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Fuel in the debate


In your generally comprehensive coverage of the US ASCE/FEMA report on the World Trade Center collapses (NCE 8 May), you describe fires as 'intense'.

The report estimates that the planes were carrying 10,000 US gallons of fuel on impact - some 32,000kg. The fire energy of jet fuel is 45MJ/kg, so the total energy was 1.45TJ (terajoules).

However, the report concludes that 10% to 30% was consumed outside in initial fireballs, and that half the remainder drained away, leaving 40% - about 0.6 TJ - to burn.

The average fire load of an office floor is 8psf of wood, which over 30,000 sqft (2,800m 2) is 110,000kg. The fire energy of wood is 13MJ/kg, so the total fire load of one office floor is 1.4 TJ. The jet fuel therefore contributed only half a floor's worth of additional fire load.

Fire protection was 'a sprayapplied asbestos-free mineral fibre material', 19mm thick, later increased to 38mm around the entire impact zone in WTC1 but only floor 78 in the impact zone of WTC2.

The report, says impacts are 'believed to have dislodged fireproofing'. The lingering concern is the assumption that the behaviour was not dependent on the specific type of fire protection installed.

Stuart Alexander, group technical co-ordinator, WSP Group, Vantage House, Weir Road, London SW19 8UX

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