super-slender bldgs. in NYC

[simplebm]

There are apparently quite a few very slender buildings being built and/or already built in New York City.
Some of these structures have height to width ratios of something like 14 to 1. Apparently they are concrete structures.
I haven't done a lot of searches but I haven't seen anything written about the foundation design. There would
seem to be tremendous uplift to resist. Has anyone seen any articles about the foundations for these super-slender buildings?

 

[bookowski]

They are caissons to rock in general.

 

[simplebm]

Thanks for the information ,bookowski. I would be interested to know the order of magnitude of net uplift to be resisted
as well as caisson diameter and anchorage.

 

[bookowski]

It would of course depend on the project. I've seen 24" diam with 750 ton (1500 kip) tension used in nyc. There was just a ctubh in nyc with several presentations on those buildings, I'd guess that there are accompanying papers in ctbuh (not free though).

 

[bookowski]

Older article ('07) but maybe still helpful for you

http://www.structuremag.org/wp-content/uploads/2014/09/C-CI-Minicaissons-Johnsen-et-al-Dec-071.pdf

 

[simplebm]

Thanks again bookowski for the info. Speaking for myself, I guess I am fearful of seeing such extremely slender buildings and wonder the advisability from
a structural engineer's viewpoint of doing such a project.

 

[bridgebuster]

I imagine they have tuned mass dampers in these buildings otherwise the person who buys the penthouse at One57 - for $130 mil - is in for a rough ride.

 

[KootK]

Some of these structures have height to width ratios of something like 14 to 1

It's worth noting that a more meaningful aspect ratio is that between the height of the building and the width of it's lateral system. For many of these buildings, the width of lateral system is, in large measure, the width of the elevator service core rather than the width of the building as a whole. Viewed in that light, the NY examples look a little closer to normal than they would using a conventional h/b ratio definition. Additionally, many of these structures make use of outrigger systems which effectively increases the with of the lateral system relative to buildings that do not incorporate these features.

None of my comments above should be construed as diminishing the achievements that some of these structures represent. NY engineers, and NY structures, are world class to be sure.

Apparently they are concrete structures. I haven't done a lot of searches but I haven't seen anything written about the foundation design.

In addition to the deep foundation technologies that bookowski mentioned, most of these structures have enough below grade levels that, when modeled appropriately, pretty much the whole building footprint can be utilized to resist overturning rather than just foundation below the superstructure lateral resisting elements. That's helpful.

Speaking for myself, I guess I am fearful of seeing such extremely slender buildings and wonder the advisability from a structural engineer's viewpoint of doing such a project.

This is quite a subjective matter as it comes down to engineering philosophy. Here's mine:

1) I use the best tools available to me to determine what is possible and what is not. I do my best to let the numbers be my guide with respect to what is possible and what is not. Not all subjective biases can be eliminated from my decision making but I do my best. We have much better tools available to us today than we had twenty years ago and, consequently, I'm not surprised to see that we're migrating towards higher aspect ratio structures.

2) Any engineer unwilling to exploit the latest tools to push boundaries is likely find himself left in the dust of his more aggressive competitors. That's just how the free market economy works and, truly, I believe that we're all better off for it. My dental hygienist used to be an architect in communist Poland. He got so disillusioned rubber stamping the same damn six story concrete office building over and over that he walked away from the profession entirely.

3) Realize that structural engineering is a reactive field. We push the boundaries until something does go wrong. Seriously. Then we circle back, figure out what did go wrong, and try not to make that same mistake again. Look at the extreme difference in lateral stiffness between the Tacoma Narrows bridge *(Galloping Girtie) and the subsequently constructed Mackinac Bridge. Eventually we will build some skyscrapers that are just too darn slender, I guarantee it. When that happens, we'll adjust, and keep moving forward. It sounds terrifying to think of things in these terms but it's the reality of our work.

4) Take some comfort in the likelihood that the first "too slender" failures will probably be serviceability issues. Uncomfortable accelerations at the upper floors etc. I see that as being a much more likely issue than actual building overturning. But then, who knows? The mass tuned dampers that bridgebuster mentioned allow designers to push the envelope even further with regard to occupant comfort. Perhaps that will push us closer to a true overturning failure.

I'm actually going to be in NY the third week of October for my first serious visit to the city. I can't wait to check out all of the infrastructure miracles, building and bridge.


I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[kipfoot]

not a technical description, but rather a basic overview:
ny times magazine

I recently visited NYC after a long absence. I was sitting in Central Park and looked up to see a pencil thin tower that I hadn't know about. It wasn't there many years ago when I lived there and it was shocking to me to see its proportions. 432 Park Ave.

engineer: Silvian Marcos - Cantor Seinuk
It has a concrete core and perimeter concrete frame
video narrated in layman's terms

 

[bookowski]

Minor note - it's not technically cantor seinuk, they're now wsp (who seems to be buying everyone - soon there will only be a few mega shops and tiny shops). And s. marcos is probably mid 80s by now and the actual engineer is some pm and a few lackeys.

But yes, the buildings on 57th are very impressive. I agree that technically its height/core but when you see them and the apparent slenderness it's definitely very cool.

 

[KootK]

@simplebm: pretty recent discussion here on tall building foundation in general: Link

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[yaschief]

Take a look at 785 8th Avenue...30'x90'x50' and 43 stories ... 18:1 ratio...my firm designed the structure.

 

[calvinandhobbes10]

Compression can be just as critical as uplift.

Believe most of these buildings use belt trusses at a few elevated, intermediate to engage the perimeter column dead loads and spread the overturning uplift/compression across the full footprint (raft plus piles). If you run a few back-of-envelope numbers for a concrete building, you'll find uplifts in bookowski's range, and also see how high the D+W compression can get.

Don't have this full paper but you can buy or preview, talks about outriggers/belts helping foundations.

Here is a blog by a designer of supertall buildings for anyone who hasn't seen it, not a lot about foundations though.

I was in NYC last year hoping to view the foundations of 220 Central Park South, but it wasn't far enough along yet.

Great topic, simpelbm.

 

[hawkaz]

Slightly off topic-
Are most buildings in New York City on piles, or are conventional spread footings used for smaller buildings?

 

[ahypek]

It depends, Manhattan has an interesting bedrock elevation profile. There are two shallow (relative) humps and then a dip in between them. If you look at it superimposed on the NYC skyline, you can see that most of our skyscrapers are sitting on this shallow part. For the most part, I deal with a lot of silty sands in Brooklyn and it's generally shallow foundations for 16 stories-ish and under. But that's if you're lucky to not have any transit authority structures nearby or problematic adjacent structures.