QUOTE(popeye47)
The official versions of FEMA is that the fires from the fuel from the planes weaken the structure beams enough to cause a pancaking effect. This is one floor giving away and collapsing on the floor below, and so forth to ground level.
Which would seem to be a far more realistic explanation than someone, somehow sneaking around the (highly secured since 1993) WTC, rigging explosives, without being seen, or anything out of the ordinary being noticed. Not to mention the fact that explosives leave trace evidence. For it not to found considering the amount that would have been needed is not very likely. For the multitude of local, state, federal and private agencies involved in the investigation of the collapse to agree to cover-up this evidence is impossible to believe.
Steel does not like extreme heat. The planes, loaded with fuel for their 3000+ mile scheduled flights, hit the tower. The fuel poured into the tower. The fuel ignited (perhaps due to sparks from the crash, or broken electric lines or a careless smoker

). The fire was intense (to say the least). It burned for some time before the buildings collapsed. The collapse happened as a result of structural failure of the steel due to the extreme heat of the fire.
As a former member of the US Navy with extensive Damage Control (AKA fire-fighting training and qualifications) I can say with absolute certainty that steel will loose structural integrity when exposed to extreme heat.
If you would like to examine the possibility I suggest you do a experiment at home. Goto the hardware store and purchase a piece of steel rebar and a propane torch (and thick, insulated leather gloves if you don't have any). Then, without applying heat, try to bend the rebar. Depending on the thickness of the bar you purchased (as well as your individual uper body strength) you might be able to do it, but it will be pretty tough. Now, apply heat to the center from the propane torch until the middle of the rebar glows red (or if you prefer, apply the heat for the length of time the fire burned in the WTC). Then (remember your gloves) try to bend the rebar again. It will be dramatically easier to do.
Recently I re-wired a home my mother purchased. #12 wire is not that pliable (though much more then say, a piece of steel rebar). I ran alot of wires through the attic (in the late spring and early summer

). I became concerned when I noticed how dramatically easier it was to manipulate the wire that had been in the hot attic for awhile vs. the wire that was elsewhere in the house. Of course the attic never got above 125 degrees. NOw, Copper is more susceptible to heat then steel, but the point is that metal is susceptible to heat, period. (BTW, to correct the problem (though it was likely not an actual safety issue) I installed an attic fan to exhaust the space of heat and keep it at or below 90 degrees).
QUOTE( From JOM (link below))
it is very difficult to reach this maximum temperature with a diffuse flame. There is nothing to ensure that the fuel and air in a diffuse flame are mixed in the best ratio. Typically, diffuse flames are fuel rich, meaning that the excess fuel molecules, which are unburned, must also be heated. It is known that most diffuse fires are fuel rich because blowing on a campfire or using a blacksmith’s bellows increases the rate of combustion by adding more oxygen. This fuel-rich diffuse flame can drop the temperature by up to a factor of two again. This is why the temperatures in a residential fire are usually in the 500°C to 650°C range.2,3 It is known that the WTC fire was a fuel-rich, diffuse flame as evidenced by the copious black smoke. Soot is generated by incompletely burned fuel; hence, the WTC fire was fuel rich—hardly surprising with 90,000 L of jet fuel available. Factors such as flame volume and quantity of soot decrease the radiative heat loss in the fire, moving the temperature closer to the maximum of 1,000°C. However, it is highly unlikely that the steel at the WTC experienced temperatures above the 750–800°C range. All reports that the steel melted at 1,500°C are using imprecise terminology at best.
Some reports suggest that the aluminum from the aircraft ignited, creating very high temperatures. While it is possible to ignite aluminum under special conditions, such conditions are not commonly attained in a hydrocarbon-based diffuse flame. In addition, the flame would be white hot, like a giant sparkler. There was no evidence of such aluminum ignition, which would have been visible even through the dense soot.
It is known that structural steel begins to soften around 425°C and loses about half of its strength at 650°C.4 This is why steel is stress relieved in this temperature range. But even a 50% loss of strength is still insufficient, by itself, to explain the WTC collapse. It was noted above that the wind load controlled the design allowables. The WTC, on this low-wind day, was likely not stressed more than a third of the design allowable, which is roughly one-fifth of the yield strength of the steel. Even with its strength halved, the steel could still support two to three times the stresses imposed by a 650°C fire.
The additional problem was distortion of the steel in the fire. The temperature of the fire was not uniform everywhere, and the temperature on the outside of the box columns was clearly lower than on the side facing the fire. The temperature along the 18 m long joists was certainly not uniform. Given the thermal expansion of steel, a 150°C temperature difference from one location to another will produce yield-level residual stresses. This produced distortions in the slender structural steel, which resulted in buckling failures. Thus, the failure of the steel was due to two factors: loss of strength due to the temperature of the fire, and loss of structural integrity due to distortion of the steel from the non-uniform temperatures in the fire.
source (emphasis mine)
Steel looses strength with heat. Add to that variations in heat, loads applied that are outside the structural design (like a large plane being added to it), losses of several structural supports (from collision with a plane) that shifts in the load, and you get a combination of factors that leads to a catastrophic structural failure like what we saw with the World Trade Center.
Note: I found a
link to the NOVA special mentioned by
Aquilla earlier in the thread (at least I think it is the same one).
Now I have personally seen steel fail in a fire. When it goes, it goes fast. One minute a steel catwalk is sound, then next minute it crumbles from under you into a twisted pile of metal that looks more like spaghetti then a structure once designed to safely hold men. Considering the fact that I was in the same room as the fire when I rode this ride (though I was wearing "turn out" gear at the time) I think I safely assume that this can happen at
relatively low temperatures.
The science behind the fact that the steel structure failed is valid. Some in the media chose to report this as the steel "melting" which is inaccurate, but the steel definitely failed due to heat combined with other factors resulting in bending, twisting, buckling, etc leading to the catastrophic collapse.
I welcome you to experiment with heat and steel (safely, and with good gloves). I look forward to hearing your findings.