What was it about the construction of the NYC World Trade Centre buildings that made them collapse so easily in case of a very hot fire?

Answer by David Kahana:

The design of the towers was unique among steel frame buildings of the time. They were built like a tube within a tube, with the floors supported by trusses that tied on to supporting columns on the outer walls as well as on to the core columns making up the inner tube, where the elevator shafts ran.

The whole design maximized the open volume inside the building, so that there would be a lot of space for offices. This was not a steel cage design like many older steel frame buildings, there were no supporting girders running through the middle of the floors and under them. There were only relatively light floor trusses to keep the floors in place. There was no masonrywork.

This big open volume of course also helped with spreading the fire and making it much worse when the attack happened. There had been fires in the buildings before which were quite serious.

Both the inner tube and the outer tube were load bearing structures, with the weight distributed roughly fifty/fifty between them and carried down to ground.

They were commercially zoned buildings so they did not have to survive for very long in a fire, it wasn’t required. They depended on fireproofing around all of the steel work to insulate the steel from the heat of a potential fire. That fireproofing was sprayed on to the members of the floor trusses and there was also fireproofing built around the core columns as well so that they could stay cool and hold their structural strength, but only for the time that was considered necessary for evacuating the people. That time was a lot less than a day. In some places the insulation was known not to be adequate and it was in fact in the process of being upgraded when the planes hit.

The impacts of the airliners cut huge gashes through the outer walls, shifting the weight that the damaged walls were bearing through the roof truss to the core columns and to the other undamaged walls, which was what the design was made to do. A lot of the insulation was stripped from the floor trusses when the airliners went through. Core columns suffered significant damage on the impact floors, some were destroyed, and insulating structures were stripped away – an open tube in the core now ran up and down across several floors where the fire began to burn. First from the fuel, then from the office materials and the materials of the airliners. All of it could burn. And steel of the now overloaded and out of true core columns was in places open to the flame. The floor trusses were also open to flame.

Given all of this damage, it did not need to be an extremely hot fire to cause the collapse, which was quick, but I wouldn’t say was exactly so easy. But these were massive and pretty hot fires. It’s sometimes hard to see from the outside shots, but they went across several floors. They were a horrible inferno in fact. They were very bad fires.

The collapse when it happened can clearly be seen to have initiated near the impact floors high in the building, and it was initiated by the loss of integrity in the core. The core was stressed by damage, by additional weight, by fire, and by the distortion of the floor trusses due to fire as well. When it fails the building core unweights again through the roof truss onto the damaged outer walls which, having been well heated and which are already curving inwards at this point in time, then finally fail under the overload and progressively unzip all the way around the building. The now mostly unsupported top then falls downward onto the lower floors which can’t take the dynamic load and it’s all over.

But you should remember that almost all of the people below the impacts in both towers were evacuated, while almost nobody above the impacts survived.

The towers stood for long enough that they could be evacuated, where there was actually a route for the people to travel down. Above the impacts the people were trapped. The stairways were cut completely in one tower at the level of impact, in the other tower one stairway of three survived, and a few people actually made it down from the zone above the impact, or so I remember.

What was it about the construction of the NYC World Trade Centre buildings that made them collapse so easily in case of a very hot fire?

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