Until this week, the most horrific live television clip most of us had ever seen was the explosion of the space shuttle Challenger. A few minutes after 9 AM on Tuesday, a new image replaced the Challenger memory. Surely, I thought, I would never witness anything more appalling than watching a commercial airliner disintegrate into the South Tower of the World Trade Center. Yet within an hour, a new world-wide standard in video-captured horror was set, as millions of us watched a second plane deliberately steered into the New York skyline.

The yet untold loss of innocent lives and the unthinkable collapse of two of the world's greatest skyscrapers strikes a personal chord of horror for the millions of us who work and live in high-rise buildings. How could such mammoth buildings just collapse? Maybe 110 stories IS too high. Are these buildings safe for my family -- for me?

I remember countless discussions with colleagues after the depraved attack on the World Trade Center in 1993. These discussions were often prideful, maybe even arrogant.

"How simplistic, how foolish!" we said, knowing that the 1993 attempt to knock over one of the Towers with a below grade explosion was almost certain to fail.

Nearly all high rise buildings easily support their own dead load (their own weight); taking out a column or two in an attempt to knock over a high rise is not unlike trying to drain a swimming pool with a tablespoon. A terrorist would have to take out a lot of columns to have a high rise collapse under its own load. Even at the Murrah building in Oklahoma City, three years later more than half the building remained standing after an entire row of support columns was obliterated.

The design load that really stresses high rises is wind (or seismic loads), not dead load. Even with extreme lateral loads imposed by wind, we could theoretically build a steel skyscraper a mile high, with current technology and methods, and still be certain of its structural integrity. A building 110 stories is not even close to pushing the theoretical structural envelope.

It wasn't the impact of the 767, which, according to Boeing, can weigh up to 412,000 pounds at take off. Both Towers survived the impact. Not even the incredible force of a 400,000-pound mass moving at several hundred miles per hour was enough to collapse these structures. Like most contemporary high rises, they were that well built.

In fact, even older buildings can readily survive a crash situation. In 1945 an Army B-25 bomber ran into the 79th floor of the Empire State Building. A total of 14 people died and the structure was quickly repaired, but no damage comparable to what has been seen in the past few days occurred.

So what happened on the 11th?

It was fire -- not just a typical building fire, but an extraordinary fire. The type of extreme heat fire that can only be ignited under exceptional circumstances -- in this case, the ignition of the 24,000 gallons of jet fuel a 767-300 can carry at take-off. The intensity of a fire created by explosion of that much jet fuel is virtually incomprehensible.

The Achilles heel of a steel building is fire. Even at relatively low fire temperatures, steel begins to quickly lose its rigidity and bearing capacity. For decades prior to the design and construction of the World Trade Center, the fire-induced failure characteristics of steel were well known. This is why nearly all high rise buildings incorporate some type of passive fire protection on the steel work -- either enclosure of the steel in concrete, or simple insulation of the steel to protect it against ordinary fires.

Here's the main point: The fire-induced collapse of a contemporary steel structure is extremely rare because the passive (and active) fire protection systems of buildings are almost always effective for at least long enough to allow full evacuation. Even in the case of the Twin Towers -- the most extreme situation imaginable -- thousands of people were able to leave before the buildings collapsed.

On the 11th, however, the extraordinary intensity of the heat, in combination with probable exposure of the steel from the impact, was just too much for the World Trade Center Towers. Once a full set of columns on the impacted floors were sufficiently heated, they failed. This appears to have given the downward momentum to the thousands of tons of "cold" materials above, which, in turn, "pancaked" the floors below.

The impact of the plane, and more importantly, the intensity of the fire caused by the extreme volume of ignited jet fuel are what finally brought down the Towers. It's not a combination of factors that can be readily duplicated.

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