Why the towers collapsed. (Letters).

SIR: Richard Bell's letter "Why the Towers Collapsed" (January-February 2002) draws ill-conceived conclusions from the facts surrounding the use of asbestos in buildings and the reasons why the World Trade Center buildings collapsed. To better understand this, it's worth considering how the Twin Towers buildings were designed.

Because the buildings were over 100 storeys tall, if traditional lift configurations had been used, over half the floor space would have been devoted to lift shafts. To improve the floor space utilisation, Otis Elevators developed a local-express lift concept which reduced considerably the size of the central lift shafts. This required strengthening the external face of the building, using closely spaced steel columns, between which the windows were placed.

Mr Bell correctly points out that it is standard engineering practice in steel buildings to spray the steel structure with fireproofing materials. Until the 1970s, the most commonly used fireproofing material was a mixture of cement and asbestos fibre. When asbestos was banned due to concerns about its causing serious diseases, substitutes for asbestos such as fibreglass and vermiculite were used. While not generally considered as effective as asbestos, the new materials passed building standards.

The fire rating of the fire protection system is usually specified in terms the number of hours of protection that will be afforded under certain fire conditions. These normally do not include being hit with aeroplanes with seventy or eighty tonnes of kerosene aboard. When the aircraft struck the buildings, slicing though the external columns, the integrity of the whole structure was damaged. Had there been no fire, it is likely that the buildings would not have collapsed. However, the Boeing 767 aircraft were carrying nearly-full fuel loads of about 90,000 litres.

The ...