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Collapse inevitable


I read with interest the article in NCE 'Twin towers 'gave out too soon' (27 September) and feel the article cannot go unchallenged.

In my opinion the towers did not in fact 'give out too soon' but, rather I extend credit to the design that allowed the towers to stand longer than reasonably could be expected after such dramatic impacts.

The flying bomb containing tonnes of aviation fuel would have, on impact, destroyed most of the building's external - and possibly many of the internal - columns at that level and perhaps even on the floors immediately above and below.

The central building core would have been extensively compromised.

Damage from most explosions is highly effective when the forces from the blast are focused and confined.

Milliseconds after impact the second massive dynamic shock of exploding fuel would have destroyed the floors and ceiling. In that area, horizontal beams, cross bracing and internal columns would have been instantly sheared, buckled and bent. Consequently, the design slenderness ratio of any remaining columns would immediately increase from a single storey to perhaps a length of two or maybe three storeys on an extensive basis.

In the ensuing blast and structural devastation, any fire resistant material would have been blown off, crushed and extensively cracked, dramatically reducing the resistance to fire of remaining intact structural members.

Unlike most fires that generally start gradually in confined isolation, this inferno would have instant temperatures of hundreds of degrees Celsius over complete floor levels, and any sprinkler systems would have been totally destroyed and inoperative.

Finally, there was the dynamic shock upon the first tower from the later impact of the plane on the second tower and the effect of its collapse.

This sequence of combined and compounded events led relatively quickly to catastrophic structural failure and a classic progressive collapse.

Melton Claughton (F), ARQ Consultants, Gwendale, Millworth Lane, Shinefield, RG2 9EN

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