simplebm said:
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.
simplebm said:
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.
simplebm said:
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.
