what was the cause of the twin towers to collapse?
139651
The twin towers did not collapsed because the stee. melted because the fire wasn't hot enough to melt steel. They collapsed becaue the steel columns that held up the buildings buckled. When steel is exposed to high temperatures for an extended period of time, its elastic modulus (or stiffness) decreases at a very fast rate. It is believed that the protective fire coating was blown off the steel at the initial impact, causing the steel to be exposed to higher temperatures for a longer period of time. \n\nThe steel columns in the building were connected by trusses that were supposed to support their own weight, the weight of the concrete floors, and the weight of all the office equipment. The high temperature damaged the material and one floor collapsed onto another. Once the first floor collapsed, all the floor above it lost their support and fell.
672221
i want a pilot to answer me.i am not a susspecting person.i have been flying single prop planes as a hobby for the last 15 years.i cant immagin how can a man who just compleated his primery flying training flies a jumbo jet,aim it to a building and hit it so accurately\nhow both the hita came in places that led to the collapse of both the towers in a short time\ni really wonder,could the engeneers who designed the tower  know that such critical points exist in their towers.i dont belive in consperacy theoty,but honestly i am unable to immagine this.i am totally against terrorisim,totally against killing even an insect.i cant immagin the savage act of killing 3000 innocent people at all.
687913
It tilts because its foundation was built on unstable soil. Construction of the tower began in 1173 and ended between 1360 and 1370. The ground beneath the tower first started to sink after the first three stories were built. \n\nIn 1990, the tower was closed for repairs. At that time, its lean had been increasing an average of 1/20 of an inch (1.3 millimeters) per year. Engineers hope to stabilize the tower's foundation and straighten it slightly to prevent it from eventually collapsing and falling over.\n\nCheck it out for yourself at :\n\nhttp://www.spartechsoftware.com/reeko/ScienceTips/LeaningPisa.htm
1027720
I'm sorry, but I have to disagree with you.  The burning fuel INSIDE the WTC generated enough oven-type temperature to weaken the steel of the building (not high enough to melt it, granted).  The rest was gravity and the inherent design of the buildings (which, BTW, were designed to pancake exactly as they did, albeit far in the future when their natural demolition time came).  The fact that temperatures over 1500 degrees were registering in the pile afterward is the same closed-oven concept- the pile kept the heat in much longer than if it was exposed to the air.
271060
Looks like an planned demolition if you ask me. \nNo building falls like that naturally, also, the center falls first so that the walls will fall inward.  \nNone of the videos show the first few seconds (you know, where you hear the blast of dynamite in the demolition).\n\nNo motive, I can think of, but very suspicious.
715923
1.  Natural disaster...for obvious reasons\n2.  Growth and new industry\n3.  Lack of jobs and mass movement of people to better areas.
877293
I appreciate this question and I will rightly scold anyone who gives it a thumbs down. After many hours of careful reading and studying, I have concluded that to deny the unanswered questions of 9/11 makes you no different than those who have denied the Holocaust. \n\nThere are many anomalies surrounding 9/11 and the truth of the matter is that depsite the so-called investigation that took place, no one has been held accountable for the failures that occurred that day. Not one person has been fired for the deaths of 3000 people which could have been prevented. \n\nThink about it. If this were a corporation, heads would have rolled by now. People would have been brought up on charges for criminal negligence. WTC 7 collpased from fire alone. No plane hit it. This is the first time in the history of architecture that you have a skyscraper (a 47-story one at that!) falling from fire alone. \n\nIf you were a engineer, wouldn't you want to know the truth why a skyscraper collapsed from fire alone? \n\nIf you were a fire-marshall, wouldn't you want to know the truth? \n\nIf you were an underwriter for an insurance company, wouldn't you want to know the truth? \n\nThe unexplained circumstances of the WTC 7 collapse was outright ignored, as were the other anomalies surrounding the collapse of the Twin Towers and the attack on the Pentagon. It was as if they had the conclusion set in stone before they gathered all the facts from that fateful day. \n\nIf you read David Ray Griffin's book, "The New Pearl Harbor," it spells out in pure science, sequential logic,  and deductive reasoning the argument that 9/11 was preventable and furthermore, that the root cause of the attacks were essentially covered-up. \n\nThe 9/11 Commission has substantially proven themselves to be more interested in ignoring the more crucial questions. They didn't want to place any blame, except on the alleged hijackers. This was their documented goal from the beginning. They protected those in power who failed to protect us that day. It is undeniable and irrefutable. Standard Operating Procedure for such emergencies were not followed by NORAD, the FBI, the NSA, the FAA, or the CIA. \n\nMore and more people are joining the truth movement. These include engineers, firefighters, police officers, architects, physicists, and other scholars. The victims of 9/11 have also embraced them. \n\nBelow are some links to some films that will enlighten you on the subject. \n\nWarning: if you have your mind made up that the government holds no responsibility whatsoever and if you unconditionally defend those in power despite overwhelming evidence that proves otherwise, don't bother clicking below. Your patriotism is the cheapest and weakest one possible. It is the type that never questions authority, a direct opposition to what the founding fathers urged all of us to do when they drafted the Constitution. \n\nLaugh, scoff, or give thumbs down if you are weak in this regard. \n\nIf you are strong, truly patriotic, and willing to face the truth no matter how painful, and if you have some time to spare, click the links below. Or at least add to favorites for future viewing. \n\nIf you are convinced ny viewing this evidence that suggests 9/11 needs further investigation, then urge your Congressional Reps and Senators to re-open 9/11. There are both Democrats and Republicans who have already joined in this effort. \n\nPeace and God Bless.
107663
The attacks started with the hijacking of four commercial airliners. With jet fuel capacities of nearly 24,000 U.S. gallons (91,000 litres) per aircraft [3], the aircraft were used as flying incendiary bombs. American Airlines Flight 11 crashed into the north side of the north tower of the World Trade Center (WTC) at 8:46:40 AM local time (12:46:40 UTC). At 9:03:11 AM local time (13:03:11 UTC), United Airlines Flight 175 crashed into the south tower, an event covered live by television broadcasters that had their cameras trained on the North Tower. American Airlines Flight 77 crashed into the Pentagon at 9:37:46 AM local time (13:37:46 UTC). The fourth hijacked plane, United Airlines Flight 93, crashed in a field near Shanksville and Stonycreek Township in Somerset County, Pennsylvania at 10:03:11 AM local time (14:03:11 UTC), with parts and debris found up to eight miles away. The crash in Pennsylvania is believed to have resulted from the hijackers either deliberately crashing the aircraft or losing control of it as they fought with the passengers. No one survived in any of the hijacked aircraft.\n\nThe fatalities were in the thousands: 265 on the four planes; 2,595, including 343 New York City firefighters, 23 NYPD police officers, and 37 Port Authority police officers in the towers and on the ground; and 125 civilians and military personnel at the Pentagon. At least 2,986 people were killed in total. In addition to the 110-floor Twin Towers of the World Trade Center itself, five other buildings at the WTC site, including WTC building 7, and four subway stations were destroyed or badly damaged. In total, on Manhattan Island, 25 buildings were damaged. Communications equipment such as broadcast radio, television and two way radio antenna towers were damaged beyond repair. In Arlington County, a portion of the Pentagon was severely damaged by fire and one section of the building collapsed.
595073
Some people think 9/11 was a purposeful choice when choosing when to attact the World Trade Center because across America we use 9-1-1 when there is an emergency.  My own opinion is that it was a coincidence.  I guess no one will ever know for sure.
90194
Are you aware that a group of islamic terrorists exploded a truck under the World Trade Center towers in 1993 while President Clinton was President?  How was THAT something that the current President Bush initiated?  The root cause of the terror that is ravagin the middle east was the 1920s in Egypt.  You should inform yourself of the issues.
71976
I believe that Bin Laden was responsible for the attacks, yes. Unfortunately, people have (a reasonable) tendency to question anything 'official' or 'government-related', so they turn to explanations involving aliens, missiles, CIA, Freemasons, and the Mossad.\n\nHere's the thing : Why the hell would a country send four airplanes to smash into their own buildings, and to kill their own people? That's just stupid.\nAnd if the government is lying, then what happened to the victims? The planes? Saying "it's a conspiracy" tells the families of the dead, and the survivors, that they died for nothing.
1139459
Yes, the jet fuel by itself would not be enough to weaken the steel.  However the burning jet fuel ingnited all of the combustible material inside of the tower, notably all of the paper.  \n\nAs an iron worker, you know that the strength of steel comes primarily from the carbon inclusions within the iron.  When there is enough heat to dissolve these inclusions, the steel becomes only as strong as iron.  When that happens, the steel/iron doesn't melt--it bends under whatever load it is carrying.  This puts added stress on the surrounding structural members, eventually leading to collapse.   \n\nAlso, as an iron worker, you know that in many modern buildings the steel is covered with some type of thermal blanketing material, to keep the heat from a fire away from the steel.  Why go to so much trouble if, as you claim, a fire cannot bring down a steel building?  \n\nAs to the black smoke:  This signifies that all of the oxygen inside the WTC was being consumed in the usual burning process, and that other combustible materials were also being consumed without oxygen.  This points to a very large fire indeed.  \n\nAs to the firefighter on the 78th floor.  The South Tower was struck askew, and there was one corner that was not on fire.  I believe that one of the stairwells was still accessible, and that people above the impact level were able to escape thru this stairwell.  So this "evidence" has no merit.  \n\nI have a graduate degree in civil engineering, and have studied the failure of many buidlings, including the failure of steel buildings during fires.  Anyone who doubts that the WTC failed due to the aircraft impacts and resulting fire should:\n1) Read up on the collapse of buildings\n2) Provide an alternate explanation how those other buildings fell\n3) Face up to whatever emotional need they have to see a conspiracy.  I say this because I have relations who "go to conspiracy"  on everything.\n\n(And you can see in the questioner's subsequent remarks that he falls into the same category as my relations.  They bring up some conspiracy theory, and then berate those who don't believe it.  It really is a strange psychopathology.  Writing this post is good exercise for me to get ready to listen to and counter the rantings of their shriveled minds.)\n\nAgain, for those of you quoting resource sites, remember that they all contain a central erroneous argument--that steel has to melt before buildings fail.
308864
It depends on what you mean by collapse.\n\nDo you mean collapse into a black hole because of its mass or something?  Never.\n\nDo you mean our social, political, and economic systems?  When there is something that undermines the current structure so much that it makes the current structure unsustainable.  \nIn order for it to collapse soon, the price of gas shooting above $5/gallon would be a good start.  \nA nuke exploding in New York or LA would shake the US and the western world to its core, and we'd probably be in martial law and a bigger conflict that WWII.  \nA nuclear war.\nA religion or belief system that completely changes our economics or societal structure.\nA catastrophic change in our environment.
1180502
Guys, this retard thinks that Americans downed the twin towers using DYNAMITE. You cannot take anything he/she says seriously.\n\nI am NOT making this up ... see it here:\n\nhttp://answers.yahoo.com/question/?qid=20060629152526AARdiv2
657905
well i can tell u that the way the wtc was build was that every floor would stay up if anything were to happen to anything else.  and also, after the 1993 bombing with the truck, every floor got fixed so that if there was a fire on one floor, it would not escalate to many other floors.  i know about architecture, and the wtc was one of the strongest build buildings in nyc, if not all the world.  for a plane to hit it and all of it just collapse is impossible.  you can find out info and watch a video by watching Loose Change.  You can google or yahoo search it.  its free to watch and it gives you insight on things the government is hiding from us.  oh yea, in short, the reason the buildings fell so fast was that there were bombs planted on many floors in the buildings, trust me, I was there when it happened.
555431
Fires cause most of the destruction after the Earthquake. So surviving buildings would probably suffer the most in the hours after the Earthquake.
605448
Because they can't jump from the Twin Towers anymore...