March 22, 2015 § 3 Comments
New York’s skyline has been in the news lately due to a handful of reed-thin additions. Rafael Viñoly’s astonishingly slender tower at 432 Park Avenue is on the jogging route between the hotel and Central Park this week, so it’s been a daily source of amazement.
As reported this week by Martin Filler in the New York Review of Books, 432 Park is just one of a number of residential towers either going up or on the books that are exploiting a handful of loopholes in New York’s zoning code and (perhaps more crucially) a seemingly limitless source of funds from the %0.01 fueling the luxury high rise market. The code has always allowed transferring air rights from one property to another, and has also permitted unlimited height for any tower not occupying more than 25% of its overall site. What’s new is the incredibly financial pressure being exerted by land values and the market for ultra-high end residences. These have pushed developers to exploit small sites with prestigious addresses to unheard-of heights. (There is, of course, a giant socio-economic story behind this that was addressed last month in the New York Times–also well worth the read).
432 Park is 1,397 feet tall–Sears territory–but its floor plates are each just 65’ square. This gives it a slenderness ratio of just over 21.5, quite literally pencil-thin. In the 1890s, a slenderness ratio of 7 was enough to give engineers fits because of lateral loading from wind. 432 Park and most other planned towers in its slenderness territory handle the loading by using super-stiff moment frames, the same principle that Sears uses for its bundled tubes. That accounts for the rigorous checkerboard pattern of windows on its facades; what you’re seeing is literally the structure, filled in with glass, and nothing more. The columns and girders are all thicker than they’d need to be for gravity loads alone so that they can make large, rigid connections to one another. Rigid connections, in turn, make the structure behave like a network, with every element being ‘recruited’ into resisting any sudden gusts of wind.
All fine and good, but the simple scale of wind loading isn’t the only problem. As architects Harrison and Abramowitz discovered when the then ultra-slim-looking Empire State Plaza buildings opened in Albany in 1966, buildings sway when subjected to constant lateral loading, and eventually they resonate. As buildings get deflected and spring back into place, they accelerate because of the constant force of wind (f=Ma, no matter what). At the right wind speed, they’ll keep going with surprising velocity. The repetitive motion isn’t necessarily a structural problem (though it can be–this was what brought down the Tacoma Narrows Bridge), but it’s a person problem. We get seasick with repetitive, lulling motions. The legend of the Empire State Plaza is that Nelson Rockefeller, then governor of New York, experienced this with predictably disastrous results upon moving in.
The standard solution for this kind of harmonic motion is a tuned mass damper–a huge weight in the upper stories of a slender skyscraper that’s allowed to slide back and forth, but that is tied to the building structure by massive springs. The inertia of the weight keeps it more or less in the same place while the building sways around it, and the springs dampen any excessive movement. Tuned mass dampers sound crazy, but they stabilize buildings ranging from the Citicorp Tower in New York to Taipei 101. (And, yes, in a retrofit, the Empire State Plaza).
What’s particularly problematic about this new generation of super-talls (can we go ahead and call them ludicrously-talls?) is that the upper floors are so small that there isn’t room for the large, massive dampers and surrounding space for them to move. One Madison Park, a mere 621’ residential tower that opened in 2010, solved this by using a “slosh tank” filled with fluid that takes the place (and space) of the springs. Other planned towers, like SHoP’s design for 111 W. 57th, tuck a tuned mass damper below setbacks and spires that narrowly taper to the advertised height–but of course this means access above the damper is next to impossible. Almost 300 feet of 111 W. 57th will be unoccupied.
So, what’s a structural engineer to do? One trend that’s utterly counterintuitive is to make these buildings heavier at their tops–to increase slab depths and column thicknesses to give the tops of towers more inertia on their own. This goes against two thousand years of conventional wisdom–after all, the Pantheon’s builders took great pains to incorporate pumice and empty clay jars in the topmost strata of its concrete to lighten the weight of the roof. But with high-strength concrete the norm, and larger column sizes in lower floors based on moment connections instead of dead load anyway, there’s no real reason not to do this. Think of an apple on the end of a yardstick–it takes a lot more effort to shake it back and forth than if you shake the yardstick alone.
Weird, right? Weird and hardly resource-efficient. But intuition only works for so long–new economic pressures, code loopholes, and material science have always combined to create uncanny structural forms; after all, many Chicagoans thought the ‘super skinny’ Reliance Building would blow over in the first good windstorm. So it’s just possible that super-skinny will be the new norm, and that we’ll continue to see reed-thin skyscrapers that conceal giant weights at their tops–whether these weights are built into the structure, or whether they’re sliding around on an upper floor. But there’s also a lot of Rube-Goldbergian cleverness to getting these towers to hold still, and there are rumors about more than one of them that even with the best-intentioned dampers and stiffness the motion at the top–the most expensive units, of course–is more than what was hoped for. F=Ma has a sneaky way of winning out.
(hat tip to argz for some good background ‘skinny’ on this topic today).