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 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.

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 = 𝐿

𝐴𝑓.

𝐴𝑓

√𝐴𝑤𝐴𝑡

• 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