The Empire State Building and the Twin Towers were all hit by aircrafts, but only one of them remain standing today. Was the collapse of the World Trade Center preventable, due to precautionary measures that designers could have taken to make the buildings more structurally sound? Or were the demise of the buildings inevitable, not being able to withstand the impact of the airplanes no matter how they were structured? Extensive research and studies have been done on the collapse of the World Trade Center and many key failures of the structures have been found. Still the question remains to why the Empire State Building remains standing with hardly any structural damage while the Twin Towers were totally destroyed. Many factors contributed to the fate of each of the buildings including the force of impact, amount and type of jet fuel, and the support structure of each building.
The Empire State Building and the Twin Towers were built and designed very differently. While the Twin Towers facing materials consisted of aluminum and steel, that of the Empire State Building consists of limestone, granite and brick. The distinctions in facing material would later prove to have an impact on durability. Another difference between the structures is the way in which the support beams were situated within the buildings. The Twin Towers were built to be economical and modern, so the designers chose to situate most of the support beams on the outermost perimeter of the buildings as well as significant support beams in the center. In this design, the floors were critical in supporting the building. This design allowed for large open spaces, and prime real estate. The Empire State Building has a more conventional structure with its beams distributed evenly across the area of the building to support its weight.
Other distinctions between the buildings include methods of fireproofing the steel support beams. The Empire State Building’s structural steel beams are encased within eight inches of limestone and slabs of concrete. This prevents excessive heating from fires. Instead, the fireproofing method used in the World Trade Center was a lightweight spray on coating of insulation.
Other fireproofing methods that the buildings featured were fireproof staircases and walls. The Empire State Building, being very compartmental within each floor was divided by thick masonry walls. This would prevent the spread of a fire and contain it to a small section of the building. The World Trade Center, having vast open spaces within each floor opted to build walls consisting of dry wall, a lightweight and fireproof material. Since there were not many sections to each floor as compared to the Empire State Building, fires were susceptible to spread more easily.
The skyscrapers’ different features would prove to work in different ways when put to the test. However, it is important to include that the pressures that the buildings’ suffered were of different measures. The Empire State Building was hit by a B-25 Bomber weighing approximately 21,500 lbs. flying at a speed of about 200 miles per hour. The Twin Towers were hit by a Boeing 767 weighing nearly fifteen times more than the B-25, and traveling almost twice as fast. In turn, the impact to the Twin Towers was 60-100 times greater. When the B-25 hit the Empire State Building, it was nearly out of fuel. This type of aircraft burned a high-octane version of gasoline, not as combustible as the fuel used in commercial jetliners. A Boeing 767 has thirty five times more fuel capacity than that of the bomber. At the time of impact it was half filled to capacity. Boeing 767s burn a type of kerosene that tends to burn more intense fires.
When the planes hit all three buildings, each was ignited in flames around the point of impact. A key factor is that since the facing materials of the Empire State Building was of stronger material and hit by a smaller plane, the plane did not penetrate very far into the building, sparing important structural support. Much of the impact was absorbed by the exterior wall. The fire in the Empire State building was contained in a small section of the building and did not burn as intensely. Since it the interior of the building was separated by heavy masonry walls, it aided in the spread of fire. Another factor was the heavily fireproofed stairways.
On the other hand, the impact of the Twin Towers caused severing of about two thirds of support columns. The inner core was severely damaged and aviation fuel saturated it. Thousands of gallons of jet fuel ignited, spreading to other combustibles including the dry wall. At the point of impact, the spray-on fireproof insulation was simply blown off by the force. This left the building, floor trusses and support beams vulnerable to the raging fires, spreading it through the vast spaces and across different floors. The impact also cut off the water supply to sprinklers used to put fires out. The dry wall used to protect the stairways was either on fire or blown off, making evacuation for people above the impact difficult or impossible. One particular positive aspect of the weak dry wall was that a man stuck in an elevator escaped by prying himself out and breaking the wall with his cleaning tool. A difference between the impacts at the World Trade Center and the Empire State Building is the size of the holes that the aircrafts caused. Since the hole in the exterior was much smaller, less oxygen could come in the building to feed the fire. It was also a very foggy day so some speculate that the damp air could have reduced the growth of the fire.
Ultimately the support system of the World Trade Center failed due to the fire. The intense temperatures weakened the unprotected steel and caused the failure of the floor trusses, a vital part its support system. Evidence revealed that most of the interior core stayed in tact while the outer walls supporting the floor trusses were significantly damaged. The failure of one floor began a domino effect to each floor below it causing the structures to collapse. This would make the World Trade Center the first building in the world to collapse due to a fire.
Major differences between the buildings that ultimately distinguished the outcome were fireproofing the support structures and how these support structures were situated across the building. These are two factors that may have prevented the collapse of the World Trade Center, had the designers and engineers calculated this. In retrospect, it is important to realize that when many people’s lives are at stake it is essential that economical and business decisions (such as vast floors for real estate; cheaper building materials) should never supersede safety.
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