
Originally Posted by
geoffm
I have to comment on this, as it is something I know a fair bit about. The WTC fires have been extensively studied and modelled, and I have talked to some of those involved in the investigation. The SFPE had an interesting presentation on the WTC collapse at the last meeting.
You are quite correct that other steel framed buildings have survived major fires without structural collapse - Broadgate being a well known example. A common feature is they didn't have a fully loaded airliner flying into them, loaded with 70,000l of fuel - many times the expected building fuel load. Around 1/3 of the fuel was lost in the fireball, with the remainder burning inside the building. and running into the floors below.
The WTC was an interesting design. The building consisted of a central core containing the services and liftshafts, with an exterior steel frame and curtain wall. The wall and the core were connected by the floor slabs, which were supported on lightweight steel trusses, bolted at each end. The fireproofing of the structural steel was originally by spray asbestos in part of the 1st tower, with spray mineral fibre/ plaster (IIRC) in the rest. Steel has the advatage fo rhigh rises that it is strong, and lightweight (comapretively). .Unfortunately it weakens with increasing temperature, with the strength starting to diminish around 200 degrees, and by 500 degrees, it is at 40% of it's cold strength. Steel framed buildings often have insulation added to the steel members so they don't reach the failure temperature. ALternatively, if the steel is large enough, it won't get hot enough to fail in the time available - for the design conditions, which don't include a huge additional fuel load.
The impact of the airliner sheared off a lot of of the outer frame, along with the fire sprinkler riser pipes, so there was no sprinkler control of the fire, although the fuel load would be far in excess of what the sprinklers could handle. The impact also knocked off the fire protection insulation - a key factor in the collapse The fire weakened the remaining connections, so that the end connections of the trusses failed and the floors collapsed, leading to the pancake stacking you saw at the end.
Buildings are designed to handle certain fire conditions, based on the expected usage, fuel load, openings, etc. Office buildings are not designed to take 5-600kph impacts combined with a full load of jet fuel, as well as the extra fuel provided by the airliner itself. The design usually takes some consideration of the benefits of a sprinkler system, and is assumes there is at least most of the applied fire protection to the structure. All fair assumtions for an accidental fire. No fire protection system can prevent arson or terrorism, except security.
The only buildings designed to handle a jet airplane fire are large hangers at airports, and they have big chemical foam monitors to drown the whole lot in foam.
Geoff
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