The big difference between the two fires was the amount of fuel (or combustible material) available. All the fuel spread and ignited at once, while the first fire ’ate’ through the material.
1st fire: The hottest are moves along the path of the fire as the fuel is spent. Thus you have a very hot zone, and cooler zones before and after the fire.
2nd fire: Maybe less intense, but equally hot over most of the floors where the explosion (and just after that) the fire was.
I don’t think that there is any question that failure was due to molten steel. Steel’s properties change with temperature change. (have a look at a barbeque grid). Using basic engineering mechanics (http://www.engineeringtoolbox.com/linear-expansion-coefficients-d_95.html), you can calculate that a 1m long section of steel, heated up by 700deg C. would expand by 9mm. Apply this over the length of your beams (about 10 metres), and you get beams the are 9cm longer
than usual. Now, these have to still fit into the existing structure (ignore the enormous forces that push columns apart now).
You will have strange forces acting on the joints, torsion, etc. The whole structure would contort and twist.
I assume that the WT buildings were bolted together. It is very likely that the rivets either just broke (failed). Once the connections fail, the best beams just come tumbling down...
This, to me is the really plausible reason for failure, if failure was due to the planes colliding.
The big difference between the two fires was the amount of fuel (or combustible material) available. All the fuel spread and ignited at once, while the first fire ’ate’ through the material.
1st fire: The hottest are moves along the path of the fire as the fuel is spent. Thus you have a very hot zone, and cooler zones before and after the fire.
2nd fire: Maybe less intense, but equally hot over most of the floors where the explosion (and just after that) the fire was.
I don’t think that there is any question that failure was due to molten steel. Steel’s properties change with temperature change. (have a look at a barbeque grid). Using basic engineering mechanics (http://www.engineeringtoolbox.com/linear-expansion-coefficients-d_95.html), you can calculate that a 1m long section of steel, heated up by 700deg C. would expand by 9mm. Apply this over the length of your beams (about 10 metres), and you get beams the are 9cm longer
than usual. Now, these have to still fit into the existing structure (ignore the enormous forces that push columns apart now).
You will have strange forces acting on the joints, torsion, etc. The whole structure would contort and twist.
I assume that the WT buildings were bolted together. It is very likely that the rivets either just broke (failed). Once the connections fail, the best beams just come tumbling down...
This, to me is the really plausible reason for failure, if failure was due to the planes colliding.