Matt, you ignorant sl... Oops. Sorry.
Matt, the physics associated with the collapse of the buildings is the extraordinarily difficult to grasp concept sometimes known as gravity.
The engineering associated with the collapse of the buildings includes statics, kinetics, thermodynamics, strength of materials, combustion, crack propogation, and on and on.
Concrete decomposes in hydrocarbon pool fires. You would know this if you had ever paid attention to the condition of a concrete road subjected to a car fire. Decomposed concrete is bad for the structural integrity of concrete buildings.
Steel loses significant amounts of its strength with increasing temperature. At 930 deg F, structural steel has down to 49% of its nominal yield strength. At 1100 deg F, it is down to 28% of its nominal yield strength. (Machinery's Handbook, 25th)
Compare that to the maximum allowable design bending stress of steel, for example. That allowable stress is 66% of the nominal yield strength (AISC Manual of Steel Construction, 8th).
Estimates of the interior temperature generated by the fires in the WTC vary, but are nearly all above 1000 deg F. Which leads to steel strengths below 49% of yield. Which is a problem for a steel structure designed assuming stresses equal to as much as 66% of yield.
At the same time that the steel is becoming weaker, it is also expanding by approximately 0.00001 inches per inch per deg F. A nominal temperature rise of 1000 deg F results in an expansion of 0.01 inches per inch. Which is about 1/8th inch per foot. Or 1/2 foot expansion for a 50-foot-long beam or truss joist.
The beams and joists in buildings are generally fixed at both ends. They certainly are not designed with 6 inches of longitudinal expansion in mind.
So either the beams expand causing the columns to which they are attached to bow outwards and collapse, or the columns resist the expansion causing the heat weakened beams to buckle longitudinally and themselves collapse.
Onto the fire-rotted concrete.
Hmm....what structural elements remain to support the steel and concrete buildings? Too bad gravity's all I'm coming up with for "physics." I bet there's something else in there that could explain it all.Comment Posted By Troll feeder On 21.08.2007 @ 22:40