Did anyone watch Nova's Why The Towers Fell?

God, was that hard to watch. Listening to the architect actually hurt me physically.
The conclusions that the report came to were not surprising. The impact weakened the structures, the fires brought them down. I found the show completely fascinating, but I was in tears the whole time.

I was riveted. The video of the plane seemingly just disappearing as it entered the building was amazing. A very good show.

I only caught the last 15 minutes or so. I saw a similar program on TLC (I think) a couple months ago.

The engineer in me was utterly fascinated by the technical aspects as to why the buildings collapsed and when they did. But the emotional side of me was almost overwhelmed–it brought up some feelings I hadn’t had in a while. I guess part of that is because I’m relatively removed from the tragedy. I saw some footage I hadn’t seen before–namely the people sticking their heads out of windows to escape the heat and smoke. I never caught that while watching CNN (thankfully, I never saw the footage of people falling to their deaths).

I felt so sorry for the structural engineer who worked on the towers (the one who’s office overlooks ground zero). I can imagine that he’s haunted by what happened every day.

Still over 7 months later, and it doesn’t seem real.

Excellent show. Not that that is at all surprising from Nova, but I definitely learned some things I would never have known otherwise.

Got very sick of that beams-falling-down animation, however.

I tried, but I had to change the channel. I couldn’t handle it.

I didn’t see the special (not being a TV watcher), but all in all I would think that most buildings would fall if someone flew commercial airliners into their sides. Whaddaya think, that had something to do with it?

I would liked to have seen that. Canada’s building codes don’t allow for structures with such sparsely-distributed load-bearing walls - the WTC towers have been mentioned specifically as a legal impossibility for us in engineering courses for years.

I’ve often wondered if it would have been ultimately better or worse if the design weren’t so “collapsable” – there may have been more casualties if the towers fell over instead of down.

Actually, an engineer interviewed for this show says that’s not possible simply due to the buildings’ size:

The rest of the interview can be found here as part of the accompanying website for the show.

I’ll always remeber the picture of the sad face of the man who was the architect for the WTCs. His window looked out on ground zero.

minty, check out the animation on that page. It’s your favorite part of the documentary.

Very depressing and disturbing. But I am always surprised when people say, “I never thought the towers would come down.” I saw the whole thing from my office window, and I assumed they would collapse, seeing the damage and the fire. When the South Tower fell, I said, “well, it’s down,” and we all had to evacuate the office (as the phone calls were just coming in about the Pentagon). By the time I’d walked a few blocks, I heard people on the street saying, “the other one’s gone, too!”

Yes. But the WTC was built specifically to withstand the impact of an airplane crashing into it (which it did!). Most other buildings would have toppled instantly. Becuase these two were designed not to fall, it gave people time to get out, something that would not have happened had they chosen to hit the Empire State Building instead.

Zev Steinhardt

According to the experts on the show the towers could have withstood both the impact and the fire if the impact had not removed all of the fireproofing on the steel.

Well, sorta. I mean, it was designed to withstand an impact with the largest airplane extant at the time of its construction, but I’ll wager that any such impact was imagined as “glancing”, not head-on and deliberate. Who could have imagined that someone would purposely accelerate into the buildings? I don’t think we can blame the architects for that oversight.

On the special I saw on TLC about the towers, here’s how they explained the fall: as zev pointed out, unlike the Empire State Building, the external steel walls, rather than an internal steel skeleton, served as the supporting structure. This allowed for very open internal floor space, which was quite attractive to potential renters, because they could arrange walls and partitions pretty much however they wanted. In addition to the external walls, there was a solid internal core that housed stairs, elevators, and other stuff. Basically, the buildings were big tubes.

Each floor consisted of concrete panels on top of a sort of lattice-like steel support frame. The steel frame was bolted to the core and the exterior walls with basic L-brackets, except they were huge and made of incredibly strong steel. The steel frames were sprayed with fireproof material. These floors didn’t support the buildings in any way, but they helped add rigidity by keeping the core and the exterior aligned.

So, a plane impacts the building broadside. It tears through the exterior wall, smashes into the core, and explodes. The explosion, coupled with its shockwave, knocked the loose fireproofing material off the steel frames under nearby floor sections. Hell, it probably knocked the material off for dozens of floors above and below the point of impact. The steel frames supporting the concrete panels are now nakedly exposed to the burning jet fuel, and the bolts and L-brackets start to weaken. It’s not hot enough to melt the steel, but at high temperatures, the steel becomes much weaker, too weak to support the weight of the floor sections. The floor panels that weren’t destroyed on impact begin to give way and collapse.

Consider now the area near the point of impact. Floor sections above and below are collapsing. Without the panels to keep things in line, a million tons of steel and concrete (the floors above the point of impact) are essentially balanced on fifty- or sixty-foot tall steel stilts. The slightest shift in weight, or a minor weakness in the support structure, and the weight becomes unbalanced. The top of the tower collapses. As it falls, the massive kinetic energy it releases causes each floor below to fail and collapse along with it, adding more mass to the falling structure. And the tower is gone.

So, that’s what happened: explosion knocks off fireproofing material, fire weakens steel under floor panels, steel fails, floor panels collapse, remaining structure can’t support weight of upper sections, it all comes down. Had there not been a large explosion to knock off the fireproofing material, it’s possible (though I frankly doubt it) that the towers could have endured the fire.

The scenario above is believed most likely by persons studying the debris. It sounds reasonable to me. And I don’t know what the architects could have done differently, except perhaps change the type of fireproofing material used.

I watched the show and checked out the PBS online materials and noticed some discrepancies between what was on TV and what they published online, in particular the transcript of the interview with Prof. Eagar. I think a fair summary of the failure scenario posited by the TV show is:

  1. The impact of the planes destroyed the fireproof drywall and blew the fireproofing off the structural steel.

  2. The burning fuel and debris raised the temperature of some of the load-bearing members to 2000 F (I believe I heard this figure mentioned), at which temperature steel loses most of its strength.

  3. The weakened steel collapsed first on the outer perimeter (in one tower) but first in the core (for the other tower), due to the difference in where the fire was the most intense, resulting in the destruction of the buildings.

But in the online interview, Eagar notes that the fire probably burned no hotter than 1300 F, which basically agrees with his detailed analysis here: http://www.tms.org/pubs/journals/JOM/0112/Eagar/Eagar-0112.html. At that temperature, steel loses only about half its strength, which is unlikely to have been enough to cause collapse, even supposing that some of the columns were cut by the impact of the planes. In his detailed analysis, Eagar goes on to say that one of the key phenomena was the expansion of the beams that were hot on the inside but cool on the outside (this in itself will cause the columns to bend, regardless of the load). So the fire really caused two different things to happen: it weakened the columns and caused the columns to bend (which makes them more susceptible to buckling). The WTC was probably more susceptible to this kind of failure than other buildings because its load-bearing columns were concentrated around the perimeter, so they always had a cool side.

I thought the TV show put far too much emphasis on the role of the floor clips. There is no reason to think that the failure initiated at the floor clips, so the only time they came into play was when the floors above began to crash down onto the floors below. Each floor has a 1300 ton design capacity with a safety factor of about 3, but ten floors crashing down from above had a dead weight of at least 45000 tons. The floor clips would have needed a factor of safety of (45000/1300)*3 = over 100 to survive. That is not a reasonable expectation.

An important question I have not seen answered is whether the fire itself would have caused the towers to collapse, regardless of whatever structural damage was done by the airplanes. If so, buildings constructed with their load-bearing columns around the perimeter could experience the same failure scenario as the WTC if a floor catches fire and the columns are exposed to the full heat of the fire.

That being said, it is unlikely that such an event could happen without a large impact or explosion to strip the insulation and blow down the internal fireproof walls. Requiring a building to withstand that kind of event would make it impossible to economically construct any more tall buildings at all.

The towers were originally constructed to survive the direct collision of a Boeing 707. The trouble is, they were hit by aircraft that have larger fuel capacities, which aircraft were nearly topped-off with fuel. There is the discrepancy that allowed the whole process to happen. No one thought of, or planned for, the higher fuel load, which created hotter, longer-lasting fires.

Thanks for the link, Red Menace.

ONCE AGAIN i’m screwed over by the inconsistant airings of NOVA by my pbs affiliate! i stumbled across the website yesterday and was looking forward to seeing the broadcast. should be repeated this saturday afternoon, so i may get to catch it.

hey, AHunter3, you might try watching a little pbs-- sounds like you could use it! :smiley:

The impression I received from last night’s Nova is that no one thought of anyfuel load and the resulting fires. One gentleman mentioned that we simply didn’t have the models back then to create fire scenarios, so it wasn’t part of the design process. I am trying to find the thread about this topic that was started back in September – a Doper had looked up fuel loads, and I think that one of the 9/11 planes carried more fuel, and the other carried less fuel, than a 707.

Overall, I was impressed by this show. I agree that we didn’t need the many repeats of the floor beams falling down footage, but other than that it was very interesting.

i watched as well. the live footage was hard to watch, the animation was much easier. it is just too soon to see the collapse in slow motion over and over and over.

the difference in failure in the towers was fasinating. the story of survival from the man who got out of tower two from above the impact was riveting. i went to the pbs site to read it in entirety.

in philly we had a building burn for hours, that did not collaspe. the steel did melt and give way in places. it was torn down about 2 years ago. the impact esp. in tower one exposed naked steel to the flaming jet fuel and resulting “building stuff” fire. with the fire protection on it, the steel has about a 4 hour fire rating.

i believe that they built a strong building for 1960 technology. that building took the impact, many buildings now would not take that kind of hit.

nice of our head of stucture to tell me the building i work in couldn’t stand the impact…