Originally Posted by dr325i
ii) THe WTC -- simplest one -- the melting point of any steel is higher than the burning temp of kerosine. SImple as that. The steel in the WTC was special kind with the melting temp increased by almost 200C. Now, the science cannot add up here that the steel melted and crashed the building.
Any reasonably sensible structural engineer could explain this to you.
The impacts themselves damaged large areas of the structure, but buildings are generally designed with a degree of redundancy.
This is to avoid something called progressive collapse. The most famous 'progressive collapse' was at Ronan Point in the UK:
This is the domino effect - at Ronan Point a small gas explosion damaged the structure, which caused all related parts of the structure to also fail.
Unfortunately, the WTC was not just structurally damaged - it was exposed to an incredibly severe fire. Kerosene might burn at lowish temperatures, but in building fires the temperature effects are cumulative. All steel buildings are at risk from fire and a fire fed by jet fuel will reach temperatures of maybe 1500 degrees.
This is not warm enough to melt steel, but steel behaves elastically in fire. Steel structures loose strength at temperatures much lower than melting point. At 1500 degrees the steel would have lost about half it's strength.
Sadly, this reduction in strength ultimately meant the steel could no longer support the required load. This was first felt in the areas where the impact had already stripped out some of the redundancy. Ultimately the structure began to fail and this failure ended in a progressive collapse.
One thing I read which I think explains why the progressive collapse occured so quickly relates to the method of fire protection to the steel structure. This was a 'loose' system, rather than an intumescent coating. Some engineers believe that a large amount of the fire protection material was dislodged in the initial impact leaving the steel in the fire 'zone' only partially protected.
Either way - as conspiracy theories go - this one is pretty easily discredited.