G J May’s letter raises some interesting points. It turns out that, while the World Trade Centre was correctly designed to withstand ‘normal’ office-type fires, it was not constructed to withstand the damage inflicted by fuel laden aircraft crashing into the towers.
The steel structure was fire-proofed by sprayed on plaster or gypsum cladding (the report refers to “pulverised wall-board material”), materials which would have enabled the steel to withstand a normal 60 minute fire test. However, the damage done to the cladding by an aircraft entering the building at normal cruising speed would undoubtedly have stripped cladding and left steel pillars exposed.
In the open air, jet fuel burns at 260 to 315 Celsius, which would hardly have affected the steel. In the semi-closed conditions of the WTC, heat would have built up and eventually reached the structure’s critical temperature. The critical temperature of a steel member is the temperature at which it cannot safely support its load – often considered the temperature at which its yield stress has been reduced to 60 per cent of the room temperature yield stress.
The article referred to by Mr May (http://www.benthamscience.com/open/tocpj/articles/V002/7TOCPJ.pdf) is most interesting, and gives rise to some interesting questions. These include: given the heat of the fire, how is it that the red and grey chips – the ‘thermitic material’ – did not ignite during the fire? This is given the fact that the chips, when heated, ignited at temperatures between 415 to 435 Celsius (fig 19). Note, a commercial ‘super-thermite’ ignited at higher temperatures between 500 and 550 Celsius (fig 29) and “Ordinary thermite ignites at a much higher temperature (about 900 °C or above).” If the ‘thermitic material’ was used to destroy the towers, how was it placed so that the crumbling of the buildings started at the levels the aircraft crashed into, yet presumably did not ignite for a substantial time. The South Tower collapsed 56 minutes after being hit, the North Tower lasted 102 minutes.
The question as to why the collapse was symmetrical is plausibly answered by reference to the structure of the buildings. Vertical loads were shared between steel pillars grouped in the core and steel pillars around the outer edge, with a large open plan office area between.
The NIST investigation concluded that sagging floors pulled on the perimeter pillars, which bowed inwards and one face collapsed. I would expect that, as soon as that occurred, the collapse ran around the outer face in milliseconds, and the building above the collapsed floor commenced falling. It crushed everything below, and then above when it hit ground, as we saw.
Intriguing is the possibility that the chips resulted from the fire. We have the combination of very hot aluminium (from the building cladding and the aeroplane), and steel from the structure. Could substantial amounts of these have vaporised and, when cooled as solid particles below the ignition temperature, adhered to each other as the chips? When thermite is used to weld steel, the products are liquid steel and aluminium oxide, which forms a ‘slag’. Sparks are certainly created, but there is no explosion, which would have been necessary to blow a very substantial amount of ‘unused thermite’ into the air.
Definitely an intriguing report and it leads to unanswered questions, but not to any speculations as to sabotage.
Dudley Horscroft
Banora Point
