World Trade Center: Reasons Behind the Failure of Building After 9/11 Attack

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The World Trade Center (WTC) complex was composed of seven buildings (WTC-1 to WTC-7) on a site toward the southwest tip of Manhattan Island. The two buildings, WTC-1 and WTC-2 built in the early 1970s, were 110 stories high, dwarfing the other skyscrapers in Lower Manhattan. Both buildings were the main focus of the WTC complex and were taller than any other building in the world at that time.

The WTC-1 and WTC-2 were unique in many ways and their construction resulted in a tremendous increase in open-plan commercial office space. Key structural systems of the WTC-1 and WTC-2 included the exterior framed-tube system, the core structure which supported gravity loads, the composite floor truss systems supported by the exterior and core columns, and the hat truss that distributed loads between the core and exterior columns at the top of the buildings.

On 11 September 2001, the north building (WTC-1) was hit by a Boeing 767, conveying 10,000 US gallons of fuel, flying at 470 mph on the north face between the 94^th and 98^th floors. After fifteen minutes, the south building (WTC-2) was hit by another Boeing 767 with 10,000 US gallons of fuel, flying at 586 mph, on the south face towards the southeast corner between the 78^th and 84^th floors.

Both WTC-1 and WTC-2 were designed to resist a two-hour fire for an open-web floor framework and three hours for the columns sections. However, the impact of the airplane was so huge that the fire protection system didn't work correctly.

The strength of steel progressively reduced as the flames seethed, and collapse became inevitable. WTC-2 collapsed initially, 56 minutes after the impact of the airplane while WTC-1 collapsed after 102 minutes.

The connection for the floor trusses, done by cleats and bolts, failed due to the loss of strength. This led to an unrestrained behavior of columns which ultimately resulted in their collapse.

A significant number of encompassing structures endured considerable damage; however, they didn't fail. The twin buildings too stood long enough for each and every individual who had an unobstructed course to get out.

This article mainly describes the design of the structure and the reasons behind the structure's failure due to the fire after getting hit by the airplanes.

1. Design of the World Trade Center 

The construction of WTC started in 1960 and was completed within five years. A novel approach was adopted for the construction of the skyscraper where lightweight materials and modular construction methods were adopted to enhance the schedule and slash the construction costs. The following points describe the design of the WTC:

1. Width and height above the ground level of each building were 61 m and 411 m, respectively. The height to width ratio was 6.8.
2. After construction, the total weight of each building was 500,000 tons.
3. Due to its height, the wind load governed the design of the structure rather than the gravity load.
4. Each building was designed for a wind speed and wind load of 225 km/h and 2 kPa, respectively. This resulted in a total lateral force of 5,000 tons.
5. A lightweight perimeter tube design was adopted to resist such a huge lateral force. The design of the perimeter tube consisted of a total of 244 exterior columns. Each column was constructed with a 0.36 m² steel box with a center to center distance of 100 cm.
6. A core wall of 27 m x 40 m was constructed inside the tube to support the dead weight of the structure.
7. Web joists of 80 cm height were provided to connect the core wall with the tube at each story.
8. Reinforced concrete slabs were constructed over these joists to establish the floors of the building. In addition, the egg-crate method (floor space of the building is constructed uninterrupted by columns or walls) was used to construct the twin buildings, which consisted of 95% of air. This explains why after the collapse of the building, only a few stories of high rubbles were deposited at the ground level.
9. The egg-crate construction method made the buildings redundant. In short, in any case, if more than one column fails together, then the load of these columns will transfer to the adjacent columns. Thus, the structure will remain stable. 
10. Before the construction of the WTC, skyscrapers were constructed using a massive amount of masonry columns spaced at 5 m center to center. The WTC was a very lightweight steel structure and its perimeter tube approach with an egg-crate method made the building one of the most resilient and redundant.

2. The Impact of Airplane on the World Trade Center

On 11 September 2001, the twin buildings of WTC were hit by the airplanes. The impact was so huge that it resulted in the collapse of the WTC. The following points describe the impact of the airplane with the WTC:

1. The buildings resisted the initial impact of the airplane because they were designed to withstand wind loads of more than 30 times the weight of the airplane. Also, the buildings were designed to resist the dead load of more than 1000 times the weight of the aircraft.
2. On the day of the attack, the intensity of the wind was very low. Therefore, the outer columns of the twin buildings were only stressed to their one-third capacity.  
3. The keel beam at the bottom of the airplane fuselage was the only component of the airplane that had more strength than the exterior columns of the WTC.
4. Undoubtedly, the impact of aircraft resulted in the failure of several columns on the exterior of the twin buildings. However, the twin buildings were highly redundant such that the structure did not collapse immediately because the loads of failed columns were transferred to the adjacent columns.
5. The main incident during the impact of the airplane was the explosion of 10,000 US gallons of fuel. This explosion ignited the fire inside the buildings. The weight of the fuel was one-third of the weight of the aircraft, the structure could have survived the impact of aircraft but the fire worsened its condition. Thus, the fire was the main factor behind the failure of the structure.

3. The Principle Cause behind the Collapse of the World Trade Center

The fire is the most misjudged aspect of the WTC breakdown. Even today, the media reports (and numerous researchers) accept that the WTC failed due to the melting of the steel, which is not a valid reason.

The main problem is that most people confuse the terms, temperature and heat. Though both the terms are related but definitely not the equivalent of each other.

In thermodynamics, the heat of a material is related to the temperature by the capacity and density of the heat. Temperature is characterized as an intensive property, implying that it doesn't change with the amount of material. Simultaneously, the heat is characterized as an extensive property, which differs with the quantity of the material.

One approach to differentiate between the two is, for instance, if an extra log is added to a fireplace, the temperature of the fire will not double and remain the same. However, the time period and the size of the fire doubles. Thus, during the impact of the airplane, there were more than 10,000 US gallons of jet fuel on the floors of the twin buildings. The amount of jet fuel doesn't relate to the temperature. The temperature of the fire was not much significant and was undoubtedly not capable of melting the steel.

Generally, the softening point of structural steel is 425⁰C. At 650⁰C temperature, structural steel loses its strength by half. However, the loss of 50% of steel strength is not enough to cause the collapse of WTC. Also, due to low wind intensity on the day of the impact, the outer columns of the twin buildings were only stressed to their one-third capacity. With this limit, the capacity of structural steel was more than 70% of its ultimate strength. Even if the strength of steel is reduced to 50%, it would resist more than two times the stresses caused by a 650⁰C fire.

The additional issue was the twisting of the steel in the fire. The temperature of the fire was not uniform all over and the outside temperature of the column sections was lower than the inside temperature due to the fire. The difference in temperature level produced the residual stresses in the structural steel, resulting in the distortions of the structural steel and the buckling of steel members.   

Thus, structural steel failed because of two variables. The first one is due to the loss of strength of steel because of the temperature of the fire. The second one is the loss of structural stability because of the twisting of the steel from the non-uniformity in the fire temperatures.

4. Was the World Trade Center Designed Defectively?

The design of the WTC was not defective. No structural engineer, even in their wildest dreams imagined 10,000 US gallons fuel on the floor of the twin buildings. Mostly, all the skyscrapers are designed to resist three hours of fire in case if the sprinkler system fails to work so that all the occupants of the building can be evacuated within these three hours. However, the twin buildings of WTC resisted the fire for one to two hours, less than the design value, because the amount of fire load was massive. The fire inside the twin buildings of WTC spread over 4,000 m² in a fraction of seconds. This would not normally happen in any office building. 

Generally, the fire would spread in one or two hours across the width and height of a building, but it was not in the case of WTC. The fire was massive and spread rapidly across the building and caused the failure of the structure.  

5. Future of Buildings After the Failure of the World Trade Center

After the WTC catastrophe, several changes in the building design codes were incorporated. For instance, the emergency communication system was upgraded. Also, separate provisions for higher stability of the emergency evacuation path were incorporated. To safeguard occupants from smoke inhalation, a few other provisions were adopted. High energy-absorbing materials were also considered in the design of tall skyscrapers.  

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