We have recently passed the first anniversary of this tragic day. The introspection of countless essays on "What does it mean?" and "Where do we go from here?" were inevitable, but we sampled them carefully.
At first, we resisted the coverage on the subject, "Why did the World Trade Center towers collapse?" Focusing just on the engineering perspective didn't seem right. It also ignores the tragedy of the attack on the Pentagon, a common failing in discussions of 9/11.
But, eventually, this one question offered another way to think about the day. By applying principles of engineering and metalworking, we can allow reason to prevail.
What happened?The planes that struck the towers, American Airlines Flight 11 and United Airlines Flight 175, both out of Boston, were Boeing 767s. Fully loaded, a Boeing 767 weighs about 200 tons, including up to 24,000 gallons of jet fuel. A tanker-trailer that you see refilling the pumps at your local gas station carries about 10,000 gallons, so each plane held the equivalent of about two tankers of fuel.
When 20,000-plus gallons of jet fuel explodes and catches fire in one spot, and when that spot is in the upper stories of an enormous high-rise, it is not difficult to imagine that this alone is the cause of the building's destruction. But it was a unique combination of design and circumstance that caused the collapse.
Some have wondered why the buildings' automatic sprinkler systems did not extinguish the fire. That would have been impossible. Water, in any quantity, is useless in a gas fire. The fuel just floats and continues to burn. And when the system is damaged, it cannot even deliver water reliably. But why would the fire have contributed to the collapse?
According to www.skyscraper.org, architect Minoru Yamasaki and engineers John Skilling and Leslie Robertson used an innovative structural model to build the towers.
The design called for a rigid hollow tube of closely spaced steel columns with floor trusses extending across to a central core. The columns, finished with a silver-colored aluminum alloy, were 18-3/4 in. wide and set 22 in. apart. The twin towers were the first skyscrapers built without masonry.
According to www.greatbuildings.com, the structural system was modeled after the IBM Building in Seattle.
The 208-ft. wide facade was a prefabricated steel lattice with columns on 39-in. centers acting as wind bracing. The central core bore only the buildings' gravity load. The outside surface of the buildings provided the wind bracing, and did not transfer the forces through the floor membrane to the core.
Office spaces had no interior columns. On the upper floors, there was as much as 40,000 sq. ft. of office space per floor. The floors were constructed of 33-in.-deep prefabricated trussed steel that ran the full 60 feet to the core, and also acted as a diaphragm to stiffen the outside wall against lateral buckling forces.
When the planes struck the towers, they destroyed a large number of vertical columns around the edge of each building and at their core. The crashes did not take out enough columns to immediately crumble the superstructures. The columns that remained could still hold up the top sections of each building.
However, the fire twisted and weakened the standing columns. Without enough support, the top of each building collapsed onto to the lower sections.
The velocity of the falling sections created an impact force that exceeded the structural integrity of the columns underneath. The buildings were crushed one floor at a time. Professional demolition crews use the same principle to allow the force of a falling building to do much of the work.
World Trade Center 1 was hit first but World Trade Center 2 collapsed first. Experts believe that, because the plane crashed lower on the building in WTC 2, the strained support columns experienced a greater load pressing down on them, and reached the buckling point more quickly than WTC 1.
The buildings fell because they began to fall. They began to fall because their innovative design could not withstand the forces of the crash. Without the unique superstructure support design, the buildings could not have been constructed at their enormous height. Yet it was their impressive height that made them targets for terrorism.
Ayn Rand considered skyscrapers to be concrete symbols of man's indomitable individualism. Mysticism and illogic don't build buildings. Thinking men and women who apply the principles of reason are the only ones who can create magnificent structures from bare patches of ground. Fanatics and ideologues are the only ones who prefer destruction to creation.
The buildings are not as important as the people who worked within their walls. Compared with the tragedy of their deaths, twisted metal is a mere inconvenience. You cannot compare the grandeur of a building with the sanctity of life.
But you also cannot consider the destruction of the World Trade Center itself to be inconsequential. Man needs his symbols as much as he needs breath. Unless the replacement buildings inspire us, the dark forces will have won a great victory.