A first reaction to the cause of collapse of the World Trade Center towers might have attributed the collapse to a section of the central core (NCE last week). But witnessing the details of the aircraft impact and the immediate events that followed, a different scenario is more likely.
The twin towers were constructed using a structural system known as the 'Hull-core system' - the most advanced system for tall buildings at the time. The buildings were, quite literally, gigantic square box beams. It is clear that the overall structural integrity of the towers rested on the resistance capacity of the outer hull.
As we all witnessed, the North Tower, or World Trade Center 1 (WTC1), was hit first.
The aircraft pierced through the outer skin and disappeared inside causing an enormous fire within the building. The intensity of heat started to melt the perimeter columns, and when sufficient columns yielded, local buckling of the perimeter columns caused the upper section of the tower to collapse within it.
The eventual collapse of both towers was caused by local yielding and buckling of the perimeter columns, followed by the progressive implosion of the floors.
In the past, although the possibility of high speed aircraft impact has been considered in every tall building design, no designer has realistically resolved the issue nor made provision for the real danger.
If a warning of the possibility of immediate collapse of the towers could have been sounded out, within the vital hour between the impact and collapse, the process of evacuation could have been speeded up saving many more lives, and the lives of the service personnel.
Dr Nutan Subedi, reader, department of civil engineering, University of Dundee, Dundee DD1 4HN
As we watched the dreadful events unfolding on Tuesday our thoughts were with all those inside. We pray that Members among them may have survived, and also for those who did not.
But surely it didn't have to be 5,000. You report that World Trade Centre Tower 1 survived 1 hour 29 minutes and WTC2 survived 51 minutes. They should have lasted at least four hours each. Many would say that for a super high-rise it should be six hours.
The buildings survived the impact of a plane, but failed under fire. Either the building codes for fire were inadequate, or the structures were not built in accordance with the codes. I prefer to believe the former.
It makes no sense for firemen to be running up the same staircase as thousands of people are running down it.
Many such architectural and engineering questions need to be asked and answered.
Codes and their supervision need urgent investigation and updating, for the safety of all those who live and work in tall buildings, both here and around the world.
After box girder and after Ronan Point we reviewed our methods and inspected our structures. Nothing less will be acceptable in this instance.
Gordon Rose, FASCE FIStructE rosecharlbert@aol. com
Comments by Waterman's managing director printed in your publication this week following the collapse of the World Trade Centre quote him as stating that the Canary Wharf Tower 'would not collapse if hit by a plane'.
This is clearly a ridiculous statement to make before all the facts are published into the extreme conditions imposed upon the twin towers by this awful event.
May I suggest that Mr Campbell publish a technical article on this event, demonstrating his superiority of knowledge over other engineers methods of construction, or perhaps it would be better for him to keep quiet during this period of reflection and analysis.
Alan Lace-Evans, Bramley Manor, Bramley, Surrey GU5 0HS
The events of last week in New York while being sad, tell us that as engineers we are not immune from circumstances and events which remind us of our human vulnerability and the challenges that we face.
I was, however, saddened by the statement made by one of our leading professionals which exposed the inherent weakness in our profession of being out of touch with reality.
If ever a terrorist organisation needs encouragement then they need not look any further than statements such as those made in your article - which were both arrogant and stupid.
Karuna Tharmananthar (M), Blackberries Cottage, Sheriffs Lench WR11 5SR
The combined effect of removing columns with the intense heat of an aviation fuel fire would undoubtedly destroy Canary Wharf. It is such arrogance that gives engineers a bad name.
John M Wilson 3 Redwood Crescent, Peel Park, East Kilbride, G74 5PR
I cannot be the only civil engineer to wonder at the ease with which the two towers of the World Trade Center in New York collapsed last week. Is there no structural integrity in today's world of cost-cutting design philosophies?
Of course, it was all about the chain reaction of vertical impact loading, not normally a design consideration. But should we not take note of this salutary experience, and hope that the construction industry contributes to ensuring that this never happens again - under any circumstances?
Mike Franklin (F), 27 Paget Rise, Abbots Bromley, Rugeley, Staffs, WS15 3EF