The official report into why the Twin Towers collapsed is due out this month, and preliminary reports could raise eyebrows because it appears to play down the effect of the Boeing 767s smashing into them.

The planes severed perimeter columns and damaged interior core columns in the 11 September 2001 terrorist attacks. However, what brought the buildings down was fire, ignited by jet fuel but fed mostly by the building’s contents and fanned by the holes in the walls and fire-induced window breakage.

With the columns weakened and fire-protection on the steel dislodged by the planes’ impact, the fire made the floors sag and pulled the perimeter columns inward, Shyam Sunder, lead investigator with the US government’s National Institute of Standards and Technology (NIST), told reporters in New York in April.

Sunder suggested that the buildings would still be standing if the fire-proofing had not been dislodged.

Using it as an example of the importance of passive fire protection provided by concrete over steel, Dr Pal Chana, technical director of the British Cement Association, said the report demonstrated the continued need for robust concrete elements, such as for stairwells, to ensure occupants can escape. “In many ways it was a normal office fire,” he said.

The buildings actually performed pretty well. Here are some of the factors that enhanced the towers’ structural performance after the plane crashes:

  • The unusually dense spacing of perimeter columns, coupled with deep spandrels, led to an efficient load redistribution after impact;
  • The robustness of the perimeter frame-tube system, and the dimensional size of the towers, helped them withstand the impact;
  • The composite floor system with open-web bar joist elements, framed to provide two-way flat plate action, redistributed loads from areas of plane damage, preventing collapse.

But Sunder described how they could have done better, and designers of tall buildings will be watching to see how building codes around the world adopt his suggestions:

  • Perimeter columns and floor framing need greater mass to enhance thermal and buckling performance;
  • Steel with improved high-temperature properties (eg yield strength and stiffness) and creep behaviour;
  • Passive and active fire protection features such as: compartmentation, heat-resistant window assemblies, fire protected lifts for fire-fighter access with continuous, redundant water supply for standpipes;
  • Remoter stairwells, and tougher stairwell enclosures.

The full NIST report is due 23 June.

Source

Construction Manager