performance based design for fire safety

7/30/2019 PERFORMANCE BASED DESIGN FOR FIRE SAFETY

1/23Advances in Structural Engineering Vol. 7 No. 4 2004 311

1. PERFORMANCE BASED DESIGNFOR FIRE SAFETYA rational approach to fire safety assessment is to relate

functional requirements, such as prevention of spreading

heat and smoke, safe evacuation and rescue etc., to fire

resistance considering both local and global stability of

structures. In a performance-based design, the designer

needs to first understand the level of performance is

expected, then to design to these levels and finally to

predict the performance that will be achieved to ensure

the reliability and robustness of the design.

At the beginning of the performance-based analysis

and design, the designer and the stakeholders should

both agree on the project scope, intent and building use.

The designer will then define and prioritize the fire safety

plans and performance criteria and define the fire safety

goals and objectives pertaining to the design intent. The

next step to follow is to determine the implementation of

the selected solutions into the project scope and execution.

The final stage is to establish the necessary verification

and quality assurance to ensure compliance with the

agreed solutions.

In general, a process for performance-based design

would involve the following steps:

1. identify goals and defining stakeholder and design

objectives

2. identify the possible fire scenarios which could

occur during the service life of the structure,

Performance Based Fire Safety Design of

Structures A Multi-dimensional Integration

J. Y. Richard Liew*

Department of Civil Engineering, National University of Singapore, Blk E1A, #05-13, 1 Engineering Drive 2, Singapore 117576

(Received 16 July 2003; Received revised form: 17 November 2003; Accepted: 18 November 2003)

Abstract: Design codes for fire safety in buildings can be either a prescriptive type or

performance-based type. It is now widely recognized that performance-based codes

provide greater advantages over the prescriptive codes in that it allows designers to use

the fire engineering methods to assess the fire safety of the structure. However, as the

assessment of the whole structure performance is not easy, most codes currently used

are still prescriptive codes or a combination of prescriptive codes and performance-

based codes. The key feature for implementing the performance-based fire design

codes is the assessment of the fire resistance of the structure. This paper provides an

overall view on performance-based code and the approaches for designing steel structure

in fire considering a multi-dimensional integration of fire engineering simulation,

emergency evacuation and structural resistance. Various fire models and heat transfer

analysis methods are reviewed and discussed. The basis to modelling of large deflection

and plasticity using appropriate stress-strain relationship at elevated temperature is

explained. Finally, structural response calculations from simplified hand calculation

method to advanced numerical procedures are presented. Future trends for research are

identified.

Key words: fire safety, emergency evacuation, explosion, fire modelling, fire protection, performance-based design, tall buildings,

plastic hinge analysis.

*Corresponding author. Email address: [email protected]; Fax: +65-6779-1635; Tel: +65-6874-2154.


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Performance Based Fire Safety Design of Structures A Multi-dimensional Integration

312 Advances in Structural Engineering Vol. 7 No. 4 2004

3. evaluate the likelihood and consequences of such

scenarios,

4. establish appropriate performance criteria by

ensuring effective evacuation, escape and rescue

and to prevent injury arising from such events,

5. predict the performance of the system based on

available engineering data, and6. ensure robustness of the designs, reliability and

durability of the protection systems, etc.

Although the governing criteria for impairment of

the main safety functions are often non-structural (for

example exposure to heat, temperature, toxic gases etc.),

it is, however, a functional requirement that the structure

would remain stable to allow adequate time for safe

evacuation and rescue. Therefore a quantitative

assessment on fire resistance of a structure is necessary

and this opens up the opportunity in using advanced

simulation tools and computational methods developed

as reported in the recent fire workshops (Liew 2002;Moss 2002).

2. EMERGENCY EVACUATIONThe objective of providing emergency evacuation is to

allow occupants to travel safely to a place of safety in

the event of fire. In general, the procedure involved in

estimating evacuation time is given below:

1. define the space domain to be analyzed

2. estimate the response time of occupants

3. calculate the travel time to a specific location

4. determine whether this location is safe5. if the specific location is safe, record the total

required safe escape time and make sure that it

does not exceed the available safe escape time,

which is determined by untenability condition.

If the specific escape route is not safe, select

another one and repeat Step 3.

A range of possible emergency scenarios may occur in

high-rise and complex buildings for which it is necessary

to develop a range of strategies for managing accidental

fires. The following points are considered when planning

for an evacuation strategy for such systems:

Risk perception in high rise/complex buildings

Types of fire scenarios that may occur and may

need different evacuation strategies

Fire scenarios that will affect building protection

strategies and the emergency plans proposed

Limitations of each emergency plans

Integration of fire service into the emergency

plan

Types of information to be provided to the

occupants

Emergency evacuation training and fire

protection maintenance plan

A major limitation on prediction of incident outcomes

for performance-based design and hazard assessment

is the lack of quantitative data on pre-movement time

and evacuation time, and occupant behaviours for

different fire scenarios and occupancies. There is a need

to study human behaviour during evacuation from a

range of occupancies. Advanced computer software isavailable to predict the evacuation time considering the

effects of exposure to fire effluence on occupant

evacuation behaviour. However, improvements are

still needed to include the effects of different warning

systems, information provided to occupants, occupant

characteristics, pre-training, building complexity and

level of fire safety management on pre-movement and

evacuation time. The September 11 incident had shown

that fire service intervention occurred while there were

many occupants still in the buildings, many were in the

process of evacuating while other remained in refuge

were trapped by fire. In order to achieve effective designfor buildings in emergency situations, it is essential to

consider occupants characteristics, their abilities to

evacuate and the effects of exposure to fire effluence on

occupant evacuation behaviour.

3. FIRE MODELLINGFire modelling is a mathematical simulation of the fire

conditions in a compartment and is capable of giving

information based on the parameters which have been

designed. The fire development in a room normally

involves three phases: pre-flashover, post-flashover andfire decay. In the pre-flashover phase, fuels begin to burn

and the gas temperature varies from one point to another

in the compartment. In the post-flashover phase, the

fire develops fully, and the gas temperature increases

rapidly to a peak value and becomes practically uniform

throughout the compartment. The fire has the most

influence on structural design because of high temperature

and radiant heat fluxes produced in this phase. In the fire

decay phase, the available fuel begins to decrease and

the gas temperature falls. There is considerable benefit

when the effects of natural fires in buildings, where the

amounts of the combustible contents are small and the

buildings are of large volume, is considered than using

the standard ISO fire.

For many years, fire engineering research has shown

that overall structure performs better than isolated

members in a fire situation. Numerous studies have been

carried out to determine the temperature reached in real

(natural) fires, to quantify the factors that govern fire

severity and to investigate the parameters that cause

structures to fail in fire. The studies show that the severity

of natural fires in building compartments is governed by

the amount of combustible material (the fire load), the


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J. Y. Richard Liew

Advances in Structural Engineering Vol. 7 No. 4 2004 313

area of the doors and windows (the ventilation), and the

thermal characteristics of the wall, floor and ceiling

materials. In addition, fire-fighting measures are also

important for the determination of fire exposure.

The following subsections discuss the various models

available for modelling fire of different complexity and

severity.

3.1. Design Fire ModelsDesign fires are derived empirically and may yield

reasonable and consistent predictions provided that the

fire conditions are similar to those in the underlying

assumptions. Standard fire curves such as ISO-834 do

not represent the real fire in a compartment and serve

only as criteria to evaluate the fire resistance capacity of

single structural members. Simplified code prescribed

method often assumes that the fire has a constant

temperature throughout the burning period.

Eurocode 1: Part 1-2 (2001) recommends equationsfor parametric fires, allowing a temperature-time curve

to be produced for any combination of fuel load, opening

factor, height of opening and thermal characteristic of

the boundary materials.

The temperature T (C) and time relation during theheating phase is given as (Eurocode 1: Part 1-2, 2001):

(1)

where t* is the fictitious time given by

t* = t. (2)

t is the time (hr) and

(3)

where b is the square root of thermal inertial of the

boundary material of the compartment and O is the

opening factor (m1/2) given by

(4)

Av is the total area of vertical openings on all walls;

heq is the weighted average of window heights on all

walls andAt is the total area of enclosure (walls, ceiling

and floor, including openings). In case of= 1, Eqn 1approximates the ISO834 standard temperature-time

curve.

Depending on whether the fire is fuel controlled or

ventilation controlled, the duration of the heating phase

tmax (hr) is given as:

(5)

qt,d is the design value of the fire load per total surface area

At of the enclosure. The recommended fire growth rate is

taken as tlim = 25 minutes, 20 minutes and 15 minutes for

slow, medium and fast growth rate, respectively.

The introduction of tlim is to avoid unrealistic short

fire duration when the ratio between the fire load and the

opening factor decreases. Any object or fire load needsa certain amount of time to burn, even if there is

unlimited presence of air (Franssen 1997).

The temperature-time curve during the cooling phase

is given by:

T = Tmax 625(t* t*max.x) for t*max 0.5

T = Tmax 250(3 t*max)(t* t*max.x)for 0.5 < t*max < 2

T = Tmax 250(t* t*max.x) for t*max 2 (6)

in which

t* = t.

t*max = (0.2 103 qt,d/O).

x = 1.0 if tmax > tlim

x = tlim./ t*max if tmax = tlim

Figure 1 shows the parametric fire curves plotted for a

range of opening factors (OF), fuel loads and materials

according to the Eurocode 1: Part 1-2 (2001). Fire curvesare produced for three fire loads, four opening factors

and two types of construction, showing the significant

dependence of fire temperature on the bounding materials.

The fire loads are 400, 800 and 1200 MJ per floor area,

for a room 5 5 m in plan and 3 m high. Feasey &Buchanan (2002) pointed out that the Eurocode equation

gives extremely fast decay rates for large openings in

well insulated compartments (e.g., OF = 0.12 in Figure 1)

and extremely slow decay rates for small openings in

poorly insulated compartments. They proposed some

modifications to the Eurocode formula to give a better

estimation of the temperature-time curve in the fire

decay phase.

3.2. Zone ModelsZone models represent more of the phenomenological

behaviour of fire. They solve the conservation equations

for distinct and relatively large regions. In each zone,

the heat balance equations are solved to generate gas

temperatures. There are several options for calculating

the heat release rate, based on ventilation control, fuel

control or the porosity of wood crib fuels. Other computer

models including ZONE, CSTBZ1, CFAST, BANZFIRE,t = Maximum 0.2 q O, tmax t,d lim[ ]

103

O = A h Av eq t

=( )

( )

O b/

. /

2

20 04 1160

T

e e et t t

=

+ ( ) 20

1325 1 0 324 0 204 0 4720 2 1 7 19

. . .. * . * *


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are summarised in SFPE (2002). Schleich (1996)

developed a realistic fire evolution model, which not

only takes into account the physical factors, but also the

influence of active protection measures in the structure.

The multi-zone fire model (Liew et al. 2002) places

heating boxes one outside another and assumes uniform

heating in each box, as shown in Figure 2. The smallest

box nearest to the heat source has the highest

temperature. The boxes further from the heat source have

lower temperature, and the farthest box has the lowest

temperature. The structural elements enclosed within

each heating box are subjected to a uniform heating

rate, which can be either constant or vary as a function

of time. The temperature-time relationship in each box



1400

1200

1000

800

600

400

200

00 50 100 150 200

Time (min)

T

emperature(C)

250 300 350

ISO 834

Concrete

GypsumOF = 0.02

400

1400

1200

1000

800

600

400

200

00 50 100 150 200

Time (min)

T

emperature(C)

250 300 350

ISO 834

Concrete

Gypsum OF = 0.04

400

1400

1200

1000

800

600

400

200

00 20 40 60

Time (min)

Temperature(C)

80 100

ISO 834

Concrete

GypsumOF = 0.08

120

1400

1200

1000

800

600

400

200

00 20 40 60

Time (min)

Temperature(C)

80 100

ISO 834

Concrete

GypsumOF = 0.12

120

Figure 1. Parametric temperature-time curves (fuel load = 400, 800, 1200 MJ/m2 floor area)

Figure 2. Multi-zone fire model


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may be obtained by calibration with fire tests or from

numerical simulation based on the theory of thermal

energy balance. The air temperature at each time step

can be prescribed.

The multi-zone fire model (Liew et al. 2002)

provides a means to calibrate actual fires. The heat

intensity and the flame size of the actual fires dependon many factors such as fire load, ventilation, active fire

control devices, and so on. If there is such a part in a

structure that is heated significantly more or less than

others, the multi-zone model could be a more suitable

fire model to simulate an open fire. The multi-zone fire

model can be used to simulate uniform heating. In this

case, only one-box is prescribed and all structural

members within this box will be heated simultaneously

under the same fire load.

3.3. Radiation Model

Radiation model may be used to simulate fire as aradiating source with the heat flux intensity defined by

its distance from the source, as shown in Figure 3. The

heat rays are emitted in all directions. The heat intensity

may be constant or vary as a function of time. The heat

flux intensity, q, received by the individual elements is

calculated as (SINTEF 1995):

(7)

where

Ei = total energy emitted from the sourceri = distance between the heat source and the midpoint

of ith element

i = angle between the ray and the element surface

normal (see Figure 3).

The air temperature can be computed by adopting the

Stefan-Boltzman formula once the heat flux intensity is

known (Yao et al. 1995):

(8)

where

= the emissivity coefficient;

qi = the heat flux intensity calculated from Eqn 7;= the Stefan-Boltzman constant of 5.67 108 W/m2K4;TK= the temperature on the spherical surface with a unit

of Kelvin (K).

The radiation model is a convenient way to propose a

simple relationship between the air temperature and its

distance from the fire source. For large and complex

structures, the radiation model is preferred as it is

relatively simpler to use for simulating an open fire.

On the other hand, the multi-zone fire model requires a

descritization of space into finite boxes. Each heating

box needs to be prescribed with an appropriate time-

temperature relationship.

3.4. Computational Fluid Dynamic (CFD)ApproachThe CFD model can be used to represent various types

of walls of different materials and the exact location

and dimensions of openings. The fire compartment to

be modelled by CFD is divided into a number of small

volumes. The fluid dynamics equations are written in

each of these small volumes, and each volume is linked

to the adjacent volumes. The heat transient problems are

expressed in differential equations, and time integrationhas to be performed by solving a large number of

equations in the time domain. The main drawback of

CFD model is that a significant number of parameters

have to be given and many of them are variable with

very little, if any, link to any physical phenomenon. It

requires an experienced user before the any meaningful

Tq

ki=

1 4

qE

r2i

i

i=4

cos

J. Y. Richard Liew


I = intensity cos()

y

x

z

radius

(x, y, z)

Intensity

Figure 3. Radiation model to simulate open fires


6/23

result can be obtained. Practising engineers often find

difficulties in understanding the hypothesis underlying

the CFD models and they have to be careful in interpreting

the results.

A large amount of results can be produced by CFD

computation. For instance, it provides the temperature

components of the velocity, the pressure, the oxygen

concentration, etc, in every volume at every time step. A

graphical representation of the results is mandatory, see

Figure 4. To some extend, the amount of results can

even create a difficulty if the interest is for the behaviourof structure. An interface is often needed to be created

between the CFD model and the structural model, because

thermal environment in the compartment calculated by

the CFD model would influence the structural element.

The main advantage of CFD models is to model

compartments with complex geometry because only the

flexibility given by the fine discretisation of the space

inherent to CFD models allows a correct representation

of this complexity. Notwithstanding the costs and

problems associated to these models, they are favoured

by researchers who use them as the design tools for fire

analysis. Their application domain is for complex

projects which are very often combined the geometrical

complexity and the financial resources which make the

CFD model an attractive choice.

4. HEAT TRANSFER ANALYSISThe process of heat transfer between a fire and a

structure can be described by the balance between the

net incident thermal radiation and convective heat flux

and the rate of heat conducted in the material. The rate

of heating of any structural member is dependent at any

time on the temperatures of both the fire atmosphere and

the member. Calculation of member temperature requires

solution in time domain via a fairly complex differential

equation. There are two kinds of heat transfer methods

used in fire engineering design: analytical method and

finite element method.

4.1. Analytical MethodDifferent standards or specifications give simplified way

to calculate the net heat flux and temperature development

in the steel member. The heat flux due to convection is

proportional to the temperature gradient between theambient gas temperature and temperature of the steel

member. The heat flux due to radiation is proportional to

the temperature gradient of the forth order of the ambient

gas temperature and the steel temperature. ECCS (1993)

uses a single expression to represent the total heat flux as

(9)

where

As = surface area of the member per unit length exposed

to heating

f= fire temperature at time t

s = temperature of the steel member

h = coefficient of total heat transfer

Based on this heat flow law, the temperature

development in unprotected steel member can be

calculated. Eurocode 3: Part 1-2 (2001) provides a rational

means to estimate steel temperature development by

considering the section factor and configuration factor

for internal steelwork and external steelwork. The

temperature increments in the structural members are

calculated over small time steps. This method is

particularly suitable for calculation using simple

spreadsheet programming.

( )Q h As f s=



8.00

Plot 3dSpeedm/s

7.20

6.40

5.60

4.80

4.00

3.20

2.40

1.60

0.80

0.00

Figure 4. Example of smoke velocity in a fire compartment from CFD model


7/23

For internal steelwork, the increase of temperature

(s) in an unprotected steel section during in a timeinterval tmay be calculated as (Eurocode 3: Part 1-2

2001):

(10)

in which

correction factor for the shadow effect of

flanges, defined as a ratio of the box value of the section

factor to the section factor

specific heat of steel [J/kgK]

density of steel [kg/m3]

section factor for unprotected steel members

Heat transfer coefficient per unit area per degree

Kelvin

t= The time interval [seconds]On the fire exposed surface the heat transfer

coefficient may be determined by considering heat

transfer by radiation and convection as

(11)

where the radiative term may be expressed as:

(12)

where

Stefan-Boltzmann constant of

resultant emissivity using f = 1.0 and

m = 0.7, where f and m are the emissivity of the fire

and the surface of the member,

f and s = Fire and steel temperature, respectively, in

Celsius.

The convective heat transfer coefficient is hnet,c =

25 W/m2K for standard fire, 35 W/m2K for natural fire

and 50 W/m2K for hydrocarbon fire.

The section factor uses the fire exposed

perimeter in calculating an appropriate value of . The

section factors for selected examples are shown in

Figure 5.

For protected steel members under fire, the protection

material of low thermal conductivity reduces the rate of

heat transfer from the fire to the steel section. The

increase in steel temperature in a time increment

due to the heat transfer from the fire through the fireprotection to the steel section may be calculated as:

(13)

in which the relative heat storage in the protection

material is given as

(14)

andsection factor for protected steel member,

where is the inner perimeter of the protection material.

specific heats of steel and protection material

thickness of fire protection material

temperatures of steel and fire at time t

increase of fire temperature during the time step tthermal conductivity of the fire protection material

densities of steel and fire protection material

The temperature development in protected steel

members can be evaluated based on the thermal

properties of the insulation materials.

4.2. Finite Element MethodFinite element method may be used to estimate the

thermal effects on the structural elements by subdividing

the structural element into a number of quadrilateral

heat transfer elements. Heat conduction, heat convection

and exchanges of radiation are calculated on the basis of

the heat transfer element. One simplified approach is to

store the temperature history in each structural member

and then calculate the equivalent nodal expansion based

on the incremental temperature change. Consistent nodal

forces are produced on an elastic element at elevated

s p, =kp =f = s f, =tp =c , cs p =

Ap

A /Vp =

=c

ct

A

V

p p

s s

p

p

/

sp f s

p p s s

f

A V )

t k )ct e=

+

/ (

( / ( )( )

/

1 31

10

ts

Am

A Vm/

r f m= =5 67 10

8. =

hnet r r f s

f s

, ( ) ( )

= + +[ ]

+ +[ ]

273 273

546

2 2

h h hnet net c net r = +, ,

hnet

hnet =A Vm/ =s =cs =

kshadow =

s shadow

m

s s net f s

kA V

ch t= ( )

J. Y. Richard Liew


B

tD

Am/V = 2(2B+Dt)/A

A = area of the steel section

D

B

Am/V = (3B+2D2t)/A Am/V = 2(B+D)/A

Figure 5. Values of parameter for use in the calculation of Section factor A m/V


8/23

temperatures due to the axial expansion and temperature

gradient increment over the cross-section. These forces

can be used in the structural analysis to determine the

responses of the structure in fire (Ma and Liew 2004).

Although the techniques for solution of the heat

transfer problem are relatively well established, several

complicating factors exist. For examples, physicalproperties such as thermal conductivity, specific heat,

emissivity/absorbtivity and heat transfer coefficients

vary with temperature. Surfaces may also be exposed

to re-radiation from other surfaces. The fraction of

the emitted radiation received is governed by the

configuration factor, which may be tedious to calculate,

especially if shadowing of other members are present.

Numerical method is often used to improve the accuracy

of the heat transfer analysis.

5. MATERIAL BEHAVIOUR AT

ELEVATED TEMPERATUREExperimental evidence shows that the stiffness and

strength of steel deteriorate at elevated temperatures.

Typical stress-strain curve of steel at elevated temperatures

is shown in Figure 6.

The stress-strain relationship at elevated temperature

does not exhibit a distinctive yield plateau. Therefore,

the yield stress, or 0.2% proof stress, which is conventional

design strength for steel at ambient temperature, loses

its relevance because of the nonlinearity of the stress

strain curve. Since fire is considered to be an accidental

situation, large plastic strains are allowed. Hence, aneffective yield stress is used, which is attainable when

the strain is considerably larger than the elastic limit at

normal temperatures. Eurocode 3: Part 1-2 (2001) adopts

a yield strain of 2% to define the effective yield

stress. The temperature dependence of the proportional

limit, the effective yield strength as well as the elastic

modulus recommended by the code is shown in Figure 7.

Creep may be of importance in a fire situation where

a cross section is subjected to high temperature (above

400C) and high stress for a long period of time. The

stress-strain curves given in the Eurocode code arebased on measurements at constant temperature within a

period of time to allow creep to take place in the tests.

Therefore, creep effect is implicitly included in the

effective yield strength used for design. In other words,

when the design is based upon code values, creep does

not need to be considered explicitly. If the temperature

and load history is such that a structural component

remains at high temperature or is highly stressed for

only a short period, the prediction using the code values

should yield conservative results.

6. NONLINEAR ANALYSIS ATELEVATED TEMPERATUREModern design standards such as Eurocodes provide

sufficient guidance to assess the fire performance of

individual members in a fire compartment of a building

framework. In the case of a braced frame in which each

storey comprises a separate fire compartment with

sufficient fire resistance, the effective buckling length of

a column may be used to compute the limit load of the

frame. However, guidance is not given for sway frames

in which storey buckling and overall stability may

dominate the design of individual member. Wang et al.(1995) provide simplified methods to analyse the

performance of steel frames. They study the effect of

continuity on the fire resistance of columns in both sway

and non-sway steel frame and suggested some restraint y,



Strain x

fy, h effective yield strength;

fp, h proportional limit;

Ea, h slope of the linear elastic range;

xp, h strain at the proportional limit;

xy, h yield strain;

x t, h limiting strain for yield strength;

x u, h Ultimate strain;

x u, hx t, hxy, hxp, h

fy, h

fp, h

Ea, h= tan a

a

Stress q

Figure 6. Stress-strain relationship of steel at elevated temperature according to European Committee for Standardisation (2001)


9/23

stiffness values at the end of the column due to the

continuity of the sub-frame from the parametric study.

General commercial programs such as ABAQUS,

ANSYS, NASTRAN, may be used for analysing

structures exposed to fires. They offer the advantages of

full validation, powerful ability to model different kinds

of problems and availability of further developmentso that they can almost suffice all needs. But these

programs are rather inconvenient to use for being both

time-consuming and complicated to operate since they

are general purpose program not specifically written to

perform analysis of structures under fire condition

which is highly non-linear and transient.

One powerful tool for analyzing large-scale steel

structures exposed to fire is to adopt a second order

plastic hinge-based analysis (Liew et al. 1998, 2002). In

this approach, it is assumed that cross section is compact

and the full plastic capacity can be achieved. At elevated

temperature the plastic strength surface should follow

the effective yield strength and its temperature reduction

curve for yield, as illustrated in Figure 7. The elastic

modulus also degrades at elevated temperature following

the temperature degradation curve for the slope of linear

elastic range as in Figure 7. Other fire effects

include thermal expansion and thermal bowing. Further

improvement to this model is to model the gradual

plastification of members cross section using a two-

surface plastic hinge method, which captures the gradual

yielding of cross sections at elevated temperature (Ma

and Liew 2004; Liew et al. 2000).

The two-surface plastic hinge model, which is

formulated based on the bounding surface plasticity

concept, represents the inelastic cross section behaviour

by considering the interaction of axial force and bi-axial

bending. The initial yield surface is assumed to be a

scaled down version of the bounding surface that is

fixed in size and translates without rotation in a stress-resultant space. The gradual translation of the initial

yield surface towards the bounding surface provides a

smooth transition from initial yield to full plastification

of cross section. Moreover, the element displacement

fields are derived from the exact solution of the fourth

order differential equation for a beam-column subjected

to end forces (Liew et al. 2000), hence it is accurate

enough to use only one beam-column element to model

the stability behaviour of column member.

At elevated temperature, the yield surface and the

bounding surface have to contract in size in order to

satisfy the yield condition. The degradation of the yield

strength is based on the effective strength concept. At

high temperature, the stress-strain relationship of steel

is highly nonlinear and does not exhibit a distinct yield

plateau. The idea of the effective strength is introduced

to define a yield plateau at a relatively high strain level.

For the two-surface plasticity model, the size of the

bounding surface corresponding to full cross-sectional

plastification, follows the reduction curve for effective

yield strength in Eurocode 3: Part1-2 (2001), as illustrated

in Figure 8. The size of the initial yield surface is

assumed to degrade proportional to the bounding surface.

kE,

ky,

J. Y. Richard Liew


Reduction factor, k

0 200 400 600 800 1000 1200

0.2

0

0.4

0.6

0.8

1.0

Temperature [C]

Proportional limitkp,h= fp,h/fy

Slope of linear elastic rangekE,h= Ea,h/Ea

Design strength for satisfying

deformation criteriakx,h= fx,h/fy

Effective yield strengthky,h= fy,h/fy

Figure 7. Reduction factor for steel according to Eurocode 3: Part 1-2 (2001)


10/23

However, it can be observed from Figure 8 that the

initial yield strength decreases at a faster rate than the

effective yield strength; therefore, the size of the initial

yield surface may be over predicted at higher temperature.

In practice, this will not have any significant effect on

the inelastic behaviour of members in fire.

The verification of the two-surface plasticity model at

ambient and elevated temperature is reported in Liew

et al. (1998) and Ma and Liew (2004). Verification studies

have been carried out on both components and frames

over a wide range of parameters including uniformly

heated members, three-side heated members with

concrete slab acting as heat sink, members with passive

fire protection and 2-D frames. Several examples are

given in the next section to illustrate the application of

the plasticity model and to study the accuracy of the

model in modelling the inelastic behaviour of frame

structures.

7. COMPARISON OF PLASTICHINGE METHOD WITH SPREAD-OF-PLASTICITY ANALYSISFigures 9 to 11 show a simply supported beam, a single

storey braced frame and a multi storey braced frame

subjected to fire. The main purposed of the study is to

compare the results obtained from fire analyses based on

the plastic hinge (P-Hinge) method and the Spread of

plasticity (S-Plastic) methods.

All the structural models are subject to ISO

standard fire. For the simply supported beam (model 1),

the member cross section is exposed to 4-side fire

(Figure 9). The temperature is assumed to be uniform

over the cross section and along the length. The second

and the third models are based on the portal frame

shown in Figure 10. In model 2, the frame members are

exposed to fire from all sides without fire protection

while in model 3, the beam is protected by concrete slab,



0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 200 400 600 800 1000 1200

Temperature [C]

Strengthdegradation

Ratio

effective yieldstrength

initial yield strength

bounding surface(200C)

initial yield surface(200C)

bounding surface(700C)

initial yield surface(700C)

Figure 8. Size of yield and bounding surface at elevated temperature according to Eurocode 3

q = 15 KN/m

Uy

Ux

254 146 UB

4.5 m

Figure 9. Model 1: simply supported beam (4-side heated)


11/23

and the beam section is 3-side heated. Model 4 is a

3-bay, 3-storey rigid frame (Figure 11), in which the

lower left corner compartment is subject to fire. S275

steel with yield strength of 275 N/mm2 is used for all the

structural members.

For model 1, only vertical deflection of the beam is

compared. For the other models, the comparison is made

both on the mid-span deflection of the beam and the

lateral deflection at the top of the heated column. The

results are shown in Figures 12 to 15. In all figures, solid

lines represent the vertical deflection of the beam, and the

dashed line means the lateral deflection of the column.

The comparison of the results indicates that the

plastic hinge method gives satisfactory results before

the formulation the first plastic hinge in the structure. If

the failure is not due to the formation of plastic hinge

mechanism, then, the plastic hinge method shows a greater

stiffness reduction than the spread-of-plasticity method

(Figures 12 and 13).

If a collapse mechanism occurs (Figures 14 and 15),

the plastic hinge method shows collapse with rush out

of deformation while the spread-of-plasticity analysis

shows ability to sustain further load with moderate

deformation. Hence the plastic hinge method may be

used to predict the collapse of structures. But it may

underestimate the post-collapse stiffness of the

structures.

For steel structures under fire attack, it is possible to

allow the structure to undergo large deformation as long

as the structure maintains stable. Therefore, when post-

collapse behaviour is needed, the spread-of-plasticity

method should be used.

J. Y. Richard Liew


Ux Uy

P = 500 kN P = 500 kNq = 25.4 kN/m

Beam section: 305*165UB40

Column section: 203*203UB52S275 steel

5.5 m

3.5 m

Columns are 4-side heated

Figure 10. Rigid portal frame: Model 2: beam is 3-side heated; Model 3: beam is 4-side heated

P = 75.5 kN P = 151 kN P = 75.5 kNP = 151 kN

5.5 m 5.5 m 5.5 m

3m

3m

3m

Beam section:305*165UB40

Column section:203*203UB52

U.D.L = 25.4 kN/m over all beams

Ux

S275 steel

Uy

Figure 11. Model4: three-bay, three-storey rigid frame


12/23

8. VERIFICATION OF NUMERICALMETHODSVerification studies are important to ensure the validity of

any analytical or numerical methods. Ma and Liew (2004)

established the accuracy of their proposed advanced

analysis method against experimental results for both

individual members and complete frames exposed to fire.

8.1. Uniformly Heated MembersCritical temperatures from the advanced analysis are

compared with those in BS5950: Part 8 (BSI 1990) for

beams under uniform distributed loads and columns

under axial loads in Table 1. Critical temperature for

beams is taken at a mid-span deflection of L/20 and

critical temperature for columns is taken at the failure

of the column symbolized by a sudden increase of

lateral deflection. The results agree well with each

other.

8.2. Three-side Heated BeamsEighteen UK standard fire tests on unprotected simply

supported steel beams supporting concrete slabs without



1600

1400

1200

1000

800

600

400

200

00 2 4 6 8 10

Time (minute)

Formulation of the plastic hinge

at the mid of the beam

PHinge

SPlasticity

uy(mm)

12 14 16

Figure 13. Deformation of structural model 2

700

100

0

1000

P-HingeS-Plasticity

Solid line: Uy

Dash line: Ux

Time (minute)

Deformation(

mm)

5 10 15 20 25

600

500

400

300

200



13/23

composite action are simulated. The test descriptions

and data are available in Wainman (1988). Table 2

summarizes the measured and calculated critical time

and temperature for each test. In general, the agreement

is good except for a few cases (such as Test 14). The

discrepancy is possibly resulted from the assumed steel

strength degradation at elevated temperatures due to

lack of material test data. Figure 16 plots the temperature

and the mid-span deflection predictions against

experimental results for test 11.

8.3. Two-dimensional FramesTwo frames (Li et al. 1997; Zhao 1995) have been

studied. The configuration of the frame and the loading

J. Y. Richard Liew


Solid line: Uy

Dash line: Ux

yielding of the column

P-Hinge

S-Plasticity

Plastic hinge of the beam

350

400

300

250

200

150

100

50

0 5 10

Time (minute)

Deformation(m

m)

15 20 25

0

50


Solid line: Uy

Dash line: Ux

P-Hinge

Time (minute)

Deformation(mm)

S-Plasticity

300

250

200

150

100

50

00 2 4 6 8 10 12



14/23

is shown in Figures 17 and 18. In both studies, the

measured temperature of each member is used in

the structural analysis. Figures 19 and 20 illustrate

the excellent correlation between the predicted and the

measured displacements.

8.4. Three-dimensional FrameA full scale fire test was carried out on a three-

dimensional steel tubular frame in SINTEF. The test

details and the verification procedures can be found in

Skallerud and Amdahl (2002). The proposed advanced



Table 2. Critical time and temperature of UK standard fire tests

Critical Time (min) Critical Temperature (C)

Test No Load Ratio Test Analysis Error % Test Analysis Error %

Test 2 0.50 22.5 20.7 8.0 660 693 5.0

Test 3 0.57 22.0 20.6 6.4 634 682 7.6Test 4 0.36 29.0 28.2 2.8 701 745 6.3

Test 5 0.61 26.7 22.9 14.2 647 694 7.3

Test 6 0.37 22.8 25.9 13.6 737 734 0.4

Test 7 0.36 22.3 24.2 8.5 731 743 1.6

Test 8 0.36 21.3 24.4 14.6 705 742 5.2

Test 9 0.37 24.2 26.4 9.1 714 734 2.8

Test 10 0.49 20.5 21.0 2.4 655 709 8.2

Test 11 0.50 21.4 20.8 2.8 683 706 3.4

Test 12 0.53 28.4 29.3 3.2 681 680 0.1

Test 13 0.40 25.1 24.3 3.2 727 736 1.2

Test 14 0.25 26.4 33.2 25.8 745 791 6.2

Test 89 0.50 20.0 22.4 12.0 651 692 6.3

Test 90 0.65 20.7 19.0 8.2 630 640 1.6

Test 91 0.34 23.0 29.5 28.3 705 742 5.2

Test 92 0.05 117.0 109.0 6.8 1061 1046 1.4

Test 93 0.09 75.0 56.4 24.8 977 932 4.6

Table 1. Critical temperature of uniformly heated members

Simply Supported BeamCritical Temperature (C) at load ratio R1

in Bending 0.2 0.3 0.4 0.5 0.6 0.74-side BS5950 715 660 620 585 555 520

heated Analysis 725 671 629 591 559 527

% difference 1.4 1.7 1.5 1.0 0.7 1.3

Critical Temperature (C) at load ratio R2

Column in Compression 0.2 0.3 0.4 0.5 0.6 0.74-side BS5950 710 655 615 580 540 510

heated Analysis 723 678 641 608 564 529

70 % difference 1.9 3.5 4.2 4.8 4.5 3.8

4-side BS5950 635 635 590 545 510 460

heated Analysis 662 649 611 568 529 478

>70 % difference 4.2 2.2 3.5 4.2 3.8 3.9

R1 = Mf/Mc R2 = F/Agpy + Mx/Mcx + My/Mcy

: slenderness ratio Mf: applied mid-span moment at fire

Mc: moment capacity at ambient temperature F: axial force

Ag: cross-section areapy: yield strength

Mx and My: applied major and minor axis moment at fire

Mcx and Mcy: major and minor axis moment capacity at ambient temperature


15/23

analysis is used for the verification. It has been found

that the correlation between simulation and test results

with respect to the mechanical response was extremely

good. The primary collapse load obtained was almost

perfectly predicted.

9. FIRE ANALYSIS OF ANOFFICE BUILDINGThis section presents a performance-based approach for

analysing a six-storey building frame subject to various

scenarios of fire attack. Advanced analysis method (Ma

J. Y. Richard Liew


Time (min)

Test (deflection)

Analysis (deflection)

Test (temperature)

Analysis (temperature)

Mid-Span

Deflection(m)

0.20

0.16

0.12

0.08

0.04

0.00

LowerFlan

geTemperature

800

600

400

200

0

0 10 20 30 40 50

Figure 16. Test and analysis results for test 11

30 30 30 30 30 30 30 kN

540 540540 540 540 540

4.2

55

4.5

7.2

column crosssection

beam crosssection

A B

100

100

100

1400mm

Figure 17. Lis frame

beam & columncross-section

82 kN 82 kN

9.45 kN

1500 mm

1500mm

DC

6.0

100

4.5

56

Figure 18. Zhaos frame

010

0

Horizontaldisplacement

(mm)

Time (min)

Node B,Analysis

Node A,Analysis

Node B, Test

Node A, Test

10

20

10 20 30

Figure 19. Analysis and test results (Lis frame)

Analysis

Test20

10

0

0 20 3010

Horizontaldisplacement at D

Vertical displacement

at CDisplacement(mm)

Time (min)

Figure 20. Analysis and test results (Zhaos frame)


16/23

and Liew 2004) is used to propose a much reduced fire

protection plan for beams and columns and prove that

the design can satisfy the performance criteria of fire

safety. The building is classified as office building as

shown in Figure 21.

9.1. Limit States DesignThe frame is designed for the strength limit state at

ambient temperature according to Eurocode 3 with the

following actions:

Permanent Dead load (Gk,1) 3.6 kN/m2

actions Gk:

Permanent imposed 1.9 kN/m2

load (Gk,2)

Variable Variable imposed 1.6 kN/m2

actions Qk: load (Qk,1)

Wind load (Qk,2) 593 kN (in

Y-direction)The beam and column sizes are indicated in

Figure 21. A36 steel (design strength of 250 N/mm2) is

used in all sections. Plastic hinge analysis method is

adopted. Each beam is modelled using 4 elements and

each column using 1 element. Wind load is simulated

by applying a point load in Y-direction at every beam-

column joint of the front elevation.

At the fire limit state, which is treated as accidental

loads in Eurocode 3 Part 1-2 (2001), the design effect of

the actions is expressed as:

Efi,d,t = Gk+ 1Qk,1 + 2Qk,2 (15)

Where 1, 2 are factors due to the probability of loadsacting individually or in combination. Depending upon

which variable load is the dominant action, two load

combinations are possible under fire limit state:

Load combination 1:

Efi,d,t = Gk,1 + Gk,2 + 0.5Qk,1 + NL (Notional Load)

Load combination 2:

Efi,d,t = Gk,1 + Gk,2 + 0.3Qk,1 + 0.5Qk,2

In load combination 1, the notional load is taken as

0.5% of the factored gravity load at each storey, applied

in Y-direction and is distributed to the beam-column

joints as point load. In both cases, the structure is subjected

to gravity load or the combination of gravity load and

wind load first, followed by fire.

9.2. Fire ModellingParametric fire recommended in Eurocode 1 Part 1-2

(2001), is used to simulate the fire in the compartment

by considering the type of building, floor layout,

realistic fire load and possible fire fighting measures.

Fire load density per floor area qf,k = 420 MJ/m2 is

adopted for common office building. The design fire

load qf,d is defined as:

qf,d = qf,k. m. q1. q2 . n (16)

where m is the combustion factor and is assumed as

0.8; q1 is the partial factor taking into account thefire activation risk due to the size of the compartment.

For floor area from 25 m2

up to 250 m2

, q1 is equal to



Z

7.315 m 7.315 m

1 2 3

X

Y

7.315m

W12 26 W12 26

W12 26 W12 26

W1253

W1287

W1253

PLAN

W12x87

W1287

W12120

X

W

1060

H=6x3.658m=2

1.948m

1 2 3

W1060

FRONT ELEVATION

Figure 21. Six-storey building frame


17/23

1.5 (in this case the floor area Af is 53.5 m2). q2 is 1.0

for occupancies such as office, residence and hotel. It is

assumed that no automatic fire suppression and

detection system is installed but an off site fire brigade

is available from which n is calculated as 0.78. Thedesign fire load qf,d is thus computed as 393 MJ/m

2,

which is equivalent to 98 MJ/m

2

per total area (qt,d). Thesurrounding surfaces of the compartment are assumed to

be normal concrete with b value of 1900 J/m2s1/2K.

Assuming an opening factor OF = 0.4, the temperature

time curve is plotted as shown in Figure 22. It can be

seen from Figure 22 that the fire curve with opening

factor OF = 0.04 are below the standard ISO 834 fire

curve, providing the possibility of reducing passive fire

protections. Two compartments are considered asshown in Figure 23. The columns in compartment 1 are

J. Y. Richard Liew


0

200

400

600

800

1000

1200

0 20 40 60 80 100 120

ISO 834

OF = 0.04

Time (Min)

Temperature (C)

Figure 22. Parametric time-temperature curves for six-storey office building frame

Firecompartment 1

Firecompartment 2

1

4

5

2

7Z

X

Y

9

40

42

43

41

Elementnumber

Figure 23. Fire compartments in 6-storey space frame


18/23

most heavily loaded and the size of columns reduces

from the 4th storey onwards.

9.3. Fire at the First Storey CompartmentIf all the beams and columns in the lower floor fire

compartment are unprotected, it is found that load

combination 2 is more severe as the structure fails at a

critical time of 33.7 minutes while it can survive the fire

under load combination 1. The deformed shape of the

structure under fire for each load combination is shown

in Figure 24.

Under load combination 1, as the beams expand

under fire, column heads are forced to open up in both

X and Y directions. When the frame is subjected to load

combination 2, the effect of wind load in Y-direction

becomes pronounced, causing the frame to deform in a

twisting mode. Despite the expansion of the heated

beams, all columns (1, 2, 4 and 5) sway to the same

direction as the wind load. The center of gravity of the

frame thus shifts to the leeward columns (4 and 5),

producing large axial force in the columns (Figure 25).

It is the failure of column 4 which triggers the collapse

of the frame under fire.

Load combination 2 is found to be most critical;

therefore subsequent analyses are carried out using only

this load combination.

Since the columns are found to be the critical

members, it is proposed that all the fire affected

columns are fully protected. Second-order plastic hinge

analysis is again carried out on the partly protected

structure, and the displacement of the column head in Y-

direction is found to be greatly reduced (see Figure 25),

in contrast to the runaway deflection of column 4 when

approaching failure for the unprotected columns.



Buckling ofColumn 4

Load Combination 1 (no collapse) Load Combination 2

Z

X

Y

Figure 24. Deformed shapes of 6-storey frame for load combinations 1 and 2

Case 1: Column Unprotected

Case 2: Column Protected

0.40

0.35

0.30

0.25

0.20

0.15

0.10

0.05

0.000 5 10 15 20

Time (min)

ColumnHeadDeflection(m)

25 30 35

1

4

Figure 25. Column 4 head displacement in the Y-direction


19/23

However, the critical time shows only marginal

improvement, from 33.7 min to 35.5 min. Beam collapse

mechanism forms at beams 7 and 9 (the smallest beams)

in X-direction due to large restraint force from the

supporting columns, causing larger mid-span deflection

as shown in Figure 26.

The next logical step is to provide fire protection to

beams 7 and 9. In this case, the building survives the

entire fire duration with plastic hinges occur at the

beams in Y-direction.

9.4. Fire at the Fourth Storey CompartmentFire is assumed to occur at the 4th storeys compartment

as shown in Figure 23. When all the members are

unprotected, extensive plastic hinges form at the four

columns, triggering the collapse of the frame during the

fire. Although the loads on the fourth storeys columns

are smaller than those on the first storeys columns, the

column size from the fourth storey onward is also smaller.

At a critical time of 33 min, columns experience runaway

deflections in both X and Y directions (Figure 27),

symbolizing the failure of the columns. Figure 28 shows

the axial force in windward and leeward columns. The

shift of the center of gravity due to wind load produces

larger axial force in the leeward column.

If a realistic fire model is considered, it is possible

to reduce the cost of fire protection. In this 6-storey

building frame, columns and beams in X-direction

require passive fire protection from 1st storey to 3rd

storey. But from 3rd storey onwards, only columns

need to be protected while all the beams can be left

unprotected. However if ISO standard curve is used

irrespective of layout of the building, fire loads and

ventilation condition, all the members in the building

need to be fire protected Further study will be carried

out on high-rise buildings where the savings in passive

fire protection may become more significant if a

realistic fire is considered.

J. Y. Richard Liew


00 5 10 15

Case 1: Column Unprotected

Case 2: Column Protected

20

Beam 7

Beam 9

25 30 35

0.05

0.1

0.15

BeamMid-SpanDeflection(m)

Time (min)

0.25

0.27

9

Figure 26. Mid-span deflection of Beams 7 and 9

Column 42

Deflection in X-direction

35302520

Time (min)

ColumnHeadDeflection(m)

1510500.35

0.30

0.25

0.20

0.15

0.10

0.05

0.00

0.05

0.10

0.15

Column 42

Column 40

Column 40

Deflection in Y-direction

40

42

Figure 27. Column head displacements in X and Y directions (unprotected)


20/23

10. FIRE PROTECTIONFire protection falls into two main categories, prevention

and protection. Preventative measures include control

of flammable inventories, control of ignition sources,

monitoring of environmental conditions leading to

initiation of alarms and automatic process control,

and fire detection systems designed to extinguish fires

immediately on detection. Protection measures fall into

two categories, passive and active. Passive measures

include fire barriers, fire resistant enclosures, fire doors,

fire retardant coatings and fire protective coatings. Active

measures include water and chemical sprays or deluges,foam dispersion and inert gas dispersal.

After the WTC incident, people expect the public

buildings and their work place to be designed to allow

safe evacuation in the event of fire or explosion. Certain

industrial buildings have stringent fire protection

requirements. For examples, oil and gas production

and processing, nuclear related product storage and

processing, chemical process and storage and key

infrastructures and transportation routes are facilities

that attracted greater risks.

From safety and licensing authorities point of view, the

structure must be capable of safe evacuation in the event

of fire. From the owners view point, the fire protection

is often an expensive statutory feature, which requires

initial capital investment. From the operator point of

view, fire protection must be maintained to preserve the

safety margins declared in the safety documentation. The

volume and cost of passive fire protection materials are

often a critical factor for consideration; therefore, there is

a need to balance these conflicting requirements when

specifying fire protection.

Fire protection can be provided as an all

encompassing scheme, or it can be functionally designed

to optimize on cost, weight and maintenance. Researchhas shown that fire can be successfully suppressed using

a properly design and maintained active protection system

without the need of passive fire protection. Evidently

the structural member affected by the fire may not be

reusable, but there is no guarantee that a fire protected

structure could be re-used anyway.

Computer models have been developed to predict

the response of structures considering fire protection

materials. This model will include the basic thermal

transmission phenomena, radiation, conduction and

convection, the temperature dependent properties ofmaterials and the location and nature of fire protection

measures. The resulting thermal histories are then

applied to the structure to predict time to collapse, or

to demonstrate the degree of collapse. From this, the

structure can be economically protected to meet the

safety requirements.

11. INTEGRATED EXPLOSIONAND FIRE ASSESSMENTOF STRUCTURESThe assessment of the response of structures to explosion

is an increasingly important factor in design, particularly

where the storage and processing of explosive materials

is concerned. Many structures are required to be blast

resistant to protect personnel and adjacent facilities, and

to reduce the possibility of escalation of events. These

structures are therefore designed to contain the effects of

explosion or to act as a significant barrier.

There is a great difference in the structural behaviour

of buildings subject to explosion and fire loads. The short

duration of explosion loading implies that the material is

strain-rates dependent, i.e., high strain rate will increase

the yield strength of steel (Izzuddin & Fang 1997). On



Column 40

00

50

100

150

200

250

300

5 10 15

Time (min)

ColumnAxialFor

ce(kN)

20 25 30 35

Column 42

40

42

Figure 28. Axial force in columns 40 and 42 (unprotected members)


21/23

the other hand, fire loading is associated with elevated

temperatures which cause thermal strains and lead to

significant deterioration in the material properties of steel.

Izzuddin et al. (2000) proposed an integrated analysis

method for explosion and fire analysis of steel structures

incorporating material models which account both for

rate-dependency and thermal property of steel.Liew and Chen (2004) presented a numerical

approach for inelastic transient analysis of steel frame

structures subjected to explosion loads followed by

fire. The proposed transient inelastic analysis can be

used effectively to solve explosion and thermal response

problems, taking account of geometric and material non-

linearities. To achieve both computational accuracy and

efficiency, an analysis procedure has been proposed in

which fire can be treated as a separate event after the

occurrence of an explosion. The fire resistance of the

structure can be evaluated by analyzing the deformed

geometry of the structure caused by the explosion. Theapproximate fire analysis method does not require time

domain solutions but it predicts higher fire resistance

than the strict inelastic transient analysis. Nevertheless,

it offers an alternate means to evaluate the performance

of structures subjected to combined scenarios of explosion

and fire at a much reduced computational cost.

Hand calculation procedure and computational

techniques have been developed with the aim of predicting

how a structure will respond under the interaction of blast

and fire. The followings summarized some of the works

that are being investigated using the explosion responsetechnologies developed.

Pseudo-dynamic or pseudo-static methods these

are the simplest to apply since they take the

explosion overpressure as a blanket loading, and

are usually combined with dynamic amplification

factors. The methods can assess both elastic and

inelastic responses and can be applied to complex

structures.

Single Degree of Freedom Method this is a

dynamic analysis technique, which predicts the

response of a structure by reducing the structure

to a simplified spring/mass system. The method

is effective for simple structures that behave in a

similar manner as to a spring/mass system. The

method can assess both linear and non-linear

responses and can be applied to simulate the

response behaviour of more complex structures.

Finite Element Analysis they can be used

effectively to solve explosion response problems,

taking account of geometric and material non-

linearities. In term of computation time usage,

balance has to be sought since there is a fine line

between a model which is detailed enough to

predict the response, and coarse enough not to

run for impracticable lengths of time. Evidently,

with the continued increase in the speed and

capacity of computers, this problem will be less

critical. The model detail is a matter to be

addressed by the analysts carrying out the work

as is the choice of solution method. The twomost common time domain solutions, implicit

integration and explicit integration can both be

used, each having their own pros and cons.

Significant experience has been gained in the

assessment of structures subjected to explosion loading

using both hand and FEA based methods. Combining

the structural response analysis with the explosion

prediction analysis, it is possible to predict and hence

optimize structural resistance.

12. CONCLUSIONSThe difference in perspective between architect andengineer is noticeable in the design and construction

process. In the conventional approach, the architect

would specify the fire designs based on prescriptive

code requirements and the engineer would design the

structures with fire protection to achieve a certain fire

rating. It becomes apparent in the recent years that

the structural engineers should directly involve in fire

engineering rather than the traditional approach of the

architect specifying the fire designs. In a performance

based design approach, the first step is to understand

what level of performance is expected, then to design tothese levels and followed by predicting the performance

that will be achieved, and finally to be able to assure the

reliability and robustness of the design in the occurrence

of an extreme event.

To carry out a quantitative assessment on the

performance of a building in fire requires the knowledge

of fire science, material properties at elevated temperature,

occupant behaviour and evacuation procedure during an

emergency situation, heat transient and structural response

phenomena, and fire protection etc. All these would

require a multi-dimensional integration approach, as

described in this paper, for performance-based design of

structures.

Fire may be treated as a building load, consistent with

the treatment of other loads in building design such that

it can be integrated with structural design in various load

combinations. Design fire scenarios can be prescribed

for standard building forms and further examined for

more complex systems. Structural design should also

consider the integration between evacuees and fire-

fighter interactions. Proper fire model with interaction

with the active and passive protection measures should

be developed, and relationship between emergency

J. Y. Richard Liew



22/23

response and fire resistance should be established so

that appropriate performance-based method can be

developed to predict the building response to extreme

events. The examples given in this paper illustrated that

saving in passive fire protection could be substantial for

high-rise and large building framework if realistic

natural fires, instead of the conventional standard fire,were considered in design.

Improved understanding of the real behaviour of

natural fire in tall buildings opens new ways of integrating

fire safety and structural design. Prescriptive codes

without considering the systems limit states behaviour,

are often quite approximate in nature. With the advance in

computing technologies, there is an increasing demand for

robust and efficient nonlinear analysis methods for

performance-based design of structures subject to fire and

explosion. Some of the works mentioned in this paper are

a step towards this development.

ACKOWLEDGEMENTSThe author would like to acknowledge the contributions

made by Dr H Chen, Dr L K Tang, Ms K Y Ma, and

Ms H X Yu for their research work on steel structures

in fire in the Department of Civil Engineering at the

National University of Singapore. The work is funded

by research grants (R264000138112) made available by

the National University of Singapore.

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J. Y. Richard Liew

Jat-Yuen Richard Liew is an associate professor and the director of the centre forconstruction materials and technology in the National University of Singapore (NUS). Hereceived his B.Eng and M.Eng degrees (Civil Engineering) from NUS and his Ph.D degreefrom Purdue University in 1992. His research interests include deployable structures, steel-concrete composite systems and fire safety design of buildings. Arising from theseworks, he has generated some 150 technical publications. These include technical articles

in journals, presentations, reports, books, and patents. He interacts closely with the steelindustry in the Asian region as a technical advisor in the areas of steel and compositestructures. He has also seen his R&D brought from the laboratory to full-scale applications.The latter include projects in airport structures, high-rise buildings, large-span andprestressed structures. He is a registered professional engineer in Singapore and achartered engineer in U.K.

performance based design for fire safety

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