Off Topic — HabitableZone

Off Topic » in reply to My understanding is...

The WTC was designed to withstand an airplane impact, BUT...
Larry J — (archived)

the standard used when the buildings were designed was a Boeing 707 sized airliner (then the biggest in operation) flying at approach speeds. They were keeping in mind the accident in 1945 when a B-25 bomber, lost in bad weather, collided with the Empire State Building*. The planes that hit the towers were much more massive than 707s, carried much more fuel, and were traveling about 3 times as fast as what the buildings were designed to withstand. The kinetic energy was probably at least 15 times what the design assumptions called for and the thermal energy from all that fuel had to be many times greater. In retrospect, it's a testiment to the stong design that they stood as long as they did.

The post collapse analysis shows that the initial impact probably blew away a lot of the fire retartant (largely asbestos, IIRC) from the structural steel. The fuel burned intensely for 5 minutes or so, starting many internal fires. The sprinkler systems were offline and many of the fire escapes were blocked. With no way to stop the fires, collapse was inevitable. I'm sure a lot of those firefighters who ran into the buildings knew it, too.

*I first heard about the impact while driving to work. All the news said was that a "twin engine" airplane had impacted one of the towers. Being a lover of aviation history, I immediately thought of that B-25 incident and wondered if it had happened again. When I got to work and called up the news stories, I saw clear blue skies and knew something was terribly wrong. It was still possible that a light plane, perhaps with an incapacited pilot, had impacted the building because nothing being reported at time said the key word, "airliner." Then the second plane hit and everyone knew...

Log in or register to post.

The whole thread (54 posts)