bump

Good post. I'd wondered about all that. Nice to have all the questions answered.

Yossef Bodansky makes a comment in his book TERROR (a report on the first World Trade Center bombing) that it is the hallmark of Iranian terrorist methodology to construct and place its bombs so as to use the engineered strength of the target building to magnify the impact of their bombs. Despite all the news reports to the contrary, the first bomb DID do severe damage to the World Trade Center. However, the tower was so well engineered that the terrorists realized it would be impossible to bring it down using a bomb they could easily place inside given the increased security measures from their failed attempt. What we see from this second successful attack is the terrorist learning curve. I pray future engineers will take this attack and use the information to build more secure towers. However, I'm not sure we will be able to do more than keep barely one step ahead of these devils. People need to realize that we are not dealing with ignorant camel drivers in this attacks but men who are scientists and engineers who are bent on using their knowledge to destory the West.

The WTC had something other buildings do not have and that was that each had an anti-sway stabilizer at the top. These were huge and extremely heavy concrete blocks, one on top of each tower which were put there to keep the natural sway to a minimum. Controlled by computers, it kept people from feeling the sway so much that it would make them sick.

Each block had to weigh more than just a few floors of concrete in order to offset the natural sway. When a plane hit just below that section it added untold tons to the structure,disconnected the computer controls and added an instability to the towers which could not keep them erect. That heavy structure at the very top is why both towers telescoped downwards. People who witnessed it firsthand said that the top of the towers seemed to have acted like pile drivers. Demolition experts have said it was the first time they have ever seen a building implode from the top. The concrete blocks at the top are what gave the collapse the initial momentum.

No one could have forseen the collapse. Osama bin Laden had no way of knowing that the towers would fall the way they did. He probably saw it on TV and thought he hit the jackpot. I am given to understand that four 50 stories buildings are being purposed to take the place of the two towers. The towers were great while they lasted.

Interesting comments on the engineering. The almost vertical collapse seemed almost surreal.

"I pray future engineers will take this attack and use the information to build more secure towers."

I'm having trouble imagining buildings that could withstand a 1-kiloton suitcase nuke.

The answer is to remove the terrorists and all who support them....root and branch.

--Boris

When the WTC was built, the "tube" design was criticized as being unusually likely to lead to total collaplse of the building, as opposed to the older method using multiple cell-like boxes of girders. It was recognized even before the WTC was built that failure of a single floor's steel trusses, which span the distance from the core to the periphery, at any level, would start a pancaking that would bring down the entire structure.

It was also recognized that failure of a significant fraction of the peripheral girder structure would also bring down the entire building. Both of these factors occured in the WTC collapses.

A further factor in the WTC collapse was that the steel in the peripheral girders and in the floor trusses above the 70th floors was not well protected against fire. At the time during construction when the height of the buildings reached 70 floors, the prohibition against using asbestos came into effect. The person responsible for installing the asbestos correctly predicted that a major fire above the 70th floor would result in collapse of the entire structure.

Going after the terrorists currently active now will reduce attacks for awhile. However, since the practice of terror is an ideal form of warfare for those without standing armies and also those nations of the political left, I'm inclined to think terrorists will be like the poor, "always with us." We can make their work harder by insuring only properly cleared people get near vulernable buildings, keeping a close tab on non-citizens within our borders, and by building buildings that are designed to minimize violent explosions. Since architects are designing ways to deal with earthquakes, they will need to think up ways to foil terrorism as well. It takes a lot of effort to pull off a terrorist attack, if they don't get a big enough bang for their buck, they'll have to find something else to vent their rage.

When the WTC was built, the "tube" design was criticized as being unusually likely to lead to total collaplse of the building, as opposed to the older method using multiple cell-like boxes of girders. It was recognized even before the WTC was built that failure of a single floor's steel trusses, which span the distance from the core to the periphery, at any level, would start a pancaking that would bring down the entire structure.

Had the more conventional 'box-of-girders' approach been used, it's quite possible the buildings would have collapsed within seconds of the impact as many vertical support girders would have been hit by large projectiles flying hundreds of miles per hour. Conventional steel structures, whether of steel or concrete, are very limitted in their ability to redistribute loads given the loss of even a small number of vertical supports. Consider the Alfred P. Murrah building. Every failed vertical support collapsed everything above and below it. Now imagine that mode of failure in the World Trade Center: seconds after the plane hits, much of the center of the building collapses. If the collapsed section is 50 feet wide, the tower would now be two towers, each 75' wide. Since each of those will only have about a third of the horizontal bracing it is supposed to have, it's unlikely to survive long before it topples over.

And that's before we even consider that the jet fuel would pose the same problems there as in the WTC as constructed.

I understand the wet asbestos method was only used on the first 40 floors of one tower. (See Milloy's junkscience.com.)

The Murrah building was supported by gravity. The floors were not strongly connected to their supporting columns. Whe the floors were momentarily lifted from their supports the supports could fall over. This effect was multiplied by gravity.

Makes you wonder how many building are selected for bombing by their design weaknesses. And whether the FBI should be investigating structural engineering students as well as pilot trainees.

Good point about the asbestos. The lack of asbestos above the 64th floors, plus the existence of those anti-sway stabilizers in the top floors need to be taken into any serious account of the structures failure.

Also, while there was that "pile driver" effect, there was also considerable dispersal of the exterior steel girders. In the photos of the collapse you can see girder segments flying outward like missiles, along with the cloud of pulverized concrete.

I happened to watch a special on cable, (History channel, TLC or Discovery, ?) and they showed how the WTC was built and why they built it the way they did. It was indeed a marvelous design. Whoever did the show was pretty thorough. Most of what I've read about it on this forum has been a discussion of whether the steel held up or not. With enough force ANY steel structure will collapse. The force at the top of the WTC was strong enough to do that. Yeah, call it putting two and two together.

The Murrah building was supported by gravity. The floors were not strongly connected to their supporting columns. Whe the floors were momentarily lifted from their supports the supports could fall over. This effect was multiplied by gravity.

Yes, the upward pressure on the third floor above the 'alcove' liftted it from the supports beneath, but the failure of the structure above that was due to the loss of its support. My point was that redundancy for missing pillars is very expensive(*) and so many buildings will be severely damaged by the loss of even one pillar, and the loss of a row of pillars (as would have resulted from a plane plowing through) would be catastrophic.

(*) The required strength of a beam or truss is proportional to the square of the length it must span. If a structure loses a single pillar, the span over that pillar doubles in length; to survive it must have a safety margin of at least 4; to survive the loss of two consecutive pillars requires a safety margin of at least 9.

While I generally agree with you about the danger of future terrorist attacks, I think you underestimate the resolve of our President and of the armed forces.

I believe (I have no evidence to back this up other than than the same news stories we all have access to) that for the first time in our lifetimes, the military has been given a mission, an objective, and the freedom and means to achieve them.

Many here have said "Release the dogs of war!", and it brings a bit of lump to my throat to read it, because I believe that is just what has happened, and I couldn't be prouder to be an American than I am in these times. Our soldiers, wherever they are, are the BEST this world has to offer, and I believe that they are not only doing it for love of country, but also because this is what God sent them to earth to do.

No doubt these are dangerous times for all free people, but it is these United States who will bring to an end these attacks. Not for all time, but certainly for the rest of our lives.

And then, as always has happened, the torch of freedom will be passed to the next generation for them to keep lit.