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 SURVIVE THE IMPACT OF A BOEING 767.
Fact. The twin towers were designed to withstand a collision with a Boeing 707.
In the early 1970's the World Trade Center's chief structural engineer, Leslie Robertson, calculated the effect of the impact of a Boeing 707 with the World Trade Center towers. His results were reported in the New York Times where it was claimed that Robertson's study proved the towers would withstand the impact of a Boeing 707 moving at 600 miles an hour. Little did he know that decades later, two aircraft, almost identical to the Boeing 707, would impact the towers.
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.
However, the actual aircraft involved in the World Trade Center impacts were only flying from Boston to Los Angeles, and consequently, would have been nowhere near fully fueled on takeoff (the Boeing 767 has a maximum range of 7,600 miles (12,220 km)). The aircraft would have carried just enough fuel for the trip together with some safety factor. Remember, that carrying excess fuel means higher fuel bills and less paying passengers. The aircraft would have also burnt some fuel between Boston and New York.
Government sources estimate that each of the Boeing 767's had approximately 10,000 gallons of unused fuel on board at the times of impact.
To give you some idea how much jet fuel this is, an 11 foot by 11 foot by 11 foot tank contains 10,000 gallons (1 US gallon = 0.13368 cubic feet). So a novel way of destroying high-rise buildings is to load an 11 foot by 11 foot by 11 foot glass tank of jet fuel into a Ryder truck, drive it into the ground floor lobby, break the glass, set light to the fuel and walk away, the high-rise should collapse in about an hour (after all, 12,000 gallons of diesel was all it took to bring down WTC 7). Look mom, no explosives needed.
Since, the Boeing 767 is much more fuel-efficient than the 707, a Boeing 707 traveling the same route would carry significantly more fuel and would therefore be a much greater danger from the perspective of a jet fuel fire.
Thus the quantity of fuel that burnt on September 11 would have been envisaged by those who designed the towers. In fact, the towers were designed to survive much more serious fires than those of September 11. Over the years, a number of other high-rise buildings have suffered significantly more serious fires, but none have collapsed (not one). Before September 11, no steel framed skyscraper had ever collapsed due to fire. However, on September 11, it is claimed that three steel framed skyscrapers collapsed mainly, or totally, due to fire.
See this article for proof that the jet fuel fires can be ruled out as the cause of the World Trade Center collapses.
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 more fuel-efficient, and the 707 is faster.
The thrust to weight ratio for a Boeing 707 is 4 x 18,000/336,000 = 0.214286.
The thrust to weight ratio for a Boeing 767 is 2 x 31,500/395,000 = 0.159494.
Since the Boeing 707 had a higher thrust to weight ratio, it would be traveling faster on take-off and on landing.
And, since the Boeing 707 would have started from a faster cruise speed, it would be traveling faster in a dive. So in all the likely variations of an accidental impact with the WTC, the Boeing 707 would be traveling faster. In terms of impact damage, this higher speed would more than compensate for the slightly lower weight of the Boeing 707.
To illustrate this point, we calculate the energy that the planes would impart to the towers in any accidental collision at their cruise speeds (and stated weights).
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 at cruise speed, a Boeing 707 would smash into the WTC with about 10 percent more energy than would the slightly heavier Boeing 767. That is, under comparable 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 has been estimated to be 470 mph = 689 ft/s.
The speed of impact of UA Flight 175 has been estimated to be 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.
Note that the speed of a projectile determines whether the impact damage is localized or spread across a large area. The faster the projectile, the more localized the damage. Common examples illustrating this effect are, the driving of a nail through a piece of wood, and the firing a bullet into a fencepost. Both are done at speed and thus do only local damage. In both of these examples, the wood just a centimeter or two from the impact point, is essentially undamaged. Similarly, the aircraft impacts were at great speed and the damage localized. This effect is
Figure 5. Results of simulation analysis of impact of a 747 jetliner crashing into a steel structure. Notice fracture of the steel column and breaking of the plane due to dynamic stresses (Graphics and analysis by MSC Software Corporation).
illustrated in the above graphic from the simulation of the crash of a Boeing 747 (maximum takeoff weight 875,000 lb, unloaded weight 670,200 lb, fuel capacity 57,285 gallons) with a steel framed building.
We are told that the "hijackers" wanted to cause maximum death and destruction, then why didn't they hijack Boeing 747s? Boeing 747s weigh more than twice as much, they can carry more than twice the fuel and travel faster than the Boeing 767. Consequently, Boeing 747s would have caused much more death and destruction than the 767s.
Also, why did the hijackers choose to hijack aircraft leaving Boston, when they could have just as easily hijacked aircraft from one of the New York city airports (LaGuardia, Newark or JFK). Hijacking aircraft from Boston, meant that they had to deviate from their designated routes, while still a long way from Manhattan. Of course, as is usual, all sorts of alarm bells would be set off as soon as the aircraft deviated substantially from their prescribed routes. Not only that, the US Air Force specialist quick response unit, the Air National Guard, would almost certainly intercept them before they reached their target (and would have assuredly shoot down the second 767, after seeing what happened to the first).
It is often claimed that the WTC was designed only to withstand the collision of a Boeing 707 that was seeking to land at one of the nearby airports, and that since such aircraft would be low on fuel, only small jet fuel fires were envisaged. However, this is an obvious lie. Why is it an obvious lie? Well, because if you take into consideration planes that are landing at an airport, then you must consider planes that are taking off, and such planes are potentially fully laden with fuel.
Since the WTC towers were designed to handle extreme wind loading (140 mph hurricane force winds) they would survive the impact of a Boeing 707 (even one that was traveling at full speed) without adding any extra features to the design (above those already necessary to handle the wind loading). All that the designers would have to consider, is the effect of a jet fuel fire from a fully fueled jet that crashed into one of the towers shortly after taking off from one of the local airports.
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 much 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.
THE BAR JOISTS (TRUSSES).
The following is a critique of an article I wrote some time ago. The original article concerned certain structural components of the World Trade Center, called trusses. Although the media has generally given them this title, the word truss refers to the diagonal reinforcement of a rectangular frame, and so can be applied to a variety of structures. The trusses referred to here, are more correctly called "open web joists" or "bar joists". The original article is in black, comment is in red.
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.
Actually, the "official" story is silent about intermediate lateral support (implicitly implying that there is no such support). Godfrey's book [1] states
Composite floors comprise 900mm deep bar joists (spaced at 2.04 m centres and braced transversely by secondary joists) and a 10 cm thick lightweight concrete slab laid on steel trough decking as permanent formwork. Composite action between the concrete and the steelwork is ensured by extending the diagonal web members of the joists through the steel decking and embedding them in the slab. Dead weight of floor 50 kg/in2, imposed load 488 kg/in2.
Each upper floor comprises 32 prefabricated units spanning between core and external columns. These units are of two sizes: 18.3 x 6.0 m along the longitudinal faces of the core and 10.7 x 4.0 m along the transverse faces. Additional beams are provided to strengthen the four corner bays.
It is not clear exactly what the phrase "each upper floor" means in this instance. It turns out that 18 floors have heavy steel beams instead of trusses. Consider the following quote from Engineering News-Record, January 1, 1970.
On the 41st and 42nd floors, both towers will house mechanical equipment. To accommodate the heavy loads, the floors are designed as structural steel frame slabs. All other floors from the ninth to the top (except for 75 and 76, which will also carry mechanical equipment) have typical truss floor joists and steel decking.
Typical office floors have 4-in. thick slabs of composite construction using top chord knuckles of the joists (trusses), which extend into the slab, as shear connectors. On mechanical floors, composite action is provided by welded stud shear connectors.
So the first 8 + 6 = 14 stories, and the 41st, 42nd, 75th and 76th floors, used solid steel beams in place of trusses. Interestingly, the FEMA report into the collapse of the towers just "forgets" to mention this (however, they did remember to point out similar specially reinforced floors in their report on WTC 7). Also, the top stories had special steel reinforcing diagonals called outrigger trusses (these were nothing like the bar joists that have also been labeled trusses).
The above photos provide "solid" evidence that the 8th, 41st and a number of sub-plaza stories, have solid steel beams, rather than trusses, supporting the floor slabs between the core and perimeter wall.
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 lateral support comes from the flimsy trusses and the lightweight concrete floors. The WTC was designed to survive a 45 pounds per square foot (220 kg per square meter), 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. Using the metric system, this is (220 x 417 x 63 x 9.8)/1000 = 56,640 kN.
The intermediate lateral support due to these specially reinforced floors does not by itself explain the World Trade Center's ability to handle the above mentioned wind-loading. Laying the building on its side and using the bridge analogy, the intermediate support reduces the span between bridge supports to around 400 feet, but even this reduced span is problematic.
The "secret" of the World Trade Center's ability to handle wind-loading is composite flooring. The media coverage of the WTC collapse portrayed the concrete slabs as, just sitting on the bar joists (trusses), and stated, or implied, that these bar joists could just fall away from the slabs, if say, weakened by fire. However, if the media tells you something, that doesn't mean that it is true. A typical example of this disinformation is the Nova interview with Thomas Eagar of MIT. Here is an animated graphic from the article (the whole interview with comment can be found here) which vividly illustrates the main lie (among the many) of the Nova article.
Composite flooring is the name given to floors where studs (called shear studs or shear connectors) are welded to the supporting joists/trusses and then concrete is poured around them, setting them solidly in the concrete slab. The joist-concrete composite slab is significantly stronger than a non-composite slab. In the case of the WTC the main double trusses used their top knuckles as shear connectors. Ordinary shear studs were used along the transverse trusses. We have the following quote from Godfrey:
"Composite action between the concrete and the steelwork is ensured by extending the diagonal web members of the joists (trusses) through the steel decking and embedding them in the (concrete) slab."
The picture on the left shows a construction worker welding studs to the floor joists (through holes in the corrugated decking). The second picture shows typical shear studs, slab reinforcing steel and (rusty) corrugated steel decking. The third picture shows construction workers pouring the concrete slab (around the shear studs, a row of which can still be seen toward the right of the photo).
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 Robertson [2] we find information concerning the World Trade Center when subjected to a 95 mph (153 kph) wind. It turns out that the static deflection is 45 inches (114 cm) from the vertical and that the building then oscillates some 33 inches (84 cms) either side of the point of static deflection (with a period of eleven seconds). Thus a 95 mph (153 kph) wind induces a maximum deflection of 78 inches (198 cms) from the vertical.
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.
Apparently, the intermediate lateral support and composite flooring was sufficient to keep the World Trade Center towers standing for the 30 years that the buildings stood.
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 the 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.
The confusion over the role of the trusses in the original version of this article, was bought about by the medias ommission of information concerning composite flooring and the existence of intermediate lateral support. Although deliberately omitted (or downplayed in the very rare cases when it was mentioned), the existence of intermediate lateral support was clearly predicted by arguments like this one. Since the number of floors providing intermediate lateral support turned out to be quite small, some other form of lateral bracing was also predicted. This turned out to be composite flooring.
First you need to know that the north tower of the World Trade Center suffered a very serious fire on February 13, 1975. You also need to know that this fire caused no serious structural damage to the tower and that no steel-framed high-rise has ever collapsed due to fire. The following is a report concerning the February 13, 1975 fire.
For the rest of the article on the World Trade Center fires, click here.
