A discussion of fire scenarios and models for steel framed enclosed multistorey balconies Gordon Cooke International Fire Safety Consultant Formerly Visiting Professor School of Engineering and Mathematical Sciences City University ID: 170540
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Slide1
Good Morning everybodySlide2
A discussion of fire scenarios and models for steel framed enclosed multi-storey balconies.
Gordon Cooke
International
Fire Safety
Consultant
( Formerly Visiting
Professor, School of Engineering and Mathematical Sciences, City University,
London)
www.cookeonfire.com
Prepared for the Institution of Structural Engineers ‘Steel in Fire‘ Forum meeting , 24 September 2013, LondonSlide3
Proprietary balconySlide4
Advantages of balconies
These includeAdding to the usable space in the dwelling
Adding to the monetary value of the dwelling
Providing a glazed space in which to enjoy the sun
protected
from wind and rain
Improving the aesthetic of an old building Slide5
Single cantilever balconySlide6
Vertical section through multi-storey balconySlide7
Horizontal section through balconySlide8
Effect of cross windSlide9
Two balcony systems
Multi-storey balconies can be added relatively easily, often with four unseen slender steel columns per balcony which extend from ground level to top of building so that the whole balcony system is self-supporting and
adds minimal
imposed dead loads to the
parent building
.
Single storey balconies
can be added to a building so that they
are:
a) supported from the external wall of the parent building with diagonal tension members, or
b) supported by an existing cantilevered floor.
In both cases the
dead load
is normally low
as the balcony system
can be lightweight
. Slide10
Functional regulations in the UK affecting balconies
Building products are governed by
regulations,
codes, and standards. The UK comprises England, Wales, Scotland and Northern Ireland
The regulations applying to new buildings and buildings subject to alteration are:
In England and Wales -The Building Regulations 2010Slide11
Functional regulation B3 (England and Wales)
B3. (1) The building shall be designed and constructed so that, in the event of fire, its stability will be maintained for a reasonable period.Slide12
Fire resistance in flats (ADB)
According to Table A2 of AD B, which applies in England and Wales, structural elements such as beams and columns within
a non-sprinklered block of flats need the following amount of fire resistance (the numbers in the first row of the table below are the height of the top floor (not the top of the building) above ground level measured in metres)
Not more than 5m
Not more than 18m
Not more than 30m
More than 30m
30 minutes
60 minutes
90 minutes
120 minutesSlide13
Importance of choice of fire scenario
Fire scenario affects amount of fire resistance of balcony structural elements
If small balcony columns are to be employed the amount of fire protection is very dependent on FR required
The section factor (A/V) needed for a bare steel I-section column needs to be less than 50m-1 to achieve 30 minutes FR for 4-sided exposure. E.g. a ‘massive’ bare solid steel column 150mm square achieved only 38 min in a FR test.
An RHS section 150mm square with wall thickness of 8mm has a section factor of 135m-1 requiring a large thickness of added fire protecting material.
BS 5950-Part 8: 2003, the ASFP ‘yellow’ book and fire protection manufacturers gives guidance on A/V values etc. Slide14
Section factor v fire resistanceSlide15
Fire safety engineering (ADB)
This can provide an alternative approach to fire safety. It may be the only practical way to achieve a satisfactory standard of fire safety in some large and complex buildings and in buildings containing different uses e.g. airport terminals. Fire safety engineering may also be suitable for solving a problem with an aspect of building design which otherwise follows the provisions of this (ADB) document.Slide16
Some questions
What fire resistance is required for balconies?Should the fire ratings in the building regulations guidance Approved Document B be adopted without question?
Could the fire severity be more (or less) than the regulatory (ADB) value?
What standardised fire models might be encountered and be appropriate?
What purpose-designed fire test rig might be suitable for approval purposes?Slide17
Possible fire models
ADBEquivalent time of fire exposure based on fire load and ventilation factorTotal engulfment by standard fire, BS EN
1365-5: 2004
External fire model, BS EN 1362-2
Jetting flames model (
Eurocode
or Law/O’Brien)Slide18
Test for external claddingSlide19
Typical fire test rig for external claddingSlide20
Some standard fire (temperature-time) exposuresSlide21
BS EN 1365-5:2004
This specifies a method for determining the fire resistance, in respect with loadbearing capacity and with no separating function, of:
balconies exposed to the fire from either outside or inside the building; and
walkways exposed to the fire from either outside or inside the building.
This European standard is used in conjunction with BS EN
1363-1 i.e. involving exposure to the standard fire resistance test exposure (ISO 834).Slide22
Bare external structural steel
Law M and O’Brien T, Fire safety of bare external structural steel, pub
Constrado
(now SCI), 1981, 88
pSlide23
Bare external structural steel
Section C Design Tables, states in C2.2 that a bare steel column opposite a window with no through draft should be at least two thirds of the window height away from the plane of the window if the limiting temperature of the steel is not to exceed 550
degC
. This is conservative and detailed calculations might show that less gap is needed but these calculations are time consuming and tedious.
In this location it is deemed to be outside the trajectory of the jetting flame. Hence for a window height of 2m the bare column should be 1.33m away. Assumes fire load density does not exceed 50 kg/m2 of floor area. Greater gap may be needed
if
through-draft
presentSlide24
External fire exposure curve.
Clause 5.1 of EN 1362-2
In some cases elements may be exposed to conditions which are less severe than when the element or structure is exposed to a compartment fire. Examples of this are walls at the perimeter of the building which may be exposed to an external fire or flames coming out of windows…
This exposure condition is only relevant to the evaluation of fire resistance of
separating
elements. Other evaluation techniques exist for the evaluation of beams and columns … Slide25
Flame temperature model
PD 7974-3: 2003 page 43 gives an equation (equation 41) for flame temperature and states that ‘the temperature of the flames at the opening can exceed the temperature of the fire within the compartment’.
This can occur when the fire within the compartment is starved of
oxygen and air is entrained outside the compartment leading to stoichiometric combustion.Slide26
Time equivalent - early equation by Law
FR =
Where
FR = equivalent Fire Resistance time, minutes
L
= total fire load of contents, expressed as kg of timber having equivalent calorific value of contents
A
f
= floor area, m
2
A
w
=
area of window opening in room, m
2
A
t
= area of walls and ceiling, excluding area of floor and ventilation openings, m
2
Slide27
Numerical example using Law equation
Compartment 5m wide, 5m deep, 3m high, one ventilation opening (glazed balcony entrance door) 2m high by 2m wide, fire load density 920 MJ/kg (90% fractile) – from BS 7974-1: 2003, cellulosic fire load 18MJ/kg.
Substituting values gives
FR =
=
60 minutes
Note. This calculation does not include a safety factor to allow for criticality of element etc.
Slide28
Current methods of deriving time equivalent
PD 7974-3: 2003 Application of fire safety engineering principles to the design of buildings, Part 3 Structural response and fire spread beyond the enclosure of origin, section 9.4.3 Equivalent time of fire exposure
Eurocode
1: Actions on structures Part 1-2: General actions – Actions on structures exposed to fire, Annex F equivalent time of fire exposure.
However the National Annex to the
Eurocode
, BS EN 1991-1-2: 2002, states that Annex F may not be used, and PD 6688-1-2: 2002 should be used as a replacement. Slide29
Time equivalent using PD 7974-3: 2003
t
e
= k
b
w
v
q (
31)
(valid for unprotected steel up to 40 minutes fire resistance) where:
t
e
= duration of time equivalence (min)
k
b
= 0.07 for typical boundary surfaces ,
eg
masonry, gypsum plaster (m
2
/MJ)
q = fire load density per unit area of enclosure surface or floor area (MJ/m
2
)Slide30
Time equivalent using PD 7974-3, continued
wv
= 1.7 H
-0.3
{0.62 +90 (0.4 – A
v
/
A
f
)
4
} (1+b
v
A
h
/
A
f
)
-1
≥ 0.5
(
32)
H= height of enclosure (m)
A
v
= area of ventilation in vertical plane (m
2
)
A
f
= floor area of enclosure (m
2
)
A
h
= area of ventilation in the horizontal plane (m
2
)
b
v
= 12.5{1 +10 (A
v
/
A
f
) – (A
v
/
A
f
)
2
} ≥ 10
(
33)
Slide31
Time equivalent using PD 7974-3, continued
For residential buildings, safety factor у
1
= 1.1 and 1.6 for height of enclosure above ground level of ≤ 20m and ≤ 30m respectively,
and
у
2
= 1.2
у
3
may be taken to be 0.6
Substituting values used in previous example gives a time equivalent of 53 min which appears sensible, but the PD states that it cannot be used when time equivalent is greater than 40 min. Slide32
Equations and numerical values taken from PD 6688-1-2 : 2007
t
e,d
=
q
f,d
k
b
w
f
limited to 30 minutes for totally unprotected structural steel
(B.1)
where
q
f,d
= fire load per unit floor area
k
b
= conversion factor = 0.09 when
q
d
is given in MJ/m
2
(
B.4a)
w
f
= ventilation factor
A
t
= total area of enclosure (walls, ceiling and floor including openings)
A
f
= floor area of compartment
For small fire compartment (
A
f
< 100m
2
) without openings in the roof Slide33
Equations and numerical values taken from PD 6688-1-2 :
2007, continuedw
f
=
O
-0.5
A
f
/A
t
(B.3)
where
O
is opening factor according to Annex A (of EN 1991-1-2 ?),
ie
O =A
v
(h
eq
0.5
)/A
t
where A
v
= total area of vertical openings on all walls
h
eq
=
weighted average of window heights on all walls
A
t
= total area of enclosureSlide34
Calculated value using PD
6688-1-2 : 2007
Substituting values gives time equivalent of 82 min. This seems high and, again, the calculation result is not acceptable because time equivalent exceeds limit of application i.e. exceeds 30 min.
Note. The limits of application in PD 7974-3 and PD 6688 -1-2 are different (40 min v 30 min) Slide35
Tentative Conclusions
The time equivalent calculation is easy to do and gives periods of fire resistance, but, for the above example compartment size and fire load density, gives results which are outside the limits of application. This applies to PD 7974-3 and BS 6688-1-2.
The external fire exposure curve is inappropriate.
The BS EN 1365-5 for balconies assumes the whole balcony is exposed to the BS EN 1363-1 standard fire – a very severe fire exposure.
The flame temperature model in PD 7974-3 is difficult to apply to flames outside the opening partly because it requires the use of flame radiation configuration factors and does not result in a period of fire resistance.
The jetting flames model (Law/
Eurocode
) involves tedious calculations and is difficult to use.
The ADB tabular values of fire resistance are convenient to use and more likely to be accepted by the building control official.Slide36
Ancient references to simple time equivalent equation
Law, Margaret. Prediction of fire resistance, Paper No2 of Fire resistance
requirments
for buildings – a new approach.
Dept
of Environment and Fire Offices’ Committee Joint Fire Research
Organisation
, Symposium No 5, London 1973 HMSO
Cooke
GME, ‘Fire Protection,’ chapter of Volume 1 of ‘Specification 85′, Published by The Architectural Press, 1985,
pp
69.
(available on
www.cookeonfire.com
website under Publications)Slide37
Are the FR requirements anomalous?Slide38
That’s it - thanks