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Steel frame towers are built very strongly. They need to withstand the pressure of gale-force winds, the violent rocking motion of earthquakes, and the ravages of time. For this reason, they are quite difficult to destroy. Airplane strikes do not destroy skyscrapers. A bomber strike to the Empire State Building during World War II did not bring down that building. The World Trade Center towers were designed to survive a strike by a Boeing 707. The 767 is more massive, so the building was stressed near its design limits. But if a failure had occurred at that moment, it would have been at the point of highest levered stress, near the base of the tower, and the tower would have fallen over like a giant tree in a forest windstorm. That, of course, did not happen. Fires do not destroy skyscrapers. No modern steel frame structure has ever been destroyed by fire, at least not until Sept. 11. How to destroy a skyscraper. So, how do you destroy a skyscraper? Suppose you need the vacant land to build another one, for example. A nuclear bomb is very effective, but it can be difficult to get permits from the city. An early invention was the wrecking ball. A huge lump of steel and lead is swung from a massive chain at high speed. With the benefit of momentum, it is able to bend or break a few girders at a time. But it would be a hopeless task to destroy a tower the size of the World Trade Center, using a wrecking ball. The most effective, cleanest, safest way to destroy a skyscraper is known as explosive demolition or implosion. The trick is to distribute explosives to the all the vertical columns of the building across at least one level. In very tall buildings it is typical to destroy more than one level, to help prevent the upper part from tipping. The explosives are detonated simultaneously, destroying the integrity of the steel frame, such that no part of the building is supported against the force of gravity. The vertical columns must be destroyed over a height of several feet, so that the building is in free fall long enough to build up substantial kinetic energy. Finally, the entire mass is pulled swiftly to earth, where gravity does the work of pounding the structure into tiny fragments of steel and concrete. The gravitational potential energy of the structure is converted smoothly and uniformly into kinetic energy, and then is available very efficiently to pulverize the fragments of the building as they impact against the unyielding earth. Explosive demolitions have a striking and characteristic appearance of smooth, flowing collapse. The World Trade Center collapses had the appearance of explosive demolitions. But were they? Let's look further... Why Did the World Trade Center Collapse?—Simple Analysis An article by this title by Zdenek P. Bazant and Yong Zhou appeared in the on-line version of Journal of Engineering Mechanics ASCE, with the first version dated 9/13/01 and revisions dated 9/22/01 and 9/28/01. With such rapid publication, it is evident that the paper must have been rushed, with no time for full peer review. Let's take a critical look at this paper. The basic theory of Bazant & Zhou is explained as follows, and sketched in their Fig. 1: In
stage 1 (Fig. 1), the conflagration caused by the aircraft fuel spilled
into the structure causes the steel of the columns to be exposed to
sustained temperatures apparently exceeding 800°C. The heating is
probably accelerated by a loss of the protective thermal insulation
of steel during the initial blast. At such temperatures, structural
steel suffers a decrease of yield strength and exhibits significant
viscoplastic deformation (i.e., creep—an increase of deformation
under sustained load). This leads to creep buckling of columns (e.g.,
Bazant and Cedolin 1991, Sec. 9), which consequently lose their load
carrying capacity (stage 2). Once more than about a half of the columns
in the critical floor that is heated most suffer buckling (stage 3),
the weight of the upper part of the structure above this floor can no
longer be supported, and so the upper part starts falling down onto
the lower part below the critical floor, gathering speed until it impacts
the lower part. At that moment, the upper part has acquired an enormous
kinetic energy and a significant downward velocity. The vertical impact
of the mass of the upper part onto the lower part (stage 4) applies
enormous vertical dynamic load on the underlying structure, far exceeding
its load capacity, even if it is not heated. This causes failure of
an underlying multi-floor segment of the tower (stage 4), in which the
failure of the connections of the floor-carrying trusses to the columns
is either accompanied or quickly followed by buckling of the core columns
and overall buckling of the framed tube, with the buckles probably spanning
the height of many floors (stage 5, at right), and the upper part possibly
getting wedged inside an emptied lower part of the framed tube (stage
5, at left). The buckling is initially plastic but quickly leads to
fracture in the plastic hinges. The part of building lying beneath is
then impacted again by an even larger mass falling with a greater velocity,
and the series of impacts and failures then proceeds all the way down
(stage 5).
In my opinion the fires had a less important role to play in the collapse of both towers than the damage from the initial impact. It took both to cause the collapse, however the fire was in no way severe enough to have caused the collapse on its own. The reasons for this opinion are as follows: 1. If the temperatures inside large regions of the building were in the order of 700+ deg C, then these regions would have been glowing red hot and there would have been visible signs of this from the outside. Also there would have been visible signs of flames. If one looks at the photos of the Cardington fire tests, the flames and glowing of the steelwork is clearly visible even in the large enclosure test where the maximum fire temperature was only 700 Deg C. In contrast, the pictures of the towers after the impacts and prior to the collapses show sign of severe burning over only relatively small regions of the tops of the towers, even pictures taken from the air looking horizontally into the impact region .... Photos of the First Interstate Bank fire in Los Angeles in the early 1990s show what appears to be greater heating effects and over larger regions than were apparent in either tower. [....] 2. When fully developed fire conditions ( temperatures of over 700 deg C) are reached within a region of a building, this results in the breaking of glass in any external windows within that region. This continuous breakage of glass as the fully developed fire spread through the floor of the First Interstate Bank, for example, was the most hazardous feature of the fire to those at ground level around the building. In contrast, once the blast and fireball effects of the impacts had subsided, there appeared to be little ongoing window breakage from either tower, either as evidenced from pictures/video footage or as reported from the ground. Significant areas of window even remained intact within the impact region .... This is further evidence that fully developed fire conditions did not spread much through and beyond the initial devastated region, following the impacts. So the basic premise of Bazant and Zhou is seriously flawed. But let's move on to their next analysis: For our purpose, we may assume that all the impact forces go into the columns and are distributed among them equally. Unlikely though such a distribution may be, it is nevertheless the most optimistic hypothesis to make because the resistance of the building to the impact is, for such a distribution, the highest. If the building is found to fail under a uniform distribution of the impact forces, it would fail under any other distribution. According to this hypothesis, one may estimate that C 71 GN/m (due to unavailability of precise data, an approximate design of column cross sections had to be carried out for this purpose). Why would all the impact forces go into the columns as compression? In a tube structure, the columns would bend outwards, placing stress on the perimeter frame and the floor trusses. The entire tube and core system is a complex box which is much stronger than the columns alone. Therefore, the number 71 GN/m is probably a very serious underestimate of the strength of the underlying building. As if to recognize the problems with Bazant and Zhou, Clifton proposes a buckling theory for the South Tower which is similar, but is based on locally focal structural defects rather than any uniform distribution of stresses, and which does not require uniformly high temperatures. Immediately after the impact the perimeter frame in the South East corner would have been severely weakened, being reduced to an unknown number of intact box columns in towards that corner on each of the two sides. However, these columns would have lost the lateral support from the floor slabs over many levels and would have had to function as isolated columns spanning multiple storey heights. They would likely have suffered blast damage and loss of alignment, however immediately following the impact they still retained sufficient compression capacity to resist their share of the loads from the 30 or so floors above the impact region. The fires started by the impact would have then progressively weakened the vertical load carrying capacity of the remaining core, causing progressively more load to have to be carried by the perimeter frame system. In my opinion, based on the footage taken of the building over that time, the fire would have had little impact on the strength and stiffness of the perimeter frames, even in the damaged corner. The stiffness of this system above the impact region would have distributed this load approximately uniformly around the perimeter frames, increasing the loading on these frames through the impact region, including on the residual columns in the damaged corner. Finally the combination of increasing compression load on these damaged columns, with second order effects from this load acting on the buckled shape of these columns over their unsupported length, would have caused their collapse. This collapse would have initiated in the damaged corner and spread rapidly over the impact region, causing the tower above to fail by toppling sideways with the floors above the impact region momentarily in an intact condition. Clifton's argument seems almost reasonable, from a structural engineering point of view. The problem is, it doesn't fit at all with the facts. It is the very opposite of the facts.
This brings up another scenario for the destruction of the towers, one which is now commonly cited only for the North tower. In this version, the floors of the building started to collapse, one on top of the other. This process would accelerate until all of the floors pancaked into the ground, leaving a weakened shell of a building above. No credible structural engineer that I am aware of, has ever proposed a mechanism for the initiation of this pancaking process in the South Tower. Because of the asymmetrical damage, if any floors collapsed they should have done so gradually, starting from one corner and working around. This would have protected the floor below from any massive build-up of kinetic energy. But from the video and eyewitness accounts, we know that the pancaking of floors did occur in the South tower, culminating in a massive blow-out at the base of the tower, which showered debris in the direction of Building 7. This crash would also have sent a tremendous shock wave up the building, which I suggest may be the event which triggered the crushing process described by Barant and Zhou and also by Clifton.
The World Trade Center was leased by Westfield America and Larry Silverstein, on April 26th, 2001. Zim Israeli Shipping moved out of the buildings around that time. With a certain amount of shuffling of tenants from floor to floor, it should have been possible for the owners to gain access to various parts of the building. Critics of the demolition theory have often remarked on the difficulty and expense of explosive demolition, requiring tens of thousands of pounds of explosives, drilling into structural members, and months of time. However, a particularly diabolical structural engineer with a clear understanding of the unique flaws of the WTC architecture, might have hatched on the plan of exploding the trusses supporting a single floor. This would have been sufficient to bring about the sequence of events which destroyed the towers, with the added benefit that if an airplane strike had occurred, the upper stories would appear to fail at the location of the strike. This theory might be confirmed by carefully observing film of the collapse, to determine the point at which the floors started to pancake. Was it at a location consistent with the air strike, or was it elsewhere? Have we been lied to? At the very least, there has been plenty of confusion and controversy. The first question was that the role of the load of fuel from the aircraft. Early reports were that the hot fire was responsible for the collapse, but other observers pointed out that no kerosene fire can burn hot enough to melt steel. In point of fact, most of the fuel in the jets was contained in their wing tanks. The thin aluminum of the tanks was pierced or stripped as the airplanes penetrated the walls of the towers, and the result was the huge fireball which was seen on national TV, where most of this fuel was burned. A hot, vigorous fire would have blown out many windows in the building and would have burned a red or white color. This was not what happened. The fire in the World Trade Center was an ordinary smoldering office fire. But let's suppose that the fire was hot enough to melt steel. What would have happened in that case? Before it breaks, hot steel begins to bend. This redistributes the forces in the structure and puts elastic stress on those parts that are still cool. The process is asymmetric, so that the structure should visibly bend before breaking. Let's suppose the structure were sufficiently weakened that it did fail catastrophically near the point of the airplane strike. In this case, the intact structure below would exert an upward force on the base of the upper story portion of the building (the part that has been broken loose), while any asymmetry would allow the force of gravity to work uninhibited on the tip of the skyscraper. Thus, the top section of the skyscraper would tip and fall sideways. This seems like common sense, and the analysis of Bazant & Zhou may not be sufficient to disprove it. The events of 9/11, summarized. Aside from explosives, there may have been more "magic" at work on 9/11, to produce the effects that were seen on the TeeVee. Taken all together, the evidence suggests very strongly that the attacks of 9/11 were fake terror, and quite possibly were a collaborative venture of the Israeli and US governments. Student pilots from Saudi Arabia and other Arab nations were enrolled in flight schools in Venice, Florida and other locations. The flight school in Venice is linked to CIA drug running operations, according to one researcher. A recently leaked document from the US Drug Enforcement Agency indicates that a number of Israeli intelligence operatives describing themselves as art students took up residences in close physical proximity to the Arabs as they moved about the country. The Arab flight students boarded the flights on 9/11. Did they intend to hijack the airliners, and if so, for what purpose? Had the Israelis played in any way the role of agent provocateur in organizing whatever was planned? It seems reasonable to conjecture that the goals of these Arabs were opposed in some way to some US Middle Eastern policy. It would be very interesting to question the Israelis regarding their knowledge of the Arab flight students. At any rate, if the Arab flight students had been ordinary hijackers, they might have taken the controls of the airplane, but their plot should have quickly been foiled for two reasons. First of all, the Boeing 757 and 767 aircraft are probably equipped with remote-controlled flight computers for purposes of hijack recovery. This was stated by a British intelligence operative and was also suggested by a former German secretary of defense. The technology needed for such systems is well known, and its utility is obvious. If these systems had been operative on 9/11, then they should have been used to take control from the hijackers. Secondly, the US air force has standard operating procedure to send jet fighters to intercept hijacked aircraft within minutes after they are reported. These fighters may be armed and are certainly very maneuverable, and an airliner cannot hope to match them. For these reasons, the Arab hijackers' mission should have been an ignominious failure. These measures (as well as pre-9/11 airport security measures) have been effective enough that hijacking has rarely been a problem for many years now. But on 9/11, the remote control systems were not used to bring the planes home, nor did fighters scramble to escort. Instead, the airplanes executed highly skilled aerobatic maneuvers (well beyond any known educational background of the Arab student pilots) and crashed into the World Trade Center towers and the Pentagon. If the remote controls were used, who was operating them? The World Trade Center towers are designed to withstand aircraft impact, which they did for about an hour. Then they collapsed directly to the ground, with remarkably little collateral damage to surrounding buildings, in a manner strikingly resembling the appearance of controlled demolitions. The US government claims that fire was responsible for the collapse, and this is certainly possible, but many reports have overstated the likely heat of the fire and the amount of fuel from the airplanes which was not consumed in the fireballs outside the towers. If explosives had been planted in the World Trade Center towers, they could have been used to trigger the collapse of the towers. Building 7 was destroyed later in the afternoon. It was never hit by any airplane, so there is no known reason (besides explosives) for it to have collapsed into rubble. However, a cloud of dust was seen in the area of building 7 immediately before the collapse of the south tower, which has not been explained. While the whole attack was going on (a period well over an hour) George W. Bush sat in a classroom and listened to a story about goats, and the US military did not respond to the first three attacks. A fourth flight was also "hijacked" that day, but it was apparently struck down by some sort of missile or bomb before crashing in Pennsylvania. Within hours, a massive media campaign to blame the attacks on Arabs and specifically on Osama Bin Laden was begun, and this campaign has continued to the present day. Our traditional American form of government, unfortunately, may not survive -- the Patriot Act appears poised to supersede the Bill of Rights. Given the many uncertainties about these events, it certainly seems that there should be more questions, more investigations, and more thoughtfulness about the responsibilities of the various parties involved. A little bit of logic will reveal that the Arabs alone could not have been solely responsible for the entire chain of events. It is equally unlikely that the Israelis could have pulled it off alone. Yet instead the US government is gathering up support for war against Middle Eastern nations, a tragic response to the enigmatic events of that day. The author has a master's degree in Electrical Engineering from Stanford University, and a Ph.D. in Psychology from the University of Oregon. Here are links: Justin
Raimondo http://www.antiwar.com Source: http://www.regena.com/demolition.htm
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