fire resistance of steel structures

Aniket Gohil MSC STRUCTURAL ENGINEERING | 1431523 | SUPERVISOR: DR. ZHAOHUI HUANG

FIRE RESISTANCE OF STEEL STRUCTURES

Detailed research in acknowledging the purpose and use of Eurocode in regards to fire resistance design with aid of designing of a steel and truss frame and testing it under elevated temperatures and fire load respectively.

CE5516 Fire Resistance of Steel Structures 1431523

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PROJECT SUMMARY

Materials and construction assemblies that include the use of fire resisting materials, measured in

terms of fire endurance time are known as fire resistance-rated-construction. These control the spread

of fire, preventing the loss of structural stability within the prescribed period, with use of fire

resistance. This is also based on occupancy of the building and fire safe objectives available.

In this project, use of finite elements software such as Vulcan and spreadsheet used to design and test

of steel and truss frames under fire and how they react under those circumstances. Spreadsheet is an

advance finite element computer programme used to develop and analyse structures and structural

elements. It is divided into different modules, each module representing an aspect of modelling, e.g.

geometry defining, adding material properties, loading, generating mesh, etc. With aid of these

modules, precise models and results are developed. Vulcan is a three-dimensional computer

programme developed at University of Sheffield. It is used for modelling steel, steel-framed

composites and reinforced concrete buildings in fire. The development of the model extends its

capability to model the dynamic and static behaviour of steel structural elements under fire. It is a

powerful tool to investigate the mechanism of the progressive collapse of an element due to local

failure.

For this particular dissertation, two programs will be used in order to understand and design for the

fire resistance of steel structures: spreadsheet and Vulcan. Various parametric analyses will be carried

out on the program Vulcan on steel members at elevated temperatures at different location within the

frame. Examining these members on Vulcan would mean that the members are analysed in holistic

manner. The tests will be carried out on both protected and unprotected steel members via both

Vulcan and the spreadsheet method. The parameters for the tests are based on fire regime and

characteristics of beam to column connectivity.

The dissertation is based on analysing 2D steel frame via Vulcan and spreadsheet. The building takes

a shape of a 2 storey non sway steel frame comprising of 5 bays, each at a distance of 9m. In the

transverse direction, there are 4 bays with a distance of 6m with a floor height of 4m. The sections for

the different elements used in the structure will be uniform. A number of five scenarios are designed

which include different fire regimes. As this analysis is not based on isolated members, different

characteristics of beam to column connectivity will be analysed including rigid and pinned

connections. For testing of isolated members, spreadsheet method will be used and cross examined on

Vulcan. The results achieved for the steel frame building for the scenarios will then be discussed in

relation to each other.


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ACKNOWLEDGMENTS

I would like to express my deepest gratitude and thankfulness to all those who helped me in all stages

to complete this dissertation. I appreciate their support and this would not have been possible without

them. I’d like to personally thank my supervisor Zhaohui Huang and for his help, motivation and

guidance. I would also like to thank my parents Rajesh Gohil and Jignasha Gohil, my sister Jagruti

Gohil and my friends Varad Gokhale, Manas Adhvaryu and Sulheman Khan for their unlimited

support.

I would also like express my deepest gratitude to my various course leaders whose knowledge in

various fields helped me to undertake a project which was not limited to a certain topic. To all my

relatives, friends and others who in a way or another shared their support, morally, financially and

physically.

Above all, thanking to the Great Almighty, the author of knowledge and wisdom, for his countless

love.

THANK YOU


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STATEMENT OF ORIGINALITY

Author’s declaration

I declare that the work in this dissertation was carried out in accordance with the requirements of the

University’s Regulations and Code of Practice for Taught Programmes and that it has not been

submitted for any other academic award. Except where indicated by specific reference in the text, this

work is my own work. Work done in collaboration with, or with the assistance of others, is indicated

as such. I have identified all material in this dissertation which is not my own work through

appropriate referencing and acknowledgement. Where I have quoted or otherwise incorporated

material which is the work of others, I have included the source in the references. Any views

expressed in the dissertation, other than referenced material, are those of the author.

SIGNED: ……………………………………………………………. DATE: ……………..

(Signature of student)


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TABLE OF CONTENTS

PROJECT SUMMARY .......................................................................................................................... 1

ACKNOWLEDGMENTS ...................................................................................................................... 2

STATEMENT OF ORIGINALITY ........................................................................................................ 3

LIST OF FIGURES ................................................................................................................................ 7

LIST OF TABLES .................................................................................................................................. 8

INTRODUCTION .................................................................................................................................. 9

1.1. Background ............................................................................................................................. 9

1.2. Aims and Objectives ............................................................................................................. 10

LITERATURE REVIEW ..................................................................................................................... 11

2.1. Fire and Testing .................................................................................................................... 11

2.1.1. Building Fires ................................................................................................................ 11

2.1.2. Compartment Fires ........................................................................................................ 12

2.1.3. Building Regulations..................................................................................................... 13

2.1.4. Standard Fire Curves and Furnace Testing ................................................................... 15

2.1.5. Parametric Fire Curves .................................................................................................. 16

2.1.6. Cardington Fire Test ..................................................................................................... 18

2.1.7. Broadgate Fire ............................................................................................................... 22

MATERIAL PROPERTIES ................................................................................................................. 24

3.1. Thermal Properties of Steel at Elevated Temperature .......................................................... 24

3.1.1. Thermal Expansion ....................................................................................................... 24

3.1.2. Volumetric Specific Heat .............................................................................................. 25

3.1.3. Thermal Conductivity ................................................................................................... 26

3.2. Mechanical Properties of Steel at Elevated Temperature ..................................................... 27

3.2.1. Stress-Strain Relationship ............................................................................................. 28

HEAT TRANSFER .............................................................................................................................. 29

4.1. Heat Transfer for Unprotected Steel Member ....................................................................... 29


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4.2. Heat Transfer for Protected Steel Members .......................................................................... 31

4.3. Section Factor ....................................................................................................................... 32

4.4. Protection Mechanisms ......................................................................................................... 33

FIRE RESISTANCE ASSESSMENT .................................................................................................. 35

5.1. According to Eurocode 3 ...................................................................................................... 35

5.1.1. Calculations for Design Data ........................................................................................ 35

5.2. According to British Standard’s ............................................................................................ 37

5.2.1. Load Ratio Method ....................................................................................................... 37

5.2.2. Moment Capacity Method ............................................................................................ 37

BEHAVIOUR OF STRUCTURAL ELEMENTS ................................................................................ 38

6.1. Beam Analysis ...................................................................................................................... 38

6.2. Column Analysis ................................................................................................................... 39

METHODOLOGY ............................................................................................................................... 40

7.1. Spreadsheet Method .............................................................................................................. 40

7.2. Vulcan ................................................................................................................................... 41

7.2.1. Computer Modelling ..................................................................................................... 41

7.2.2. Capabilities and Limitations ......................................................................................... 41

PROCEDURE ....................................................................................................................................... 43

8.1. Proposed Plan ........................................................................................................................ 43

DESIGN OF STEEL FRAMED BUILDING ....................................................................................... 45

9.1. Primary Beam ....................................................................................................................... 46

9.2. Column .................................................................................................................................. 48

ANALYSIS OF 2D FRAME ................................................................................................................ 49

10.1. Scenario 1: FIRST FLOOR – Bay 1 – ISO Fire with Rigid and Pinned Connections ...... 50

10.2. Scenario 2: FIRST FLOOR – Bay 1 – Parametric Fire with Rigid and Pinned

Connections....................................................................................................................................... 53

10.3. Scenario 3: FIRST FLOOR – Bay 3 – ISO Fire with Rigid and Pinned Connections ...... 55


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Connections....................................................................................................................................... 58

10.5. Scenario 5: FIRST FLOOR – Bay 1 – 3 – Simultaneous ISO Fire and Parametric Fire

with Rigid Connections ..................................................................................................................... 60

CONCLUSION ..................................................................................................................................... 62

FUTURE RECOMMENDATIONS ..................................................................................................... 64

REFERENCES ..................................................................................................................................... 65

APPENDIX I: Spreadsheet FiRE.xls for Determining Critical Temperature ....................................... 68

APPENDIX II: Scenario 1 of 6 (1 of 4 input models) .......................................................................... 70


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LIST OF FIGURES

Figure 1: Schematic of Compartment Fire Growth............................................................................... 13

Figure 2: Building regulations approved document B (Volume 1 and 2) ............................................. 14

Figure 3: Time Temperature Curves according to Eurocode ................................................................ 15

Figure 4: ISO 834 Standard Fire Curve ................................................................................................ 16

Figure 5: Floor layout of Cardington test building ............................................................................... 19

Figure 6: Loading of Cardington steel framed building........................................................................ 19

Figure 7: Office layout for Cardington Fire Test .................................................................................. 21

Figure 8: Broadgate fire damages ......................................................................................................... 23

Figure 9: Thermal elongation of steel at elevated temperatures (European Committee for

Standardisation, 2005) .......................................................................................................................... 24

Figure 10: Specific heat of steel as a function of the temperature ........................................................ 26

Figure 11: Thermal conductivity of steel as a function of the temperature (European Committee for


Figure 12: Reduction factor for stress-strain steel at elevated temperatures ......................................... 27

Figure 13: Stress-strain relationship for steel at elevated temperatures (European Committee for


Figure 14: Stress-strain curve for strain illustrating yield and proof strength ...................................... 28

Figure 15: Heat transfer in an unprotected steel member ..................................................................... 29

Figure 16: Deformation (left) thermal stresses due to fire (right) ......................................................... 29

Figure 17: Heat transfer in protected steel ............................................................................................ 31

Figure 18: Section factor for protected and unprotected steel members (Haller & Cajot, 2006) ......... 32

Figure 19: Critical temperature for simple steel members based on standard fire test ......................... 36

Figure 20: Failure mechanism for simply supported beam (left) and continuous beam (right) ............ 38

Figure 21: Plan view of the building ..................................................................................................... 45

Figure 22: Side view of the building .................................................................................................... 46

Figure 23: Standard fire curves for protected and unprotected primary steel beams ............................ 47

Figure 24: Parametric fire curve for both protected and unprotected beam .......................................... 47

Figure 25: Simply supported beam model ............................................................................................ 49

Figure 26: Bay 1 exposed to ISO fire conditions .................................................................................. 50

Figure 27: Temperature deflection graph of bay 1 (node 82) with isolated member subjected to ISO

fire ......................................................................................................................................................... 50

Figure 28: Time deflection graph of bay 1 with isolated member subjected to ISO fire at node 82 .... 51

Figure 29: Temperature - axial force graph for beam to column connections at bay 1 ........................ 51

Figure 30: Time - axial force graph for beam to column connections at bay 1 .................................... 52


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Figure 31: Temperature deflection relationship of bay 1 and an isolated member subjected to

parametric at node 82 ............................................................................................................................ 53

Figure 32: Time deflection graph of bay 1 with isolated member subjected to parametric fire at node

82 .......................................................................................................................................................... 53

Figure 33: Temperature - axial force graph for beam to column connections at bay 1 subjected to

parametric fire ....................................................................................................................................... 54

Figure 34: Time - axial force graph for beam to column connections at bay 1 subjected to parametric

fire ......................................................................................................................................................... 54

Figure 35: Bay 3 exposed to ISO fire ................................................................................................... 55

Figure 36: Temperature deflection relationship of bay 3 and an isolated member subjected to ISO fire

at node 132 ............................................................................................................................................ 55

Figure 37: Time deflection graph of bay 3 with isolated member subjected to ISO fire at node 132 .. 56

Figure 38: Temperature deflection graph of bay 3 with isolated member subjected to ISO fire at node

132 ........................................................................................................................................................ 56

Figure 39: Time deflection graph on bay 3 with isolated member subjected to ISO 834 fire .............. 56


132 ........................................................................................................................................................ 58

Figure 41: Temperature deflection graph of bay 3 with isolated member subjected to parametric fire

at node 132 ........................................................................................................................................... 58


132 ........................................................................................................................................................ 59

Figure 43: Bays 1-3 exposed to ISO fire ............................................................................................... 60

Figure 44: Temperature deflection graph of bay 1 0 3 and isolated member subjected to

ISO/parametric fire conditions ............................................................................................................. 60

Figure 45: Time deflection graph of bays 1 - 3 subjected to ISO and parametric fire .......................... 61

Figure 46: Temperature axial force graph of beam to column connection at bay 1-3 subjected to

ISO/parametric fire ............................................................................................................................... 61

LIST OF TABLES

Table 1: Fires by location and type, Great Britain, 2000/01 - 2013/14 [1] ........................................... 11 Table 2: Floor Loading Details ............................................................................................................. 19 Table 3: Thermal properties of common fire protection materials (ECCS Technical Committee 3,

1995) ..................................................................................................................................................... 33 Table 4: Load reduction factor based on partial safety factor according to EC3 .................................. 36 Table 5: Critical temperature values for primary beam members ......................................................... 46 Table 6: Fire resistance and protection steel column ............................................................................ 48


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1 INTRODUCTION

1.1. Background

Construction of a structure consists of many stages and layers, one of which is fire protection. Fire

protection is a key element in all structures. Current fire protection strategy integrates a combination

of active and passive fire protection measures. Active fire measures considers the use of fire alarms,

sprinklers, fire extinguishers, etc. that require either human involvement or automatic activation,

which helps control the fire and its effects during fire. Passive fire measures are built into the

structural system by the use of materials in construction of a building, dimensions of building

components and fire compartmentation.

Materials and construction assemblies that include the use of fire resisting materials, measured in

terms of fire endurance time are known as fire resistance-rated-construction (Clayton, 2012). These

control the spread of fire, preventing the loss of structural stability within the prescribed period, with

use of fire resistance. This is also based on occupancy of the building and fire safe objectives

available.

It is now commonly known that to design fire resistance of a beam it is the period of time that it can

maintain a deflection less than span/20 under standard ISO 834 fire condition. If tested without

protection, a usual beam would result in a fire protection of 15 to 20 minutes with its deflection

reached when the temperature is around 550° to 700° C depending on the load.

When designing a building, design practice codes and standards such as Eurocodes and ISO standards

are considered. Recommendations and guidelines are also provided by the local government and

Secretary State of Committee, called “Building regulations”. This consists of regulation determination

for: internal and external fire spread (B3 and B4 respectively of building regulations) and, access and

facilities for the fire service (B5 of building regulation) (BBC, 2014). Eurocode is a structural design

code covering – basis of design, actions on structures, design of elements in structures (concrete, steel,

composite concrete and steel, timber, masonry and aluminium) along with seismic and geotechnical

design.


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In this project, use of finite elements software such as Vulcan and Spreadsheet/CAE are used to

design and test of steel and truss frames under fire and how they react under those circumstances.

Spreadsheet/CAE is an advance finite element computer programme used to develop and analyse

structures and structural elements. It is divided into different modules, each module representing an

aspect of modelling, e.g. geometry defining, adding material properties, loading, generating mesh, etc.

With aid of these modules, precise models and results are developed. Vulcan is a three-dimensional

computer programme developed at University of Sheffield. It is used for modelling steel, steel-framed

composites and reinforced concrete buildings in fire. The development of the model extends its

capability to model the dynamic and static behaviour of steel structural elements under fire. It is a

powerful tool to investigate the mechanism of the progressive collapse of an element due to local

failure.

Fire resistance is an important part in building and structure design. With following Eurocodes and

building regulations, user safety is considered utmost. Along with that, structural life and fire spread

hazards are also considered (Collette, 2007). In case of fire, the ability of loaded structural material to

retain its strength can provide extra time for users in evacuation. It is a challenging task for fire

engineers in delivering structural element that is able to withstand high fire loads for a longer period.

1.2. Aims and Objectives

Aims: The main aim of this individual research project is to acknowledge the purpose and use of

Eurocode in regards to fire resistance design with aid of designing of a steel frame and testing it under

elevated temperatures and fire load respectively.

Objectives: In order to achieve the set aims to this individual research project and project, the

following objectives must be accomplished:

Clearly understand the use of Eurocode 3 and Eurocode 1 in relation to building fire design,

putting it in practice to design a steel and truss frame.

Calculating protected and unprotected steel under fire.

Involving the use of Vulcan programme to understand the behaviour of steel frame at elevated

temperatures and truss frames under fire, failure point for one and progressive column and

analysis of two dimensional steel frame structure in fire.

Comparing and analysing the achieved results to design fires, such as: Eurocode and ISO 834.

Presenting how to minimise the risk of fire and fire spread with use of passive fire protection

measures.


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2 LITERATURE REVIEW

In increase of extreme fire disasters, growth of safety from fire and of life and the developments in

current design standards, the field of fire safety is expanding day by day. Codes and standards provide

guidelines, regulations and techniques to improve fire resistance. With a combination from technical

advances, increased research, and the necessity to predict behaviour of structures, analysation of both

thermal and structural behaviour under fire conditions must be evaluated.

2.1. Fire and Testing

2.1.1. Building Fires

Fire has always been a risk to human life. There have been many scenarios where human life has been

left in danger due to building fires. The aim of fire safety design is to lessen the chances of fire

occurring in buildings. According to data collected by the government, fire scenarios have been

recorded each year.

Table 1: Fires by location and type, Great Britain, 2000/01 - 2013/14 [1]

107.3

85.5 59.0 39.1

2.7

17.2

102.2 66.9

29.5

221.8

251.3

116.3

13.6 7.6 7.7 0

50

100

150

200

250

300

350

400

Nu

mb

er o

f fi

res

(th

ou

san

ds)

Total Buildings Dwellings Other residential Non-residential

Outdoors Secondary fires Chimney fires


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As seen above in Table 1, the total number of primary fires recorded in 2000/01 was 209,400. At

present the total number of primary fires was decreased by 42% and resulted in 88,500. Similarly,

following the trend, total secondary and chimney fires in 2000/01 was 221,800 and 13,600

respectively which was decreased by 52% and 57%, resulting in 116,300 and 7,700 fires respectively.

As seen throughout the chart, there is a strong decreasing trend in all three sectors: primary, secondary

and chimney fires.

2.1.2. Compartment Fires

Compartment fires relate to the essence of fire growth. In this situation, a “compartment” is any

confined space that has the ability to control the ultimate air supply and thermal environment of fire.

These factors control the spread and growth of fire, its duration and its burning rate (Quintiere, 2006).

During a compartment fire, a Flashover stage is introduced where a rapidly occurring transitional

event leads in the development of compartment fire. Building Research Establishment (BRE) (2005)

states that a factors affecting compartment fires are method of storage, size of the compartment and

ventilation for air supply in the compartment. A flashover is when there is a rapid increase of heat in a

compartment leading to drastic spread of developed fire. Compartment fires are divided into two

stages; pre and post flashover. A pre flashover phase has a bigger influence on human life safety

where the rapid growth of fire and the upper layer of gas is recorded. On the other hand post flashover

fires have influence on structural integrity where a high turbulent flow of gasses and high

temperatures are recorded.

Additionally, post flashover fires are also considered as fuel controlled fires. This scenario is similar

to that of fuel burning in open air with enhancement and from radiant feedback from hot upper layer

of gas. In a fuel controlled burning, all of the heat is released inside a room, avoiding the flames to

extend out the windows like ventilation controlled fire. Figure 1 describes the schematic of

compartment fire growth, defining the flashover stages and ignition, heating and cooling phases. For a

structural design, it is vital to consider both the growth and decay phase of fire.

According to Eurocode, the fire resistance determination in this work is related to ventilation

controlled fire. Ventilation controlled fire depend largely on the shape and the size of the ventilation

opening (International Standard Organization, 1999). According to Corbett (2009), the ventilation

controlled fire in compartment fires is limited by the volume of cool air that can enter and volume of

hot gases that can leave the room. While there is insufficient air flow inside the room, all the gases

tend to not burn out. However these gases extend outwards through the window mixing with outside

air and additional combustion takes place.


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Using the data and the figure displayed above a computer model can be designed in order to provide

aid to fire analyst. This computer model will reveal information like the time of flashover, heat release

rate, fuel available for flashover, ventilation required to prevent flashover and temperature profiles of

the compartment. However it is also vital to understand that this is a computed result from fire

modelling and can be used as ‘reliable estimates’. Every fire is different in some way providing

different outcomes to one another.

2.1.3. Building Regulations

Building regulations are compulsory standards of design and construction that every building has to

achieve. In order to progress onto constructing a designed structure, one has to get an approval from

the Parliament. Laws and regulations which apply to design and construction are set out in Building

Act 1984[2]

.

Currently there are 14 sections to the building regulations, where each one is guided by an approved

document. The approved document provides guidance in meeting the legislations, not a set of rules to

follow. In an approved document, the legislation applied is described in detail followed by a number

of means which must be fulfilled to satisfy the regulations. It is important to comply with the Building

Regulations and there are many ways to do so. In order to comply with the legislation, “Deemed to

satisfy” provision (attached in the approval document) must be achieved.

Building Regulations Approved Document B (fire safety) complies with this particular dissertation.

Part B of the building regulations consists of five aspects of fire safety (HM Government, 2006):

Figure 1: Schematic of Compartment Fire Growth


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B1 Means of warning and escape

Section B1 of the regulations ensures that in the event of fire in a building, it is easy for a person

inside the building to be notified with means of fire alarm and have a satisfactory standard of

means of escape.

B2 Internal fire spread (linings)

Section B2 ensures that the internal linings inside the building do not

support the rapid spread of fire.

B3 Internal fire spread (structure)

Section B3 ensures that in the event of fire, the spread of fire must be

slow and in cavities and voids with the provision of fire resisting

building materials and must include partitions where necessary. The

building must also ensure that no premature collapse should occur.

B4 External fire spread

Section B4 ensures that in the event of external fire, the spacing between buildings must

discourage the spread of fire. This can be controlled by the number and size of the opening on

boundaries.

B5 Access and facilities for the fire service

Section B5 ensures that the building should be designed in such a way that during the event of

fire, fire brigade can easily access the facility to control the fire and rescue anyone caught on fire.

For this particular project, Section B3 is highly related; stating that in the event of fire, the structure

should not easily lose its stability. In order to comply with the standard, load bearing must be added to

the structure so it maintains its ability for a reasonable amount of time.

According to British Standard 6336, there are three ways in which a structure can resist failing in the

event of fire:

Insulation – resisting heat transmission

Integrity – resisting plumes of flames and smoke

Stability – resisting structural failure

Stability is crucial for structural members while integrity and insulation are both important for walls,

flooring and ceiling/roof.

Figure 2: Building

regulations approved

document B (Volume 1 and

2)


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2.1.4. Standard Fire Curves and Furnace Testing

ASTM E119 was one of the first tests to be published about establishing a fire resistance rating

system for steel members through prescribed method. It was called “Standard Test Methods for Fire

Tests of Building Construction and Materials”. Eurocode had since been developed from the basis of

ASTM E119 to develop fire resistance rating tests.

2.1.4.1. Eurocodes

Eurocode 1 Part 2 provides detailed information for design fire of both standard and parametric fire

used in calculations with Eurocode methods. Formulas are provided for three different types of fire

curves: standard, external and hydrocarbon.

Figure 3 shows the time temperature curve according to Eurocode. Using three of the formulas

(formula 01, 02 and 03) for fire curve where, “Θg” represents the gas temperature in degree Celsius

and “t” represents time in minutes.

It can be seen from figure 3 that data for hydrocarbon and external fire are similar in shape.

Hydrocarbon fire temperatures tend to be 75% higher than external fire. The standard fire curve is

similar in shape and values when compared to other standard curves like ISO.

Θg = 20 + 345 log10 (8t + 1) [°C] 01

The external curve is intended for the outside of the separating external walls. When this wall is

exposed to the external cloud of fire coming either from the inside or adjacent to the respective

external wall (Franssen, et al., 2006). Formula 2 defines about the external fire curve.

Figure 3: Time Temperature Curves according to Eurocode


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Θg = 660 (1 – 0.687e-0.32t

– 0.313e-3.8t

) + 20 [°C] 02

In terms of hydrocarbon fire, the following formula is used for to understand the effects of

hydrocarbon fire.

Θg = 1080 (1 – 0.325e-0.167t

– 0.675e-2.5t

) +20 [°C] 03

2.1.4.2. ISO 834

The standard fire curve presented by the International Standard Organisation, ISO 834 is achieved by

using formula 04 producing chart showed in figure 4.

Tg = To + 345 log10 (1 + 8t) 04

2.1.5. Parametric Fire Curves

Along with the use of standard fire curves, many codes and standards are now making use of

parametric fire curves. Parametric fire method is used as a method to approximate post flashover

compartment fires. Parametric fire curve is similar to that of compartment fire with almost same

requirements; the size of compartment, fuel load, ventilation condition and thermal properties of

compartment wall and ceiling (University of Manchester, 2007). When compared to standard fire

curve, parametric fire curves provide a more realistic estimate of the temperature within a

compartment which is more advantageous in structural fire design of steel members. The analysed

results assume that the temperature in the fire compartment is uniform. According to BS EN 1991 – 1

Figure 4: ISO 834 Standard Fire Curve


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– 2 (2002), the method in achieving natural fire model is divided into two categories: simplified fire

models and advanced fire models.

For simplified fire models, Annex A in BS EN 1991 – 1 – 2 contains guidelines in achieving

parametric fire curve in compartment fires. The equation used by the standard is given by Formula 05.

Θg = 20 + 1325 (1 – 0.324e-0.2t*

– 0.24e-1.7t*

– 0.472e-1.9t*

) [°C] 05

Where,

t* = t × Γ Γ = [O/b]2 / (0.04/1160)

2

t = time b = thermal absorptivity for total enclosure

O = opening factor of fire compartment.

Under Annex A of EN 1991 – 1 – 2, there are two restrictions given by;

1. The time-temperature curves are valid for compartment up to 500m2 without any roof openings and

a maximum floor size of up to 4m. It is also assumed that the fire load of the compartment is

completely burned out.

2. If the fire load densities are specified without any specific consideration to combustion behaviour

(see annex E), this approach will then only be applied to fire compartments with mainly cellulosic

type fire loads.

From the equation for parametric fir curve shown in formula 5, the most influential factor is its

opening factor, “O”. It is included in almost all parametric fire curve equations and is used to govern

the behaviour of fire. The opening factor represents the amount of ventilation depending on the

opening area in the compartment walls, on height of the openings and on the total area of enclosure

surface. The value of “b” is introduced in the parametric fire equation to account for the multiple

layers present in the enclosure surface. If however there are not multiple layers then the value of b can

be taken as 1. To calculate the value for b;

b = √(ρcλ)

With many variables and multiple equations needed to create the heating and the cooling phase of the

fire, parametric fire, as described by EN 1991, is said to be a challenging curve for use in design and

practice.

In case of advance fire models (BS EN 1993 – 1 – 2, 2005), the following of the three models should

be used:

1. One-zone models – a uniform time dependent temperature distribution in the compartment.


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2. Two-zone models – upper layer with time dependent thickness and time dependent uniform

temperature and lower layer with time dependent uniform and lower temperature.

3. Computational Fluid Dynamics – CFD models give the temperature evolution in the compartment

in a time and space dependent manner.

2.1.6. Cardington Fire Test

It is vital to understand the behaviour of fire to steel structures, in order to attain information for its

behaviour; a full-scale fire test was carried out in a research facility in the UK. Between 1995 and

1997 a full-scale fire test was conducted on a constructed steel structure at BRE test facility at

Cardington, UK. The test was directed by British Steel Swinden Technology Centre. The aim of the

test was to understand and develop numerical calculation procedures that guide in predicting the

structural behaviour of steel framed building under fire.

The large scale test comprised of four fire tests which were carried out on different parts of the

building in order to understand various facets of structural behaviour. The building was designed as

per the use of an office building and was tested in an open plan.

2.1.6.1. Facility

Four tests were conducted on an eight storey composite frame building within the BRE facility at

Cardington, UK. The frame constructed was of composite steel and concrete to meet the national code

“British Standards”, BS 5950. The steel frame building was also checked for compliance with

provision of EC3 ENV 1993 – 1 – 1 (pre standard for Eurocode 3 during the test phase). One of the

most important things that needs to be considered is that the building was tested under normal

commercial pressure, which would mean that the data produced are realistic and can be applied/

compared to a real structure in order to understand the behaviour of steel in actual fire conditions.

Cardington fire test structure constructed is a braced frame including three core; one central elevator

shaft and two stair wells at either end of the building. Figure 5 defines the layout of the structure,

showing that it consists of five 9m bays along the elevation and 6m-9m-6m bays across the gables

(British Steel plc, Swinden Technology Centre, 1999). The total area of the layout is 45m x 21m.


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For simplicity in design and the aim to reduce costs, only 4 types of beams were used (British Steel

plc, Swinden Technology Centre, 1999): 9m secondary beam of grade S275 with 305x165x40 UB,

9m primary beam of grade S275 with 610x229x101 UB, 6m primary beam of grade S355 with

356x171x51 UB and 9m perimeter beam with grade S355 with 356x171x51 UB.

Due to moderation in design load specification, only 1/3rd

of the definite imposed load was applied

during the tests. The imposed load was implemented using sandbags. On all floors except roof, a total

of 2.4 kN/m2 was used with use of 12 sandbags (demonstrated in Figure 6) around the area of 9mx6m,

each weighing around 1.1 ton. Full details of floor loading are described in Table 2.

Load Case Value (kN/m2)

Composite Slab 2.06

Steel Sections 0.25

Raised Floor 0.4

Services 0.25

Ceilings 0.15

Partitions 1.0

Imposed 0.83 (1/3rd

of design load)

Table 2: Floor Loading Details

2.1.6.2. Tests Conducted

During the time frame of 1993 – 1995, four full scale fire tests were conducted on Cardington test

steel frame building. Each fire test was designed to understand structural behaviour in different ways.

Figure 5: Floor layout of Cardington test building

Figure 6: Loading of Cardington steel framed building


Page 20

Tests conducted with details about its objective, description, fire description and the measurement

needed are all defined below. The main focus of the test conducted was, after completion of the

project, the temperature data obtained will be used in understanding and designing of numerical

calculation procedures (British Steel plc, Swinden Technology Centre, 1999).

1D – Restrained Beam

The aim of this test was to understand the structural deformation when a beam is heated and restrained

by a composite slab spanning in two directions, with surrounding steel at ambient temperature.

The restrained beam test was carried out on the 7th floor of the building using a 305x165x40 UB

spanning 9m. In order to achieve a uniform temperature result, a gas fired furnace of 8mx3m was

incorporated in the design and built up to the underside of the composite floor. In order to minimise

heat losses, ceramic fibre collars were fitted at the ends of the beam as they passed through the

furnace wall.

Measurements which were conducted from the test are: temperature, strain gauge measurement,

vertical deflection and rotational measurements.

2D – Plane Frame

The aim of this test was to understand the behaviour of series of beams and columns supported by the

fourth floor by taking a 2D slice across the building. Fire protection also played a necessary role at the

connections.

Understanding the importance of fire protection, all columns of fourth floor were lightly protected up

to a height of 200mm below the connection. However the beams (beam to beam and beam to column)

remained totally exposed to fire. To heat up the structure, at fourth floor a gas fired furnace of

21mx4m was constructed to form a 2.5m corridor across the building.


vertical deflection, column displacements and rotational measurements.

3D – Corner

The aim of this test was to understand the behaviour of the complete composite floor system,

particularly the importance of membrane action under fire.

This test was conducted on first floor of the building; a compartment of 80m2 area was built in one

corner of the structure. To ensure no load bearing function was provided from gable end walls and

wind posts, all of the restrained ties were removed. In addition, the slots in wall construction were

also provided below the beams to ensure that no additional support to the floor slab was given. This

would prove that the walls of the first floor or the entire structure were nothing more than a non-


Page 21

loadbearing for containing the fire. Ventilation with a 7m wide opening was also provided and

partially covered by insulated screen. Although the ventilation conditions were pre calculated,

opening factor of 0.031m1/2

, the screen allowed fractional support to the burning rate temperature

within the compartment. To test the structure with extreme temperature of around 1000°C, the fire

load was increased to 45kg with an increase in the opening factor of 0.034m1/2

.

Based on the structural behaviour to fire in 2D – plane frame test, 25mm of ceramic fibre blanket was

used on all columns and beam to column connections. However, fin plate connections and both

primary and secondary beams remained exposed to fire.


vertical deflection, column displacements and rotational measurements.

Office Fire (demonstration)

The aim of this test was to conclude the research from previous studies in a more realistic fire

scenario and at the same time assessing other structural behaviour not addressed previously.

A compartment of 18mx10m was constructed in the form of a realistic office. The compartment was

fitted with modern furniture that would be found in a real office along with placing (figure 7),

computers and filing system while making use of the same proportion of wood and plastic (this

information was achieved from a survey carried out in modern office accommodation). The fire

loading of 45.6kg was applied from the total of wood/m2 of floor area. Similarly to previous tests,

columns and beam to column connections were protected with 25mm of ceramic fibre and the beams

(primary and secondary) were left unexposed to fire.

Measurements which were conducted from the test are: temperature, strain gauge measurement and

vertical deflection.

Figure 7: Office layout for Cardington Fire Test


Page 22

2.1.6.3. Effect on Fire Protection

The tests conducted on the steel framed building at Cardington, UK described above play a critical

step in structural fire protection of steel members in various ways. These ways were described by

Wang (2002):

1. It is important to examine the use of fire protection on steel members. It is imprtant to know that if

chosen to add fire protection to steel members, in a real construction, this would add labour costa and

extend the time in construction.

2. Research attained from Cardington fire test can be used as knowledge and can also be used as

comparison data to newley designed building for the performance of unprotected steel members.

The results attained from Cardington Fire tests display how structural steel members react under fire

conditions. This can aid engineers in using the data in creating design fires and conditions in

accurately depicting fire occurring in multi storey offices.

All of the data that was recorded at Cardington is now available online by British Steel. With easier

access, engineers can now progress in analysing and understanding of individual steel members along

with the entire steel structure under fire conditions.

2.1.7. Broadgate Fire

A fire was developed during the construction of a fourteen storey building called Broadgate Building

in 1990. During the fire, smoke detectors were not operational and sprinkler systems were not in

operation. It was till such extent that, protection of beams and connections were yet to be applied as

well as fire protective cladding had not been applied to the columns. The entire structure was covered

in plumes of fire causing over £25 million worth of damage, of which only £2 million worth of

damage was caused to the structural steel frame (Wang, et al., 2013).

2.1.7.1. Fire

The Broadgate Fire happened during the construction of a fourteen storey steel framed building. The

structure inhered of steel columns with composite beams, supporting a composite long-span lattice

truss. According to the design, fire protection of the building was to provide for standard fire of 90

minutes, however, with on-going construction, the protection to the structural elements was yet to be

applied. With lack of active measures, the developed fire spread quickly at very high temperatures

causing severe damages.


Page 23

According to the reports published and articles provided by Wang, et al., the fire started on the first

floor. The temperatures recorded were over 1000°C in some parts of the floor and the maximum steel

temperature achieved was just below 600°C.

2.1.7.2. Implications

Observing the damages caused by fire at Broadgate building, two theories can be removed (Wang, et

al., 2013).

Theory 1: During the fire scenario, it is known that the fire load was very less as there was no

imposed load acting on the building; the temperatures experienced by the steelwork would have been

fairly lower than the critical temperature of steel which would mean that there would not have been

any structural element failure. However, in this case there was a massive structural element failure;

distortion and local buckling (bottom flanges and webs near the support) was suffered by steel beams.

Failures and damages were also suffered by structural columns (Figure 8a). Due to column failure, the

load bearing capacity was reduced, causing vertical deformation (Figure 8b).

Theory 2: Even after such damages caused on the first floor of the building, the structure was stable

and all the floors retained their durability, preventing the spread of fire to other floors of the building.

This could be because the structure redistributed its loads from the failed elements to other parts of the

structure.

(a) Buckled column and beam deformation

(b) Floor deformation

Figure 8: Broadgate fire damages


Page 24

3 MATERIAL PROPERTIES

Properties of steel results from both its me chemical composition and its thermal condition, this could

relate to how the steel has been manufactured, including fabrication process. The standards for the

product define the limits for quality, composition and performance and these limits are used by

structural engineers in designing structures. This chapter evaluates the chemical and thermal

properties of structural steel relating to high temperature.

3.1. Thermal Properties of Steel at Elevated Temperature

3.1.1. Thermal Expansion

Thermal properties of steel define that steel expands at high temperature. High temperature in steel

can be experienced when there’s fire. With high temperature, steel loses its strength (load beating

capacity). Although steel temperatures vary according to the density, thermal conductivity and

specific heat, these differences still do not hold any effect on the strength of steel. Figure 9 define the

steel temperature vs thermal elongation, of how strength of steel is decreased with an increase in

temperature (EN 1993 – 1 – 2: 2005).

Figure 9: Thermal elongation of steel at elevated temperatures (European

Committee for Standardisation, 2005)


Page 25

The coefficient of thermal expansion can be used in order to work out the expansion, using Formula

06:

Δl/l = α(T) ΔT 06

Where,

Δl = change in length α(T) = coefficient of thermal expansion

l = initial length ΔT = change in temperature

3.1.2. Volumetric Specific Heat

Specific heat relates to a materials capability to absorb heat. Specific heat for steel is a function of

temperature and is independent of the composition of steel (Farmahini, 2013). The specific heat, “cs”,

in terms of Eurocode (European Committee for Standardisation, 2005) can be achieved using the

Formula 07 given below.

cs = 425 + 7.73 (10-1

)T – 1.69 (10-3

)T2 + 2.22 (10

-6)T

3 [J/kg°C] for 20 ºC ≤ T ≤ 600 ºC 07a

cs = 666 + 13002 / (738 – T) [J/kg°C] for 600 ºC < T ≤ 735 ºC 07b

cs = 545 + 17820 / (T-731) [J/kg°C] for 735 ºC < T ≤ 900 ºC 07c

cs = 650 [J/kg°C] for 900 ºC < T ≤ 1200 ºC 07d

Where,

T = temperature of steel, °C

For simplicity in calculations, specific heat could be considered to be independent of temperature,

giving an average value for cs as 600 J/kg°C.

The variation of specific heat from the formula above when compared to the temperature of steel

provides with graph shown in Figure 10. As seen from the graph shown in Figure 10, it can be seen

that there is a sharp peak at 730°C. According to Eurocode 3, this is because of metallurgical change

in the steel crystal structure.


Page 26

3.1.3. Thermal Conductivity

Thermal conductivity relates to the rate at which a material conducts heat. In terms of steel, it all

depends on the temperature and the composition. In comparison to other properties described above

(thermal expansion and volumetric specific heat), thermal conductivity is affected by the

microstructure of steel. According to Eurocode 3, structural steel’s thermal conductivity can be

approximated using the graph shown in Figure 11.

The accurate thermal conductivity of steel can be attained using Formula 8. However, for simpler

calculations, thermal conductivity, λa can be taken as 45 W/mK.

Figure 10: Specific heat of steel as a function of the temperature

Figure 11: Thermal conductivity of steel as a function of the temperature

(European Committee for Standardisation, 2005)


Page 27

λa = 54 – 3.33 x10-2 θa [W/mK] for 20 ºC < θa ≤ 800 ºC 08a

λa = 27.3 [W/mK] for 800 ºC < θa ≤ 1200 ºC 08b

Where,

θa = steel temperature [°C]

As noted from Figure 11, there is linear decrease in thermal conductivity as the temperature increase

from 0°C to 800°C and remains constant afterwards.

Thermal conductivity of steel is higher to that of concrete. It is recorded that at room temperature,

thermal conductivity of steel is about 50 W/mK whereas for concrete it is 2 W/mK. Conductivity of

steel is uniform in normal sized sections, however, temperature differences can occur in large sections

or sections which are able to lose heat, e.g. top flange in contact with concrete slab.

3.2. Mechanical Properties of Steel at Elevated Temperature

Creep is the biggest concern in the mechanical properties of steel. If the steel temperature reaches

over 450°C, increase in both the stress and the temperature influences steel to deform. Mechanical

properties of steel change when strength and the stiffness is reduced in steel due to an increase in

temperature. Eurocode 3 defines the temperature dependence in detail using the chart shown in Figure

12. The reduction factor, kθ, is used to determine the resistance of steel to tension, compression,

moment and shear force.

Figure 12: Reduction factor for stress-strain steel at elevated temperatures


Page 28

3.2.1. Stress-Strain Relationship

According to Eurocode 3, the stress-strain relationship chart shown in Figure 13 can be used to

determine the strength and the deformation properties of steel at elevated temperatures. This applies

when the steel heating rates are between 2 K/min and 50 K/min.

From the figure shown above, the elements which are unknown:

fy,θ = effective yield strength εy,θ = yield strain

fp,θ = proportional limit εt,θ = limiting strain for yield strength

Ea,θ = slope of the linear elastic range εu,θ = ultimate strain

εp,θ = strain at proportional limit

According to Kodur & Franssen (2010), proof strength can be used in place of yield strength of steel

at elevatd temperatures. The proof strength of steel can be taken as the point from the stress-strain

curve with a line intersecting at 1% strain. Thi is well defined in the chart shown in Figure 14.

Figure 13: Stress-strain relationship for steel at elevated temperatures (European

Committee for Standardisation, 2005)

Figure 14: Stress-strain curve for strain illustrating yield and proof strength


Page 29

4 HEAT TRANSFER

4.1. Heat Transfer for Unprotected Steel Member

If a structure was to go under a transient temperature increase with unprotected members, it could

result in a structural member reducing strength. At this high temperature, steel tends to expand and

lose strength causing it to buckle or fail in some cases.

If steel were to have high diffusivity, uniform temperatures exist through the member, whereas steel

with low diffusivity can expect a gradient temperature. Thermal gradient in a structural member will

cause the member to expand unevenly with compression stresses in the hotter section and tension

stresses in the other parts of the section.

The uneven expansion of the member will cause deformation, making the material swell towards the

source of the heat. In scenarios where deformation is restrained or be able to move, the member will

generate additional stresses (as seen in Figure 16) and add those to the external loads (as seen in

Figure 17).

Figure 15: Heat transfer in an unprotected steel member

Figure 16: Deformation (left) thermal stresses due to fire (right)


Page 30

During fire, the rise in temperature for steel members depend on the duration of heating,

thermophysical properties of fire and steel members. For this project, Vulcan and design spreadsheet

will be used to design for fire resistance of steel structures; in this program, three forms of heat

transfer are used to evaluate the steel temperature. For design spreadsheet, heat transfer can be

accounted with use of equations.

Formula 9 provides a method to work out the temperature increase in unprotected steel in time step t

(≤ 5 seconds):

a.t = (ksh/caa) × (Am/V) × hnet.d × t 09

Where,

ksh is the shadow effect caused by the local shielding of radiative heat transfer, due to shape of the

steel profile (“⌶” profiles have shadow effects and “◻” profiles don’t). ksh can be calculated using

Formula 10:

ksh = 0.9 × (Am/V)box / (Am/V) 10

According to Eurocode 1 (2002), thermal actions are achieved by net heat flux to surface member.

The net heat flux can be determined by heat transfer through convection and radiation and can be

achieved with Formula 11.

ḣnet = ḣnet,c + ḣnet,r [W/m2] 11

Convection, in terms of fire situation, relates to plume from fire moving past transferring heat to a

cool solid object. Convection is a complex problem to study in terms of fire spread and one can only

assume the flame spread parameters. The net convection can be predicted by using Formula 12:

ḣnet,c = αc × (g – m) [W/m2] 12

Radiation, in terms of fire situation, relates to transfer of heat energy via electromagnetic waves.

These waves can be absorbed by any object that is in the right sight of the heat source. The net

radiation per unit surface area can be predicted by using Formula 13:

ḣnet,r = 5.67 x10-8

× Φεres [(g + 273)4 – (m + 273)

4] [W/m

2] 13


Page 31

4.2. Heat Transfer for Protected Steel Members

In order to calculate the heat stored in protective layer for the member, φ, Formula 14 should be used:

φ = [(cpp) / (caa)] × dp × (Ap/V) 14

According to Eurocode 3, the temperature rise in steel in time step t (≤ 30 seconds) can be calculated

using Formula 15:

a.t = (λp / dp)/(caa) × (Ap/V) × (1/(1 + φ/3)) × (g.t – a.t) t – (eφ/10

– 1) g.t 15

The relationship between heat stored in protective layer and the temperature rise in steel relies on

hypothesis that there is no thermal capacity in the insulation. This would mean that transfer of heat to

the member would take longer time. Protective layer generally used contains moisture which causes a

level of increase in heat rate at around 100°C.

In heat transfer for protected steel members, temperature dependent property consists of thermal

conductivity, specific heat and density of insulating materials; of these specific heat and the density

do not change significantly. Thermal conductivity, however, does increase when the temperature goes

from 100°C to 1000°C. The process is very complex and in order to achieve the final thermal

conductivity, calculations need to be run through computer program. For simpler calculations, average

value for conductivity can be used based on the mean temperature of insulation when steel member

reaches maximum safe temperature, or, safety factor can be introduced with maximum acceptable

temperature of around 600°C.

Figure 17: Heat transfer in protected steel


Page 32

4.3. Section Factor

Section factor introduced in calculations for both protected and unprotected steel members, Am/V,

refers to the rise in temperature of steel section. The formula “Am/V” is where Am is the surface area

of a member through which heat is flowing to the steel section and V is mass of steel. During fire

scenarios, increase in Am would mean that there is rapid increase in temperature for the member,

inversely; increase in V would mean that there is slow increase in temperature. So, the lower the

section factor of a member, the lower the rate of increase in temperature.

In most cases, section factor for most members is somewhere between 10 and 300. If in cases where

the section factor is more than 300, then the temperature of the member is same as the furnace gases

and if lower than 10 then temperature is varied in the member. Figure 18a shown below displays the

section factors for selected types of unprotected members and Figure 18b displays the section factors

for selected type of protected members.

(a)

(b)

Figure 18: Section factor for protected and unprotected steel members (Haller & Cajot, 2006)


Page 33

4.4. Protection Mechanisms

Fire protection mechanisms are used in structures to delay rise in temperature in members when

exposed to fire. In order to achieve the best protective mechanism, a designer must be aware of the

rating of the used factor, such as the properties of the protective materials and the temperature it can

withhold. This usually can be achieved via testing of the structural mechanism, thus validating if it

meets the set criteria by the client and the designer.

The protection mechanisms commonly used in structures are listed below:

Insulating – These are materials with low conductivity adding minimal weight to the

structural member. Usually consisting of SFRM (Sprayed Fire Resistive Material), mineral

fibreboard and ceramic wool.

Absorbing – These are materials that release water of crystallisation in order to reduce the

pace at which the material heats up. Usually gypsum or concrete based products.

Intumescent – These are materials that are applied in the form of coating. Different to

insulating materials, these protective materials tend to expand when exposed to high

temperatures and act as an insulation layer. Materials like this are expensive however provide

great results, such as reduced weight, durability and aesthetics.

The thermal properties of protective mechanisms described in Table 3 below:

Material Unit mass

[kg/m3]

Moisture

content

[%]

Thermal

conductivity

[W/mk]

Specific heat

[J/kgK]

Spray-on

- mineral fibre

- vermiculite cement

- perlite

300

350

350

1

15

15

0.12

0.12

0.12

1200

1200

1200

High density sprays

- vermiculite (or perlite) and cement

- vermiculite (or perlite) and gypsum

550

650

15

15

0.12

0.12

1100

1100

Boards

- vermiculite (or perlite) and cement

- fibre silicate or fibre calcium silicate

- fibre cement

- gypsum boards

800

600

800

800

15

3

5

20

0.20

0.15

0.15

0.20

1200

1200

1200

1700

Compressed fibre boards

- fibre silicate, mineral wool, stone wool

150

2

0.20

1200

Concrete

- lightweight concrete

- concrete bricks

- bricks with holes

- solid bricks

2300

1600

2200

1000

2000

4

5

8

-

-

1.60

0.80

1.00

0.40

1.20

1000

840

1200

1200

1200

Table 3: Thermal properties of common fire protection materials (ECCS Technical Committee 3, 1995)


Page 34

Most SFRM protective layers make use of cementious or fibre materials to protect steel from heating.

Mineral fibre and vermiculite cement are most commonly used materials on steel columns, beams and

joists. Materials like these are commercially sold and have proprietary formulations; therefore, it is

crucial that designers must follow manufacture’s recommendations for application.

Protective mechanism like intumescent coatings is largely advantageous. Coatings used have an

appearance of thick film or paint and when these materials are exposed to fire, it chars, foams and

expands to form an insulating layer for the steel member. In order to retain this formed layer,

reinforcing is essential at sharp corners, e.g. flange tips.

Gypsum boards are largely used in protection of members too. These consist of non-combustible

cores and paper to form sheets which are available in range of sizes depending on the member or

recommendation. There are various types of gypsum boards available, special fire resistant types are

best for fire protection.

Mineral fibre products are generally more expensive than other SFRM products but are easy to install

and takes minimal maintenance. Unlike absorbing materials, these products do not consist of wet

process and are therefore easy to maintain too. Materials like these are generally used in projects

where speed and dry process is of important. These materials are supplied with outer sheathing of

either aluminium foil or similar products. Mineral fibre products are fixed to steel members in variety

of methods, such as wire ties, screws, bonding agent, etc.

Concrete protection is largely used in hollow steel members. The biggest advantage in using this

system is that no advance fire protection material is necessary and the load bearing for the member

will be increased (Steel Construction Institute, 2012), however, the member gets bulky and adds more

weight to the structure.


Page 35

5 FIRE RESISTANCE ASSESSMENT

5.1. According to Eurocode 3

In order to meet the fire requirements, designers have to follow the given guidelines in Eurocode 3.

This is measured in terms of time. The value of time given via calculation mean that the fire resistance

will protect the structural member from standard fire until that time after which the member will fail.

The time requirements depend on the number of floors in the building, type of occupancy, the fire

load and the amount of active measures in the building.

Fire resistance assessment in terms of Eurocode 3 is based on either standard fire tests in a furnace or

calculations. For this particular dissertation, a graphical rendition of a member resistance is designed

as regards to the temperature; critical load temperature can then be worked out for the given load for

non-composite beams with lateral restraint and tension members.

5.1.1. Calculations for Design Data

The mechanical properties of steel and the protection mechanism change according to the

temperature. In fire scenarios, the effect of temperature on a structural member is constant. As

member temperature increases, the resistance will lose and on resistance failure, the member will fail.

This is called critical temperature, θa,cr. Critical temperature relies upon the member’s ability to

withstand high temperatures in fire conditions. It is independent to the size of the member or the rate

at which member heats up.

In order to achieve the critical temperature, utilisation factors of the member is involved and it is

calculated with measured yield stress. Utilisation factor is the ratio of design loading in fire to design

loading at ambient temperature. It can be calculated with Formula 16:

µ0 = Efi,d / Rfi,d,0 16

Where,

Efi,d = effects of action for fire design, Rfi,d,0 = resistance of isolated member for ambient temperature


Page 36

For simpler calculation, to achieve µ0, Formula 17 can be used:

µ0 = ηfi × (γM,fi / γM,1) 17

Where,

ηfi = (γGA Gk + ψ1,1 Qk,1) / (γGA Gk + γQ,1 Qk,1)

The simpler formula (Formula 17) is said to be traditional formula, as ηfi is calculated as the amount

of design loading at ambient temperature. The partial safety factor for steel is 1,0.

At ambient temperature, permanent loads and combination factor or variable loads are: 1.35 and 1.5,

respectively. Whereas in fire design, permanent loads and combination factor or variable loads are 1.0

and 0.5, respectively. Table 4 shows the load reduction factor based on partial safety factor according

to EC3.

Qk,1 / Gk 1 2 3 4

ηfi 0.53 0.46 0.43 0.41

Table 4: Load reduction factor based on partial safety factor according to EC3

Shown below in Figure 19 is a graph of critical temperature vs degree of utilisation, µ0, for simple

steel members based on standard fire test.

The curve achieved in the graph is based off formula from EC3 (Eq 4.22), where the critical

temperature is expressed as a function of the degree of utilisation μ0, for class 1, 2 and 3 steel

sections:

Θa,cr = 39.19 × ln × [(1/(0.9674 (µ0)3.833

) – 1] + 482 18

The formula used can only be applied to non-composite members with section classification of class

1, 2 or 3 sections subjected to tension or bending that are restrained to prevent instability effects.

Figure 19: Critical temperature for simple steel members based on

standard fire test


Page 37

5.2. According to British Standard’s

According to British Standards, in order to determine the fire resistance of a member, BS5950-8 shall

be used and worked out using the load ratio method. Load ratio method is independent in comparison

to other fire resistance methods as it only requires the temperature in the critical section of the

member with the strength reduction of the member in fire scenarios. The fire temperature in the

compartment or in the member is achieved from the fire test data or thermal model. Moment capacity

method is also provided by BS5950-8 but however is sophisticated than the load ratio method and is

generally used on complex structures where temperature data is available.

5.2.1. Load Ratio Method

The load ratio method describes all the combined effects acting on the structural member in fire

conditions. Load ratio for members in bending would be the applied moment on the member at fire

limit state divided by the moment capacity of the member. It is assumed that the member is not

subjected to second order effects resulting from deflection.

Load ratio provides the designer with stress in the member at the fire limit state relative to the design

strength of the member. The higher the load ratio, the higher the retention of strength in member

during fire. Therefore, the temperature of the critical element to resist the applied load will be lower.

5.2.2. Moment Capacity Method

Carrying out small scale fire test on a structural member of a complex structure to obtain thermal

profile has proven to be inexpensive. Using the data obtained from the test, its fire resistance can be

evaluated by considering its strength properties in the section at elevated temperatures. This is known

as the Moment Capacity method and can be applied to composite beams, shelf angles and slim floors.

It is the foundation of the method in determining the resistance of composite floors in fire.

6 BEHAVIOUR OF STRUCTURAL ELEMENTS

A fire in a compartment can affect the entire structure, its members and its mechanical, physical,

chemical and thermal properties. A steel framed structure subjected to fire relies on a number of

properties: degradation of materials at high temperatures and restraint stiffness of the structure around

fire. Elevating temperatures in a structure are the reason for deflections and failures. Failure of a

member links to failure of a structure. Failure of the member relies upon the size and shape of the

member, the location of the member, the location of fire and the type of protective mechanism

applied.

6.1. Beam Analysis

The occurrence of beams with lateral buckling at elevated temperature is very rare as most beams are

braced by floor system (Bennetts & Thomas, 2002). Bennetts and Thomas’ research findings were

based off the data produced by Cardington Fire Test Building of eight storey steel framed building

subjected to full scale fire test. It was recorded by the group that, unprotected steel beams designed to

be composite linked with composite floor slab in fire perform better than an isolated beam subjected

to fire. With this research, it can be noted that protection for some structural members can be avoided

as they provide a better result overall, also saving time and cost of the project.

Lewinger, et al. (1999) designed and tested a three storey steel framed building under fire conditions

and reviewed the difference between design moment strength and ultimate moment strength and that

the moment capacity of the structure need to be evaluated as the reserve moment capacity. The results

for reverse moment strength were analysed and later discussed that after two hour of fire resistance,

the reverse moment strength with a yield stress of 250 MPa or 345 MPa is less than the temperature

moment strength. This measn that the higher the steel performance, the better its durability under fire

and ambient temperatures. Therefore the overall fire resistance of the structure can be improved with

the use of high performance steel. Shown below in figure is the failure mechanism of simply

supported beam and continuous beams.

Figure 20: Failure mechanism for simply supported beam (left) and continuous beam (right)


Page 39

6.2. Column Analysis

According to Bailey (1999), the internal and external columns at Cardington Fire Test facility are

subjected to high moments which are caused by the expansion of connecting beams during fire test. It

was also sated in the report by Bailey that if the moments were included withihn the member in the

design process, then calcaulations would prove that the member was subjected to failure due to local

plasticity. The research further stated that the instability in column was affected by the heating rates in

beam to column, cross seciton and span of the beams, end rigidity of heated column and axial load at

the column. It was also proven that, cross section of column, connection rigidity of beam to column

and the horizontal restraint to the heated beams had shown to have nominal effect on behaviour of

column during fire.

Going back to Bennetts and Thomas (2002) research on Cardington Fire Test data, the reduction in

the failure temperature for steel column varies with its slenderness, for example the failure

temperature of a bulky column with slenderness ratio of 40 will be reduced by 100°C. Both Bennetts

and Thomas’ and Bailey’ research prove similar point of where columns are affected with expansion

of connecting beams in fire. The research further studies that due to the expansion of the beams

linking to expansion of floors, significant bending moments were established withi the columns. Other

factors to consider when analysing the failure of column is the end connection, size of column to

beam connection and the temperature of the column. It should also be noted that the presence of

thermal gradient in column reduces the failure temperature even more, while uniform distribution

proves beneficial.


Page 40

7 METHODOLOGY

For this particular dissertation, two programs will be used in order to understand and design for the

fire resistance of steel structures: spreadsheet and Vulcan. This chapter will provide with brief

information about the test methods, its capabilities and limitations.

7.1. Spreadsheet Method

The results achieved form the finite element program, Vulcan will be used in comparison to Eurocode

3 calculation method. This will be done with aid of excel spreadsheet filed called FiRE.XLS which is

generally used to predict the temperature of a steel member using heat transfer theory to understand

and estimate the heat energy transferred to the member. The methods to achieve heat transfer to

protected and unprotected members are generally the same, with use of different formulas presented

in Chapter 4 to allow for the effect of protection on the rate of heating for steel member.

With use of the spreadsheet, the temperature of the beam can be calculated between intervals, by

considering the energy transferred to the beam during the previous time step. The duration of the time

step does not affect the calculated temperature however only defines the temperature of steel member

at a particular time.

The spreadsheet assumes that the structural steel member is of constant thickness and does not have

thermal gradient. This is governed by the section factor of the member. This is called a lumped mass

approach where no regard for the actual geometry of the cross section is given. Constant value for the

thermal properties of steel, such as specific heat and density are used in the spreadsheet to simplify

the method and number of variables.

Thermal properties and geometry of the steel member are averaged values and will not exactly be the

same when constructed in place. This would prove that accuracy is a must, if however, errors occur,

then other factors such as the temperature of the fire and the member interaction with fire may not

exactly be the same. This however is very rare to occur in a real design and construction of structure.

It is known that the spreadsheet data provides higher temperatures than other finite element programs,

such as Spreadsheet, the results achieved are acceptable to be used in design of a compartment or

when it is a four sided exposure and when analysing the temperature elevation of a simply supported

members.


Page 41

7.2. Vulcan

7.2.1. Computer Modelling

It is highly expensive to perform structural fire test, in order for designers to understand and design

for fire, computer simulations are now highly used. The data achieved from the tests performed in

these programs are used in understanding and designing for structural members in elevated

temperatures. This type of method is becoming readily available over the recent years and engineers

are now using these programs to design structural members. For this project, Vulcan finite element

program will be used to design a for fire resistance.

Vulcan is a non-user friendly program which works from a textual input file. It precisely describes the

structure / compartment to be modelled as a series of node connected by beam to column, shell or

spring elements, each by its specific geometry and material property. A heating regime and

temperature increments are prescribed in the program. The file is then reformulated to make it more

user friendly and improve its flexibility for future enhancements.

Vulcan takes an input file, processes its nonlinear finite element analysis and creates an output file of

results. The input filed can be analysed in series of batch file, allowing parametric studies to be

performed easily. This output file can then be interrogates in spreadsheet program.

7.2.2. Capabilities and Limitations

With advantages in use of finite element programs to understand its capabilities, there are a few

limitations too. The beam to column elements in Vulcan are presented as two node line elements with

each note consisting of eight degrees of freedom. The degree of freedom for this element represents

the strain and displacement in each of the three dimensions together with three selected derivatives of

these degrees of freedom (which signifies either direct-strains or shear-strains) with twisting and

warping. The eight degrees of freedom are transformed into eleven global co-ordinates. In order for a

model to be solved it is therefore necessary that three degrees of freedom are constrained at each

node. This can be done either manually by applying external boundary conditions or via being

constrained by other elements.

Limitations in Vulcan are that, only I shaped section can be defined in the elements. The only way

other shaped elements can be approximated is by defining I section with similar cross section

properties similar to the ones of desired shape. Tapered elements can also be approximated by

dividing long element along its length into sub elements, which is in smaller cross sectional area.

Spring elements have the same degrees of freedom as beam-column elements (Huang, 2011),

however, spring elements ae normally used to represent semi rigid connections so their rotational


Page 42

stiffness properties are modified in an analysis to simulate behaviour of moment connection with

temperature dependent stiffness and capacity. The author can then reformulate the input file to specify

each spring element as one of a choice of pre-defined types. Based off the results from Cardington

Test Fire, pinned, rigid and semi rigid elastic characteristics have been also been defined along with

two temperature dependent characteristics representing full and partial depth end plate connections.


Page 43

8 PROCEDURE

In this dissertation various parametric analyses will be carried out on the program Vulcan on steel

members at elevated temperatures at different location within the frame. Examining these members on

Vulcan would mean that the members are analysed in holistic manner. The tests will be carried out on

both protected and unprotected steel members via both Vulcan and the spreadsheet method. The

parameters for the tests are based on fire regime and characteristics of beam to column connectivity.

The dissertation is based on analysing 2D steel frame via Vulcan and spreadsheet. The building takes

a shape of a 2 storey non sway steel frame comprising of 5 bays, each at a distance of 9m. In the

transverse direction, there are 4 bays with a distance of 6m with a floor height of 4m. The sections for

the different elements used in the structure will be uniform.

8.1. Proposed Plan

In order to design for fire resistance, the proposed plans are that before the testing begins, the section

size of the members will be selected prior to designing, the members will be analysed according to

Eurocode 3 via the spreadsheet. The fire resistance time for the elements will be evaluated and the

hand calculations will be proposed for both protected and unprotected steel members. Both types of

members will be tested against standard and parametric fires to observe how these members react to

various fire conditions.

Simply supported beams will then be tested in Vulcan at elevated temperatures. The analysis will only

be carried out on unprotected beams exposed to ISO and parametric fire. The entire frame will then be

tested on Vulcan; the frame will be divided into segments to allow variation of temperatures, stress

and strain.

A number of five scenarios are designed which include different fire regimes. As this analysis is not

based on isolated members, different characteristics of beam to column connectivity will be analysed

including rigid and pinned connections. For testing of isolated members, spreadsheet method will be

used and cross examined on Vulcan. The results achieved for the steel frame building for the

scenarios will then be discussed in relation to each other. The scenarios are mentioned on the next

page.


Page 44

List of 5 scenarios for the proposed steel frame building:

Scenario 1: FIRST FLOOR – Bay 1 will be exposed to ISO 834 time-temperature fire regime with use

of rigid and pinned beam to column connection.

Scenario 2: FIRST FLOOR – Bay 1 will be exposed to parametric time-temperature fire regime with

use of rigid and pinned beam to column connection.

Scenario 3: FIRST FLOOR – Bay 3 will be exposed to ISO 834 time-temperature fire regime with use

of rigid and pinned beam to column connection.

Scenario 4: FIRST FLOOR – Bay 3 will be exposed to parametric time-temperature fire regime with

use of rigid and pinned beam to column connection.

Scenario 5: FIRST FLOOR – Bays 1-3 will simultaneously be exposed to ISO 834 time-temperature

fire regime with use of rigid beam to column connection.


Page 45

9 DESIGN OF STEEL FRAMED BUILDING

In this chapter, steel frame building will be designed also evaluating elements and section sizes.

Spreadsheet program will be used to determine critical temperature of the elements according to

Eurocode 3. As mentioned in previous chapter, this is a non-sway building with 5 bays, each at 9m

and in the transverse direction, 4 bays, each with 6m and a floor height of 4m. Figure 21 shows the

side view of the building. As seen from the figure, line of symmetry has been added in the side view

of the building at half way point, this allows for a repetitive frame of the structure to be analysed in

Vulcan more manageably.

Secondary beams are attached to the building along the longitudinal axes, with them being placed

halfway between each transverse bay. With attachment of secondary beams, it is assumed that

primary beams are laterally restrained and in torsion at the mid span. The plan view of the building is

shown in Figure 22.

Figure 21: Plan view of the building


Page 46

9.1. Primary Beam

A primary beam with a section size 610x305x179 UB has been selected and verified for the

calculation analysis. The member has been categorised as Class 1 section; cross-section that can form

a plastic hinge with rotational capacity from plastic analysis without reduction of the resistance. In

finite element analysis, this beam will be exposed to three sided heating to simulate the condition of a

concrete slab on top of the member. Fire resistance and protection of primary beam is shown in Table

5.

Fire resistance and protection of primary beam

(Class 1 Cross-Section)

fy = 355N/mm2 Design Bending Moment, Msd = 1548.2 kNm

Permanent action, Gk = 12.2 kN/m2 Resistance bending moment, MRd = 1630.9 kNm

Variable action, Qk = 23 kN/m2 Shear capacity, Vpl, Rd = 1613.7 kN

Combination factor, Ψ1.1 = 0.5 Design moment in fire, Mfi,sd = 719.9 kNm

Qk,1/Gk = 1.9 Adaptation factor, k1 = 0.85

Reduction factor, ηfi = 0.465 Adaptation factor, k2 = 1

Critical temperature, θcr = 629.7°C

Table 5: Critical temperature values for primary beam members

The critical temperature achieved for an unprotected beam exposed to ISO fire is calculated to be

658.659°C at a time of 9.33 minutes.

Standard fire curves calculated for protected and unprotected beams are shown in graph displayed in

Figure 23.

Figure 22: Side view of the building


Page 47

In order to protect the structure from fire, fire resistance shall be applied to extend the time until

structural failure. This also saves lives of people inside the building. For primary beam to achieve 60

minutes of fire protection and as per Eurocode 3, sprayed mineral protection of 10mm was used. After

the application, the temperature of primary bean reduced to 628.06°C.

Comparing the temperature data achieved from both protected and unprotected beams, it is not an

improvement in terms of failure time for the beam. This could be due to the time temperature curve

provided by the spreadsheet. The failure of the beam at relatively low time is due to the design fire

quickly reaching high temperatures as shown in Figure 24.

0

200

400

600

800

1000

1200

1400

0 600 1200 1800 2400 3000 3600 4200 4800 5400 6000 6600 7200

Te

mp

era

ture

(o

C)

Time (sec)

Starndard Fire

Gas Temp Protected Unprotected

0

200

400

600

800

1000

1200

1400

0 600 1200 1800 2400 3000 3600 4200 4800 5400 6000 6600 7200

Te

mp

era

ture

(o

C)

Time (sec)

Parametric Fire

Gas Temp Protected Unprotected

Figure 24: Parametric fire curve for both protected and unprotected beam

Figure 23: Standard fire curves for protected and unprotected primary steel beams


Page 48

For parametric fire, primary beam’s temperature is nowhere close to the critical temperature and is

found to be at 133.3°C at the 60 minute mark. In fact, the highest temperature achieved by the

protected beam in parametric fire is 234.8°C at 32.25 minutes. Therefore it is vital to make use of

unprotected primary beams in Vulcan analysis and to keep columns protected in order to extend the

column failure time.

It is interesting to note the characteristics of fire interacting with protected and unprotected beams. At

45 minute mark the fire temperature of protected beam is higher (190.3°C) than that of unprotected

beam (114.1°C). This is because the application of protection layer to the beam increases its overall

heat capacity of the member and gradually dissipates the heat.

9.2. Column

A column with a section size of 203x203x71 UC has been selected and verified for calculation

analysis. According to Eurocode 3 the member is classes as Class 1 section; cross-section that can

form a plastic hinge with rotational capacity from plastic analysis without reduction of the resistance.

Throughout the testing the column will be exposed to a four sided fire exposure with one hour

duration during the Vulcan analysis process. Fire resistance and protection of the column is shown in

Table 6.

Fire resistance and protection of column

(Class 1 Cross-Section)

fy = 355N/mm2 Total design axial compression, Nsd = 1696.2 kN

Combination factor, Ψ1.1 = 0.8 Effective length factor = 0.7

Qk,1/Gk = 1.9 slenderness, λ = 52.9

Reduction factor, ηfi = 0.600 Normalised slenderness, λ/λ1 = 0.692

Buckling resistance, Nb,Rd = 1754.87 kN Normalised slenderness at θa, λθ = 0.785

Design loading in fire, Nfi,sd = 1018.3 kN

Buckling resistance at time t with

uniform θa, Nb,fi,t,Rd = 1932.8 kN

Table 6: Fire resistance and protection steel column

As mentioned above in primary steel analysis, all columns will be protected and as per column

analysis, it usually buckles at around 406°C. Temperature achieved from steel column at 60 minute

mark is 607.2°C for ISO fire and for parametric fire, the temperature achieved in steel column at 60

minute mark is 215°C. As seen from the temperatures, the column is sufficient to remain stable. Even

though the temperature achieved for column from ISO fire exceeds the limit, it would mean that the

beam would face higher temperature and is predicted that the beam will fail much earlier than the

column.


Page 49

10 ANALYSIS OF 2D FRAME

When designing for fire, it is essential that critical members of the structure are evaluated for fire

resistance and these members should be tested for ISO 834 fire time temperature curve. It is not

economical to perform fire test for each member of the structure but with help from finite analysis

software, Vulcan, structural analysis simulation can be run under more realistic boundaries, providing

results faster and more accurate. This type of analysis also demonstrates the non-uniform temperature

distribution within the member section.

From the primary beam analysis, the section size chosen is a 610x305x179 UB with a span of 9m.

The beam is simply supported from both ends with one end of the beam free to move in longitudinal

direction. The beam will only be exposed to three sided heating and will be subjected to both standard

and parametric fire conditions.

For Vulcan, the beam is modelled as a 13 nodes, each spaced at a distance of 750mm. Three node

beam elements will be used for this analysis which means that the beam will result in six elements as

shown in Figure 25. The analysis will be followed for both protected and unprotected beams. The

permanent and variable actions for primary beams are 12.2 kN/m2 and 23 kN/m

2 respectively.

In order to understand the analysis thoroughly, deflections attained by the beams within a structure

will be compared to the deflection of the isolated beams for both ISO and parametric fire test. As

mentioned priorly, all beams will be unprotected and column protected. Beams will be exposed to

three sided heating whereas; columns will be to four sided heating. The fire duration during testing is

designed to for 60 minutes.

1 2 3 4 5 6

1 2 3 4 5 6 7 8 9 10 11 12 13

Figure 25: Simply supported beam model


Page 50

10.1. Scenario 1: FIRST FLOOR – Bay 1 – ISO Fire with Rigid and Pinned

Connections

The first scenario is based on ISO 834 fire with rigid and pinned beam to column connection for bay

1. This scenario focuses on achieving ISO 834 time temperature for two cases. Pinned beam to

column connections are only considered for scenario 2, 3, and 4 where individual compartments are

tested.

Critical temperature achieved for the beam in this structure is reached at the mid span of 450mm as

per the ‘span/20’ rule set by BS5950 – 8. Beams with same connection types have closely the same

temperature deflection curve whose results and data are relatively related. So, whether the beam is

exposed to ISO fire or parametric fire, the deflection temperature achieved will be the same; the only

difference will be between time temperature relationship. The deflection temperature for simply

supported beam is also plotted for comparison.

According to the results achieved, the pinned connection and simply supported beams reach their

critical temperature at 485°C whereas rigid connections reach their critical temperature at 660°and

628°C for ISO and parametric fire respectively. The simply supported beam reaches highest

deflection of 1065mm before failing. Simply supported and pinned connection starts to lose their

strength at 220°C after which the point increases dramatically. The transition for rigid connection is

not as gradual; the connection starts to lose its strength at 510°C onwards.

Figure 26: Bay 1 exposed to ISO fire conditions

0 100 200 300 400 500 600 700 0

-200

-400

-600

-800

-1000

-1200

Temperature Deflection Relationship

Def

lect

ion

(m

m)

Temperature (°C)

Span/20

Rigid

Pinned

Simply Supported Beam (ISO)

Figure 27: Temperature deflection graph of bay 1 (node 82) with isolated member subjected to ISO fire


Page 51

It should be noted that there can never be a perfect rigid or pinned connection. The data presented is

just a comparison between the two rather than predicting what will happen to the connection itself. It

should also be noted that rigid connections’ deformation process occurs in such a manner that the

stress occurring is distributed along the span of the beam and the cooler areas of the structure provides

restraint so that the beam does not fail before reaching its critical temperature.

It is evident that with use of ISO fire, structural elements heat much quicker than that of parametric

fire. There are various ways possible to model for parametric fire, so, reaching the temperature peak

could take much longer than that of ISO fire. It must be noted that data generated by ISO fire are not

realistic and therefore, a designer must pay enough research time in producing more realistic

parametric fire. Eurocode 3 mentions that with ISO fire, an unprotected beam can take between 6.52

minutes – 9.25 minutes to reach its critical temperature. According to the results achieved from the FE

analyses, an unprotected beam can take between 7 minutes – 11.9 minutes to reach its critical

temperature.

From Figure 29, it can be noted that the axial force gradually increases and then restores its force by

declining. The axial force for rigid connection is noted to be at 68 kN at 498°C. The beam then fails in

tension mode.

0 200 400 600 800 1000 1200 0

-200

-400

-600

-800

-1000

-1200

Time Deflection Relationship

Def

lect

ion

(m

m)

Time (sec)

Span/20

Rigid

Pinned


Figure 28: Time deflection graph of bay 1 with isolated member subjected to ISO fire at node 82

0 100 200 300 400 500 600 700

80

20

0

-80

40

60

-20

-40

-60

Temperature – Axial Force Relationship

Axi

al F

orc

e (

kN)

Temperature (°C)

Rigid (ISO)

Pinned (ISO)

Figure 29: Temperature - axial force graph for beam to column connections at bay 1


Page 52

There are various ways in modelling for beam to column connections. This is compared by Huang

(2011) where he has compared his achieved results with Block, et al.’s (2007). The results have been

compared for a compresive component based models where in Block et al.’s model, the connection

used is represented as assembly of springs which represents individaul components of the connection,

such as tension bolt rows, shear bolt rows and compression parts of beam and column’ web and

flange. In comparison form Huang and Block et al.’s work, for this analyses two noded spring

element is used in which the stiffness and the strength is determined based on each component of the

connection. Therefore, the results achieved for this connection is predicted to be better when

comparted to prediction by Block et al.’s model.

80

20

0

-80

40

60

-20

-40

-60

0 200 400 600 800 1000 1200

Time – Axial Force Relationship

Axi

al F

orc

e (

kN)

Time (sec)

Rigid (ISO)

Pinned (ISO)

Figure 30: Time - axial force graph for beam to column connections at bay 1


Page 53


Connections

As stated above, the results achieved for temperature deflection relationship is the same as that for

scenario 1 for ISO fire due to the use of same connection. As also stated above, the stiffness of the

pinned column can be modelled in various ways. However, with pinned connection, it can be noted

from the results shown in Figure 31 that there is warning before the connection fails compared to

simply supported beam where the deflection is not as sudden.

Comparing the critical temperature achieved for the simply supported beam with results calculated

from Eurocode 3, the results are noted to be very close. However, this should not be used as a general

rule as this needs more results and data confirmation in order to be approved for the suggestion. Since

the utilisation factor achieved is only for isolated members, this can be modified in order and the

result can be based on the structures as part of the whole building.

0 100 200 300 400 500 600 700 0

-200

-400

-600

-800

-1000

-1200


Def

lect

ion

(m

m)

Temperature (°C)

Span/20

Rigid (parametric)

Pinned (parametric)

Simply Supported Beam

(parametric)

Figure 31: Temperature deflection relationship of bay 1 and an isolated member subjected to parametric at node 82

0

-200

-400

-600

-800

-1000

-1200


Def

lect

ion

(m

m)

Time (sec)

Span/20

Rigid (parametric)

Pinned (parametric)


(parametric)

0 200 400 600 800 1000 1200

Figure 32: Time deflection graph of bay 1 with isolated member subjected to parametric fire at node 82


Page 54

The data show in the graph in Figure 33 indicate that tensile axial force acting on the connection

behaviour develops more significant as the temperature increases. The curve achieved for the pinned

connect ion beam in Figure 31 follows the same pattern of rigid connection beam from scenario 1,

however, it only manages to reach an axial force of 29 kN. Beams with these connections tend to fail

at higher tensile force when compared to rigid connection.

As seen from the results, parametric fire scenario evaluates that internal connections take longer to

reach the maximum compressive value than ISO fire scenario (Scenario 1). If parametric fire was

modelled in a way that if the higher temperatures were to take longer time to reach the maximum

temperature then the diagram would be expected to elongate. It can be noted that rigid connections

take higher loads at elevated temperatures and do not fail as easily as pinned connections.

0 100 200 300 400 500 600 700

80

20

0

-80

40

60

-20

-40

-60


Axi

al F

orc

e (

kN)

Temperature (°C)

Rigid (parametric)

Pinned (parametric)

Figure 33: Temperature - axial force graph for beam to column connections at bay 1 subjected to parametric fire

80

20

0

-80

40

60

-20

-40

-60

0 200 400 600 800 1000 1200


Axi

al F

orc

e (

kN)

Time (sec)

Rigid (ISO)

Pinned (ISO)

Figure 34: Time - axial force graph for beam to column connections at bay 1 subjected to parametric fire


Page 55

10.3. Scenario 3: FIRST FLOOR – Bay 3 – ISO Fire with Rigid and Pinned

Connections

This scenario is designed to understand how beams respond to fire with more axially loaded columns.

Structural members tend to obtain a higher critical temperature with higher rotational restraint. It is

known that the cool adjoining members restrain the thermal elongation along with the end rotations of

a beam at elevated temperature. Therefore, there is significant restoring effect from rotational restrains

for beams at elevated temperatures.

According to the results achieved, it can be seen that for rigid connection beam, the critical

temperature achieved is 650°C. The beam shows longer deflection in comparison to scenarios 1 and 2.

The rigid connection beam fails at 730°C with maximum deflection of 1080 mm. On the other hand,

pinned connection beam has a critical temperature of 485°C and fails at 543°C which is improved in

comparison to pervious scenarios. The results also prove that the pinned connection beams fail at

lower temperature in comparison to rigid connection beams.

Figure 35: Bay 3 exposed to ISO fire

0

-200

-400

-600

-800

-1000

-1200


Def

lect

ion

(m

m)

Temperature (°C)

Span/20

Rigid (ISO)

Pinned (ISO)


0 100 200 300 400 500 600 700

Figure 36: Temperature deflection relationship of bay 3 and an isolated member subjected to ISO fire at node 132


Page 56

According to the results shown for temperature axial force relationship in Figure 38, both the

connections experience gradual increase in axial forces with sudden decline. This could be due to the

connections experiencing a greater tensile force as they are slowly deflected. The temperature axial

force relationship for rigid connection beam are almost four times greater than that of pinned

connection beam.

0

-200

-400

-600

-800

-1000

-1200


Def

lect

ion

(m

m)

Time (sec)

Span/20

Rigid (ISO)

Pinned (ISO)

Simply Supported Beam (ISO))

0 200 400 600 800 1000 1200

Figure 37: Time deflection graph of bay 3 with isolated member subjected to ISO fire at node 132

0 100 200 300 400 500 600 700

280

70

0

-280

140

210

-70

-140

-210


Axi

al F

orc

e (

kN)

Temperature (°C)

Rigid (ISO)

Pinned (ISO)

Figure 38: Temperature deflection graph of bay 3 with isolated member subjected to ISO fire at node 132

280

70

0

-280

140

210

-70

-140

-210

0 200 400 600 800 1000 1200


Axi

al F

orc

e (

kN)

Time (sec)

Rigid (ISO)

Pinned (ISO)

Figure 39: Time deflection graph on bay 3 with isolated member subjected to ISO 834 fire


Page 57

The connection in this scenario show more realistic restrained conditions due to the placement of two

protected columns at either end of the tested beam. The connections were subjected to axial

compression force due to thermal expansion of the beam at high temperature at the beginning of the

test which then concluded in reduction of axial compression force and changing to tensile axial force

as the beam had lack of stiffness at experienced high temperatures with catenary’s action resulting

from the large deflections. The tensile axial forces are expected to increase too if the test was run

during the cooling phase of the fire.


Page 58


Connections

The compressive axial forces in the heated beams were generated at the first stage of fire due to the

restrain of columns. Therefore, the compressive axial forces at bay 3 are twice the forces recorded at

bay 1.The axial tensile force were developed in the later stages due to catenary action. It is clear from

the achieved results that the axial tensile forces at both the positions mirror their compressive axial

force counterparts.

The restraint provided by the columns has insignificant influence on the catenary action of the beams.

If however, the beam does not reach its critical temperature then the FE analysis to generate axial

forces which were generated during the cooling phase of fire. Tensile forces will be generated in

beams due to thermal shrinkage and this will be greater in magnitude than tensile forces generating in

beams with catenary position (Bailey, et al., 1996).

Looking at the graph drawn from the results achieved, the temperature at which the rigid connection

beam fails is at 730°C which is 100°C more than the failure temperature for the connection at bay 1.

Having experiencing these high temperatures with connections experiencing tensile forces, structural

0

-200

-400

-600

-800

-1000

-1200


Def

lect

ion

(m

m)

Time (sec)

Span/20

Rigid (parametric)

Pinned (parametric)


(parametric)

0 200 400 600 800 1000 1200 1400 16000


0 100 200 300 400 500 600 700

280

70

0

-280

140

210

-70

-140

-210


Axi

al F

orc

e (

kN)

Temperature (°C)

Rigid (parametric)

Pinned (parametric)

Figure 41: Temperature deflection graph of bay 3 with isolated member subjected to parametric fire at node 132


Page 59

members in bay 4 are less likely to fail. With constant heat acting towards the beam, catenary actions

are slowly developed, the tensile forces begin to decrease.

The pinned connection beams for both ISO and parametric fires reach their highest compressive

internal forces at time interval of 6 and 10 minutes with temperatures of 410°C and 415°C,

respectively. The tensile forces achieved in bay 4 are 115 kN and 120 kN at a temperature of 510C°

which is more than double in comparison to that of bay 1.

0 200 400 600 800 1000 1200 1400 1600 1800

280

70

0

-280

140

210

-70

-140

-210


Axi

al F

orc

e (

kN)

Time (sec)

Rigid (parametric)

Pinned (parametric)



Page 60

10.5. Scenario 5: FIRST FLOOR – Bay 1 – 3 – Simultaneous ISO Fire and

Parametric Fire with Rigid Connections

In this scenario, the entire floor will be subjected to ISO fire with only rigid connections. The analysis

that will be carried out will be compared to the results achieved in previous scenarios. It is expected

that the beams will reach their critical temperature and failure point at a lower temperature in

comparison to single compartment fires with lower mid span deflection at failure point, like previous

scenarios.

From the results achieved in FE analysis, the temperature achieved for bay 1 stands at 625°C with the

beam failing at temperature and deflection of 640C and 835 mm respectively. Simulation for bays 2

and 3 are shortened as it is expected that due to bay 1 failing first, critical temperatures for other bays

cannot be obtained. All beams show to fail at a temperature of 639°C due to the failure of beam at bay

1. Higher deflection is achieved for other bays in comparison to bay 1 and follow the same deflection

curve for bay 4.

The results achieved from the FE analysis show that connections reach a higher axial forces at low

temperatures compared to individual compartment fires. On average, the compressive force achieved

is 43 kN which is more than that of individual compartment fire, at an averaged reduction of strength

at 240°C. As following the pattern, there is a gradual increase in compressive force which is followed

by restoration of that force as it starts to experience tensile force.

Figure 43: Bays 1-3 exposed to ISO fire

0

-200

-400

-600

-800

-1000

-1200


Def

lect

ion

(m

m)

Temperature (°C)

Span/20

Bay 3

Bay 1


0 100 200 300 400 500 600 700

Figure 44: Temperature deflection graph of bay 1 0 3 and isolated member subjected to ISO/parametric fire conditions


Page 61

Understanding individual compartment fires for each bays, the connection for the bays fail with an

increase of tensile force. For bay 1 and 2, the failure of bay was occurred with internal tensile force of

-50 kN and -29 kN, respectively. On the other hand failure of bays due to internal compressive force

for bays 3 and 4 occurred at 28 kN and 70 kN, respectively. Therefore, for individual compartment

fire, the connections fail at a progressively compressive as opposed to tensile region. This is caused

by the elongation of beams due to thermal expansion.

Looking at the results demonstrated in Figure 45, the internal forces acting on the beam to column

connections depend on the fire characteristics in terms of temperature and time. So, whether the

connection is exposed to either of the fire regimes, same internal forces would give same

temperatures; the only difference would be when the temperature is reached. With the difference in

the characteristics for ISO and parametric fires, axial forces develop quicker in ISO fires than

parametric fire.

Looking at the generated graphs and the results projected from FE analysis, the end of the curves

remain identical for a short period of time, which is a very strange pattern. This could be due to the

fact that Vulcan runs numerous iterations at the failure temperature which is then plotted with a time

temperature fire produced via the spreadsheet. Therefore, time axial force and temperature axial

forces for bay 2 and 3 are incomplete due to the simulation stopping after the failure of beam at bay 1.

0

-200

-400

-600

-800

-1000

-1200


Def

lect

ion

(m

m)

Time (sec)

Span/20

Bay 3 (parametric)

Bay 1 (parametric)

Bay 1 (ISO)

Bay 3 (ISO)

0 200 400 600 800 1000 1200

Figure 45: Time deflection graph of bays 1 - 3 subjected to ISO and parametric fire

0 100 200 300 400 500 600 700

400

100

0

-400

200

300

-100

-200

-300


Axi

al F

orc

e (

kN)

Time (sec)

Bay 3

Bay 1

Figure 46: Temperature axial force graph of beam to column connection at bay 1-3 subjected to ISO/parametric fire


Page 62

11 CONCLUSION

It is vital to understand the importance of fire resistance of steel structures and it is still based on the

performance of isolated members in fire test. Procedures provided by the design codes do not capture

the true performance of the whole frame. The future of structural fire design needs to be evaluated in

terms of the whole structural performance during fire, including natural fire exposures, calculations

for heat transfer and structural behaviour for the entire frame; understanding how the interaction

between all elements from the heated parts to the cooler parts in the structure.

Various types of grading methods were used throughout this project, out of which standard fire

proved to be very useful for protection materials; however, one has to understand its limitations as

means of representing reality. The results achieved from the fire tests undertaken prove promising but

one must not rely just on computed data as there are other influences and interactions in a real

structure to expect a simple under presentative test to provide the answer.

Critical temperature in terms of Eurocode 3 relates to load utilisation factor at the start of the heating

process and boundary conditions for beams are same as the room temperature. While the utilisation

factor is computed at the very beginning, BS EN 1991 – 1 – 2 assumes that both axial forces and

moment contributions remain constant during the heating phase. On the other hand finite element

analysis shows that the internal forces change substantially with change in temperature. Therefore,

numerical approached provided by deign codes (BS5950 – 8 and BS EN 1991 – 1 – 2) are insufficient

as they are only provide results for simply supported members exposed to ISO 834 fire.

Making use of both ISO time-temperature curve and parametric fire curve, it is evident that the

elements heat up quicker in an ISO time-temperature curve than that of parametric curve. There are

various ways in modelling for fire resistance therefore; heating an element may take longer to peak. It

can be noted that results achieved from ISO fire are not realistic, so more research needs to be done in

creating more realistic parametric time-temperature curve.

Critical temperatures according to Eurocode 3 fall short of critical temperature achieved. However,

results achieved for the simply supported beam from finite element model was close to Eurocode 3

data. The results achieved from Vulcan prove that the beam can obtain high temperatures and yield in

subtle manner rather than failing right away.


Page 63

Adding of internal forces and strains for a heated beam with moments acting on each end and axial

load added to the beam, it demonstrates that external moments can reduce the critical temperature of

beam to column connections. Rigid beam to column connection tend to have a higher critical

temperature value than that of pinned or isolated. Thus, calculation of load utilisation factor must

include the external moment for rotationally restrained beam, when calculating for beam critical

temperature.

As noted from the results, the compressive axial force at the very last internal bay is much higher than

that of force at edge bay. This was due to restraint of the columns with compressive axial force in the

early stages of fire. The tensile forces in the later stages were because of catenary action. Similarly to

compressive axial force, the axial tensile forces increase from edge bay down to internal bay. The

restraints provided from columns and types of connections used have a huge influence on catenary

action of beams.

The results achieved from finite element analyses prove promising with regards to structural response

and critical temperature of the beam. There is a strong interaction between structural members proven

by the analyses due to material deterioration and thermal expansion of members. Boundary conditions

must also be incorporated in the model as these affect the overall behaviour of members at high

temperatures.

There is a huge development of internal force within the building structure, the analyses has shown on

how the structure was able support its loads at elevated temperatures. The results achieved show that

the thermal effects and bowing are major issue. This was due to the thermal gradients that govern the

response of the structure for the range of temperature in fire. The internal forces developed exceed

that of imposed loads. The results also show that with part of the structure being subjected to

enormous thermally imposed loads causing loss of strength, the cooler parts of the structure

redistribute the stresses to maintain structural stability.


Page 64

12 FUTURE RECOMMENDATIONS

The main aim of this project was to understand structural behaviour to elevated temperatures.

However, if the project were to be done again, research into structural behaviour during the cooling

phase will also be considered as it is believed that the failures caused in Broadgate Fire occurred

during its cooling phase. Computer analyses provide evidence that during the cooling phase of the

structure, high axial tensile forces are developed which causes the structure to fail. Therefore,

structural integrity can be maintained when it’s under fire but lose it all during its cooling phase.

In terms of fire design, compartment fires are localised, however, there is a possibly of when fire

spread may develop where fire barriers have failed and the fire doors are left open. Modelling for

situations like this is more realistic when compared to simultaneous fire across a building floor. This

can be adopted in future study in order to compare the data that can be achieved for deflections of mid

span beam to the data achieved in this dissertation. Further studies can be done with use of different

time temperature curves and relationships between adjacent bays.

Investigating bays 2 and 3, similar mid-span displacement have been achieved, however, the axial

forces generated do not show similar results like prior. Further parametric study needs to be done on

different fire and structural scenarios in order to confirm that the similar results achieved for mid-span

displacement is not coincidental before concluding. Also, the 2D frame can be converted into a 3D

frame to generate more realistic results including continuous floor slabs which prove to have a

significant effect on the structural behaviour in fire.


Page 65

13 REFERENCES

[1]

https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/399294/FSGB_2013-

14_Time_Series_Tables_1a_-_16.xlsx

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---

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Bailey, C. J., 1999. The influence of the thermal expansion of beams on the structural behaviour of

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https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/399294/FSGB_2013-14_Time_Series_Tables_1a_-_16.xlsx

http://www.planningportal.gov.uk/buildingregulations/


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Eurocodes/Eurocode-1/

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Building Research Establishment, 2005. The integrity of Compartmentation in Buildings During a Fire,

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International Standard Organization, 1999. ISO 834-1 Fire-resistance tests - Elements of building

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Page 68

APPENDIX I: Spreadsheet FiRE.xls for Determining Critical Temperature


Page 69


Page 70

APPENDIX II: Scenario 1 of 6 (1 of 4 input models)

<HEADER>

2D STEEL FRAME (Case 1 of 4 - BAY 1 - ISO FIRE w/ RIGID CONNECTION)

{HEADER}

<VERSION>

6

{VERSION}

<PROGRAM CONTROL>

0 0 0

1.000 1.000

1 0.000 0.000 0.0003 1.000

10 0

{PROGRAM CONTROL}

<STRUCTURE INFORMATION>

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500 0.0 10 11

1 1 1 1

{STRUCTURE INFORMAITON}

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19 3750 0 4000

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22 6000 0 4000

23 6750 0 4000

24 7500 0 4000

25 8250 0 4000

26 8999 0 4000

27 9000 0 4000

28 9001 0 4000

29 9750 0 4000

30 10500 0 4000


Page 71

31 11250 0 4000

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33 12750 0 4000

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41 18000 0 4000

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46 21000 0 4000

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Page 72

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Page 73

141 21750 0 12000

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217 3750 0 20000

218 4500 0 20000

219 5250 0 20000

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221 6750 0 20000

222 7500 0 20000

223 8250 0 20000

224 8999 0 20000

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226 9001 0 20000

227 9750 0 20000

228 10500 0 20000

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232 13500 0 20000

233 14250 0 20000

234 15000 0 20000

235 15750 0 20000

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239 18000 0 20000

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Page 75

261 1500 0 24000

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3

3 3 3

2 2 2

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460 200000 1

-35 0.25 0.2

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544.5 211.9 211.9 21.3 21.3 12.7 10 9 9

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209.6 205.2 205.2 14.2 14.2 9.3 10 9 9

{NEW BEAM SECTINO}

<CONNECTION INFORMATION>


Page 76

1

1

1

4 2 2 2 2

388.0 494.0 210000.0

412.0 545.0 195000.0

388.0 494.0 210000.0

412.0 545.0 195000.0

275.0 387.0 210000.0

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600.0 827.0 210000.0

780.0 1082.0 210000.0

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290.0 300.0 8.5 14.0 27.0 11300.0 182600000.0 208.0

300.0 150.0 7.1 10.7 15.0 5380.0 83560000.0 557000.0 248.6 278.6

380.0 200.0 16.0

50.0 100.0 100.0 30.0 190.0 54.65 289.65

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90.0 245.0 20.0 22.0 45.0 1

90.0 245.0 20.0 22.0 45.0 1

50.0 245.0 20.0 22.0 45.0 1

0.0 6.283

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290.0 300.0 8.5 14.0 27.0 11300.0 182600000.0 208.0

300.0 150.0 7.1 10.7 15.0 5380.0 83560000.0 557000.0 248.6 278.6

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50.0 245.0 20.0 22.0 45.0 1

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290.0 300.0 8.5 14.0 27.0 11300.0 182600000.0 208.0

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Page 77

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Page 78

34 7 0 60.0

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Page 79

1 111111

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0 0

{BOUNDARY CONDITIONS}

<JOINT LOADS>

0 0 0 0 0 0 0

{JOINT LOADS}

<TEMPERATURE DATA>

1 1 22.2000 22.2000 22.2000

2 1 21.1000 21.1000 21.1000

3 1 20.0000 20.0000 20.0000

{TEMPERATURE DATA}

{END OF FILE}

fire resistance of steel structures

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