World Trade Center Construction

In order to understand how the World Trade Center collapsed we need to understand their structure. To save money and maximize floor space the two towers were built with a tube structure instead of the more common lattice structure. The tube structure was economically favorable because the structure required half the amount of steel compared to the lattice structure and created more floor space (one acre for every floor) without compromising the strength of the structure. This design also makes the building more resistant to strong winds. This was particularly important in New York City where winds created more lateral load on the building than seismic activity. The World Trade Center was built to sustain thirteen thousand tons of wind. The
wind caused the building to shake which needed to be minimized to make the building usable. Architects also creatively used the shaking for heating. It could also withstand a plane crash from a Boeing 707 flying low at a relatively slow speed. As the name suggests a building built with a tube structure is shaped like a tube with an inner core and and an outer wall. The load bearing is on the outer wall in this structure which is made of a mesh prefabricated steel panels. The core contained elevator shafts, stairs, and the sprinkler system and hoses. The building was too tall for the current elevator system to be economically favorable, so architects needed to get creative about how to solve this problem. They figured out they could treat the tower as three separate forty story buildings stacked on top of each other. Each section had an elevator system and to get to the top you needed to switch elevators at what was referred to as sky lobbies. Another problem faced by the construction team was the Hudson River that ran along the World Trade Center. They would need to build down to the bedrock and whenever they dug the hole would fill with water. To solve this dilemma the construction crew dug a hole twenty two feet by three feet and filled it with slurry. The denser slurry sunk to the bottom and plugged the holes. Then cement was poured into the hole with the slurry and again being denser the cement sunk to the bottom. This process was repeated one hundred and fifty two times to create a waterproof “bathtub” of concrete panels around the World Trade Center complex.
The first terrorist attack on the World Trade Center was on February 26 in 1993.
A little after noon, during the lunch break, a van containing a bomb parked in the parking garage under the North tower. The van was located under the perimeter wall. The attack was supposed to destroy that portion of the base of the North tower and cause it to fall onto the South tower destroying them both. If successful the attack would have killed as estimated forty thousand people. Thankfully the terrorists underestimated the strength of the building and the bomb only destroyed part of the parking garage killing six and injuring one thousand and forty two people. The core acted as a chimney and sucked smoke up into the higher
floors.
The North tower was the first to be attacked at 8:46 a.m.
on September 11, 2001. A Boeing 767 filled with fuel for a transatlantic flight flying at four hundred miles per hour hit the tower straight on. The increased speed and mass created more kinetic energy than the building was originally designed to handle. Despite not being built to withstand such force the building remained standing for an hour and forty two minutes until finally collapsing at 10:28 a.m. when the core failed. The plane hit floors ninety three through ninety eight demolishing the core and cutting off any escape route for the people on the floors above the crash. The fuel from the plane fed fires that heated the steel frame of the building. While the fire was nowhere near hot enough to melt steel it did cause the steel to soften and be distorted. This caused the floor to sag and eventually give way and the upper floors slammed into the lower ones causing the tower to collapse straight
down.
The next attack on the South tower occurred at 9:03 a.m. and remained standing for fifty six minutes until 9:59 a.m. The second plane, another Boeing 767, hit the side of the South tower at an angle flying at five hundred miles per hour. The second plane hit lower the South tower than the first plane did on the North tower. Floors seventy eight through ninety three were hit and included a sky lobby on floor seventy eight. In the South tower the core was mostly destroyed but there was one stairway still intact but still only four people from above the floors of impact and fourteen from the floors of impact managed to escape. Since the plane hit the side of the South tower the exterior wall was no longer able to support the upper part of the building and the tower tipped.
The attacks on the World Trade Center successfully destroyed both towers and all other buildings in the World Trade Center complex killing almost three thousand people. The method and probability of the towers collapsing has been hotly debated but by studying the physics of the behind each collapse it is possible to show that the collapses could in fact be caused by the plane crashes. In the North tower the core failed after fire heated and subsequently softened the steel causing the floors to collapse on one another. The South tower was hit on the side at an angle and caused the building to fall. There are also debates as to if the structure of the buildings accelerated their collapse.