How many people normally worked in the twin towers

After the steel structure was in place, the crew attached the outer "skin" to the perimeter -- anodized aluminum , pre-cut into large panels. This continued, section by section, as the towers climbed higher and higher. The crew lifted the steel sections into place using four large cranes four per tower , mounted to long steel structures fitted inside the tube structure. The cranes could actually lift themselves higher, using heavy hydraulics, as the floors were finished. While the crew kept building upward, other workers started to flesh out the floors below, down to installing blinds and painting the walls.

A number of businesses actually moved into their new WTC offices years before the towers officially opened. In addition to utilizing an unconventional structural design, the Twin Towers were also an aesthetic departure from the older buildings in New York. Most of the city's skyscrapers have a "wedding cake" shape, with larger sections on the bottom tapering off into smaller sections on top.

This was partly due to the prevailing architectural style in the first half of the 20th century, but it was also a result of New York's zoning restrictions. In order to ensure that walls of skyscrapers didn't block all light from reaching the street, the city passed a resolution in dictating that all skyscrapers would have an overall pyramid shape. A new resolution in shifted the focus, regulating overall height rather than shape.

The new restrictions dictated a maximum number of floors, based on the district of the building and the total area of the plot. The Port Authority was allowed to build such tall towers because they had a huge plot of land with a large, open-area plaza.

From its conception all the way through its completion, the WTC project was wildly unpopular with many New Yorkers. The grand opening was certainly a celebratory day for the Port Authority, design team and construction crew, but the WTC complex had a long road in earning the city's acceptance.

Over the next decade, it did just that, and then went on to win over the rest of the country. Boosted by prominent appearances in several movies, such as the "King Kong" remake , Woody Allen's "Manhattan," and the "Superman" movies, the Twin Towers gained widespread recognition as a piece of New York. The towers' fame was also fueled by several notable stunts.

In the years after the WTC's completion, skydivers successfully parachuted from the top of the towers, climbers scaled the building and a French acrobat walked back and forth between the buildings on a tightrope.

In only a few years, the distinctive image of the Twin Towers was a staple on New York postcards, T-shirts and advertisements. The buildings had evolved into a proud symbol of the city, securing their place as an American icon. The towers also won New Yorkers over by giving them a new view of their city. Visitors could climb to the top of WTC 2, the South tower, for a breathtaking view of the skyline from the outdoor observation deck. On a clear day, it was possible to see more than 40 miles 64 km in every direction.

Visitors with a larger budget could enjoy the view from a more elegant setting, the "Windows of the World" restaurant at the top of WTC 1, the North tower. When the observation deck and restaurant opened, even staunch WTC critics showed up to check out the view. Most New Yorkers and most Americans were familiar with the towers mainly from the outside, but the thousands of people who worked in the towers had a very different perspective -- they appreciated not only the monumental size of the buildings, but also the dizzying variety of activity going on inside.

The WTC supported approximately businesses with a collective 50, employees. This included offices for banks , law firms, brokerage houses, television stations, publishers, charitable organizations and airlines , among many other things. Additionally, the towers included nine chapels of various faiths. On a typical business day, as many as , visitors from all over the world passed through the complex. With the huge range of activity going on, the towers were almost a city unto themselves.

On the morning of September 11, , immediately after terrorists struck the Twin Towers, it looked as though the buildings might remain standing.

While the plane crashes had taken huge chunks out of both towers, the overall structure seemed to be intact, at least to the observers on the ground and the millions of Americans watching the catastrophe on television.

Based on video, eyewitness accounts and debris analysis, the team formed a likely hypothesis of what happened, which they made public in April The following is a summary of the FEMA evaluation teams' findings, which were largely along the same lines as the numerous media evaluations in the weeks after the attacks.

Amazingly, the initial damage to the support structure was not enough to topple the building. The report, as well as a number of prominent engineers, have claimed that the majority of skyscrapers on the planet would have collapsed within seconds of such a collision.

But the collisions did divert the entire vertical load of the buildings to the remaining columns, significantly increasing the structure's stress level. Without any additional loads on the support structure, the report claims, the towers could have stayed up indefinitely.

But the extreme heat of the fire, which might have been in excess of 2, degrees Fahrenheit 1, C at some points, exerted tremendous stress on the perimeter columns, the core columns, and the floor trusses in between them. The main factor was really the size of the fire -- the total area it covered.

Building fires typically start with a small fire -- say a burning cigarette on a stack of papers -- which gradually spreads through a larger area. In that situation, the fire is most intense where it has the most fuel stuff that can burn , and it significantly weakens the support structure only at those most intense points.

If a fire starts in the northwest corner of a skyscraper floor, by the time the fire reaches the southeast corner, the starting fire at the starting point will have burned through most of the fuel, and the fire will not be as intense. The result is the fire doesn't put maximum strain on the total support structure all at once. It strains different parts of the support structure in turn, over time. In the case of the World Trade Center, the burning jet fuel spread the fire across several floors in a matter of seconds.

This massive fire put exceptional strain on the structure at nearly all points on those floors. Additionally, the report suggests that the force of the collision removed much of the fire-resistant material sprayed on the steel, making the structure more susceptible to heat damage.

The heat expanded, twisted and buckled the steel support structure, gradually reducing the building's stability. Any number of things could have happened during this period.

For example, connections between vertical columns and floor trusses probably broke, dropping sections of floor on lower levels and breaking connections between the core and the perimeter wall, possibly causing columns along the perimeter to buckle outward. Every broken connection or buckled length of steel added to the force acting on connected steel segments, until the entire structure was weakened to the point that it couldn't hold the upper section of the building.

When this happened, the top part of each building collapsed onto the lower part of the building. Essentially, this was like dropping a story building on top of another building. Before the crash, this upper structure exerted a constant downward force -- its weight -- on the superstructure below. Obviously, the lower superstructure was strong enough to support this weight.

But when the columns collapsed, the upper part of the building started moving -- the downward force of gravity accelerated it. The momentum of an object -- the quantity of its motion -- is equal to its mass multiplied by its velocity. So when you increase the velocity of an object with a set mass, you increase its momentum. This increases the total force that the object can exert on another object. To understand how this works, think of a hammer. Resting in your hand, it doesn't hurt you at all.

But if you drop it on your foot, it can do some damage. Michael Lomonaco, the chef at Windows on the World, would have normally been at work by , but he stopped to get new eyeglasses in the shopping concourse under the World Trade Center; he survived, while 72 of his co-workers were killed. I could see the time was I remember thinking, Gee, I have plenty of time to get back downtown before the event starts. I was talking to one of my colleagues in London when I heard the plane go over.

She would later rejoin the firm after the attacks. The comedian Seth MacFarlane also had a ticket returning home after performing a gig in Rhode Island, but the travel agent had mistyped the time on his itinerary, and he showed up just a few minutes too late to catch the plane.

Later, at the Pentagon, the third hijacked plane hit a wedge of the building that had been upgraded to the highest security standards—meaning it was both well protected and largely vacant. In both the Pentagon and New York, fate played a key role in the escapes. The person that sat to my right, the person that sat to my left apparently went out the door and took a right, and they went into the E-Ring, where they apparently perished. A decision to go in one direction or another was very important.

Percentage of those living within a one mile radius from the Twin Towers suffering from Post Traumatic Stress Disorder: Percentage of this year's 14 Pulitzer prizes won by the New York Times: Number of countries in which the star-studded telethon, 'America's Tribute to Heroes', was shown on 21 September including Afghanistan : Number of months the release of Collateral Damage was delayed on account of Warner Bros.

Number of English language books published on the subject of 11 September: Percentage of fund used for cash assistance and services such as grief counseling for families of victims and survivors: Quantity, in pounds, of food and supplies supplied by 11 September Fund at Ground Zero: 4.

Number of hot meals served to rescue workers by 11 September Fund: , Number of articles written over the last year in the UK press mentioning 11 September: 27, Many people delayed their evacuation to seek out extra information, whether by speaking with colleagues, making phone calls, sending emails or searching online for news updates. Many lives were saved by the brave leadership of people who took control of the situation, urging others to evacuate and helping those who needed assistance.

My PhD research revealed these were typically people who were used to taking charge: high-level managers, fire wardens and people with military experience.

Evacuees faced a dangerous and claustrophobic journey down to ground level. Some people recalled having to leave the stairwell either because of overcrowding, being told to do so by fire or building officials, or because they needed a rest.

Other problems included poor lighting, not knowing which direction to go, and finding the route unavoidably blocked by people with permanent or temporary disabilities.

Not everyone was so lucky.