Possumblog

Oh, and another thing.

There has been a minor uproar lately due to allegations by some guy that the World Trade Center was actually demolished by a secret government conspiracy. Well, you know, some people will believe anything. But in case you'd like a little insight on the whole thing from the point of view of someone who doesn't cover his head in tinfoil, you might want to read what I wrote about the WTC collapse back on Friday, August 23, 2002 (if the page won't scroll as you reach the bottom, press your F11 key twice):

Part Two of my continuing ed coursework for yesterday took me to the other side of town to the Richard M. Scrushy Center for the Study of Richard M. Scrushy at HealthSouth headquarters. The presentation was sponsored by the Structural Engineers Society of Alabama, and included not only the lecture by Dr. Corley but a video presentation by Mr. Leslie Robertson who was the principle engineer on the World Trade Center.

There was an article about this conference in The Birmingham News this morning, but I refuse to link to it simply because the reporter must have listened to a different presentation than I did, or simply did not understand what he was writing about. As with most news stories I have read about this subject, there was little attempt to educate but much on trying to see if someone can be blamed. In fact, the writer of the article himself points to this in the very last sentence in the article (this’ll be the only part I quote)

[…] Corley said his team's report has been criticized by some because they did not point fingers and place blame for the collapse. He said that wasn't his team's mission.

"Frankly, we don't think there is any fault," Corley said. "The hijackers are the ones at fault. They get all of our blame."

Amen.

And in spite of how horrendously terrible this attack was, it could have been far worse had it not been for one man’s acrophobia. More on that later. So, now, on to my small part of trying to make some sense of this.

As I mentioned, the first part of the presentation was a videotape of a talk given by Mr. Robertson (Click on his name to go to his firm’s message about the attack). I am not sure when the video was made, but it was billed as his first address to an audience since the attacks. I wish it had been done with a bit more forethought—it had the look and sound quality of a bootleg grade school recital tape; lots of out of focus shots, wandering framing, him having a coughing attack and gulping water right into the lavalier microphone he was wearing, folks walking in front of the camera.

He gave a good overview of the construction concept and methods, and spoke about the work his firm did on the building after the first attack back in 1993—he was referencing a slide show which most of the time was out of frame, except for when the camera would whip around to the screen. When it came time for the part about the collapse, the entire chunk of his talk and the slide show had been edited out due to some not-quite-well-explained reasons dealing with the slide images not being able to be released to the general public. It just went straight to his question and answer session at the end, which had a few technical questions, and then one more:

[Off camera-almost inaudible] ‘Is there anything you wish you had been able to do differently?’

He paused.

“I wish,” he paused again.

Choking on his words, he slowly and quietly said, “I wish…I could have…made it stand up.”

The audience in the video was silent, as were those watching the video in our meeting room.

It made my eyes burn, and my throat ache when he said that, and it does so now when I sit and type this.

I know from the muffled sniffs from the men further back in the room that I was not the only one who felt that terrible pang.

This is the side one normally doesn’t see within the staid world of welds and bolts and mass and force, but there are few people who are so acutely aware of the consequences of a potential failure in their work. If a doctor fails, a patient can die. If we fail, thousands can die. Engineers and architects do our best to anticipate the unexpected, to ask questions from different angles, to be thorough in our preparations, and above all protect the health, safety, and welfare of the people who will use our buildings.

All of the blamemongering in the world, all the heated editorials, all the jackassed stupidity of the Usenet, will never change that. You can’t make the designers and builders feel any worse, nor will you be able to magically eliminate future attacks or revoke the laws of nature.

The second portion of the presentation was Dr. Corley’s review of his assessment team’s report to FEMA. This report is available online at the FEMA website, but at the moment is appears they are having some technical difficulties (or my computer is screwed up). Luckily, it is also available over on the House Committee on Science website, which can be accessed here. It is a BIG book, close to 300 pages divided into eight chapters, and each chapter averages over a MB, although some of the more photograph intensive ones are closer to three MB.

Before you read anything else on the World Trade Center (including my own stuff), before you go popping off on MeFi about who should have known what about what, if you really want to learn something, go read the report first. It is very well-written with a good mix of understandable general language, technical data and photographs. It has background information on the project, design criteria, general information about construction and building codes, and a detailed chronology. Not only are the Twin Towers analyzed, but all of the buildings of the complex and those adjacent that were damaged.

It is far better to read that than any bit of commentary I might write in this silly blog. And just like Dr. Corley was quoted as saying, this report is an examination of the performance of the buildings under extraordinary circumstances. If you’re looking for fodder for your favorite conspiracy theories, you would do much better just to go ahead and make stuff up. You won’t find any help in it.

Have you read it? Don’t go any further! Go read it now. Okay, finished? Good.

Now, a few of my thoughts—

First, the thing I keep seeing discussed ad nauseum is ‘if it was designed to get hit by a plane, why did it fall?’

A lot of the misunderstanding seems to revolve around whether things should be designed for all possibilities, or for the most probable circumstances. Folks, the only way to design for all possible attacks would mean that each one of use would have to live in a nuclear-biological-chemical resistant structure, and that every person would have to be widely dispersed to minimize possible deaths. This is a fine and dandy approach if you live in some alternate universe, but here, the most sensible thing is to work from the most likely occurrences.

In the end, the most prudent course of action was to design for something within the most probable realm, and in this case the only similar incident occurred during World War II when an off-course B-25 struck the Empire State Building. The WTC designers concluded that the most likely way in which an aircraft would hit the towers would be if it were lost in heavy fog and low on fuel and flying at landing speed. The aircraft chosen was the most common type then flying in the area, the Boeing 707, which had a gross weight of 263,000 and a landing speed of 180 miles per hour. In the case of what actually happened, 767-200ER aircraft, each weighing 274,000 pounds, struck the towers at speeds of 470 and 590 (!) miles per hour. Given that force rises exponentially with velocity, it is a testimony to the robustness of the structural system of the buildings that they were not immediately destroyed by the impact. The study points out that on the impact faces of the building, more than 2/3 of the supporting exterior columns were destroyed, yet the load on the remaining columns only rose to their theoretical capacity.

Had there been no fire, the buildings would have remained standing.

As I mentioned at the first, this incredible structural performance had much to do with the way in which the floors were interlocked and tied to the exterior structural skin, which was made up of built-up segments of steel plate arranged as an array of continuous square tubes. Each column was only 3 feet, 4 inches apart from its neighboring column, one reason for which was that the lead architect on the project, Minoru Yamasaki, had a fear of heights. Mr. Yamasaki wanted to have window framing no further apart than he could comfortable grasp with two hands. The solution chosen was essentially to make the window framing part of the structure itself. (Dr. Corley said he had heard this story several times, but finally was able to confirm it in conversations with members of the Yamasaki firm.)

The redundancy of these structural members and the way in which they tied back into the central core contributed to the tremendous strength of the towers. In spite of the high loss of life, it could have been far worse—at the time of the impact the Port Authority estimated the population of the complex at 58,000. The strength of the building allowed enough time for able-bodied persons below the crash levels to evacuate before the buildings fell.

It was the fire though, and the inability to fight it, that set up the circumstances of the collapse. Surprisingly, the fuel on the airplanes was not a significant source of fuel except for the first 3 to 9 minutes. At least a third of the fuel burned up in the atmosphere in the form of the huge fireballs which shot out of the sides of the buildings. After about 9 minutes, the fuel had been totally consumed. However, before it was gone, it set fire to everything else within the crash area, and it was this fuel load of paper and furniture and equipment that produced the fire which finally weakened the structure enough to cause collapse. The energy of this fire was estimated in the report to be equal to the power generated “by a large commercial power generating station.”

Due to the impact of the planes cutting off main supply lines of water, none of the sprinkler systems could operate, and the impact dislodged fireproofing sprayed on the structural steel in critical points, exposing the steel to continuous heat far above design temperatures, for a far longer time. Just as the impact alone did not destroy the towers, it is conceivable that a large fire on multiple floors might not have destroyed the building had the main water supply not been cut and had the integrity of the fireproofing not been compromised.

The combination of the multitude of events and circumstances, however, was too great to prevent failure.

So what does this say about the way in which the buildings were designed, and how such buildings should be designed in the future? From the report (Chapter Eight)

Buildings are designed to withstand loading events that are deemed credible hazards and to protect the public safety in the event such credible hazards are experienced. Buildings are not designed to withstand any event that could ever conceivably occur, and any building can collapse if subjected to a sufficiently extreme loading event. Communities adopt building codes to help building designers and regulators determine those loading events that should be considered as credible hazards in the design process. These building codes are developed by the design and regulatory communities themselves, through a voluntary committee consensus process. Prior to September 11, 2001, it was the consensus of these communities that aircraft impact was not a sufficiently credible hazard to warrant routine consideration in the design of buildings and, therefore, the building codes did not require that such events be considered in building design. […]

During the course of this study, the question of whether building codes should be changed in some way to make future buildings more resistant to such attacks was frequently explored.

Depending on the size of the aircraft, it may not be technically feasible to develop design provisions that would enable all structures to be designed and constructed to resist the effects of impacts by rapidly moving aircraft, and the ensuing fires, without collapse. In addition, the cost of constructing such structures might be so large as to make this type of design intent practically infeasible.

Although the attacks on the World Trade Center are a reason to question design philosophies, the BPS Team believes there are insufficient data to determine whether there is a reasonable threat of attacks on specific buildings to recommend inclusion of such requirements in building codes. Some believe the likelihood of such attacks on any specific building is deemed sufficiently low to not be considered at all. However, individual building developers may wish to consider design provisions for improving redundancy and robustness for such unforeseen events, particularly for structures that, by nature of their design or occupancy, may be especially susceptible to such incidents. Although some conceptual changes to the building codes that could make buildings more resistant to fire or impact damage or more conducive to occupant egress were identified in the course of this study, the BPS Team felt that extensive technical, policy, and economic study of these concepts should be performed before any specific code change recommendations are developed. […]

In short, the WTC was properly designed given the state of knowledge in 1966, when the design process was first begun. Could things have been done differently? Yes, although it’s not clear if the outcome would have been any different. Should things be done differently now? Yes, and they already are, due to the constantly changing nature of the building code writing process. Should we still build skyscrapers? The United States is full of tall buildings. To build no more would be short-sighted if the economic conditions which drive the construction of tall buildings remain functioning. The alternative to building up is building out, and I suppose that were the disincentives great enough, out would be where we would go. The economics of this should reflect the fact that a repeat occurrence of this sort would be highly unlikely since we have now decided that swarthy sorts who only want to learn to fly and not land a jumbo jet and who pay in cash are probably not a very good security risk. (But we dare not say that for fear of hurting the feelings of someone.)

A better question is whether we will concede that anything over one story tall is just automatically going to be fodder for infantile-minded murderers who want to knock our blocks down like petulant bullies, or whether we will hold them accountable for their actions and make their cost of doing business too high. I sincerely hope that we decide that we make it expensive for others to attack us, rather than burdening ourselves with the cost of defending ourselves from being attacked. Do we really want the equivalent of herding ourselves through metal detectors, raised to an enormous scale, just to buy a little perceived security?

Seems like the money would be better spent eliminating the threat rather than hardening the target.

Just my two cents worth.

Posted by Terry Oglesby at June 16, 2005 09:18 AM