F. Lightweight

concrete has insulating properties, and transmits heat at a slower rate than

normal weight concrete with the same thickness, and therefore generally

provides increased fire resistance.

200 93.3

400 204.4

600 315.6800 426.7

1000 537.8

1200 648.9

1400 760.0

1600 871.1


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Figure 2 Effect of high temperature on the modulus of elasticity of concrete.

Figure 2 shows the effect of high temperature on the modulus of elasticity of

concrete. The figure shows that the modulus of elasticity for concretes

manufactured of all three types of aggregates is reduced with the increase in

temperature. Also, at high temperatures, creep and relaxation for concrete

increase significantly.

Steel

Reinforcing steel is much more sensitive to high temperatures than concrete.


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Figure 3. The effect of high temperature on the yield strength of steel.

Figure 4. The effect on the modulus of elasticity.

As indicated in the figures, hot-rolled steels (reinforcing bars) retain much of their

yield strength up to about 800 F, while cold-drawn steels (pre stressing strands)

begin to lose strength at about 500 F. Fire resistance ratings therefore vary

between pre stressed and non pre stressed elements, as well as for different

types of concrete.

Temperature Distribution Inside Concrete Elements

To use high temperature property information for various building materials in

predicting performance, it is necessary to know the temperature distribution in

the members of a structure exposed to fire. Due to the transient nature of heat

transmission the temperature distribution in concrete elements exposed to fire is

non-linear. Temperature distribution information has been developed for

concrete members made with different types of concrete.

Slabs

There is a need for simpler methods of estimating the temperature of steel in

structural concrete members to be available which do not require detailed heat

transfer calculations or computers. Such methods will of course not be as

accurate as the numerical procedures. A common way of providing the

temperature data is by graphical presentation of the form shown in Figure 13 for


29/54

concrete slabs (ACI, 1981). The temperature at a depth in the slab can be read

directly off the figure for the required period of exposure. This data is derived

from measurements taken during large numbers of fire resistance tests by the

Portland Cement Association (Abrams and Gustaferro, 1968). Temperature design

data of this sort can either be empirically based (i. e. , derived from the results ofstandard fire tests) or analytical (generated from theoretical models).

An empirical expression for the variation in the temperature within a normal

weight siliceous aggregate concrete slab is given by Purkiss, Claridge and Durkin

(1989) as:

T = 558 loglOt - (6.82 y + 373.77)

where

T = temperature in slab (0C) (for 250 < T < 950)

t = time (min) (for 30 < t < 240)

y = distance from fire-exposed face (mm)

Temperature distributions in concrete slabs or flat panels at selected distances

from the exposed surface during fire exposure are shown for structural concretes

made with siliceous, carbonate, and lightweight aggregates in Figs. 4, 5, and 6,

respectively. Similar information is available for other types of concrete and for

combinations of materials. Slab thickness does not significantly affect

temperatures in the materials except for very thin slabs or when the

temperatures are less than about 200 C (400 F).


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shown in Figs. 7 and 8, respectively. Similar distributions for selected exposure

periods have been prepared for all 32 beams of the test program.Results shown

here are taken from ACI 216.

Fig 1 Fig 2 Fig 3


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Structural behaviour of Flexural elements

Load Combination and Resistance Factors for Fire Exposure.

All major codes includes a on load combinations for extraordinary events, such as

fire, explosions and vehicular impact. This load combination recognizes the small

probability of such occurrences by utilizing load factors which are lower than for

normal load combinations. It is opined that no resistance factors need to be used

on the resistance side of the equation. Additionally, the structure only needs to

withstand the fire without failure and, thus, no service- ability criteria should beapplicable.(By Socrates A. Ioannides , Sandeep Mehta).

1.2D + Ak + (0.5L or 0.2S)

where: D = Dead Load

L = Live load

S = Snow Load

Ak = Load effect resulting from extraordinary event

Also as the safety factors are already incorporated into the fire resistance period

and the normal design loads, in fire design it is usual to allow the load factors and

design live loads to be reduced(C Wade).

2.5 Simply supported beam under fire conditions

When a simply supported beam, as shown in Figure 2.2, is exposed to fire, it will

expand outwards and gradually deflect downwards. The expansion is due to the

thermal elongation of the materials. The deflection results from the non-linear

temperature gradients that form across the cross-section of the beam and lead

the beam to bow thermally. The high temperatures at the bottom and the sides of

the beam will cause the tensile reinforcement to lose its flexural strength as well

as the concrete in the compression zone to lose its compressive strength. This


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reduction in the flexural strength of the beam will result in increasing deflections.

When the applied bending moment exceeds the residual strength of the beam, a

plastic hinge will form and failure will occur.

Experimental tests on simply supported members have been carried out by the

Portland Cement Association. In three different studies the fire resistance of

prestressed concrete members has been investigated.

Study A investigated the influence of thickness of concrete cover of the

prestressing steel strands (Car-62). The tests showed that an increase in covering

did increase the fire resistance, but not proportionally.

Study B studied the influence of aggregate and load intensity (Sel-64). It was

shown that beams made of concrete containing expanded shale aggregates

exhibited longer fire endurance than did beams of normal weight aggregate

concrete. Beams made of normal weight aggregate mainly failed in tension as the

steel elongated at critical amounts, whereas lightweight concrete members

showed a compression failure at the top of the beams as their compression zones

were subjected to higher temperatures due to the longer fire endurance. Heavier

loading led to a more rapid midspan deflection and failure occurred at lower

average temperatures, and so shorter endurance times, than did the lighter

loading.

In study C, the type of reinforcement, the type of bond and the influence of

aggregate were investigated (Gus-71). Beams with reinforcing bars showed longer

fire endurance than those with post-tensioned high-strength alloy bars or cold

drawn wire. General effects of the type of bond and the aggregate could not be

obtained in those tests.

Structural behavior For a simply supported reinforced concrete slab, with the

underside of the slab exposed to fire, the bottom of the slab will expand more

than the top resulting in a deflection of the slab [12]. The tensile strength of the


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concrete and steel near the bottom of the slab will decrease as the temperature

increases. When the strength of the steel at elevated temperature reduces to the

stress in the steel due to loads, a structural end point can be expected to occur.

The nominal moment strength can be expressed by the following

equation:(Gustaferro)

Mn=AsFy(d-a/2)

where

As = the area of the reinforcing steel

fy = the yield stress of the reinforcing steel

d = the distance from the centroid of the reinforcing steel to the extreme

compressive fiber

a = the depth of the equivalent rectangular compressive stress block at ultimate

load and is equal to As fy/O.85fc'b where fc' is the compressive strength of the

concrete

b = the width of the slab/beam. If the slab is uniformly loaded, the moment

diagram will be parabolic with a maximum value at midspan:

M= w = dead plus live load per unit of length

l = span length. It can be assumed that during a fire the dead and live loads

remain constant. However, the material strengths are reduced so that the

retained nominal moment strength is:

Mn Q=As FyQ(d - aQ/2)

in whichQsignifies the effects of elevated temperatures. Note that As and d are

not affected, but fy Qis reduced. Similarly ao is reduced, but the concrete strength

at the top of the slab fc' is generally not reduced significantly. If, however, the

compressive zone of the concrete is heated, an appropriate reduction should be

assumed. Flexural failure can be assumed to occur when Mn Q is reduced to M.


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From this expression, it can be noted that the fire endurance depends on the

load. intensity and the strength-temperature characteristics of steel.

Estimating structural fire endurance

Figure 9 shows the fire endurance of simply supported concrete slabs as affectedby type of reinforcement, type of concrete, moment intensity and the thickness of

concrete between the center of the reinforcement and the fire exposure surface

(referred to as "u" or the "cover"). If the reinforcement is distributed over the

tensile zone of the cross section, the value of u is the weighted average of the u

distances of the individual bars in the tensile zone with the weighting included to

consider the effect of using bars of different diameters. The graphs in Fig. 9 can

be used to estismate the fire endurance of simply supported concrete beams by

using "effective u" rather than "u". Effective u accounts for beam width byassuming that the u values for corner bars are reduced by one-half for use in

calculating the average u.

Fig. 9. Fire endurance of concrete slabs as influenced by aggregate type,

reinforcing steel type, moment intensity, and u.

* = Asfy/bdfc'. (


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2.6 Effect of axial or thrust restraint.

Axial restraint can have a significant influence on the fire performance of concrete

beams. It results when a heated member is restrained from thermal expansion by

a more rigid surrounding structure and thus compressive axial forces develop in

the beam.

Figure 2.3 shows the effect of axial restraint on a simply supported concrete beam

restrained by rigid supports as stated by Buchanan (Buc-01). Due to the heating of

the beam an axial thrust T develops, which can be considered as external

prestressing. It can be seen that the applied moment M*fire can be resisted,

although the flexural moment capacity at elevated temperatures Mf may besmaller than M*fire. This is due to the additional bending moment T*e induced in

the structure by the axial force, where e is the eccentricity between the line of

action of the thermal thrust and the centroid of the compression block near the

top of the beam, as shown in Figure 2.4.


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The total flexural resistance Rfirecan thus be calculated as:

RFire = Mf+ t*e

It can be seen from equation (2.5) that the positive effect of the thermal thrust is

strongly dependant on the position of the axial force. The additional moment may

become negative if large deflections occur or the axial force acts at the top of the

beam. Consequently, the resulting moment will tend to deflect the beam

downwards. Unless T is large enough to induce sufficient compressive stress to

counteract the tensile stresses caused by T*e and the applied moment, structural

failure will occur earlier (Car-65).


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The position of the line of thrust can only be located accurately for specific

support conditions where the line of thrust is well defined due to the method ofconstruction.

Figure 2.5 (a) to (c) show such determinate support conditions which mainly exist

in precast concrete construction. Figure 2.5 (d) represents a situation, such as

cast-in-place concrete, where the position of the thrust line is not clearly defined.

Fire tests have shown, that when only minimal thrust occurs, the thrust line is

near the bottom of the member throughout the fire exposure. For highly

restrained members the thrust line will be at the bottom of the member at the

start of the fire, with the position rising slowly during the fire.

In order to develop the beneficial effects of axial restraint, the surrounding

structure has to provide sufficient strength and stiffness to restrain the thermal

elongation. According to Gustaferro (Gus-86), the thrust forces that occur can be

quite large but are always considerably less than that calculated by use of elastic

properties of concrete and steel together with appropriate coefficients of


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expansion as at high temperatures, creep and stress relaxations play an important

role.

Experimental investigations of the effects of axial restraint on fire exposed

concrete members have first been executed by the Portland Cement Association

(Sel-63). A series of double-tee shaped specimens were exposed to the American

standard fire ASTME119. Each specimen was permitted to expand a given amount

and then further expansion was prevented. Issen (Iss-70) reported that all tested

beams supported their load longer than would have anticipated for simply

supported beams. The tests have also shown that the maximum thrust for a given

allowed expansion is proportional to the heated perimeterand the concretes

modulus of elasticity, where the heated perimeter is defined as the perimeter of

the cross-section of the specimen, perpendicular to the direction of the thrust,

which is exposed to fire. Based on the test results, a step-by-step method

incorporating several nomograms was developed to estimate the thrust

requirements for a given fire endurance for simply supported beams (PCI-77).

The applicability of this approach to slabs and beams other than those tested has

not been demonstrated. Besides, Anderberg and Forsn (And-82) have shown,

using the non-linear finite element program CONFIRE, that the PCI method over-

predicts the developing axial force. Wade (Wad-91) recommends that the positive

effects of thermal restraint be disregarded where the location of the thrust is

difficult to determine.

Numerical analyses on the effect of a moveable line of thrust on the behaviour of

one-way concrete slabs have been carried out by Lim (Lim-03). Lim showed that

the slab behaviour is very sensitive to the position of the line of thrust. If the

position of the line of thrust is located much above the soffit of the slab, the slabs

will rapidly undergo large deformations and sag into a catenary, imposing axial

tensile forces at the supports. The analyses have also shown that even if the line

of thrust is located close to the soffit, the slab can still deform into a catenary if

there is insufficient horizontal axial restraint (Lim-03) (Lim-04).

2.7 Effect of Rotational Restrain

Rotational Restraint (Figure 2), on the other hand, produces negative end

moments which also reduce the positive moment at mid-span. The negative end

moments can be resisted either by reinforcing in the slab (which remains cooler)


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or by the simple beam connections and the capacity of the steel section itself for

negative moment at elevated temperatures.

Figure 3 shows the required flexural strength (Mu ) and nominal flexural strength

(Mn) for an unrestrained beam before and after fire. Notice that before fire there

exists some negative nominal flexural strength (possibly less than the positive due

to longer unbraced flange lengths) in the beam, but it drops to zero at the ends

because of the absence of connection capacity. Also, notice that both Mu and Mn

have reduced after fire. Mu reduces due to reduced load factors and Mn due to

lower capacity (see discussion in the following section).


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Rotational restraint (Figure 5) results in shifting the moment diagram by imposing

negative moments equal to the flexural strength of the restraint (connection

capacity or composite action). In the case of composite action, notice the

additional negative nominal flexural strength resulting from the existence of

reinforcing in the slab.

2.7 Effect of continuity

Continuous flexural elements have a considerably greater fire resistance than

simply supported elements. Their superior performance is due to beneficial

changes in the moment distribution that take place in response to fire exposure,

and their higher level of redundancy against failure (Har-93) (Buc-01) (Gut-80).

Figure 2.6 shows the centre beam of a member that is continuous over several

supports. When it is heated, the beam wants to deflect downwards due to the

temperature gradient which produces different amounts of thermal elongation on

the top and the bottom of the beam. This thermally induced curvature results in auniform negative bending moment Mthermal along the length of the beam.

Consequently the support moments M-cold of the beam increase and the bending

moment at the span M+ cold decreases. This is beneficial to the beams fire

endurance, as the moment is reduced at the midspan where the beams flexural

capacity is reduced faster due to higher temperatures in the steel bars. The

M-cold

M+ cold

Mthermal

M-cold + Mthermal

M+ cold + Mthermal


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Structural behavior

Figure 10 shows a continuous beam, whose underside is exposed to fire. The

bottom of the beam becomes hotter than the top and tends to expand more than

the top. This differential heating causes the ends of the beam to tend to lift from

their supports thus increasing the reaction at the interior support. This action

results in a redistribution of :moments, i.e., the negative moment at the interior

support increases while the positive moments decrease. During the course of a

fire, the negative moment reinforcement remains cooler than the positive

moment reinforcement because it is better protected from the fire. Thus, some

increase in the negative moment can be accommodated. However, the redistribu-

tion that occurs may be sufficient to cause yielding of the negative moment

reinforcement. The resulting decrease in positive moment means that the

positive moment reinforcement can be heated to a higher temperature before


46/54

failure will occur. Thus, it is apparent that the fire endurance of a continuous

reinforced concrete beam is generally significantly longer than that of a similar

simply supported beam loaded to the same moment intensity.

Detailing precautionsAgain, the amount of redistribution that occurs is sufficient to cause yielding of

the negative moment reinforcement. By increasing the amount of negative

moment reinforcement, a greater negative moment will be attracted, so care

must be exercised in designing the member to assure that flexural tension will

govern the design. To avoid a compressive failure in the negative moment region,

the amount of negative moment reinforcement should be small enough so that

o- i.e. As'fy/bd'fc' is less than about 0.30 even after reductions due to

temperature in fy and fc' are taken into account. Furthermore, the negative

moment reinforcing bars must be long enough to accommodate the completeredistributed moment and change in the location of inflection points. It is

recommended that at least 20% of the maximum negative moment

reinforcement in the span extend throughout the span.


The charts in Fig. 9 can be used to estimate

the fire endurance of continuous beams andslabs. To use the charts, first estimate

the negative moment at the supports taking into account the temperatures of the

negative moment reinforcement and of the concrete in the compressive zone

near the supports. Then estimate the maximum positive moment afterredistribution. Entering the appropriate chart with the ratio of that positive

moment to the initial positive nominal moment strength, the fire endurance for

the positive moment region can be estimated. If the resulting fire endurance

is considerably different than that originally assumed in estimating the steel and

concrete temperatures, a more accurate estimate can be made by trial and error.

Usually such refinement is unnecessary. It is also possible to design the

reinforcement in a continuous beam or slab for a particular fire endurance period.

From the lowermost diagram of Fig. 10 the beam can be expected to collapse

when the positive nominal moment strength M& is reduced to the value indicatedby the dashed horizontal line, i.e., when the applied moment at a point x1 from

the outer support, M,> = M&. For a uniform applied load w,


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And

also

x0 = 2x1

For a symmetrical interior bay,

X1=

Thus, substituting for x1,

or

FIRE ENDURANCE OF FLOORS AND ROOFS IN WHICH RESTRAINT TO THERMAL

EXPANSION OCCURS

Structural behavior

If a fire occurs beneath a small interior portion of a large reinforced concrete slab,

93 the heated portion will tend to expand and push against the surrounding part

of the slab. In turn, the unheated part of the slab exerts compressive forces on

the heated portion. The compressive force, or thrust, acts near the bottom of the

slab when the fire first occurs, but as a fire progresses the line of action of

the thrust rises. At high temperatures, creep and stress relaxation play an

important role. Nevertheless, the thrust is generally great enough to increase the

fire endurance significantly. In most fire tests of restrained assemblies, the fire

endurance is determined by temperature rise of the unexposed surface ratherthan by structural considerations even though the steel temperatures often

exceed 800 0(3 (1500 F). The effects of restraint to thermal expansion

can be characterized as shown in Fig. 11. The thermal thrust acts in a manner

similar to an external prestressing force, which, in effect, increases the positive

nominal moment strength.


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Fig. 11. Moment diagrams for axially restrained beam during fire exposure. Note

that at 3 h MnQ is less than M and effects of axial restraint permit beam to

continue to support load.


The increase in nominal moment strength is similar to the effect of "fictitious

reinforcement" located along the line of action of the thrust. It can be assumed

that the "fictitious reinforcement" has a strength (force) equal to the thrust. By

this approach, it is possible to determine the magnitude and location of the

required thrust to provide a given fire endurance. The procedure for estimating

thrust requirements is:

(1) determine temperature distribution at the required fire test duration;

(2) determine the retained nominal moment strength for that temperature

distribution;

(3) if the applied moment, M, is greater than the retained moment capacity M,e,

estimate the midspan deflection at the given fire test time (if M, is greater than

M no thrust is needed);

(4) estimate the line of action of the thrust;

(5) calculate the magnitude of the required thrust, T;

(6) calculate the "thrust parameter", T/AE, where A is the gross cross-sectional


49/54

area of the section resisting the thrust and E is the concrete modulus of elasticity

prior to fire exposure;

(7) calculate Z defined asZ = A/s in which s is the "heated perimeter" defined as

that portion of the perimeter of the cross section resisting the thrust exposed to

fire;(8) use Fig. 12 with the appropriate thrust parameter and Z value and determine

the "strain parameter", A/;

(9) calculate A by multiplying the strain parameter by the heated length of the

member; and

(10) determine if the surrounding or supporting structure can support the thrust T

with a displacement no greater than A.

The above explanation is greatly simplified because in reality restraint is quite

complex, and can be likened to the behavior of a flexural member subjected to an

axial force[13,14].


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Multi-course assemblies

Floors and roofs often consist of concrete base slabs with overlays or

ndercoatings of other types of concrete or insulating materials. If the fire

endurances of the individual courses are known, the fire endurance of the

composite assembly can be estimated from the formula:

R = (R1.59

+ R2.59

+ ... + Rn.59

)1.7

where R = fire endurance of the composite assembly in minutes, and R1, R2, Rn =

the fire endurances of the individual courses in minutes.


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2.8 Shear

Shear failure is not usually a problem in fire exposed concrete structures, with the

exception of precast pre-tensioned slabs with narrow webs (Buc-01). Tests by Linet al. (Lin-88) on highly loaded reinforced concrete beams showed that shear

cracks developed during the fire exposure before flexural cracks occurred. But as

the latter extended rapidly, all the beams tested failed in flexure rather than in

shear.

Eurocode 2 (EC2-02) also states, that shear failure is very uncommon, and thus

provides calculation methods that are stated as not having been fully verified.

Normal temperature design methods are recommended, using the reduced cross

sections and material properties obtained with the simplified calculation methods

stated in section 2.3.1. If there is no shear reinforcement provided, or the shearcapacity relies mainly on the reduced tensile strength of the concrete, the actual

shear behaviour of the concrete at elevated temperatures must be considered.

Franssen and Brul (Fra-97) assumed that the shear resistance of concrete is less

affected by the temperature than the compressive strength. Thus, they showed

that the concrete contribution to the shear strength reduces more slowly than the

contribution of steel stirrups or any prestressing forces.

Desai (Des-98) has proposed a similar design rule for estimating shear capacities

of rectangular beams exposed to fire. The method takes the contribution of the

concrete and the links into account. Additionally, the provision of a centralreinforcing bar as a part of the web reinforcement is proposed. The contribution

of this to the shear strength could be useful for beams with smaller dimensions.

The method showed good agreement with test results, but is only applicable to

rectangular beams.


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elevated CF PU =ultimate tensile strength of pre stressing steel.

To calculate the reduced moment capacity due to fire, the value of F or and b usually remain unchanged

at elevated temperature where the F PS at the applicable temperature is used. The values of As, d

compression zone is protected from the fire (e.g., by a ceiling/floor slab), except that if the compression

zone of the concrete is heated above 760C (1400F), the concrete above this temperature should be

ignored in the calculation, and reduced values of ftc, b and d (ftC6, be and do) should be used. The

subscript 6 indicates the effect of high temperature.

Figure 18 shows the applied moment and moment capacity for a simply- supported beam with a

uniformly distributed load. Collapse is presumed to occur when the reduced moment capacity at mid-

span reaches the value of the applied moment with the formation of a plastic hinge at mid-span.

Purkiss et a1 (1989) also describe a simple method for calculating the fire resistance of simply supported

one-way spanning slabs which takes into account the load level. They also considered the effects of

variations in concrete strength, steel strength and temperature profiles and concluded that the largest

effect was due to variations in the calculation of temperature profiles, a lesser effect due to variation in

steel strength and a negligible effect due to concrete strength.

To calculate the reduced moment capacity due to fire, the value of F or

and b usually remain unchanged at elevated temperature where the

F PS at the applicable elevated temperature is used. The values of As, d

compression zone is protected from the fire (e.g., by a ceiling/floor slab), except that if the compression

zone of the concrete is heated above 760C (1400F), the concrete above this temperature should be


53/54

ignored in the calculation, and reduced values of ftc, b and d (ftC6, be and do) should be used. The

subscript 6 indicates the effect of high temperature.

The procedure outlined in CEB (1987) is similar but varies in the following respects: (1) Concrete heated

to above 500C is ignored in the calculation of loadbearing capacity (e values of b and possibly d need to

be reduced as shown in Figure 16), while concrete with lower temperature can be assumed to retain its

ordinary room temperature strength. Thus a step function for concrete compressive strength is assumed

changing from 1 to 0 at 500C. (2) It is noted that using practical design curves for steel strength, as

described earlier in this report, can lead to structural design which is too conservative, therefore a

critical stress approach is recommended in which F Y in the above

expression is replaced with the critical stress given in Figure 17 (CEB, 1987) as a function of steel

temperature and a cross section parameter

Continuous Beams and Slabs

A beam continuous over its supports possesses a much greater fire resistance than if simply supported.This is because restraint against rotation provided at the supports causes a redistribution of the applied

moments, increasing the negative moment at the supports as the positive moment decreases due to

elevated temperature. See Figure 19. The fire will tend to have a greater effect in reducing the positive

moments rather than the negative, since the positive moment reinforcement is more exposed to the fire

than the negative. Gustaferro and Martin (1977) indicate the procedure that should be followed for

checking the strength of a continuous beam. The procedure is summarised here (refer to Figure 20 for

moment diagrams and meaning of symbols).

Given a preliminary design of beam - 1.

Determine the positive moment capacity, Mg + at time required using equation [8] .

2.

Determine the required negative -moment capacity (after moment redistribution).

At interior support of an end bay, Mg- = 'rwL2 - wL2 ./2~~+

/wL2 [ 101 At support of symmetrical intermediate bay, Mg = wL2/8 - M~+ [ll]

-3.

Determine the amount of negative reinforcement (or prestressing steel) needed to provide the required

negative moment capacity using equation [8] .

4. Determine the position (maximum value) of the inflection points, (Xo) and thus the necessary lengths

of reinforcement.

Within an end bay, Xo = L - 2Me - /wL 1121 Within a symmetrical intermediate bay, Xo = 4L - 4 J~M~+/w

[ 13 I


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For maximum value of Xo, the minimum value of the service load (w) should be used. The negative

reinforcing bars must be long enough to accommodate the redistributed moments and the change in

the position of inflection points. It is recommended that at least 20% of the maximum negative moment

reinforcement be extended throughout the span.

5. Ensure that flexural tension governs design.

To avoid a compressive failure of the concrete, the negative moment reinforcement should be small

enough so that:

-

AS/bgdgftc. Readers are referred to the CEB publication for further detailed information.

Figure 18 shows the applied moment and moment capacity for a simply- supported beam with a

uniformly distributed load. Collapse is presumed to occur when the reduced moment capacity at mid-

span reaches the value of the applied moment with the formation of a plastic hinge at mid-span.

Purkiss et a1 (1989) also describe a simple method for calculating the fire resistance of simply supported

one-way spanning slabs which takes into account the load level. They also considered the effects of

variations in concrete strength, steel strength and temperature profiles and concluded that the largest

effect was due to variations in the calculation of temperature profiles, a lesser effect due to variation insteel strength and a negligible effect due to concrete strength.

seminar original

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