Why did the World Trade Center towers collapse?

The collapse of the World Trade Center twin towers on September 11, 2001, was as sudden as it was dramatic. The complete destruction of these massive buildings shocked almost everyone. Even today, there is widespread speculation that the buildings were structurally deficient, that the steel columns melted, or that the firefighting equipment failed to function. To separate fact from fiction, we tried to quantify various details of the collapse.

World Trade Center towers – design and construction

They were designed and built from the mid-1960s to the early 1970s. They represented a new approach to skyscrapers in that they were to be very light and involved modular construction methods, with the aim of speeding up the program and to reduce costs.

Each tower had a height of over 411 m above street level and 21 m below this level. The buildings had a height-width ratio of 6.8. The total weight of the structure was approximately 500,000 t, but wind pressure, rather than gravity, dominated the design. The building is a huge canvas that must withstand a hurricane of 225 kilometers per hour. The WTC towers were designed to withstand a wind load of 2 kPa – a total lateral load of 5000 t.

Preliminary reports noted how well the buildings withstood the initial impact of the aircraft. However, when it is recognized that the towers were more than 1,000 times the mass of the aircraft and were designed to withstand a constant wind load of 30 times the weight of the aircraft, this ability to withstand the initial impact is hardly surprising .

While the impact of the aircraft undoubtedly destroyed many columns in the WTC, the number of columns lost in the initial impact was not large, and the loads were transferred to the remaining columns. Of equal or even greater importance was the explosion, which occurred during the impact. The fire that followed was clearly the main cause of the collapse.

Fire resistance of steel

It is known that structural steel begins to soften around 425 ° C. And it loses about half of its strength at 650 ° C. But even a 50% loss of strength is still insufficient to explain the collapse of the WTC.

The additional problem was a deformation of the steel in the fire. The temperature of the fire was not uniform everywhere. Considering the thermal expansion of steel, a temperature difference of 150 ° C. From one point to another will produce residual stresses at yield level. This produced deformations in the structural steel. Thus, the failure of the steel was caused by two factors. The first is the loss of resistance due to the temperature of the fire. And the second is the loss of structural integrity due to the deformation of steel subjected to non-uniform temperatures.

The World Trade Center was not designed poorly. No designer of the WTC anticipated, nor would they anticipate. For a Molotov cocktail of such size to reach one of the floors of the building. Skyscrapers are designed to withstand a fire for three hours, even if the sprinkler system does not work. This length of time should be long enough to evacuate the occupants of the building. The WTC towers lasted less than designed because the fuel load was so high. There are no normal fires in an office building that would cover 4,000 square meters of space in a few seconds, as the fire developed in the WTC. Usually, it will take up to an hour to spread over the entire surface. We were dealing with a very large fire that progressed rapidly.

The two towers gave way due to the very strong impact from the fire

Almost every large building has a design that allows for the loss of a primary structural element, such as a column. However, when several members fail, the loads eventually affect the adjacent members and collapse occurs as a domino effect.

How the transverse beams of the two major floors affected by the fire gave way. And the outer pillars began to move outwards. The floors above them also fell. The floor below (with the capacity designed to support 1300 t) could not support the approximately 45,000 tons collapsed from the upper floors. This caused a domino effect that resulted in buildings collapsing in ten seconds. With a speed of about 200 kilometers per hour.

Article source: http://www.tms.org/pubs/journals/jom/0112/eagar/eagar-0112.html