Good Morning everybody
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, London
Proprietary balcony
Advantages of balconies
These include
• Adding 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
Single cantilever balcony
Vertical section through multi-storey
balcony
Horizontal section through balcony
Effect of cross wind
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 selfsupporting 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.
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 2010
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.
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 minutes
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.
Section factor v fire resistance
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.
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?
Possible fire models
• ADB
• Equivalent time of fire exposure based on fire
load and ventilation factor
• Total engulfment by standard fire, BS EN
1365-5: 2004
• External fire model, BS EN 1362-2
• Jetting flames model (Eurocode or
Law/O’Brien)
Test for external cladding
Typical fire test rig for external cladding
Some standard fire (temperature-time)
exposures
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).
Bare external structural steel
•
Law M and O’Brien T, Fire safety of bare external structural steel, pub Constrado
(now SCI), 1981, 88 p
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 present
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 …
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.
Time equivalent - early equation by
Law
• FR =
𝐴𝑓
𝐿
.
𝐴𝑓 √𝐴𝑤 𝐴𝑡
• Where
• FR = equivalent Fire Resistance time, minutes
• L = total fire load of contents, expressed as kg of timber
having equivalent calorific value of contents
• Af = floor area, m2
• Aw = area of window opening in room, m2
• At = area of walls and ceiling, excluding area of floor
and ventilation openings, m2
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 =
43 (5𝑥5)
2𝑥2 (4𝑥5𝑥3−2𝑥2+5𝑥5)
= 60 minutes
• Note. This calculation does not include a safety
factor to allow for criticality of element etc.
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.
Time equivalent using PD 7974-3: 2003
te = kb wv q
(31)
• (valid for unprotected steel up to 40 minutes
fire resistance) where:
• te = duration of time equivalence (min)
• kb = 0.07 for typical boundary surfaces ,eg
masonry, gypsum plaster (m2/MJ)
• q = fire load density per unit area of enclosure
surface or floor area (MJ/m2)
Time equivalent using PD 7974-3,
continued
• wv = 1.7 H-0.3{0.62 +90 (0.4 – Av/Af)4} (1+bv Ah /Af)1 ≥ 0.5
(32)
• H= height of enclosure (m)
• Av = area of ventilation in vertical plane (m2)
• Af = floor area of enclosure (m2)
• Ah = area of ventilation in the horizontal plane
(m2)
• bv = 12.5{1 +10 (Av/Af) – (Av / Af)2 } ≥ 10
(33)
Time equivalent using PD 7974-3,
continued
• For residential buildings, safety factor у1 = 1.1
and 1.6 for height of enclosure above ground
level of ≤ 20m and ≤ 30m respectively, and у2 =
1.2
• у3 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.
Equations and numerical values taken
from PD 6688-1-2 : 2007
• te,d =qf,dkbwf
limited to 30 minutes for totally
unprotected structural steel
(B.1)
• where qf,d = fire load per unit floor area
• kb = conversion factor = 0.09 when qd is given in MJ/m2
(B.4a)
• wf = ventilation factor
• At = total area of enclosure (walls, ceiling and floor
including openings)
• Af = floor area of compartment
• For small fire compartment (Af < 100m2) without
openings in the roof
Equations and numerical values taken
from PD 6688-1-2 : 2007, continued
• wf = O-0.5Af/At
(B.3)
• where O is opening factor according to Annex A
(of EN 1991-1-2 ?), ie
• O =Av(heq0.5)/At
• where Av = total area of vertical openings on all
walls
• heq = weighted average of window heights on all
walls
• At = total area of enclosure
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)
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.
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)
Are the FR requirements anomalous?
That’s it - thanks