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Good Morning everybody

1A discussion of fire scenarios and models for steel framed enclosed multi-storey balconies. Gordon CookeInternational 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, London2Proprietary balcony

3Advantages of balconiesThese includeAdding to the usable space in the dwellingAdding to the monetary value of the dwelling Providing a glazed space in which to enjoy the sun protected from wind and rainImproving the aesthetic of an old building

4 Single cantilever balcony

Considerations. Fire within balcony flat, fire in flat below, extended MOE (9m rule), poss of collapse of balcony structure onto fire fighters5Vertical section through multi-storey balcony

6Horizontal section through balcony

7Effect of cross wind

With cross wind. Can be accounted for in model but difficult with full size fire tests unless done in the open on windy day8Two balcony systemsMulti-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. 9Functional regulations in the UK affecting balconiesBuilding products are governed by regulations, codes, and standards. The UK comprises England, Wales, Scotland and Northern IrelandThe regulations applying to new buildings and buildings subject to alteration are: In England and Wales -The Building Regulations 2010

In Scotland - The Building (Scotland) Regulations 2004 In Northern Ireland The Building Regulations (Northern Ireland) 2012In each case there is official guidance to show how functional regulations can be satisfied. In England and Wales the official guidance documents are called Approved Documents(AD). ADs likely to have an impact on balconies would be ADA Structure, ADB fire safety, ADE sound insulation, ADF ventilation, ADK protection from falling.

10Functional 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.

11Fire 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 5mNot more than 18mNot more than 30mMore than 30m30 minutes 60 minutes 90 minutes120 minutesNormal in all codes for FR to increase with height. 12Importance of choice of fire scenario Fire scenario affects amount of fire resistance of balcony structural elementsIf small balcony columns are to be employed the amount of fire protection is very dependent on FR requiredThe 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.

13Section factor v fire resistance

14Fire 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.15Some questionsWhat 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?This would affect the amount of fire protection needed to keep the limiting temperature of steel members within safety which in turn affects aesthetics, complexity, ease of installation and costs.

16Possible fire modelsADBEquivalent time of fire exposure based on fire load and ventilation factorTotal engulfment by standard fire, BS EN 1365-5: 2004External fire model, BS EN 1362-2Jetting flames model (Eurocode or Law/OBrien)

17Test for external cladding

18Typical fire test rig for external cladding

Typically designed to measure the propagation of fire up cladding systems, not for checking fire stability of structural members19Some standard fire (temperature-time) exposures

20BS 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; andwalkways 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).

21Bare external structural steel

Law M and OBrien T, Fire safety of bare external structural steel, pub Constrado (now SCI), 1981, 88 p

. 22Bare external structural steelSection 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

23External fire exposure curve.Clause 5.1 of EN 1362-2In 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 24Flame temperature modelPD 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.

25Time equivalent - early equation by Law26Numerical example using Law equation27Current methods of deriving time equivalentPD 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. 28Time equivalent using PD 7974-3: 2003te = 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)

Bracketed numbers are equation numbers in the BS29Time equivalent using PD 7974-3, continuedwv = 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)30Time equivalent using PD 7974-3, continuedFor residential buildings, safety factor 1 = 1.1 and 1.6 for height of enclosure above ground level of 20m and 30m respectively, and 2 = 1.23 may be taken to be 0.6Substituting 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. Back to the drawing board31Equations and numerical values taken from PD 6688-1-2 : 2007te,d =qf,dkbwflimited to 30 minutes for totally unprotected structural steel (B.1)where qf,d = fire load per unit floor areakb = conversion factor = 0.09 when qd is given in MJ/m2 (B.4a) wf = ventilation factorAt = 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

32Equations and numerical values taken from PD 6688-1-2 : 2007, continuedwf = O-0.5Af/At (B.3) where O is opening factor according to Annex A (of EN 1991-1-2 ?), ieO =Av(heq0.5)/Atwhere Av = total area of vertical openings on all walls heq = weighted average of window heights on all walls At = total area of enclosure

33Calculated 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) 34Tentative ConclusionsThe 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.35Ancient references to simple time equivalent equationLaw, 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)

36Are the FR requirements anomalous?

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Thats it - thanks

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