Does anyone know anything about structual engineering?
I was more than suprised when the towers came down. I didn't think that an impact of a plane could so weaken the building, that it would completely collapse.

What??? A 747 weighs something like 120 tons. I'm sure the planes involved in this were much lighter. How fast were they going at impact?? 250??300??400??500?? mph. Their weight and their speed equals an awful lot of force. Not to mention they were apparently cross-country flights with plenty of fuel to explode. I was more surprised that the impact didn't immediatley drop part of the building.

Looking at the collapses it looks like the tops of the building (above the impacts) fell straight onto the rest of the building. Any idea how many tons of concrete and steel 20 or more stroies of those buildings weigh?? The top falling straight down caused the floors below  to collapse as they collapsed they added to the falling weight meaning they were impacting the lower floors with sequentially more weight. As each floor failed the top of the building added to it's mass and free fell appx 10' before hitting the next floor and crushing it.  

The WTC's were unique in their design. They were designed to bear the building load with the 4 walls alone.  There was no internal structure to support the load.  With the planes hitting they removed one whole load supporting element.  I suspect the first building to fall was hit worse and could stand anymore alone.  When it fell, it undermined the foundation of the second building and it fell.  Bad day.

I'm not a structural engineer but I am a mechanical engineer.  Its obvious that intense heat would weaken structural members.

God help the families of those killed and give strength to those who are left to deal with the carnage.

357mag

I know a little about structural Engineering.
To answer the question about the Trade Center Building collapsing. Yes, the shock from the impact and explosions and fire is definately enough to bring down both buildings.

It takes some time for enough steel girders to fail, from the moment of the impact, girders would have started to fail. Once a certain threshold number of structual supports have failed then you see a massive collapse of the entire structure.

High Rise buildings are simply incapable of withstanding such impacts.

I am an architect.  Here is what the folks in my office think:

We think the WTC is (was?) a steel frame building.  Steel structures do not withstand fire very well.  Most steel frame structures have spray-on fireproofing that is the consistancy of moist clay.  This fireproofing is only good for a certain duration of fire, then it is kaput.  It is also possible that the impact shook the fireproofing loose enough to ruin it's integrity.

The duration of time between impact and catastrophic failure seemed too long to mean the collapse was caused directly by the jet crash.

Texason AIA

(Not bothering to spell check because the net is too damn slow right now)

After the first plane hit, I was watching it from my roof.  I feared then that it would collapse and even mentioned it to a friend.  Another guy on the roof, an architect, said they'd be fine and shouldn't collapse.  As soon as he said that...BOOM - the first tower came crashing down.  They're big buildings but they got hit with one hell of a lot of force and the heat generated by the explosion and fire just made things worse.

You dont need a structural engineer to answer this one!  You drink out of soda cans dont you?

Have you ever stepped on top of an empty soda can on one foot?  Then have somebody tap the side of it and it crushed under your weight?

That's exactly what happened to the towers... plane hit one side of tower which compressed the other over time the weight of the building was too much as the structure fatigued and it collapsed.

M.

The soda can is a pretty good example.  But the fire was the real trigger. When heated steel has the srenth of wet spegetti.