Collapse of the World Trade Center Towers, 9

continued

4. When the North Tower finally collapsed, the collapse started from the top down onto the impact region. If the fire in this region had been very severe at the moment of collapse, then I would have expected to see a significant burst of fire and burning debris expelled from all around the perimeter of the impact region as it was compressed by the collapse. In reality, the footage of the collapse does not show much flame issuing from the impact region as it is compressed by the collapse.

5. It is reasonable to assume that the force of the impact and subsequent fireball would have stripped the passive fire protection from most if not all of the steel members that remained in place within the impact region. If this is the case and the fire had been as severe as some have stated, the buildings would not have remained standing for as long as they did. Left unprotected, elements of any steel members exposed to severe fire conditions would have quickly reached temperatures close to the fire temperatures. We know this from the large-scale real fire tests conducted in recent years, in which the bottom flange and webs of unprotected beams and columns exposed to the fires reach 90% or more of the fire temperature and closely follow the fire time-temperature curve. The floor beams and exterior columns of these towers were relatively light members and would have heated up rapidly. Unprotected core columns would have heated up more slowly, but even in that case would not have survived 1 ¾ hours of severe fire exposure prior to collapse, especially if they had also been distorted by the impact. This is because the columns would have tried to expand with the heating and, being unable to do so by the surrounding cold building, would have instead buckled sideways leading to further loss of load carrying capacity. That was probably the mode of failure of any damaged core columns following the initial impact, however the fact that the buildings survived as long as they did after the impact indicates to me that most of the members that survived the initial impact were not then heated to very high temperatures.

The effect of the initial impact was so severe that the fire did not have to do much additional damage to collapse the buildings. It is likely that the fire was a more significant contributing factor for the South Tower, however for the North Tower this is less obvious and it was probably progressive shear failure of the system connecting the floors to the perimeter frames that initiated the final collapse.

In fact it is likely we will never be able to establish with absolute certainty the exact sequence between impact and collapse in either case. The above are simply my best guesses based on the information available and my experience of steel building behavior in extreme events.

A Personal Footnote

The two towers were exquisitely well engineered and, as we now know, very well built. It is a tribute to those involved in their design and construction that they absorbed the massive impact from the planes and remained standing long enough to allow many occupants to escape. It is however sobering to reflect on the 5000+ that have been killed in the attack and destruction of these magnificent buildings and the suffering that this has caused and will continue to cause for a long time to come.

Although I did not know anyone directly affected by the attacks, they have left a deep impact on me. As an engineer, part of my response to this is to try and provide my hopefully educated best guesses as to what may have happened to the two towers from the time of impact to collapse. The details given above come from my background of 17 years experience and practice in the research and education into steel building behavior and design, especially research into their behavior under severe earthquake and fire attack.

I hope that as much as possible can be learned from this tragedy so that whatever practical steps can be taken to lessen the vulnerability of this type of building to similar attacks (or accidents) in the future can be implemented.

Reference

1. Godfrey, GB ( Editor); Multi-Storey Buildings in Steel, Second Edition; Collins, London, England,1985, ISBN 0 00 383031 4

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