Fire protection of cantilevered external steel balconies in multistorey flats - a case study.

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2 Fire protection of cantilevered external steel balconies in multistorey flats - a case study. A presentation prepared for April 2015 meeting of the `Steel in Fire Forum, Institution of Structural Engineers, London. Gordon Cooke BSc PhD CEng MICE FIFireE Chartered Civil Engineer and Fire Safety Consultant gordon@cookeonfire.com (Formerly Visiting Professor, School of Engineering and Mathematical Sciences, City University, London)

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4 The problem Fire enforcement notice served Client trying to resolve two issues: 1. if external balcony steelwork needs fire protecting 2. does timber cladding of top storey need coating to reduce flame spread

5 Plan of building showing location of flats, balconies and stairs. Location of balconies in open air and positions of staircases at ends of accommodation meant that means of escape were good, although there were a few locations where the flats were in dead ends. Key Stair Balcony Flat

6 There a Typical layout of a flat

7 Fire scenario

8 Structural Consulting Engineers calculations In a 2 page report the Consulting engineers interpreted the regs to require 60 min fire resistance for the balcony steelwork. My experience suggested the unprotected steelwork had less than 30 min They decided they would use BS 5950 Part 8 to calculate Limiting temp and Design temp for different times of exposure (30, 45, and 60min).

9 Engineers calculation results Acceptable if limiting temperature > design temperature

10 Engineers conclusions It is clear from the above calculations that the balconies perform well enough (not including the decking) to be safe for use as a fire escape for a period in excess of 30 min but not likely 60 minutes. However, it is possible that the building was designed in such a way that the balconies are not necessarily included as part of the overall escape strategy and that they may only require 30 minutes fire resistance or even none at all. We are not familiar enough with this building to make that determination and suggest that a fire consultant be approached to advise further.

11 Typical balcony cantilever.

12 Balcony continuity

13 Timber-slatted balcony walkway

14 My approach I considered it unreasonably onerous to assume the steel was exposed to ISO 834 conditions. If any calculation method should be employed it should use the classic jetting flames method of Law and O Brien. The Law work showed that if the external steel was spaced away from the facade by at least 2/3rd of the window height then the steel would not reach approximately 550 degc, the commonly accepted failure temperature criterion for structural steel.

15 My approach (cont d) If a balcony collapsed in a fire it would not affect the stability of the building. Severe fire would create catenary effect in balcony. Occupant safety OK Some property damage, fire-fighting and FDA considerations.

16 Fire safety strategy There did not appear to be a fire safety strategy for the building and I recommended that one be prepared and submitted to the LFB for comment and approval. It seemed that a stay-in-place strategy might have been adopted as the fire alarm was said to be disabled. Strategy and fire protection is mutually dependent.

17 Cost savings The client mentioned that the savings gained by not having to use intumescent or other passive fire protection for the balcony steelwork was in excess of 300,000. However the implications of a stay-in-place strategy were that local fire detection and alarm would be needed in some areas where there was an unavoidable dead end condition

18 The timber cladding issue. Was Class 4 surface spread of flame acceptable? I considered it was as there were no storeys above the cladding and lateral flame spread is minimal. If anything it was a property loss issue.

19 Unprotected steel columns in the undercroft car park. I deemed it unnecessary to fire protect these owing to the open-sided environment.

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