From  http://www.nerdcities.com/guardian

THE WORLD TRADE CENTER DEMOLITION.
On the 11th September, 2001, three steel framed skyscrapers, World Trade Center One, World Trade Center Two and World Trade Center Seven, collapsed entirely. Other than structures bought down in controlled demolitions, these three buildings are the only steel framed skyscrapers, in the entire history of high rise buildings, to have suffered total collapse. World Trade Centers 3, 4, 5 and 6 also suffered significant damage, but none of these suffered the total collapse seen in World Trade Centers 1, 2 and 7 (in fact, these other buildings showed amazing survivability given that they were repeatedly hit by hundreds of tons of pieces of World Trade Centers 1 and 2, which on impact were traveling at well over 100 miles per hour).

On the 23rd July, 2001, just seven weeks previous, the Port Authority of New York and New Jersey signed a deal with a consortium led by Larry Silverstein for a 99 year lease of the World Trade Center complex. The leased buildings included WTCs One, Two, Four, Five and 400,000 square feet of retail space. The Marriott Hotel (WTC 3), U.S. Customs building (WTC 6) and Silverstein's own 47-story office building (WTC 7) were already under lease. Silverstein is seeking $7.2 billion from insurers for the destruction of the center. One would estimate that the chances of the insurers paying out anything at all, are close to zero.

It should be emphasized that World Trade Center Seven suffered total collapse. World Trade Center Seven was neither hit by an aircraft nor by falling debris from the twin towers. If the claim that it was destroyed by fire were true (it is not) then it would be the only steel framed skyscraper ever to have collapsed exclusively due to fire. Although the WTC Seven collapse warrants the writing of a book, we will deal only with the collapses of WTCs One and Two.

THE WTC WAS DESIGNED TO SURVIVETHE IMPACT OF A BOEING 767.
Fact. The twin towers were designed to withstand a collision with a Boeing 707.

The maximum takeoff weight for a Boeing 707-320B is 336,000 pounds.
The maximum takeoff weight for a Boeing 767-200ER is 395,000 pounds.

The wingspan of a Boeing 707 is 146 feet.
The wingspan of a Boeing 767 is 156 feet.

The length of a Boeing 707 is 153 feet.
The length of a Boeing 767 is 159 feet.

The Boeing 707 could carry 23,000 gallons of fuel.
The Boeing 767 could carry 23,980 gallons of fuel.

The cruise speed of a Boeing 707 is 607 mph = 890 ft/s,
The cruise speed of a Boeing 767 is 530 mph = 777 ft/s.

So, the Boeing 707 and 767 are very similar aircraft, with the main differences being that the 767 is slightly heavier and the 707 is faster.

In designing the towers to withstand the impact of a Boeing 707, the designers would have assumed that the aircraft was operated normally. So they would have assumed that the aircraft was traveling at its cruise speed and not at the break neck speed of some kamikaze. With this in mind, we can calculate the energy that the plane would impart to the towers in any accidental collision.

The kinetic energy released by the impact of a Boeing 707 at cruise speed is
= 0.5 x 336,000 x (890)^2/32.174
= 4.136 billion ft lbs force (5,607,720 Kilojoules).

The kinetic energy released by the impact of a Boeing 767 at cruise speed is
= 0.5 x 395,000 x (777)^2/32.174
= 3.706 billion ft lbs force (5,024,650 Kilojoules).

From this, we see that under normal flying conditions, a Boeing 707 would smash into the WTC with about 10 percent more energy than would the slightly heavier Boeing 767. That is, under normal flying conditions, a Boeing 707 would do more damage than a Boeing 767.

In conclusion we can say that if the towers were designed to survive the impact of a Boeing 707, then they were necessarily designed to survive the impact of a Boeing 767.

So what can be said about the actual impacts?

The speed of impact of AA Flight 11 was 470 mph = 689 ft/s.
The speed of impact of UA Flight 175 was 590 mph = 865 ft/s.

The kinetic energy released by the impact of AA Flight 11 was
= 0.5 x 395,000 x (689)^2/32.174
= 2.914 billion ft lbs force (3,950,950 Kilojoules).

This is well within limits that the towers were built to survive. So why did the North tower fall?

The kinetic energy released by the impact of UA Flight 175 was
= 0.5 x 395,000 x (865)^2/32.174
= 4.593 billion ft lbs force (6,227,270 Kilojoules).

This is within 10 percent of the energy released by the impact of a Boeing 707 at cruise speed. So, it is also a surprise that the 767 impact caused the South tower to fall.

Overall, it comes as a great surprise that the impact of a Boeing 767 bought down either tower. Indeed, many experts are on record as saying that the towers would survive the impact of the larger and faster Boeing 747. In this regard, see professor Astaneh-Asl's simulation of the crash of the much, much larger and heavier Boeing 747 with the World Trade Center. Professor Astaneh-Asl teaches at the University of California, Berkeley.

Although the jet fuel fires have been ruled out as the cause of the collapses, it should still be pointed out that the fuel capacities of the Boeing 707 and the Boeing 767 are essentially the same. And in any case, it has been estimated that both UA Flight 175 and AA Flight 11 were carrying about 10,000 gallons of fuel when they impacted. This is well below the 23,000 gallon capacity of a Boeing 707 or 767. Thus the amount of fuel that exploded and burnt on September 11 was envisaged by those who designed the towers. Consequently, the towers were designed to survive such fires. It should also be mentioned that other high-rise buildings have suffered significantly more serious fires than those of the twin towers on September 11, and did not collapse.

THE "TRUSS THEORY" IS LUDICROUS.
The truss theory is the absurd belief that the only support (between the central core and the perimeter wall) for the concrete floor slabs, was lightweight trusses. It was invented to explain away what were obviously demolitions and has become the "official" dogma. The central core, perimeter wall and the mythical trusses are all introduced in the next section. There you will find out their dimensions, their numbers and their supposed usage. After reading the rest of this article you should return to this section and (with improved understanding) read it again.

According to the "official" story, there is no significant lateral support for the walls (against wind loading) between the ground and top floors. This is like a bridge with a 1,300 foot span between supports. Even though the tube structure of the perimeter wall was designed for maximum rigidity (within the given weight specifications) the 1,300 foot span between supporting pillars, meant that even this very rigid design would sag in the midsection under wind loading, just like a bridge with such a span. In a typical steel framed building the span between pillars is only 12 feet (one floor) and such a problem does not arise.

The World Trade Center towers were like huge sails in the wind. These sails had to be able to resist the 140 mile per hour winds of a hurricane. Such hurricane force winds exerted a large (some 6000 tons) lateral force on the building. This lateral force is called the wind loading (or force of the wind) on the building. According to the "official" story, the only possible intermediate support comes from the flimsy trusses and the lightweight concrete floors. The WTC was designed to survive a 45 pounds per square foot, wind loading. This translates to a 12 x 207 x 45/2000 = 56 ton force on each of the floor segments. What this 56 ton force on each floor segment means, is that if one was to lay the World Trade Center on its side and use the pull of gravity as a substitute for the push of the wind, then each of the 110 floors would need to be loaded with a 56 ton block of steel (so the entire wall would have to support 110 such blocks of steel, that is, 110 x 56 = 6160 tons in total).

The fact that the tubular structure of the walls is very rigid, does not stop the central core from needing to bend when the walls bend. This means that the walls have to transmit the full force of the wind to the core, so that the core will flex to the same extent as the walls (this is obvious, otherwise if the walls flex while the core does not, the floor slabs would, by definition, be crushed). Again, it is important to note that the rigidity of the walls does not protect the central core from the full force of the wind, what it does, is it limits the distance that the walls (and hence the whole structure) can bend. The more rigid the design the less it tilts in the wind.

In strong winds the midsection of the windward wall will be pushed several feet towards the core. In a typical steel framed building of WTC type design, heavy steel beams transmit the wind loading to the core, which then bends together with the walls. However, in the WTC (as described in the "truss theory") the trusses and floor slabs are too weak to transmit this force to the core without buckling, so the core will stay in its original position as the wall advances to it. This will crush the trusses and floor slabs, leading to the collapse of many floors. Since this did not occur during the 30 years in which the buildings stood, we must assume that the "official" story is false. To see how utterly ridiculous the "official" story is, lets calculate the lateral loading (wind loading) that each one of these trusses was expected to resist. Consider, a one floor segment. Here, we have 30 trusses and a slab of concrete supporting 56 tons. That is about 2 tons per truss and piece of slab. If you balanced a 2 ton block of steel on top of one of these flimsy 60 foot long trusses and (a 60 foot long by 6 foot 8 inches wide by 4 inches thick) slab of concrete, we all know what would happen - the truss and slab would buckle and collapse.

Another point to consider, is that if the walls alone handle lateral loading, then the pressure on the windward wall must be transmitted via the corners to the remaining walls (this transmission of loading to the other walls is what gave the WTC its rigidity) but corners are far too weak to handle this task alone.

Although the "truss theory" is ludicrous, it has been pushed by many "experts". It should be noted that it is inconceivable that these experts did not know that it was false.

WHERE IS THE STEEL?
Since the trusses are incapable of resisting the wind loading, we know that the "official" explanation of the WTC collapse is false. If the floor joists (supports) were not the claimed trusses, then what were they? They had to be strong enough to support the floor slab and stiff enough to resist the wind loading. In fact, they had to be large steel beams. This is not to say that trusses were not used at all in the construction, but just that (contrary to the "official" line) the main floor joists were steel beams and not trusses.

The above argument using wind loading is certainly enough to tell one that trusses were not really used as the floor joists, but there are also other ways to determine this. Another approach is adopted in this section. We will:

Calculate the weight of steel theoretically used in the construction of one of the towers assuming that the floor joists were trusses.
Compare the result of this calculation to the 96,000 tons of steel known to have been used in the construction of each of the towers.
Note that the calculated weight of steel is only 67 percent of the required 96,000 tons.
Conclude that the 32,000 tons of steel unaccounted for, is due to the fact the the floor joists were actually weighty steel beams and not flimsy trusses (and thus that the official story is a lie spun to explain away what were obviously demolitions).
Calculate a rough cross-sectional area for the steel beams that did serve as floor joists.



.... SNIP ....
For the missing section see The World Trade Center Demolition.



CONCLUSION.

Impacts of the magnitude of those that occurred on September 11 were considered by the designers of the twin towers and the towers were designed to survive them.
The possibility of a jet-fuel fires the size of those that occurred on September 11 were considered by the designers of the twin towers and the towers were designed to survive them.
In order to explain why the towers collapsed, where other steel framed buildings would have survived, the WTC conspirators invented the "truss theory".
The "truss theory" is seriously flawed. It cannot explain how the perimeter wall transmits wind loading to the central core.
The "truss theory", if accepted, leads to a 33 percent underestimate of the amount of steel in the towers. That is, the "truss theory" does not account for the whereabouts of 32,000 tons of steel (of 96,000 tons) used in the construction of each of the towers.
The "truss theory" is a lie that has been spun to convince a gullible public, that what appeared to be the controlled demolitions of three of the World Trade Center buildings, were actually natural consequents of the aircraft strikes and not controlled demolitions at all.
There are photos showing large steel girders positioned where the "official" line states that only (double) trusses should be.
In all, one has to conclude that the "truss theory" is false and that those who push it are part of a large conspiracy to deceive the American people.

From
 http://www.nerdcities.com/guardian
The World Trade Center Demolition.