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Integration of Structural
Engineering into Fire
Engineering Design



LEUNG Siu
-
man

Chief Structural Engineer

Buildings Department


Building (Construction) Regulation 90


Fire Resisting Construction

Every building shall be designed and constructed so as to



(a)
Inhibit the spread of fire within the building and to nearby buildings by dividing the building into
compartments;

(b)
Provide adequate resistance to the spread of fire and smoke by the separation of different uses in
a building by compartment walls and floors and by the separation of the building from any
adjoining building or site;

(c)
Maintain the stability of the building in case of fire; and

(d)
Provide adequate resistance to the spread of fire over the roof of one building to another having
regard to the position of the building.


In other words, every building shall be designed and constructed so

as to provide adequate resistance to spread of
fire

and
smoke

and

to maintain its
stability
.

High casualty fire incidents in HK

-
Garley Building (1996) : 40 fatalities

-
Mei Foo Sun Chuen (1997) : 9 fatalities


Practice Note for AP & RSE 204 issued in 1998 :

-
to provide guidance on fire engineering approach
to meet the fire safety objective and performance
requirements of B(C)R90


Fire Engineering Design


Active Fire Services Installation



Passive Fire Design



Fire Safety Management System



Total Fire Safety in Buildings


Smoke: A deadly hazard to life in a fire

Fire hazard chart

Some measures in Fire Engineering Design to
minimize the hazard caused by smoke

Active Fire Services
Installation

Smoke control system

-

to control the spread of smoke

Passive Fire Design

Appropriate selection
of building lining
materials

-

difficult to ignite

-

do not release vast quantity


of heat and smoke

-

have low rate of flame spread

Fire Safety
Management System

Fire Safety Manager

-

to ensure escape routes are


known by all occupants and


free from obstructions

-

that fire drills are practiced on


a regular basis

Three dominant criteria to ensure that the fire resistant compartment
is maintained so as to allow sufficient time for safe evacuation and
rescue operation


The “Fire Resistance Period” concept


Insulation (I)

to prevent developing excessive temperature on the unexposed
surface of the building element;


structural
Integrity (E)

to maintain the separating function in preventing spread
of flame and smoke;


ability for
Load
-
Bearing (R)
structural element to support the load under fire.

Structural Fire Engineering generally
comprises the consideration of three aspects :




Modeling of possible fire scenarios



Calculation of heat transfer to the structure



Assessment of the structural response at


elevated temperatures

Example of a restrained partition compartment wall

The 3m high concrete compartment partition wall, which is

constructed of d=100mm thick and restrained both at the top by a

fire protected steel beam and the bottom by the concrete floor slab,

is subjected to a natural fire condition with the fire exposed wall

surface at a temperature T
0

higher than the fire unexposed wall

surface.

Assuming

there

is

negligible

flexible

movement

at

the

top

and

bottom


restraints,

the

restrained

bow

(or

lateral

thermal

displacement)

y
R

[
1
]




2

H


T
0

gap






H



For

T
0

=

900


C

;



=

9
.
0

x

10
-
6

/

C
;

E

=

20
,
000

N/mm
2

H

=

3000
mm

;

the

gap

at

the

top

of

compartment

wall

=

10
mm



y
R

=

132
mm



The

restraint

stress


R


E

(


T
0

gap

)



H


R

=

95

MPa




(i
.
e
.

which

is

much

greater

than

the

crushing

strength

of

partition

wall

of

20

MPa)


[
1
]

O’Connor

D

J,

Structural

Engineering

design

for

fire

safety

in

buildings
.

The

Structural

Engineer/

Volume

73
/

No
.

4
,

21

February

1995




Sensitivity study of parameter on integrity of compartment wall under fire condition

Parameter

Variation/ % change

Y
R(mm)
/ % change


R (MPa)
/% change


Gap

10
mm

132

95

20
mm (+100%)

72 (
-
45%)

29 (
-
69%)

30
mm

-

-


H

3
m

132

95

6
m (+100%)

306 (+132%)

129 (+36%)

9
m (+200%)

479 (+263%)

140 (+47%)

12
m (+300%)

651 (+393%)

145 (+53%)


T
0

600
o
C (
-
33%)

87 (
-
34%)

41 (
-
57%)

900
o
C

132

95

1200
o
C (+33%)

165 (+25%)

149 (+57%)

(N.B. The basic case
in bold

: Gap = 10mm, L = 3m and T
0

= 900
o
C)


From the sensitivity analysis, the following findings are observed:
-

a.
Sufficient gap size would significantly reduce the restraint stress in the compartment wall

b.
The increase in compartment wall height would increase the restraint stress mildly whereas
the increase in bow deflection is much greater

c.
The change in temperature would significantly change the restraint stress in the
compartment wall


Example of a platform supported by steel hanger rods

According to BS 5950 Part 8, the limiting temperature of steel
members in tension is as follow:

Description
of member

Limiting temperature(
o
C) at a load ratio of

0.7

0.6

0.5

0.4

0.3

0.2

0.1

Members in
tension: all
cases

460

510

545

590

635

690

770

Table 1

The forces taken by each steel rod at fire limit state calculated by
using a finite element program SAFE, the structural capacities of
steel rods at ambient temperature provided by the manufactures,
the calculated load ratio of the steel rods, and the respective
limiting temperatures derived by interpolation of Table 1 are as
follow:

Size of
steel rods

Forces taken
by each rod at
fire limit state
(A)

Structural
capacities at
ambient
temperature (B)

Load ratio

= (A)/(B)

Limiting
temperature

76
mm

560
kN

1725
kN

0.33

622
o
C

95
mm

1210
kN

2695
kN

0.45

568
o
C

Deficiency


The platform is supported by hanger rods which can

only take tensile loads. Excessive relative elongation

of individual rods heated up under fire may render

these affected rods losing their supporting action (i.e.

no longer in tension). This will cause a load

redistribution and increase the load on the other

remaining hanger rods jeopardizing their original fire

limit state designed for.

Common Problems


FRP ratings are mainly evaluated for individual
building elements only

Structural aspect concentrated on strength and relied
on FRP ratings as guideline on the use of respective
element and material types

Large deformation and plastic strains allowed in fire
limit state design, but may hinder the overall fire
performance of the compartment

Thermal analysis relating to structural response of
building elements ignored or not properly considered




Structural Fire


Engineering Engineering

Gap


Factors to consider for Fire Engineer & Structural Engineer

(a) The selected fire cases should be critically reviewed to ascertain if they generally


cover the worst scenario.

(b) The derivation of the fire load should be conversant with the local condition rather


than simply refer to overseas fire load data.

(c)
Some unprotected steel members might be subjected to a temperature close to the
limiting temperature in case of fire. However, the structural design of the steel
structure should take into account the stress reduction factor of steel at elevated
temperature.

(d)
The derived surface temperature of the material subjected to elevated temperature has
to account for the heat effects of thermal conductivity of materials, which may further
raise the surface temperature.

(e)
The behaviour of natural fire set up of an open frame structure in the laboratory may
be different from the real fire on the spot. In particular, the prevailing wind and other
actual building layout, which may accelerate and increase the temperature of the
structural material, have to be taken into consideration.

(f)
Calculation of fire resistance ratings, based on full compartment burn
-
out.

(g)
Development of suitable acceptance criteria for the design and use of timber, steel,
concrete and glazing and their material limitation.

Conclusion


Importance of other design parameters
affecting the structural design in
performance base approach to be observed


Design criteria should be incorporated in
the coming Code of Practice (Fire
Engineering Design)

-
Thank You
-


Q & A