The report expresses surprise at the ability of the towers to withstand the initial impact of the planes, which it says shows the remarkable strength of the structures. It also points out that the spectacular fireballs caused little structural damage, rather disposing of a third of the 38,000 litres of fuel carried by each aircraft.
The report, commissioned by the Federal Emergency Management Agency and the American Society of Civil Engineers, put most of the blame for the collapse on the intense heat created by the burning of the remaining fuel, aided by the enormous amounts of furniture and computer equipment inside the towers.
It said the temperature in the impact area reached 2000°C and generated a comparable amount of energy to a nuclear power station.
Investigators said anti-fire devices in the towers, such as the fireproofing surrounding the steel columns and sprinkler systems, were dislodged or damaged by the impact of the planes. Stripped of fireproofing, the steel columns heated up swiftly, eventually becoming "as soft as liquorice".
Stripped of fireproofing, the steel columns heated up swiftly, eventually becoming as soft as liquorice
US government report
Investigators said the failure of the systems raised questions about the safety of tall buildings generally, although they added that the extraordinary circumstances of the attack made the wider significance of the failure difficult to gauge.
The report added that building codes and engineering practices should be studied extensively to consider changes. A final version of the report may recommend specific changes.
Experts said the repercussions of the disaster for the construction industry could include new Building Regulations, a new structural design code for tall buildings and more and wider fire escapes.