Why the towers fell.
April 30, 2002 3:58 PM   Subscribe

Why the towers fell. PBS is airing a special episode of Nova about the science behind while the World Trade Center towers collapse. Nova's reputation for converting esoteric science & engineering into understandable explanations for the layman should make the show something to watch. 7PM EDT/PDT on most PBS stations. Set your Tivos.
posted by Argyle (23 comments total)
I'm in EDT, and it's supposed to show here at 8pm.
posted by RunsWithBandageScissors at 4:35 PM on April 30, 2002

Also seems to be at 8 PDT here in San Francisco (KQED-9)
posted by vacapinta at 4:45 PM on April 30, 2002

Has anyone else read this (first hand account from 84th floor survivor) yet? These stories are absolutely gut-wrenching. And I bet there are hundreds more like it. Regardless of what has happened and is still happening with respect to terrorism, and all that jazz, there are small stories that absolutely fascinate me. Stories like this always remind me to head over to fray.
posted by tru at 4:57 PM on April 30, 2002

if i may, i once posted a link about the architecture of the wtc and its contribution to the tragedy.
posted by moz at 5:14 PM on April 30, 2002

I am so uninterested in watching this, however interested and well done it may be. Can you say: DEPRESSION?
posted by ParisParamus at 5:28 PM on April 30, 2002

Seems a very strange and unappropriate subject for a science show. We are so emotionally distanced from this subject that we can watch a whole show about it?
posted by xammerboy at 6:55 PM on April 30, 2002

It was very informative.
posted by corpse at 7:17 PM on April 30, 2002

The show was typical NOVA, which is to say, good. A lot of the information it put forth probably was familiar to those of us who've taken an interest in why the buildings fell, but it was still a worthwhile program. The animations showing the kind of damage the airplanes inflicted on the buildings, as well the enhanced video segments, were particularly striking. One image in particular stuck out: Just as the first tower started to fall, a video captured the outer walls blowing out as the internal floors collapsed.
posted by mrbula at 7:31 PM on April 30, 2002

We are so emotionally distanced from this subject that we can watch a whole show about it?

I'm not getting you. What reaction are we supposed to have?
posted by rodii at 7:35 PM on April 30, 2002

What is wrong with some people? I mean.. yes, sympathy and sadness for what has happened. But this is pathetic crying over spilled milk. It has happened, just like any other tragedy in the centuries humans have been on this planet. To dwell on it is fruitless. One should try to learn all he can about it.. not block it out and not think about.

But I suppose humans are naturally very resistant to change, so therefore all I have said, is also fruitless.
posted by spidre at 8:32 PM on April 30, 2002

It was harder to watch than I thought. One thing they brought up, which has always really bothered me: the Port Authority told people in the south tower to not evacuate, to return to their offices because the fire was all in the north tower. And people obeyed - they turned around and went back to die. I know the authorities didn't grasp what had just happened (remember early reports that it was a Cessna or something), but still - the complex had been attacked before, in 1993. You would think that you should evacuate the whole area if anything violent happened.
posted by crunchburger at 8:50 PM on April 30, 2002

crunchburger - I dunno. There is already a serious catastrophe in the building next door - the last thing you need is another 6,000 people milling around outside getting in the way of emergency workers and being hit by falling debris etc. It was a perfectly reasonable call to make given what they knew at the time.

It was an interesting show, well put together and with minimal sensationalism. I don't know that I'm glad that I saw it, but I definitly feel enlightened. If I lived on the East Coast and knew people who used to work in the WTC I might feel differently, so YMMV.
posted by jaek at 9:15 PM on April 30, 2002

The main gripe I had with this presentation was the insinuation that the engineers could have made the towers stronger, but failed to do that. I felt the insinuation wasn’t to go so far that the engineers were somewhat culpable, but I was a little uncomfortable. The basic tenant of engineering is to make a structure as efficient as possible both in terms of material and cost. I felt this wasn’t dwelled upon enough to reinforce the fact NO ONE thought of this catastrophe nor should this extreme type of catastrophe be considered in structural design – unless we like to live in 5 story buildings.

The WTC was a wonderful example of structural efficiency in terms pre-fabricated panels (the Empire State Building also uses prefab components but with a standard steel structure) and material use. Many of us here in the architecture camp were interested in the failure of the WTC (after our shell-shock was over) because many of the 1970’s-80's high-rises after the WTC were constructed with the “doughnut” configuration to allow for maximum floor area. The data from the failure will help many structural engineers in their design process. Unfortunately much of the data and ideas that architects and structural engineers depend on are from structures that fail. Casts a nice long shadow on work, doesn’t it?
posted by plemeljr at 9:23 PM on April 30, 2002

I thought it was Nova's superb work as usual. It emphasized the effects of the archetectural/engineering decisions more than the human drama. Although things like the story about the trapped elevator occupants chipping their way through the drywall to safety did both.

While I had read some of the overall theorizing before, this was the clearest explanation yet.

What the 1:21 video clip on the web site does not make clear, but the show itself did, is that the two buildings failed in some different ways.

Plemeljr, as a non-architect or engineer, to me they seemed to be saying, "we think the design held up well (with some notable exceptions) BUT not everyone agrees." Seeing as the final report is being released today, they didn't have the advantage of waiting for more reaction.

I suppose it will take some time for consensus to be reached. This will be studied for years, I'm sure.
posted by pmurray63 at 9:45 PM on April 30, 2002

Amplifing my previous post, the ASCE web site notes that it will present its findings to the House Science Committee at noon ET Wednesday. And the National Institute of Standards and Technology (NIST) plans an even bigger, two-year investigation once Congress coughs up some funding. Here's a Washington Post story with more details.
posted by pmurray63 at 9:54 PM on April 30, 2002

For those of us who missed it the first time, Why the Towers Fell will be rebroadcast a few times later this week. Your local listings may vary, but PBS plans to replay the Nova epsisode on the afternoon of the 3rd, around 2am on the 5th, and at noon on the 7th.
posted by ZachsMind at 2:24 AM on May 1, 2002

there'll be more of this sort of thing, not less, as time goes on and people get a little distance. it's turning into History before our very eyes. I think that's good, necessary, and inevitable. (Although it's got to be very tough for those who lost someone that day, or were otherwise in some way close to what happened.)

However. An article in the Washington Post this morning that says ABC is planning to devote the entirety of Sept. 11, 2002 to 'tribute' coverage. This too was inevitable, but I'm really sort of looking forward to the first anniversary with dread. It's going to be a hard day.
posted by Sapphireblue at 9:22 AM on May 1, 2002

The basic tenant of engineering is to make a structure as efficient as possible both in terms of material and cost.

Not really. This is always superseded by considerations of safety and risk. Structures are often 2 or 3 times stronger or more stable than they "have" to be (under some analysis of the risks, which may turn out to be inadequate). The beginning of the NSPE code of ethics says:
Engineers, in the fulfillment of their professional duties, shall:
1. Hold paramount the safety, health and welfare of the public.
But your point that this was an unanticipated failure mode is quite correct. This wasn't like a Hyatt Regency disaster, where an inadequate, unsafe design was subtituted for the original, which would have been quite safe. The WTC towers performed completely up to spec, but the conditions were unimagined.
posted by rodii at 9:49 AM on May 1, 2002

For those who like to see the original source documents of the ASCE/FEMA report, rather than depend upon news summaries, they are available at the organizations' respective web sites:

- Executive Summary (PDF | Word) (7 pages)
- Chapter 8: Observations, Findings and Recommendations (PDF | Word) (13 pages)

The ASCE web site says there will be an AOL chat tonight at 9 pm ET.
posted by pmurray63 at 10:35 AM on May 1, 2002

rodii - 1. Hold paramount the safety, health and welfare of the public.

Absolutely correct. I was a bit obtuse in my discussion of efficiency/cost/materials. I thought I had included life-safety (#1 for us in the architectural profession, too), but alas not. Let me talk about factors of safety, though.

Pre-1940’s (approx) the standard factor of safety was around 1.5x-2x and most of that was a guess so the factor of safety was really around 2x-3x. Then it was decided that we knew more about materials and it was lowered to 1.25x-1.5x-design load. Then around 1960-1970 when we started designing building really different a-la WTC, Lever House (NYC), PSFS (Philly), etc, there were some concerns about general safety and engineering so the factor of safety was raised back to around 1.5x-2x, Engineers are a cautious lot, so in reality for most buildings the safety factor runs around 2.0x-2.25x the design load.

A side note (and to relate it to rodii’s Hyatt Regency disaster), the CitiCorp Building is an interesting case study. Because of the site conditions, the building had to be lofted up on four piers, but the piers were not at the corners but in the middle of the shaft. Well, one little change order that dealt with beam connections. I think it went from a full-penetration weld (very strong) to a bolted connection (strong but in shear - think tearing or side force - is relatively weak). So the owner had to go in and weld steel plates at all of the structural connections so that the force could transfer. This illustrates how important all decisions are in engineering, and the off-hand change can vastly change how a structure operates. That is why engineers use the factor of safety.
posted by plemeljr at 7:56 AM on May 2, 2002

Very interesting. I'll look into some of that. It's also interesting to me how different fields consider different safety factors appropriate. Bridge and dam designers I think often use 3x, whereas designers of chemical plants, cars, etc. all have their own standards of safety. A friend of mine who is a former shuttle astronaut says that in the months leading up to the Challenger disaster, the shuttle was using
"negative" (i.e., below 1x) safety factors--that is, they were flying the shuttle with some elements spec'ed to be below the design parameters--otherwise they would never have gotten the thing up. (This is the kind of thing that makes an engineer's blood run cold. I don't remember the details, but presumably these weren't mission/safety-critical elements. But still.)

It's not a completely rational process in that often (always, really) design decisions are made without having all the data, and every social group (civil engineers, structures people, architects, ChE's, etc. have their own internal process for arriving at the "right" figure, and (as Henry Petroski has discussed) many times understanding failures, sometimes spectacular ones, is the key step in refining them.
posted by rodii at 8:50 AM on May 2, 2002

For the record, I came across a previous historic event in New York where a plane hit a building. The Empire State Building was struck by a B-25 bomber back in 1945, killing fourteen and injuring scores of others. Compared to the Nine Eleven tragedy, it doesn't seem like much, but the reason for that is because the Empire State Building was made prior to the use of glass and lighter materials on the outside of a building's structure. The walls of the Empire State Building are steel and brick, and therefore were better able to withstand the impact.

Though the WTC met and in some cases exceeded modern day requirements for building construction, there is perhaps a need to revisit the way we used to make buildings, and why. It just goes to show, they don't make 'em like they used to.
posted by ZachsMind at 1:52 AM on May 5, 2002

Zachsmind - You're comparing apples and watermelons
The B-25 grosses at 28,000 pounds, and the 767s at roughly 250,000, depending on the model. Roughly ten times the weight, so ten times the kinetic energy. Taking a wild guess at the speed at impact, assuming the B-25 hit at 150 knots and the 767s at 600 knots, that's four times the speed. Yes, redline indicated airspeed for a 767 is under 400, but I doubt they were paying much attention to the manual, or limitations, and they were MOVING. Energy varies with the square of the speed, so that's 16 times the energy from speed, or a total of 160 times the energy of the B-25 that slammed into the Empire State Building.
And that isn't even considering the fuel capacities:

normal total fuel load of 974 US gallons
Maximum Fuel Capacity: 23,980 US galllons

The collapse of the World Trade Center towers was a heat-induced failure of the load bearing materials, meaning the steal got to hot and buckled.

The load bearing structure of the Empire State Building is also steel. It also would have failed, if the building had remained standing after impact, which is not at all certain.
posted by NortonDC at 7:16 AM on May 5, 2002

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