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The Super Slender Revolution

New York is once again leading a revolution in high-rise design, but its newest skyscrapers are distinguished not only by their height but by their extraordinary slenderness. In the last five years, a crop of pencil-thin towers has begun to spring up on tiny plots, many clustered around the edges of Central Park in Manhattan’s Midtown.

New York’s building codes consider a building to be slender when its height is more than seven times its width. But WSP | Parsons Brinckerhoff is now delivering buildings that double and even triple this. One of the first examples of the trend is the just-completed One57, designed by French architect Christian de Portzamparc. It has become the tallest residential building in the city at 306 m, with a slenderness ratio of 8:1. But it will very shortly be overtaken by 420 m-high 432 Park Avenue, where construction is around three-quarters complete, and which boasts a ratio of 14:1. This will be followed by 56 Leonard Street, due to complete in 2016, MoMA Tower, 15 Hudson Yards, and the needle-like 111 West 57th Street, which has a mindboggling height-to-width ratio of 24:1.

Unlike the majority of New York's towers, these buildings are not commercial offices but luxury residential developments. “The key difference is that the floorplates are not as big as conventional offices, and because these are luxury apartments, developers want the bulk of them to be physically located above neighbouring buildings, so that they have a view,” explains Jeff Smilow, director of building structures at WSP | Parsons Brinckerhoff in the USA. “That's given rise to these very slender ratios.”

The key challenge for engineers is to manage the building’s response to the wind, and specifically its acceleration – in a car, you only sense the movement when accelerating or braking, not when travelling at a constant speed. “A person coming home to the 50th floor in a building does not expect to move in the wind,” says Silvian Marcus, director of building structures at WSP | Parsons Brnckerhoff in the USA. “It's impossible to stop the building from moving, but you can control the movement so that the majority of people will not feel it.”

Residential buildings have to perform much better than commercial buildings, adds Smilow. “With a commercial building, you can evacuate if there's a hurricane, but in a residential building people live in it and they have to feel safe in it regardless of the weather. The criteria are much more stringent.”

But engineers must do this without many of the traditional tools at their disposal. An office tower’s concrete core is a traditional source of stiffness, but this too is much smaller in a residential building. Maximizing usable space is even more important in such slender towers, and unobstructed views are a key selling point. This means that simply beefing up the structure is not an option either.

“Brute force does not work here,” says Smilow. “We have fewer engineering opportunities so we have to be more innovative. We have to work with the wind instead of fighting it, and we have to understand the behaviour and the response of the building.”

One option is to adjust the shape of the building to make it more aerodynamic, introducing openings to allow the wind to pass through or adding curves at critical locations along the facade to minimize the “vortex shedding” response which causes high acceleration. WSP | Parsons Brinckerhoff works closely with architects to refine the shape of the building, using wind tunnel analysis. There are also a range of structural systems that can improve the building’s stiffness without obstructing the layout, as can the use of high-strength concrete. “The goal is to find out what works, what shape responds best to the wind and reduces the acceleration, not just to add structure unnecessarily.”

Such tall buildings will also require a damper to slow the movement, but even here the structural engineers seek to minimize material: “We always feel we've reached our goal when we can eliminate the damper. We can’t do that in very tall buildings, but we can use a less expensive type of damper.”