Science in Africa

Engineering aspects of the WTC Twin Towers

Graham Shepherd, Rhodes University

Graham Sheperd gives some insight into the engineering aspects of the World Trade Center Twin Towers ahead of a lecture at the National Festival of Science, Engineering and Technology, Africa's largest science festival to be held in March in South Africa.

On September 11th 2001 the world watched in horror as the unthinkable happened. Two jetliners hijacked and slammed at high speed into two of the most recognizable buildings in the world, the World Trade Centre Twin Towers of downtown Manhattan's financial hub, followed by the total collapse of both mighty towers a short time later.

Created in the 1960s (and completed in the early seventies) as a landmark piece of renewal of a somewhat rundown part of lower Manhattan, funded by the Port Authority of New York and New Jersey, the towers were the career masterpiece of Japanese-American architect Minoru Yamasaki and brilliant young engineer Leslie E. Robinson.

The towers were of colossal proportions, by any standards, even those of New York's Manhattan Island, home of some of the world's tallest and best known skyscrapers. The towers, called North Tower and South Tower, stood 417 and 415 metres tall, respectively. In addition, North Tower supported a 108 metre high radio and TV broadcast tower on its roof. Each tower used enough steel to build a Nimitz class aircraft carrier, in its construction.

The towers were built up from prefabricated welded steel sections which all had to arrive on site in exactly the correct order, as storage of such a vast amount of steel would not be possible in the space available. Computers were used to keep track of the process: a first in the building industry. Each tower had a cross sectional area of over 4000 square metres and was constructed in a very unique way with all the load bearing vertical members concentrated in the exterior walls and in a very strong central "core" which carried the lift system of 99 lifts per tower. Otis elevators designed a system of express and local lifts unique to the WTC towers at the time. One caught an express to a sky lobby situated either on the 44th floor or 78th floor. This greatly reduced the space devoted to lifts, thereby increasing the percentage utilization for office space.

With the structural backbone situated in the core and in the exterior walls, it was possible to rent a whole floor of the WTC and be able to see 64 metres in two directions if one stood in the corner of one's space. There were no further columns in the intervening space! Of course you could order your floor partitioned into smaller offices in any way you wanted, but the partitions were only partitions and bore no load.

In this Science festival talk we will see exactly why the towers were designed in this way. We will use simple concepts of applied physics to see why Leslie Robinson's worst nightmare in the design was not earthquake or even the huge weight of the building. It was quite simply the wind! We will see the unique solution he devised to create an enormously strong but light tower, with enough built in redundancy to withstand a 240 km per hour wind! (Try to imagine the effect of a wind of this speed blowing against huge steel aluminium and glass "sail" 417 meters high and 64 meters wide!).

We will consider the effect of the plane crashes on the towers. Each plane was a wide cabin jetliner loaded with enough fuel to fly to the America West Coast, 5000 km away. Each was flying at very high speed at the moment of impact. At the time of the design of the towers the largest commercial plane flying was the Boeing 707. Leslie Robinson designed his towers to withstand being hit by a 707. But the scenario of his design was very different from what happened on September 11th : He envisaged a 707 lost in fog looking for the airport, low on fuel at the end of its flight, with a pilot not daring to go faster than the stalling speed of 280 km per hour under such dangerous conditions. The planes which hit the towers were estimated to be doing between 750 and 950 km per hour, respectively! Their destructive power can be shown to rise with the square of speed, so you can see that this event was about an order of magnitude worse than Robinson had imagined.

The initial collisions did tremendous damage to the various different elements of the buildings, which we will analyze in detail, but still the building stood. One of the reasons why they stood was without doubt the huge resistance to the wind that had been built in.

Further devastation followed as fires of enormous proportion, triggered by the burning jet fuel spilled at the crash sites began to rage without any hope of being checked. More damage to various building elements occurred. We analyze the probable form of this damage with the help of engineers with specialist knowledge of fire damage.

After standing for 56 minutes and 102 minutes, respectively, tower two (South tower) and tower one (North tower), in that order, both collapsed. We analyze what particular system went critical "first", for each tower and we see that the nature of collapse was significantly different for the two towers.

One collapse was initiated at the crash site floors; we will see that the total collapse was completely unavoidable. We consider the amazing "ear-witness" report of a person who survived the collapse of South Tower, from floor 22 and tie in what he heard and felt with what the engineers know must have happened. (His survival-he regained consciousness to find himself lying on top of the ten storeys high rubble pile which was once South Tower-was without doubt one of the most miraculous escapes of the day.

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