Structural Marvels Make New York’s Super-Slender Towers Possible
New York building codes consider a building to be slender when its height is more than seven times its width at the narrowest point. But the recent crop of luxury residential towers in the city’s Midtown go far beyond this.
20 July, 2016
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In 2014, One57 raised eyebrows with an aspect ratio of 8:1, becoming the tallest residential building in the city at 1,005ft (306m). It has been quickly overtaken in both senses by 432 Park Avenue, its 1,396ft (426m) height 15 times the width of its base. There are more under construction: 56 Leonard (10.5:1), 30 Park Place (12:1) and 53W53 (13:1). But 111 West 57th Street beats them all with a staggering aspect ratio of 24:1. Rising from a plot of just 80ft by 590ft, it is 1,435ft tall (437.5m).
“It’s almost three times more slender than what used to be considered a slender building,” says Silvian Marcus, director of building structures at WSP in New York, the structural engineer on the project. “It’s going to be a world record. But we are working on others that are going to be even more slender.”
There’s something unique about living in midtown and having that perfect view.
Simon Koster, JDS Development Group
The Logic of Slender Towers
The logic of slender buildings is to derive the greatest possible value from expensive, constrained city-centre plots. So it’s no surprise that the super-slender revolution began in New York, a pioneering skyscraper city with some of the most sought-after real estate on the planet and a clamouring global customer base. “There’s something unique about living in Midtown and having that perfect view,” says Simon Koster from JDS Development Group. “Everyone understands the beauty and value of being able to build something like this in this location.”
Where these towers differ from previous generations is that they are not commercial but residential – typically, each floor comprises a single luxury apartment. This only adds to the challenge for the structural engineer. High-rise engineering is a very specialized discipline, and many of the technologies used in conventional low-rise construction would be too heavy, inefficient and expensive if applied to tall buildings.
432 Park Avenue
Keeping Buildings from Moving in the Wind
But super-slender changes the game yet again. There is no theoretical limit to how tall a building can be, but the taller you go, the stiffer and stronger the structure needs to be. A skinny building will also need to be stronger than a broad one to withstand wind and seismic forces – but the structural components must also be kept as unobtrusive as possible to maximize usable space and uninterrupted views. A building’s core is an important source of stiffness but this can be proportionally smaller in a residential tower than a commercial one. Conversely, the higher a tower and the more people living there, the greater the elevator capacity needs to be, and the amount of mechanical, electrical and plumbing infrastructure increases.
“All of this can swell the core and encroach on living space,” says Fatih Yalniz, vice president of building structures at WSP in New York, who carried out analysis to establish the most effective frame for 111 West 57th Street.
Hundreds of metres above the ground, wind flows are completely different – more akin to what an airplane would experience. In managing the building’s response, their goal is not to eliminate all movement, but to control the pace of acceleration. Marcus likens it to being in a car: passengers only sense the movement when accelerating or braking, not when travelling at a constant speed.
111 West 57th Street
“It’s impossible to stop a building from moving, but we can control that movement so the majority of people will not feel it.” As residential buildings, these towers must meet more stringent standards than offices: there is no question of evacuating an apartment tower in the event of a hurricane, and people need to feel safe and comfortable in their homes no matter the weather. “A person coming home to the 50th floor does not expect to move in the wind,” he points out. “Slender towers have such a small plan area, we have to mobilize everything possible to achieve that.”
Every Building Is Unique
As in the last golden age of New York skyscrapers, every developer wants to set their building apart with an iconic design as they compete to attract the attention of a very discerning global client base. “Every building is unique and it needs a unique structural concept,” says Hezi Mena, senior vice president of building structures at WSP | Parsons Brinckerhoff in New York. “There is no set formula.”
There are however some proven strategies, which Mena and his colleagues have refined as they have achieved increasingly slender designs. The first step is to stiffen the structure. For this reason, super-slender towers are invariably concrete rather than steel. Using concrete for the core of the building and connecting shear walls enables the engineers to minimize structural elements within the apartments themselves, giving the occupants maximum flexibility on layouts and preserving those all-important views.
But strength alone is not enough. “Brute force does not work here,” says Jeff Smilow, executive vice president of building structures at WSP in New York. “We have fewer engineering opportunities so we have to be more innovative. We have to work with the wind instead of fighting it. 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.”
Like the Shock Absorber in a Car
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 façade to minimize the ‘vortex shedding’ response which causes high acceleration. WSP works closely with architects to refine the shape of a building, using wind tunnel analysis.
The engineers can also reduce the acceleration of the movement by using a damper, similar to the shock absorbers in a car. This is a heavy weight placed high up in the building, either solid, in the case of a tuned mass damper, or liquid, as with a slosh damper. As the building moves, the weight moves too but more slowly – thus slowing the acceleration of the tower.