Tweet
Re: Amazing 9-11 Documentary - A Must See
That's exactly it. We do more to ourselves every single day than any terrorist organization has done to us. Stop a team of terrorists and you might save 3,000 lives. But make a smarter driver and you save 50,000 lives a year. But we all know that will never happen. Frustrating.
But this digresses from the main point: how can a building burn for hours and not collapse? And why did the WTC seem to fail so easily?
Fire resistance of a structure depends on what it's made out of and how it's designed. The architects who designed the WTC believed that a jet airliner could hit the building and it would survive. And they were right. The planes hit and the buildings continued to stand tall. The design featured multiple steel columns around the perimeter and within the core of the buildings which, despite the sudden loss of dozens of columns, successfully distributed the load to surviving columns.
However, the engineers didn't consider the effect of a post-crash fire which continued to weaken the structure. And with the core of the building severely damaged, critical firefighting systems were knocked out.
While steel might melt at about 1300 deg. C, it generally loses about half its strength at 600 deg. C. Something that could withstand a ton of load now becomes capable of carrying only 1000 pounds. And this doesn't take into account the fact that failure doesn't need to be catastrophic. Plastic deformation takes place as heat-induced ductility increases. It doesn't have to break. It only has to bend far enough to fail.
Floors above the fire started to sag and eventually disconnect from the steel columns. One floor falls onto the other, increasing the load by a factor of two. Then another floor fails. Once floors started to collapse, a chain reaction started. That's a tremendous amount of weight and a heck of a lot of inertia to resist.
Buildings are designed to tolerate a certain amount of static and dynamic load. In the WTC, floor failure increased static loads on the lower floors. And once a mass large enough started to move, even the intact floors couldn't resist the dynamic loads being applied. Internally, floors crashed downward, columns lost the cross-ties provided by the floor joists, and the structure quickly telescoped into itself.
Demoliton experts use these fundamentals to their advantage. Their goal is not to blowing a building to bits, but to induce sufficient structural failure and weakness to get it to start moving. Once it begins to collapse, gravity and inertia does the rest.
So the weak point was that the WTC was built almost entirely from steel columns and steel beams. No structural concrete surrounded any of the major vertical elements in an effort to save weight. It was vulnerable to any uncontrolled fire.
Masonry, on the other hand, can tolerate a great deal of heat and remain standing. Look at old WWII photos of cities that were firebombed and you'll notice a lot of fireplaces and chimneys still standing, as well as major parts of some churches or cathedrals.
There was a structure fire at the Ilikai back in the 1980's. By the time the fire was extinguished, nothing remained within except some ashes. But the basic structure remained sound.
As for the plane crashing into the Empire State Building: It was hit by a B-25, (not a B-52) weighing about 30,000 pounds and with a cruise speed of about 250 knots. It carried less than 700 gallons of avgas, max. It was in the process of landing and was probably going slower and had a lot less fuel aboard when it got into the fog and hit the building.
In the Empire State Building, masonry surrounds the basic steel frame. Concrete provides insulation from heat, as well as considerable compressive strength that isn't lost except when subjected to heat -- even at temperatures far in excess of what is normally encountered in a typical structure fire.
A Boeing 767 typically weighs 350,000 pounds and cruises in excess of 400 knots. It carries almost 24,000 gallons of jet-A fuel that also has 11 percent more energy density than avgas. The WTC was hit by something that was almost a dozen times heavier and probably going two times faster than what hit the Empire State Building. Double the speed, quadruple the energy. Multiply the weight by 12, throw in 35 times more fuel and it's a staggering amount of energy. Had a 767 intentionally hit the Empire State, you can bet the results would have been catastrophic, too.
And that's why the Empire State Building is still standing. The circumstances were not even close to that of the WTC.
Originally posted by scrivener
But this digresses from the main point: how can a building burn for hours and not collapse? And why did the WTC seem to fail so easily?
Fire resistance of a structure depends on what it's made out of and how it's designed. The architects who designed the WTC believed that a jet airliner could hit the building and it would survive. And they were right. The planes hit and the buildings continued to stand tall. The design featured multiple steel columns around the perimeter and within the core of the buildings which, despite the sudden loss of dozens of columns, successfully distributed the load to surviving columns.
However, the engineers didn't consider the effect of a post-crash fire which continued to weaken the structure. And with the core of the building severely damaged, critical firefighting systems were knocked out.
While steel might melt at about 1300 deg. C, it generally loses about half its strength at 600 deg. C. Something that could withstand a ton of load now becomes capable of carrying only 1000 pounds. And this doesn't take into account the fact that failure doesn't need to be catastrophic. Plastic deformation takes place as heat-induced ductility increases. It doesn't have to break. It only has to bend far enough to fail.
Floors above the fire started to sag and eventually disconnect from the steel columns. One floor falls onto the other, increasing the load by a factor of two. Then another floor fails. Once floors started to collapse, a chain reaction started. That's a tremendous amount of weight and a heck of a lot of inertia to resist.
Buildings are designed to tolerate a certain amount of static and dynamic load. In the WTC, floor failure increased static loads on the lower floors. And once a mass large enough started to move, even the intact floors couldn't resist the dynamic loads being applied. Internally, floors crashed downward, columns lost the cross-ties provided by the floor joists, and the structure quickly telescoped into itself.
Demoliton experts use these fundamentals to their advantage. Their goal is not to blowing a building to bits, but to induce sufficient structural failure and weakness to get it to start moving. Once it begins to collapse, gravity and inertia does the rest.
So the weak point was that the WTC was built almost entirely from steel columns and steel beams. No structural concrete surrounded any of the major vertical elements in an effort to save weight. It was vulnerable to any uncontrolled fire.
Masonry, on the other hand, can tolerate a great deal of heat and remain standing. Look at old WWII photos of cities that were firebombed and you'll notice a lot of fireplaces and chimneys still standing, as well as major parts of some churches or cathedrals.
There was a structure fire at the Ilikai back in the 1980's. By the time the fire was extinguished, nothing remained within except some ashes. But the basic structure remained sound.
As for the plane crashing into the Empire State Building: It was hit by a B-25, (not a B-52) weighing about 30,000 pounds and with a cruise speed of about 250 knots. It carried less than 700 gallons of avgas, max. It was in the process of landing and was probably going slower and had a lot less fuel aboard when it got into the fog and hit the building.
In the Empire State Building, masonry surrounds the basic steel frame. Concrete provides insulation from heat, as well as considerable compressive strength that isn't lost except when subjected to heat -- even at temperatures far in excess of what is normally encountered in a typical structure fire.
A Boeing 767 typically weighs 350,000 pounds and cruises in excess of 400 knots. It carries almost 24,000 gallons of jet-A fuel that also has 11 percent more energy density than avgas. The WTC was hit by something that was almost a dozen times heavier and probably going two times faster than what hit the Empire State Building. Double the speed, quadruple the energy. Multiply the weight by 12, throw in 35 times more fuel and it's a staggering amount of energy. Had a 767 intentionally hit the Empire State, you can bet the results would have been catastrophic, too.
And that's why the Empire State Building is still standing. The circumstances were not even close to that of the WTC.
BTW: I am only a liberal in certain things. I am more of a fiscal conservative (live within your means and balance the budget, especially when it comes to governmental spending)
Comment