Reply With Quote
It appears some cannot grasp this.
The softened steel couldn't support the weight above it. This cannot be argued.
It appears some cannot grasp this.
That it can't be refuted scientifically doesn't mean that it can't be argued.
Pardon my scoffing at things like "mathematical improbabilities." I'm not sure that mathematical calculations concerning material failures and the impact of same on large vertical structures ever accounted for the dynamics occuring when large jet planes loaded with highly-flammable fuel fly at ridiculous velocities into such structures and the resulting fallout on buildings in close proximity, which are struck by flaming debris and compromised.
Then why is this the first steel frame building in the history of the world to melt?
Somehow I missed seeing the building actually melt. Did it end up just puddle on the ground?
Marvin Bush like coincidence
Anyone who thinks jet fuel can soften steel is borderline re .
Your man love stalking of mouse is getting old, let it go brah!
Keep trollin', trollin', trollin'...
When I was in the service many times we used jet fuel in our heaters that were made from steel not one time did any of them get soft. Some of you have your heads so far up your ass's it's a wonder how you can all breath.
Anyone who thinks Microsoft Word was available in 1970 is a re .
Un huh, how many jet engines melt when jet fuel is used to power them?
Your unbelievable amount of user names is getting old, get a job............brah.
And my car uses fuel from oil just like jet fuel and it is made out of steel and yet my car has never gotten soft and collapsed on the way for me to take the GED which I failed.
Thank you, you just proved it was a conspiracy by George W. Bush, the neocons, and the oil companies. They rigged the missiles with special jet fuel that melts steel.
PIZZZZZZOOOOOWWWWWWNNNNNNEEDDDDDD!!!!!!!
THE JET FUEL; HOW HOT DID IT HEAT
THE WORLD TRADE CENTER?
The Federal Emergency Management Agency (FEMA) report into collapse of the WTC towers, estimates that about 3,500 gallons of jet fuel burnt within each of the towers. Imagine that this entire quan y of jet fuel was injected into just one floor of the World Trade Center, that the jet fuel burnt with perfect efficency, that no hot gases left this floor, that no heat escaped this floor by conduction and that the steel and concrete had an unlimited amount of time to absorb all the heat. With these ideal assumptions we calculate the maximum temperature that this one floor could have reached.
"The Boeing 767 is capable of carrying up to 23,980 gallons of fuel and it is estimated that, at the time of impact, each aircraft had approximately 10,000 gallons of unused fuel on board (compiled from Government sources)."
Quote from the FEMA report into the collapse of WTC's One and Two (Chapter Two).
Since the aircraft were only flying from Boston to Los Angeles, they would have been nowhere near fully fueled on takeoff (the aircraft have a maximum range of 7,600 miles). They 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.
"If one assumes that approximately 3,000 gallons of fuel were consumed in the initial fireballs, then the remainder either escaped the impact floors in the manners described above or was consumed by the fire on the impact floors. If half flowed away, then 3,500 gallons remained on the impact floors to be consumed in the fires that followed."
Quote from the FEMA report into the collapse of WTC's One and Two (Chapter Two).
What we propose to do, is pretend that the entire 3,500 gallons of jet fuel was confined to just one floor of the World Trade Center, that the jet fuel burnt with the perfect quan y of oxygen, that no hot gases left this floor and that no heat escaped this floor by conduction. With these ideal assumptions (none of which were meet in reality) we will calculate the maximum temperature that this one floor could have reached. Of course, on that day, the real temperature rise of any floor due to the burning jet fuel, would have been considerably lower than the rise that we calculate, but this estimate will enable us to demonstrate that the "official" explanation is a lie.
Note that a gallon of jet fuel weighs about 3.1 kilograms, hence 3,500 gallons weighs 3,500 x 3.1 = 10,850 kgs.
Jet fuel is a colorless, combustible, straight run petroleum distillate liquid. Its principal uses are as an ingredient in lamp oils, charcoal starter fluids, jet engine fuels and insecticides.
It is also know as, fuel oil #1, kerosene, range oil, coal oil and aviation fuel.
It is comprised of hydrocarbons with a carbon range of C9 - C17. The hydrocarbons are mainly alkanes CnH2n+2, with n ranging from 9 to 17.
It has a flash point within the range 42° C - 72° C (110° F - 162° F).
And an ignition temperature of 210° C (410° F).
Depending on the supply of oxygen, jet fuel burns by one of three chemical reactions:
(1) CnH2n+2 + (3n+1)/2 O2 => n CO2 + (n + 1) H2O
(2) CnH2n+2 + (2n+1)/2 O2 => n CO + (n + 1) H2O
(3) CnH2n+2 + (n+1)/2 O2 => n C + (n + 1) H2O
Reaction (1) occurs when jet fuel is well mixed with air before being burnt, as for example, in jet engines.
Reactions (2) and (3) occur when a pool of jet fuel burns. When reaction (3) occurs the carbon formed shows up as soot in the flame. This makes the smoke very dark.
In the aircraft crashes at the World Trade Center, the impact (with the aircraft going from 500 or 600 mph to zero) would have throughly mixed the fuel that entered the building with the limited amount of air available within. In fact, it is likely that all the fuel was turned into a flammable mist. However, for sake of argument we will assume that 3,500 gallons of the jet fuel did in fact form a pool fire. This means that it burnt according to reactions (2) and (3). Also note that the flammable mist would have burnt according to reactions (2) and (3), as the quan y of oxygen within the building was quite limited.
Since we do not know the exact quan ies of oxygen available to the fire, we will assume that the combustion was perfectly efficient, that is, that the entire quan y of jet fuel burnt via reaction (1), even though we know that this was not so. This generous assumption will give a temperature that we know will be higher than the actual temperature of the fire attributable to the jet fuel.
We need to know that the (net) calorific value of jet fuel when burnt via reaction (1) is 42-44 MJ/kg. The calorific value of a fuel is the amount of energy released when the fuel is burnt. We will use the higher value of 44 MJ/kg as this will lead to a higher maximum temperature than the lower value of 42 (and we wish to continue being outrageously generous in our assumptions).
For a cleaner presentation and simpler calculations we will also assume that our hydrocarbons are of the form CnH2n. The dropping of the 2 hydrogen atoms does not make much difference to the final result and the interested reader can easily recalculate the figures for a slightly more accurate result. So we are now assuming the equation:
(4) CnH2n + 3n/2 O2 => n CO2 + n H2O
However, this model, does not take into account that the reaction is proceeding in air, which is only partly oxygen.
Dry air is 79% nitrogen and 21% oxygen (by volume). Normal air has a moisture content from 0 to 4%. We will include the water vapor and the other minor atmospheric gases with the nitrogen.
So the ratio of the main atmospheric gases, oxygen and nitrogen, is 1 : 3.76. In molar terms:
Air = O2 + 3.76 N2.
Because oxygen comes mixed with nitrogen, we have to include it in the equations. Even though it does not react, it is "along for the ride" and will absorb heat, affecting the overall heat balance. Thus we need to use the equation:
(5) CnH2n + 3n/2(O2 + 3.76 N2) => n CO2 + n H2O + 5.64n N2
From this equation we see that the molar ratio of CnH2n to that of the products is:
CnH2n : CO2 : H2O : N2 = 1 : n : n : 5.64n moles
= 14n : 44n : 18n : 28 x 5.64n kgs
= 1 : 3.14286 : 1.28571 : 11.28 kgs
= 31,000 : 97,429 : 39,857 : 349,680 kgs
In the conversion of moles to kilograms we have assumed the atomic weights of hydrogen, carbon, nitrogen and oxygen are 1, 12, 14 and 16 respectively.
Now each of the towers contained 96,000 (short) tons of steel. That is an average of 96,000/117 = 820 tons per floor. Lets suppose that the bottom floors contained roughly twice the amount of steel of the upper floors (since the lower floors had to carry more weight). So we estimate that the lower floors contained about 1,100 tons of steel and the upper floors about 550 tons = 550 x 907.2 ≈ 500,000 kgs. We will assume that the floors hit by the aircraft contained the lower estimate of 500,000 kgs of steel. This generously underestimates the quan y of steel in these floors, and once again leads to a higher estimate of the maximum temperature.
Each story had a floor slab and a ceiling slab. These slabs were 207 feet wide, 207 feet deep and 4 (in parts 5) inches thick and were constructed from lightweight concrete. So each slab contained 207 x 207 x 1/3 = 14,283 cubic feet of concrete. Now a cubic foot of lightweight concrete weighs about 50kg, hence each slab weighed 714,150 ≈ 700,000 kgs. Together, the floor and ceiling slabs weighed some 1,400,000 kgs.
So, now we take all the ingredients and estimate a maximum temperature to which they could have been heated by 3,500 gallons of jet fuel. We will call this maximum temperature T. Since the calorific value of jet fuel is 44 MJ/kg. We know that 3,500 gallons = 31,000 kgs of jet fuel
will release 10,850 x 44,000,000 = 477,400,000,000 Joules of energy.
This is the total quan y of energy available to heat the ingredients to the temperature T. But what is the temperature T? To find out, we first have to calculate the amount of energy absorbed by each of the ingredients.
That is, we need to calculate the energy needed to raise:
39,857 kilograms of water vapor to the temperature T° C,
97,429 kilograms of carbon dioxide to the temperature T° C,
349,680 kilograms of nitrogen to the temperature T° C,
500,000 kilograms of steel to the temperature T° C,
1,400,000 kilograms of concrete to the temperature T° C.
To calculate the energy needed to heat the above quan ies, we need their specific heats. The specific heat of a substance is the amount of energy needed to raise one kilogram of the substance by one degree centigrade.
Substance Specific Heat [J/kg*C]
Nitrogen 1,038
Water Vapor 1,690
Carbon Dioxide 845
Lightweight Concrete 800
Steel 450
Subs uting these values into the above, we obtain:
39,857 x 1,690 x (T - 25) Joules are needed to heat the water vapor from 25° to T° C,
97,429 x 845 x (T - 25) Joules are needed to heat the carbon dioxide from 25° to T° C,
349,680 x 1,038 x (T - 25) Joules are needed to heat the nitrogen from 25° to T° C,
500,000 x 450 x (T - 25) Joules are needed to heat the steel from 25° to T° C,
1,400,000 x 800 x (T - 25) Joules are needed to heat the concrete from 25° to T° C.
The assumption that the specific heats are constant over the temperature range 25° - T° C, is a good approximation if T turns out to be relatively small (as it does). For larger values of T this assumption once again leads to a higher maximum temperature (as the specific heat for these substances increases with temperature). We have assumed the initial temperature of the surroundings to be 25° C. The quan y, (T - 25)° C, is the temperature rise.
So the amount of energy needed to raise one floor to the temperature T° C is
= (39,857 x 1,690 + 97,429 x 845 + 349,680 x 1,038 + 500,000 x 450 + 1,400,000 x 800) x (T - 25)
= (67,358,330 + 82,327,505 + 362,967,840 + 225,000,000 + 1,120,000,000) x (T - 25) Joules
= 1,857,653,675 x (T - 25) Joules.
Since the amount of energy available to heat this floor is 477,400,000,000 Joules, we have that
1,857,653,675 x (T - 25) = 477,400,000,000
1,857,653,675 x T - 46,441,341,875 = 477,400,000,000
Therefore T = (477,400,000,000 + 46,441,341,875)/1,857,653,675 = 282° C (540° F).
So, the jet fuel could (at the very most) have only added T - 25 = 282 - 25 = 257° C (495° F) to the temperature of the typical office fire that developed.
Remember, this figure is a huge over-estimate, as (among other things) it assumes that the steel and concrete had an unlimited amount of time to absorb the heat, whereas in reality, the jet fuel fire was all over in one or two minutes, and the energy not absorbed by the concrete and steel within this brief period (that is, almost all of it) would have been vented to the outside world.
"The time to consume the jet fuel can be reasonably computed. At the upper bound, if one assumes that all 10,000 gallons of fuel were evenly spread across a single building floor, it would form a pool that would be consumed by fire in less than 5 minutes"
Quote from the FEMA report into the collapse of WTC's One and Two (Chapter Two).
Here are statements from three eye-witnesses that provide evidence that the heating due to the jet fuel was indeed minimal.
Donovan Cowan was in an open elevator at the 78th floor sky-lobby (one of the impact floors of the South Tower) when the aircraft hit. He has been quoted as saying: "We went into the elevator. As soon as I hit the button, that's when there was a big boom. We both got knocked down. I remember feeling this intense heat. The doors were still open. The heat lasted for maybe 15 to 20 seconds I guess. Then it stopped."
Stanley Praimnath was on the 81st floor of the South Tower: "The plane impacts. I try to get up and then I realize that I'm covered up to my shoulder in debris. And when I'm digging through under all this rubble, I can see the bottom wing starting to burn, and that wing is wedged 20 feet in my office doorway."
Ling Young was in her 78th floor office: "Only in my area were people alive, and the people alive were from my office. I figured that out later because I sat around in there for 10 or 15 minutes. That's how I got so burned."
Neither Stanley Praimnath nor Donovan Cowan nor Ling Young were cooked by the jet fuel fire. All three survived.
Summarizing:
We have assumed that the entire 3,500 gallons of jet fuel was confined to just one floor of the World Trade Center, that the jet fuel burnt with perfect efficency, that no hot gases left this floor, that no heat escaped this floor by conduction and that the steel and concrete had an unlimited amount of time to absorb all the heat.
Then it is impossible that the jet fuel, by itself, raised the temperature of this floor more than 257° C (495° F).
Now this temperature is nowhere near high enough to even begin explaining the World Trade Center Tower collapse.
It is not even close to the first critical temperature of 600° C (1,100° F) where steel loses about half its strength and it is nowhere near the quotes of 1500° C that we constantly read about in our lying media.
"In the mid-1990s British Steel and the Building Research Establishment performed a series of six experiments at Cardington to investigate the behavior of steel frame buildings. These experiments were conducted in a simulated, eight-story building. Secondary steel beams were not protected. Despite the temperature of the steel beams reaching 800-900° C (1,500-1,700° F) in three of the tests (well above the traditionally assumed critical temperature of 600° C (1,100° F), no collapse was observed in any of the six experiments."
Quote from the FEMA report (Appendix A).
Recalling that the North Tower suffered no major structural damage from the intense office fire of February 23, 1975, we can conclude that the ensuing office fires of September 11, 2001, also did little extra damage to the towers.
Conclusion:
The jet fuel fires played almost no role in the collapse of the World Trade Center.
So, once again, you have been lied to by the media, are you surprised?
At least I don't use them to stalk other men. If you want my so bad?
be a man and PM me.
Pw3nd X 1,000.000
FEMA failed to mention that each passenger was forced to eat C-4 as their in-flight meal.
Check your PMs!
johnsmith don't hit on all the men save some for me!
I think if we keep this thread going, we can find the answers.
Here's a question. In the phone calls from the passengers before impact, they all said that the turist were wielding box cutters.
So, where are all these boxes?
http://www.tms.org/pubs/journals/JOM...agar-0112.html
Logo
The following article appears in the journal JOM, 53 (12) (2001), pp. 8-11.
Feature: Special Report
Why Did the World Trade Center Collapse? Science, Engineering, and Speculation
Thomas W. Eagar and Christopher Musso
OTHER ARTICLES IN THE WTC SERIES
Why Did the World Trade Center Collapse? Science, Engineering, and Speculation by Thomas Eagar and Christopher Musso
Better Materials Can Reduce the Threat from Terrorism by Toni G. Maréchaux
An Initial Microstructural Analysis of A36 Steel from WTC Building 7 by J.R. Barnett, R.R. Biederman, and R.D. Sisson, Jr.
News & Update
There have been numerous reports detailing the cause of the World Trade Center Tower collapse on September 11, 2001. Most have provided qualitative explanations; however, simple quan ative analyses show that some common conclusions are incorrect; for example, the steel could not melt in these flames and there was more structural damage than merely softening of the steel at elevated temperatures. Some guidelines for improvements in future structures are presented.
INTRODUCTION
The collapse of the World Trade Center (WTC) towers on September 11, 2001, was as sudden as it was dramatic; the complete destruction of such massive buildings shocked nearly everyone. Immediately afterward and even today, there is widespread speculation that the buildings were structurally deficient, that the steel columns melted, or that the fire suppression equipment failed to operate. In order to separate the fact from the fiction, we have attempted to quantify various details of the collapse.
The major events include the following:
* The airplane impact with damage to the columns.
* The ensuing fire with loss of steel strength and distortion (Figure 1).
* The collapse, which generally occurred inward without significant tipping (Figure 2).
Each will be discussed separately, but initially it is useful to review the overall design of the towers.
THE DESIGN
The towers were designed and built in the mid-1960s through the early 1970s. They represented a new approach to skyscrapers in that they were to be very lightweight and involved modular construction methods in order to accelerate the schedule and to reduce the costs.
To a structural engineer, a skyscraper is modeled as a large cantilever vertical column. Each tower was 64 m square, standing 411 m above street level and 21 m below grade. This produces a height-to-width ratio of 6.8. The total weight of the structure was roughly 500,000 t, but wind load, rather than the gravity load, dominated the design. The building is a huge sail that must resist a 225 km/h hurricane. It was designed to resist a wind load of 2 kPa—a total of lateral load of 5,000 t.
In order to make each tower capable of withstanding this wind load, the architects selected a lightweight “perimeter tube” design consisting of 244 exterior columns of 36 cm square steel box section on 100 cm centers (see Figure 3). This permitted windows more than one-half meter wide. Inside this outer tube there was a 27 m × 40 m core, which was designed to support the weight of the tower. It also housed the elevators, the stairwells, and the mechanical risers and utilities. Web joists 80 cm tall connected the core to the perimeter at each story. Concrete slabs were poured over these joists to form the floors. In essence, the building is an egg-crate construction that is about 95 percent air, explaining why the rubble after the collapse was only a few stories high.
Figure 1
Figure 1. Flames and debris exploded from the World Trade Center south tower immediately after the airplane’s impact. The black smoke indicates a fuel-rich fire (Getty Images).
Figure 2
Figure 2. As the heat of the fire intensified, the joints on the most severely burned floors gave way, causing the perimeter wall columns to bow outward and the floors above them to fall. The buildings collapsed within ten seconds, hitting bottom with an estimated speed of 200 km/h (Getty Images).
The egg-crate construction made a redundant structure (i.e., if one or two columns were lost, the loads would shift into adjacent columns and the building would remain standing). Prior to the World Trade Center with its lightweight perimeter tube design, most tall buildings contained huge columns on 5 m centers and contained massive amounts of masonry carrying some of the structural load. The WTC was primarily a lightweight steel structure; however, its 244 perimeter columns made it “one of the most redundant and one of the most resilient” skyscrapers.1
THE AIRLINE IMPACT
The early news reports noted how well the towers withstood the initial impact of the aircraft; however, when one recognizes that the buildings had more than 1,000 times the mass of the aircraft and had been designed to resist steady wind loads of 30 times the weight of the aircraft, this ability to withstand the initial impact is hardly surprising. Furthermore, since there was no significant wind on September 11, the outer perimeter columns were only stressed before the impact to around 1/3 of their 200 MPa design allowable.
The only individual metal component of the aircraft that is comparable in strength to the box perimeter columns of the WTC is the keel beam at the bottom of the aircraft fuselage. While the aircraft impact undoubtedly destroyed several columns in the WTC perimeter wall, the number of columns lost on the initial impact was not large and the loads were shifted to remaining columns in this highly redundant structure. Of equal or even greater significance during this initial impact was the explosion when 90,000 L gallons of jet fuel, comprising nearly 1/3 of the aircraft’s weight, ignited. The ensuing fire was clearly the principal cause of the collapse (Figure 4).
THE FIRE
The fire is the most misunderstood part of the WTC collapse. Even today, the media report (and many scientists believe) that the steel melted. It is argued that the jet fuel burns very hot, especially with so much fuel present. This is not true.
Part of the problem is that people (including engineers) often confuse temperature and heat. While they are related, they are not the same. Thermodynamically, the heat contained in a material is related to the temperature through the heat capacity and the density (or mass). Temperature is defined as an intensive property, meaning that it does not vary with the quan y of material, while the heat is an extensive property, which does vary with the amount of material. One way to distinguish the two is to note that if a second log is added to the fireplace, the temperature does not double; it stays roughly the same, but the size of the fire or the length of time the fire burns, or a combination of the two, doubles. Thus, the fact that there were 90,000 L of jet fuel on a few floors of the WTC does not mean that this was an unusually hot fire. The temperature of the fire at the WTC was not unusual, and it was most definitely not capable of melting steel.
In combustion science, there are three basic types of flames, namely, a jet burner, a pre-mixed flame, and a diffuse flame. A jet burner generally involves mixing the fuel and the oxidant in nearly stoichiometric proportions and igniting the mixture in a constant-volume chamber. Since the combustion products cannot expand in the constant-volume chamber, they exit the chamber as a very high velocity, fully combusted, jet. This is what occurs in a jet engine, and this is the flame type that generates the most intense heat.
In a pre-mixed flame, the same nearly stoichiometric mixture is ignited as it exits a nozzle, under constant pressure conditions. It does not attain the flame velocities of a jet burner. An oxyacetylene torch or a Bunsen burner is a pre-mixed flame.
In a diffuse flame, the fuel and the oxidant are not mixed before ignition, but flow together in an uncontrolled manner and combust when the fuel/oxidant ratios reach values within the flammable range. A fireplace flame is a diffuse flame burning in air, as was the WTC fire.
Diffuse flames generate the lowest heat intensities of the three flame types.
If the fuel and the oxidant start at ambient temperature, a maximum flame temperature can be defined. For carbon burning in pure oxygen, the maximum is 3,200°C; for hydrogen it is 2,750°C. Thus, for virtually any hydrocarbons, the maximum flame temperature, starting at ambient temperature and using pure oxygen, is approximately 3,000°C.
This maximum flame temperature is reduced by two-thirds if air is used rather than pure oxygen. The reason is that every molecule of oxygen releases the heat of formation of a molecule of carbon monoxide and a molecule of water. If pure oxygen is used, this heat only needs to heat two molecules (carbon monoxide and water), while with air, these two molecules must be heated plus four molecules of nitrogen. Thus, burning hydrocarbons in air produces only one-third the temperature increase as burning in pure oxygen because three times as many molecules must be heated when air is used. The maximum flame temperature increase for burning hydrocarbons (jet fuel) in air is, thus, about 1,000°C—hardly sufficient to melt steel at 1,500°C.
Figure 3
Figure 3. A cutaway view of WTC structure.
Figure 4--Web Link
Figure 4. A graphic illustration, from the USA Today newspaper web site, of the World Trade Center points of impact. Click on the image above to access the actual USA Today feature.
But it is very difficult to reach this maximum temperature with a diffuse flame. There is nothing to ensure that the fuel and air in a diffuse flame are mixed in the best ratio. Typically, diffuse flames are fuel rich, meaning that the excess fuel molecules, which are unburned, must also be heated. It is known that most diffuse fires are fuel rich because blowing on a campfire or using a blacksmith’s bellows increases the rate of combustion by adding more oxygen. This fuel-rich diffuse flame can drop the temperature by up to a factor of two again. This is why the temperatures in a residential fire are usually in the 500°C to 650°C range.2,3 It is known that the WTC fire was a fuel-rich, diffuse flame as evidenced by the copious black smoke. Soot is generated by incompletely burned fuel; hence, the WTC fire was fuel rich—hardly surprising with 90,000 L of jet fuel available. Factors such as flame volume and quan y of soot decrease the radiative heat loss in the fire, moving the temperature closer to the maximum of 1,000°C. However, it is highly unlikely that the steel at the WTC experienced temperatures above the 750–800°C range. All reports that the steel melted at 1,500°C are using imprecise terminology at best.
Some reports suggest that the aluminum from the aircraft ignited, creating very high temperatures. While it is possible to ignite aluminum under special conditions, such conditions are not commonly attained in a hydrocarbon-based diffuse flame. In addition, the flame would be white hot, like a giant sparkler. There was no evidence of such aluminum ignition, which would have been visible even through the dense soot.
It is known that structural steel begins to soften around 425°C and loses about half of its strength at 650°C.4 This is why steel is stress relieved in this temperature range. But even a 50% loss of strength is still insufficient, by itself, to explain the WTC collapse. It was noted above that the wind load controlled the design allowables. The WTC, on this low-wind day, was likely not stressed more than a third of the design allowable, which is roughly one-fifth of the yield strength of the steel. Even with its strength halved, the steel could still support two to three times the stresses imposed by a 650°C fire.
The additional problem was distortion of the steel in the fire. The temperature of the fire was not uniform everywhere, and the temperature on the outside of the box columns was clearly lower than on the side facing the fire. The temperature along the 18 m long joists was certainly not uniform. Given the thermal expansion of steel, a 150°C temperature difference from one location to another will produce yield-level residual stresses. This produced distortions in the slender structural steel, which resulted in buckling failures. Thus, the failure of the steel was due to two factors: loss of strength due to the temperature of the fire, and loss of structural integrity due to distortion of the steel from the non-uniform temperatures in the fire.
THE COLLAPSE
Nearly every large building has a redundant design that allows for loss of one primary structural member, such as a column. However, when multiple members fail, the shifting loads eventually overstress the adjacent members and the collapse occurs like a row of dominoes falling down.
The perimeter tube design of the WTC was highly redundant. It survived the loss of several exterior columns due to aircraft impact, but the ensuing fire led to other steel failures. Many structural engineers believe that the weak points—the limiting factors on design allowables—were the angle clips that held the floor joists between the columns on the perimeter wall and the core structure (see Figure 5). With a 700 Pa floor design allowable, each floor should have been able to support approximately 1,300 t beyond its own weight. The total weight of each tower was about 500,000 t.
As the joists on one or two of the most heavily burned floors gave way and the outer box columns began to bow outward, the floors above them also fell. The floor below (with its 1,300 t design capacity) could not support the roughly 45,000 t of ten floors (or more) above crashing down on these angle clips. This started the domino effect that caused the buildings to collapse within ten seconds, hitting bottom with an estimated speed of 200 km per hour. If it had been free fall, with no restraint, the collapse would have only taken eight seconds and would have impacted at 300 km/h.1 It has been suggested that it was fortunate that the WTC did not tip over onto other buildings surrounding the area. There are several points that should be made. First, the building is not solid; it is 95 percent air and, hence, can implode onto itself. Second, there is no lateral load, even the impact of a speeding aircraft, which is sufficient to move the center of gravity one hundred feet to the side such that it is not within the base footprint of the structure. Third, given the near free-fall collapse, there was insufficient time for portions to attain significant lateral velocity. To summarize all of these points, a 500,000 t structure has too much inertia to fall in any direction other than nearly straight down.
Figure 5
Figure 5. Unscaled schematic of WTC floor joints and attachment to columns.
WAS THE WTC DEFECTIVELY DESIGNED?
The World Trade Center was not defectively designed. No designer of the WTC anticipated, nor should have anticipated, a 90,000 L Molotov tail on one of the building floors. Skyscrapers are designed to support themselves for three hours in a fire even if the sprinkler system fails to operate. This time should be long enough to evacuate the occupants. The WTC towers lasted for one to two hours—less than the design life, but only because the fire fuel load was so large. No normal office fires would fill 4,000 square meters of floor space in the seconds in which the WTC fire developed. Usually, the fire would take up to an hour to spread so uniformly across the width and breadth of the building. This was a very large and rapidly progressing fire (very high heat but not unusually high temperature). Further information about the design of the WTC can be found on the World Wide Web.5–8
WHERE DO WE GO FROM HERE
The clean-up of the World Trade Center will take many months. After all, 1,000,000 t of rubble will require 20,000 to 30,000 truckloads to haul away the material. The asbestos fire insulation makes the task hazardous for those working nearby. Interestingly, the approximately 300,000 t of steel is fully recyclable and represents only one day’s production of the U.S. steel industry. Separation of the stone and concrete is a common matter for modern steel shredders. The land-filling of 700,000 t of concrete and stone rubble is more problematic. However, the volume is equivalent to six football fields, 6–9 m deep, so it is manageable.
There will undoubtedly be a number of changes in the building codes as a result of the WTC catastrophe. For example, emergency communication systems need to be upgraded to speed up the notice for evacuation and the safest paths of egress. Emergency illumination systems, separate from the normal building lighting, are already on the drawing boards as a result of lessons learned from the WTC bombing in 1993. There will certainly be better fire protection of structural members. Protection from smoke inhalation, energy-absorbing materials, and redundant means of egress will all be considered.
A basic engineering assessment of the design of the World Trade Center dispels many of the myths about its collapse. First, the perimeter tube design of the towers protected them from failing upon impact. The outer columns were engineered to stiffen the towers in heavy wind, and they protected the inner core, which held the gravity load. Removal of some of the outer columns alone could not bring the building down. Furthermore, because of the stiffness of the perimeter design, it was impossible for the aircraft impact to topple the building.
However, the building was not able to withstand the intense heat of the jet fuel fire. While it was impossible for the fuel-rich, diffuse-flame fire to burn at a temperature high enough to melt the steel, its quick ignition and intense heat caused the steel to lose at least half its strength and to deform, causing buckling or crippling. This weakening and deformation caused a few floors to fall, while the weight of the stories above them crushed the floors below, initiating a domino collapse.
It would be impractical to design buildings to withstand the fuel load induced by a burning commercial airliner. Instead of saving the building, engineers and officials should focus on saving the lives of those inside by designing better safety and evacuation systems.
As scientists and engineers, we must not suc b to speculative thinking when a tragedy such as this occurs. Quan ative reasoning can help sort fact from fiction, and can help us learn from this unfortunate disaster. As Lord Kelvin said,
“I often say . . . that when you can measure what you are speaking about, and express it in numbers, you know something about it; but when you cannot measure it, when you cannot express it in numbers, your knowledge is of a meager and unsatisfactory kind; it may be the beginning of knowledge, but you have scarcely, in your thoughts, advanced to the stage of science, whatever the matter may be.”
We will move forward from the WTC tragedy and we will engineer better and safer buildings in the future based, in part, on the lessons learned at the WTC. The reason the WTC collapse stirs our emotions so deeply is because it was an intentional attack on innocent people. It is easier to accept natural or unintentional tragedies; it is the intentional loss of life that makes us fear that some people have lost their humanity.
References
1. Presentation on WTC Collapse, Civil Engineering Department, MIT, Cambridge, MA (October 3, 2001).
2. D. Drysdale, An Introduction to Fire Dynamics (New York: Wiley Interscience, 1985), pp. 134–140.
3. A.E. Cote, ed., Fire Protection Handbook 17th Edition (Quincy, MA: National Fire Protection Association, 1992), pp. 10–67.
4. A.E. Cote, ed., Fire Protection Handbook 17th Edition (Quincy, MA: National Fire Protection Association, 1992), pp. 6-62 to 6-70.
5. Steven Ashley, “When the Twin Towers Fell,” Scientific American Online (October 9, 2001); www.sciam.com/explorations/2001/100901wtc/
6. Zdenek P. Bazant and Yong Zhou, “Why Did the World Trade Center Collapse?—Simple Analysis,” J. Engineering Mechanics ASCE, (September 28, 2001), also www.tam.uiuc.edu/news/200109wtc/
7. Timothy Wilkinson, “World Trade Centre–New York—Some Engineering Aspects” (October 25, 2001), Univ. Sydney, Department of Civil Engineering; www.civil.usyd.edu.au/wtc.htm.
8. G. Charles Clifton, “Collapse of the World Trade Centers,” CAD Headlines, tenlinks.com (October 8, 2001); www.tenlinks.com/NEWS/special/wtc/clifton/p1.htm.
Thomas W. Eagar, the Thomas Lord Professor of Materials Engineering and Engineering Systems, and Christopher Musso, graduate research student, are at the Massachusetts Ins ute of Technology.
For more information, contact T.W. Eagar, MIT, 77 Massachusetts Avenue, Room 4-136, Cambridge, Massachusetts 02139-4301; (617) 253-3229; fax (617) 252-1773; e-mail [email protected].
Not me. I'm here to watch johnsmith get ass raped by mouse
There are currently 1 users browsing this thread. (0 members and 1 guests)
Posting Permissions
