The Nearly Fatal Design Flaw That Could Have Sent the Citigroup Center Crumbling

[M II A II R II K]

The Nearly Fatal Design Flaw That Could Have Sent the Citigroup Center Skyscraper Crumbling


AUGUST 15, 2014

BY JASON CARPENTER



Read More: http://www.6sqft.com/the-nearly-fata...per-crumbling/

Additional: http://99percentinvisible.org/episod...ral-integrity/

When it comes to skyscrapers, we put a lot of trust in architects. We have to trust that they know what they’re doing, and these seemingly impossible buildings are safe to be in and around.

- It’s even harder to trust what used to be known as the Citicorp or Citigroup Center, now 601 Lexington Avenue, whose bottom floors are like four stilts, holding 50 stories of building above them. It looks like a strong wind would blow the whole structure over. And when the building was constructed in 1977, before some emergency repairs, that was true.

- The problem was discovered in 1978, when structural engineer William LeMussurier’s staff had a discussion with a Princeton University civil engineering student named Diane Hartley. Hartley claimed, correctly, that the building was unsafe, due to an unusual weakness to winds hitting the corners of the building.

- Even worse, a construction error changed the original design’s welded joints to bolted joints, weakening the entire building. A tuned mass damper was the only thing keeping the building intact, and it required electricity to function. If the electricity were to go out, a sufficiently powerful storm could blow the building over. An emergency repair welded steel plates over the bolted joints, making the building safe again. Had Hurricane Ella made landfall that year, the story might have been very different.

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• Video Link

[volguus zildrohar]

I saw this story on a documentary a few years ago. A massive disaster that would have been.

To quote Ian Malcolm, "God help us, we're in the hands of engineers."

[dchan]

We should always remember that new engineering designs are often flawed.

Though engineers have done rigorous calculations, testing, and other "worst case scenario" analysis to predict how their new designs will behave, they cannot possibly anticipate everything that the real world will throw at their designs. The fact is that until a design becomes reality, it remains only an "on-paper" theory. Only by turning a design into a real full-sized product, having that product withstand years/decades of real-world abuse, and duplicating the designs in other products can the engineers see how their designs truly behave in the real world.

It doesn't matter what the product is. This point applies to any new engineered product, whether it be new cell phone designs, new car designs, or new structural building designs.

The big difference, of course, is that cell phones and cars can be tested thoroughly using working full-sized prototypes before they're released onto the market. Buildings and other large structures, on the other hand, are far more expensive to build. Their prototypes are supposed to be fractional-sized mockups, which usually undergo wind tunnel and seismic tests. But since these mockups are much smaller, they encounter far different conditions and forces than the actual full-sized structure will encounter.

So in essence, the actual building/structure becomes the test prototype. You can say that the Citicorp employees working in the Citicorp Center were essentially test subjects for the new design.

"Normal" designs are usually safer because they have gone through decades of duplication, rigorous testing, and engineering analysis. All of this real-world testing experience is transformed into engineering and safety codes, which then dictate how similar designs should be built in the future.

[chris08876]

I wonder how this tower would fare if a large Earthquake struck near NYC as history has shown that they have occur. Hopefully one doesn't occur but I'd be curious how the engineering would hold up.


http://upload.wikimedia.org/wikipedi...aultSystem.png

[dchan]

Strong earthquakes are taken into consideration when designing buildings in the Northeast. It's part of the building code(s), and all new buildings must be built to withstand a certain factor-of-safety above the force wrought upon it by an earthquake of magnitude X (X being whatever is stated in the building codes).

The building's tuned mass damper would also help it resist earthquakes (but, as stated in the video, only if electricity is still on).

[Londonee]

Joe Morgenstern wrote an incredible article on it in The New Yorker probably 20 years ago called The 59-Story Crisis. Google it, worth a read. Fascinating case study on business ethics.

[aaron38]

Isn't this a good example why buildings should stick to tried and true engineering, instead of going off on wild cantilevers and such? Sure it looks cool to have half a building sticking out unsupported. But that also means that half the building IS sticking out unsupported.

A building meant to last 100-200 years shouldn't depend on a handful of welds on a transfer floor. Don't try to be cleaver, put columns under it.

[dchan]

Originally Posted by aaron38

Isn't this a good example why buildings should stick to tried and true engineering, instead of going off on wild cantilevers and such? Sure it looks cool to have half a building sticking out unsupported. But that also means that half the building IS sticking out unsupported.

A building meant to last 100-200 years shouldn't depend on a handful of welds on a transfer floor. Don't try to be cleaver, put columns under it.

Generally, yes. As I stated in my post, "tried and true engineering", as you call it, has been tested by multitudes of buildings built over decades. Their designs have been written down into code books, which dictate to engineers the right materials and dimensions to use to provide adequate factor-of-safety in their designs.

But the Citicorp Building was a special case because it had a church sitting on one corner of the lot. The code books won't tell you how to design a skyscraper in such cases. It's up to architects to work around such conundrums, and structural engineers to design a structural system that will work with the architect's designs.

Let's not forget that engineering concerns are usually a mere side note in the construction of new buildings. Economics and demand are the number one factors that drive the creation and design of skyscrapers.

We all know that the Home Insurance Building in Chicago was considered the first skyscraper to use a steel skeleton structure. But don't think that this building was an absolute game-changer that prompted all skyscraper designs to follow suit, because it most certainly wasn't. First off, most of the building's skeleton was actually wrought and cast iron, not steel. But more important is that during the same time period, there were many ideas in how to design the structures of these new skyscrapers.

The traditional method, the "tried and true", if you will, was the masonry bearing wall system. This structure relied on heavy bearing walls, exterior and interior, to support the weight of the building. The "tried and true" aspect of these buildings was that if their structure was primarily made from masonry, they were essentially fireproof.

So if bearing walls were "tried and true", what prompted the switch to steel skeleton structure in skyscraper designs? You might think that wind-resistance engineering played a role, given that the flexible steel skeletons allowed it to resist wind far better than unreinforced masonry. But you'd be wrong. Back then, nobody had ever built anything tall and slim enough to require wind-resistance engineering. In any case, the sheer mass and weight of bearing wall skyscrapers of that time period allowed them to resist winds quite well (remember that the buildings weren't that tall yet).

The answer? Economics. Elevators at that time weren't the reliable modern convenience of today. They were scary newfangled contraptions mostly used to lift freight up and down. So ironically, the most valuable space in skyscrapers was actually in the lower floors, with ground floor space being the most valuable of all.

But which floors in bearing wall skyscrapers had the least amount of usable interior space? The lower floors, with the valuable ground floor space having the least space of all. And as skyscrapers became taller, this problem became worse.



This conundrum prompted building developers to seek a solution that would allow them to create as much usable interior space in the ground floor as possible. Enter the steel skeleton ... or rather, structures that predated the true steel skeleton.

Skyscrapers of this period that were built using these transitional structures were no longer "tried and true". Just like in the Citicorp Center, the occupants of these buildings were essentially test subjects of how well engineered these buildings truly were. But unlike in the Citicorp Center, there were some unhappy endings with these transitional skyscrapers.

As I mentioned before, the "tried and true" engineering aspect of buildings was mainly about their fire resistance (the main engineering focus of that period, and a very important one today as well). The cast irons, wrought irons, and steels at that time were all advertised by their manufacturers as being "fireproof" like masonry, but it was clearly not true.

We all know of steel's poor performance in intense heat from the WTC towers collapse. But steel's performance in fire in relatively benign compared with cast iron. Back then, cast iron was already known to be weak in tension and brittle, just like masonry. It was mostly used for vertical columns in these transitional structure skyscrapers. Unfortunately, it differs from masonry in that it greatly expands in heat. As a result, when the columns expanded during a fire, their connections to the floor beams broke apart (because cast iron is brittle), and the whole structure would collapse without much warning.

But without these "test subjects", if you will, we would never have transitioned as quickly to using true steel skeletons for skyscrapers. And if economics didn't play a role in the transition, we may have likely seen some wind-related failures of bearing wall skyscrapers that had been built too tall. Therefore, though you might think of "tried and true" as the most logical way to design buildings, don't forget that the same thought process may hinder the engineering and design innovations that can be used to create better structures.

[Allan83]

If we never tried anything new we'd still be living in caves.