Tuesday, August 01, 2006

By popular demand it seems

LOTS of questions for me to answer, it seems. I'll try to be concise, while expanding where I feel it necessary.

The short story is this; the explanation that Dr.Strangegun gives of how the WTC towers were built, and why they collapsed is largely correct. However, this doesn't get people very far, because most just don't understand how a building stands up. They need to have it explained in terms that they can relate to. So...

Where to begin?

First. One has to understand that, given the premiums on real estate in Manhattan, there is a market force at work that necessitates building vertically, instead of laterally. Given that, and the need for maximizing available leasing space after completion, a decision was made early on to minimize the intrusiveness of the structural system on the programmatic space (this is an architectural term referring to the question, "how are we going to use the space?"). Since the use of the space was a decision of the type of business to be conducted (that being offices with lots of white-collar workers with a high turnover rate), and leasing is done by the square foot, the need to put up simple demising walls, wherever needed, is paramount, hence the need for open floors.

Now one has to also understand that because these buildings were going to be 1300 feet, tall that building them to stand wasn't as much of a concern (we had previously built stuff almost as tall) as affording to build them that tall, all the while providing the owner or operator with the best possible solution for their leasing needs. This is important to note, because it lead to certain economies of construction style and scale. With offices and cubicles and secretaries and mailroom folks futzing about, a method for building the floors was devised that created just enough strength, a tuned floor, to support both the dead load (items that were carried by the building that were stationary, like the weight of the building itself (duh?) and others that are usually picked out of a common code book or table), and the live load (that would be the stuff inside that DID move around, like people, desks that aren't bolted to the floor, wind, snow on the roof, RAIN) and any other transient loads that the design engineer could either think of, or was, again, assigned to in a code book. Important note here: LIVE LOADS are considerably more dangerous, since moving the load about, on a floor let's say, can lead to spots of highly concentrated loading, or point loads as they are known (imagine moving half of the office personnel to ONE room, having them all stand in the same spot and you get the picture, you have to account for an unknown) which can stress structural members non-uniformly, a situation that gets engineers uptight.

The floors were, generally, a thin slab of concrete that was poured over a type of corrugated steel deck (still a standard in the construction industry, BTW) that was supported by a series of bar joists. These joists are made from a thin bar or angle that is bent and then welded to flanges at either end, themselves being a type of steel angle that is not much larger than the bars. This produces an open web design, which decreases structural weight (dead load) that is then transferred to the exterior columns and inner core. This design is rather reductive, eliminating a lot of the redundancy, and weight, that was an element of earlier designs, such as the Empire State Building.* This system was very efficient, as it could be built almost identically and used the same elements, generally, from floor to floor (contributing to that whole scale thing). Again, an efficient design for carrying the design loads, meeting with the technology, labor and technique of the era. This overall efficiency translated into an affordable building; one that could be counted on to recoup the costs, in terms of dollars, to build it. This is what we refer to as good life cycle costs in the industry.

The floors were, in short, designed to carry a very specific, and light, load, though having large expanses of open area with lots of live loads contributed to a certain amount of over-design. A good thing.**

Please understand that the building was NOT poorly engineered, nor cheaply constructed. It was merely engineered to accomplish a specific goal. When the design loads were violated, there were consequences. In other words, one could conceivably design for what did happen, but no one would actually be able to afford it. It would just be too damned expensive.

The reason for describing all this stuff with the floor and its loading is because that was a large contributing factor to the collapse.

Second. The fire. Suffice it to say that the fires did their number on the structures. Please understand that these fires were bigger than most people can comprehend. They engulfed several floors of each building, instantly, with these floors being a couple of hundred feet in each direction. That plan area is enormous. Imagine a building taking up most of a football field, and imagine several, maybe even as many as 10 floors, of fire engulfing everything. That is what really did the towers in. A sustained fire, with the building content contributing fuel, combined with the length of the burn caused the steel to get soaked with heat. Those thin bar joists definitely were the first structural elements to fail, the exterior columns at the initial impact notwithstanding. When the bar joists started failing, each adjacent joist took on additional load, until a floor failed, dumping its load to the next floor, along with its fuel. This probably accounts for some of the reports of additional "explosions" that were heard long after the initial impacts. The columns at the interior were quite large, for obvious reasons, and therefore were able to withstand heat by virtue of both additional mass, and by conducting some of the heat up and down its length. This is probably another reason the towers stood as long as they did, it took a long time for them to reach the critical temperature before permanent change took place. In short, by design, the floors carry only so much load each. They are not designed, unlike the columns which get thicker, deeper and more massive as the load accumulates at the bottom, to carry the loads from each additional story above. It just isn't something that one designs for. Ever.

Flying a plane, a large commercial jet, into a building, and expecting it to survive isn't typically something in your design criteria (it actually was in this case, but a smaller craft size was used), but remarkably, both towers took the impacts with no catastrophic damage. Let me repeat that, the impacts did little damage to the structure. The reason can be largely attributed to the fact that ALL skyscrapers are designed to accept extremely high wind loads, like those associated with hurricanes. Straight-line winds from thunderstorms or tornadic activity has the potential for higher wind speeds, albeit for shorter typical durations. These loads are applied to the model across the entire surface of each direction, yielding a massive potential lateral load. The plane, while quite massive and moving at almost 500 mile per hour, didn't have enough momentum to topple the building. That would be expected, as the building had the aircraft outweighed by many hundreds, if not thousands of times, similar to what happens when a light projectile at high velocity hits something that is much bigger and heavier, but stationary. Think 125 grain .357 hollowpoints at 1400fps. The projectile breaks itself apart when it hits the immovable object, but the body is NOT knocked down. RADICAL fragmentation of the jet is what took place, spilling its fuel load across several floors.

Third. The design standards have change since then, but not the academics. By this I mean that the values used to determine column and beam sizes and floor thickness are constantly being futzed with, with respect to the applicable code, which changes with the local municipal authority where the building is to be constructed. That is, the design standards in New York are different from those in Chicago which are different from those in San Francisco. This results in different values for member sizes, though usually not by much, for different types of structures that get the same thing accomplished, building UP.

What can be done?

Certain things can be done to offset some of what did happen. Reinforced concrete shear walls are almost always used for the inner core on these types of buildings these days. It adds mass to the building and gives excellent fire resistance to the stairs that are usually contained within, but I must admit to having doubts about even these stopping a 767 traveling at cruise speed. Considering that, I doubt that it would have bought any time for the building occupants who were above the impact zones. The devastation was just too high.

Structural fireproofing was added, if I recall, after there was a small fire not long after the towers opened. This is of some use in most cases, as it does insulate the steel from the effects of heat, and allows the sprinklers to douse the fire and wet the content of the effected area. It wasn't any use with what happened that day, and hopefully the reason is obvious.

Sprinklers. In short, they had no chance. The risers were, without doubt, severed upon impact, and couldn't feed the heads. It wouldn't have mattered with the amount of jet fuel that was dumped, as this type of fire can't be extinguished by that method, and that is because it wasn't designed to. Office buildings aren't usually filled with stuff that requires a carbon dioxide dump, or that fancy foam that they use at the airport. The idea is to get the content of the typical office wet (that would be the furniture, the carpet and PAPER) so as contain the fire until the professionals can get there and finish the job.

WTC 7

Again, in short, it DID get hit with a LOT of debris, both from the aircraft and the collapsing towers. There seems to have been little, if any, effort made to save it, as all the attention was placed on the north and south towers, where there was more at stake in terms of lives. It WAS on fire, and in a big way. Photos of it confirm that the whole building eventually was burning, though progression throughout the day was probably the case. Amateurs will inevitably argue this point with me. A building that has a 500 foot column of smoke coming out of it IS ON FIRE, and not just smoldering. I doubt that the sprinklers were even operating considering that there probably wasn't much head pressure in the immediate vicinity (this is no stretch).

Conclusion

The reason the towers, and similarly WTC 7, tended to fall straight down is that the loads from each collapsing floor overloaded the one below. Strangely, if the columns could have been somehow divorced from, A) the distortion forces of the heat, and B) the overloaded floors, the columns that constituted the outer structural "tube" would not have collapsed, since, as mentioned earlier, the columns are designed to carry these accumulated loads. This is a serious oversimplification, and made only for conceptual purposes. It's like making a perfectly frictionless surface, or perpetual motion machine. All of it works on paper, not in reality.

What couldn't be expected, however, is the floor loadings to become so unpredictable, what with the severed columns creating asymmetrical loads; point loads on floors which then tended to overload the adjacent exterior column(s). You just can't create a stable model around these criteria. The exterior walls started to deform from the bar joist pulling inward, the east wall of the south tower and the south face of the north tower followed.

The toppling myth

There exists a myth about this toppling thing. It is really not that hard to understand why these buildings fell straight down. If you drop a rock, it falls straight down, at a perpendicular angle to the ground. Gravity works the same way, every time, UNLESS AN OUTSIDE FORCE ACTS UPON IT. To make a 1300 foot tall building, that weighed as much as either of the towers, topple would have constituted a force that couldn't be readily delivered to the structure. It was simply too massive, too heavy, to push over. It HAD to fall straight down. In fact, the only way one could have gotten it to topple would have been with a 'controlled demolition' of the type that these conspiracy nuts are suggesting. THE ONLY WAY. END OF STORY.

The Murrah Building in OKC

This is another subject altogether, and constitutes a comparison that just can't be made. This is truly apples and oranges. Suffice it to say that the Murrah building was constructed with precast, prestressed concrete members in combination with some cast-in-place concrete on site. These two building types react quite differently to the types of events that each were exposed to, which were different. The Murrah was shorter and the building suffered catastrophic damage from the initial explosive forces, whereas it was the massive fire that took down the towers.

There are currently few buildings of the height of the WTC towers that use concrete as the primary structural element. Oddly enough it is the Petronas Towers in Kuala Lumpur, Malaysia, currently accepted as the world's tallest buildings I believe, which use concrete almost exclusively. It was a decision based on available materials, but required a very exotic mix with a certain silica additive to get the required compressive strength from the mixture. Steel is usually easier and cheaper to acquire and has properties that are far more forgiving for building at such massive scales. Steel structures tend to flex, and are forgiving of the idiosyncrasies associated with the typology of the skyscraper. I can't make an accurate prediction of what would have happened had the target somehow been these structures on that day, but my wild-assed guess is that it wouldn't be much, if any, better results for surviving.

* Though similar in that they both used steel extensively in their designs, the ESB used much more redundancy, both in total cross-section of the steel as well as the spacing of the steel, in almost every direction (remember, this was in the early days of going this high, they were all erring on the side of extreme caution). The additional weight and tighter plan spacing adds ballast to the structure, ensuring that the structure doesn't sway, or deflect, nearly as much during high lateral loads, like those associated with high winds. The steel joining techniques were very different in the early skyscraper days as well, with extensive use of hot rivets placed in plates, that were pounded into shape, resulting in a different type of joint, that bears its load differently than modern bolted connections (LOTS of esoteric stuff about this that is just not very germane to the discussion).

** This may have contributed to the time it took for the fires to bring down the buildings, as loadings from certain failures might have been carried for a short time by the structure below.

Recommended reading: Any layperson wishing to read about how structures work, or more importantly, how structures fail, should do themselves a favor and read an excellent book on the subject by Mario Salvadori, Why Buildings Fall Down, which is both informative and entertaining, if one can believe that reading about such subject matter can be.

For the inevitable trolls

This explanation is FAR from exact and surely leaves out some material, and instead generalizes on several relevant points about the conditions of the situation. If you disagree with it, in part or all, A) you are wrong, and B) I don't give two licks of a wet-dead-rats-ass! Keep it to yourself. If you want to comment in a sane fashion and/or have another question, just ask, I'll try to explain it.

6 Comments:

Blogger BobG said...

Excellent analysis and explanation.

8:51 AM  
Blogger Porta's Cat said...

thanks!

11:44 AM  
Blogger DirtCrashr said...

Yes, thanks!

5:43 PM  
Blogger Tam said...

Re: The Toppling Myth.

That's what I have the hardest time getting across to people...

The building's structure is designed to support its own weight... ...VERTICALLY.

If you could somehow knock a wedge out of the middle of a hundred-story structure, and set the top fifty stories falling sideways as a unit (a major accomplishment in and of itself), then once the upper structure had swung only a few degrees past vertical, its own weight would stress it in directions it was never intended to handle. If you could take a gigantic chainsaw to a skyscraper, it would collapse very nearly straight down: The support columns that perform so admirably in the vertical would be rather quickly overwhelmed by any shift towards the horizontal, and gravity would pull everything straight towards the ground in the perpendicular path that Ma Nature and Isaac Newton are so very, very fond of...

10:45 PM  
Blogger B&N said...

Tam, you're spot on.

That is exactly how it would happen.

8:38 AM  
Anonymous Steve said...

Well-stated, it's amazing the way some of the bs explanations have spred. Example, the one about "steel not reaching its melting point".. Makes me insanely mad. Steel, under load, partially destroyed by impact, doesn't need to reach MP to fail.

It's amazing that people might be willing to go to "culture war" based on their own darned stupidity.

8:32 PM  

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