General Question

gajdab's avatar

Is there a standard time ratio for how long it takes to build the foundation of a skyscraper, as opposed to the superstructure?

Asked by gajdab (8 points ) November 1st, 2009

I realize it may vary based on the amount of stories. Perhaps there is an equation that takes into account the amount of stories.

I am using the information as a learning metaphor, and have always observed that the foundation of a building take a long time to build, and the superstructure sometimes seems like it goes up overnight. Is that in fact true from an architectural/building standpoint?

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4 Answers

PandoraBoxx's avatar

Interesting question, and not one that I can answer directly except to add that a geological survey of the site is an important component of building the foundation, and the depth of the underground structure depends on how long it takes to find solid structure upon which to build.

Which, in a learning metaphor, would relate to the importance of survey classes, and having a good understanding of the subject from a broader perspective.

grumpyfish's avatar

Sort of. Pouring a foundation takes the following (rough) steps:

- Excavating
– Pouring footings
– Waiting for the footings to set
– Laying all of the conduit/etc that goes in/under the foundation
– Constructing the entire rebar structure for the foundation
– Building the concrete forms
– Pouring the concrete (which for a skyscraper, could take a while because it may need to be poured in stages)
– Letting it set.

Steel erection takes incredibly little time, because once things are bolted together they’re done—there’s no pre or post work (since the pre-work is generally done offsite).

Once the steel goes up, it’s again a slow process.

JeanPaulSartre's avatar

Super depends on what’s under that foundation, in many ways I hope more time goes into that than goes into the structure itself, since it’s literally all riding in that.

freerangemonkey's avatar

“I realize it may vary based on the amount of stories. Perhaps there is an equation that takes into account the amount of stories.”

Um, yes, definitely. There are dozens of equations. But there is waaaaay more to it than that.

The answer to this question depends on a number of factors. Namely, as you mentioned, the height or number of stories of the superstructure, the subsurface soil conditions, the type of construction (steel or concrete), local surroundings, even how you define the term “skyscraper”...

Most “skyscrapers” will require the driving of many deep piles. Often hundreds of them to a depth of hundreds of feet deep. This task alone can take months. If it is a short skyscraper, the time to drive the piles can easily exceed the time to build the superstructure. However, in a shorter “skyscraper”, it is possible that the pile-driving task will exceed the superstructure construction.

In general, however, I don’t think this rule holds up. A recent skyscraper in San Francisco, the One Rincon, Tower One, rises 60 stories to a height of about 650ft. For the bulk of the superstructure construction, the concrete subcontractor was making 3-day floor cycles, meaning that every three days they installed reinforcing and placed the concrete for one floor. I think this was their peak production rate, and it is entirely likely that the early floors went much slower, as did the completion of the roof, as both of these areas are irregular, while the middle floors are all identical plates (more or less). Nonetheless, if we assume that they were able to complete two floors per week, then it took thirty weeks to complete the concrete core. I know that the exterior glazing and cladding also lagged the concrete considerably, so I think it’s fair to say that the “superstructure” took close to a year to complete. I have a very difficult time believing that the foundation took this long, though it is possible.

Keep in mind that this is not considered a tall skyscraper, either. The tallest buildings in San Francisco are about 25% taller than this one. New York and Los Angeles both have buidings twice as tall as this, and Chicago has even taller buildings still.

And here’s the kicker, One Rincon, being a residential condo building, had to be concrete for a variety of reasons related to residential construction, whereas most of these taller offices are steel superstructures. Steel is considerably slower to erect than concrete. As such, I believe most of the tallest skyscrapers took considerably longer to complete the super- as opposed to the sub-structure.

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