[This engineering report on 9/11 is very interesting. Jan]

New York – The twin towers of the World Trade Centre would probably be standing today, if the impact of the planes used in the September 11, 2001 attack had not destroyed fireproofing material, experts said on Tuesday.

After what it described as the most detailed examination of a building failure ever conducted, the US Commerce Department’s National Institute of Standards and Technology (Nist) said it would be suggesting major changes to the way skyscrapers are built and managed.

In assessing the events that led to World Trade Centre’s collapse, the Nist report said the structural impact of the planes and subsequent jet fuel-ignited, multi-floor fires were not in themselves enough to bring the towers down.

‘The floors weakened and sagged from the fires’
“The reason the towers collapsed is because the fireproofing was dislodged,” said Shyam Sunder, lead investigator for the Nist building and fire safety investigation into the disaster.

If the fireproofing had remained in place, Sunder said, the fires would have burned out and moved on without weakening key elements to the point of structural collapse.

He drew an analogy with the 2003 Columbia space shuttle disaster when the absence of a small piece of insulation foam – knocked off during launch – allowed fire to seep into the shuttle’s entire wing span during re-entry with catastrophic results.

New alternatives to traditional fireproofing should be explored, Sunder said, citing a paint-like substance which, if applied in sufficient layers, would stick “even if a plane hit it”.

Nearly 3 000 people were killed in the attack on the World Trade Centre. Roughly 17 000 people were in the skyscrapers at the time of the attack, and Nist estimated that the death toll would have been closer to 14 000 if the two towers had been filled to their 50 000-person capacity.

The report said each jet severed perimeter columns, damaged interior core columns and dislodged fireproofing as they penetrated the buildings. The weight carried by the severed columns was spread to others.

Fires caused by the jet fuel were fed by the building contents and oxygen entering through breached walls and windows.

“The floors weakened and sagged from the fires, pulling inward on the perimeter columns,” the report said.

“Floor sagging and exposure to high temperatures caused the perimeter columns to bow inward and buckle – a process that spread across the faces of the buildings.

“Collapse then ensued,” it said.

In examining the emergency services response on September 11 and the evacuation procedures, the Nist report echoed other probes in highlighting a lack of co-ordination and poor communications equipment.

The report cited one senior emergency services officer inside the north tower of the trade centre as saying he would have known more about what was going on if he had been watching it on television.

“The lack of timely information sharing and inadequate communication capabilities, likely contributed to the loss of emergency responder lives,” the report concluded.

The evacuation of the twin towers has generally been called a success, with 87 percent of the occupants – including more than 99 percent of those below the floors hit by the planes – managing to get out.

The south tower collapsed 56 minutes after impact and the north tower in 102 minutes.

The Nist report noted that a full-capacity evacuation would have taken around four hours – a fatal length of time.

Occupants were often unprepared for the physical challenge of evacuating from higher floors. “It’s pretty demanding, especially if you want to do it fast,” Sunder said.

The Nist report, running to about 10 000 pages, is still in draft form, with a final version, complete with definitive findings and recommendations, to be released in September. – Sapa-AFP

Source: Independent Online (IOL)
URL: http://www.iol.co.za/index.php?set_id=1&click…/p>