In the supertall era, is the sustainable skyscraper a myth?

The illustrative and alliterative Tall Trends of 2019, the latest look at the global state of tall towers by the Council on Tall Buildings and Urban Habitat (CTBUH), suggests that the age of super-tall towers and expanding skylines is just beginning.

In 2019, 26 supertalls, which the organization defined as buildings measuring 300 meters (984 feet) or taller, were completed, besting the previous record of 18 set in 2018. An additional 100 skyscrapers between 200 meters (656 feet) and 300 meters or more completed last year, a slight dip from the previous year, when 146 opened, but part of the decade-long trend of more and more such buildings taking shape in more and more corners of the globe. In 2020, the organization estimates between 115 and 145 200-meter-plus towers will finish, 17 to 30 of which will be supertalls.

This new generation of towers, which represent the utilization of cutting-edge technologies, showcase great feats of engineering. But in a world slowly responding to climate change, can this type of construction, which requires massive amounts of energy and materials, ever approach sustainability?

The answer, according Daniel Safarik, editor at the CTBUH, is complicated, mostly because of a lack of data. Many designers, construction firms, and building owners don’t share information about materials inputs, or the performance of these towers over time. The CTBUH is interested in the answer, Safarik says, and helps to conduct sustainability research. But part of being more sustainable is changing not just what we know about these buildings, but the way the industry and culture evaluate, and elevate, skyscrapers.

“Displaying buildings with height markers only, like they’re in a police lineup, talks about just a small part of the story of these buildings,” he says. “I think we can do a lot better in this industry.”

CTBUH

Can better building techniques and materials make a difference?

Many of the latest generation of tall, high-tech skyscrapers exemplify improvements in design and building technology. The use of parametric design, algorithmic software to create more efficient blueprints, as well as new material technology, including the nascent growth of tall timber structures, means new buildings can be constructed with less and less concrete and steel. Increased use of new tall timber construction methods—architects at Perkins + Will have said it’s theoretically possible to build as tall as 80 stories⁠—also raises the possibility of using a renewable, locally-sourced material for construction.

One of the biggest challenges in designing tall and super-tall structures has always been counteracting wind forces; computer-aided design has helped structural engineers and architects design unique structural solutions, and novel shapes, that do just that, while using less material overall.

But despite some reduction in total materials used, buildings still require immense amounts of energy to construct—a material cost often referred to as embodied carbon—and to operate. Much of the recent push toward greener buildings focuses on the latter, the operational cost (both proponents of the Green New Deal, and many Democratic presidential candidates, have suggested the country makes big investments in energy retrofits and more stringent, sustainable building codes). For instance, New York’s Empire State Building underwent a green retrofit over the last decade that reduced energy use by 38 percent, and the city just passed its own Green New Deal, the Climate Mobilization Act, that requires buildings of more than 25,000 square feet—which are responsible for 30 percent of the city’s carbon emissions—to conduct retrofits, such as new windows and insulation, to make those buildings more energy-efficient.

Safarik says that the Tianjin World Financial Center, a 75-story supertall which was completed last year in Tianjin, China, exemplifies many new trends in sustainable construction. Designed by Skidmore, Owings & Merrill to meet LEED Gold standards, the tower, positioned across the Hai River from a high-speed rail and transit hub, has a gently sloping, crystallized facade, and curves designed to reduce “vortex shedding,” which sharply decreases the wind forces impacting the tower.

According to a soon-to-be-released study by SOM published by CTBUH, the tower utilizes an innovative design and support system. Steel plate shear walls—structural elements that transfer lateral force—helped absorb the forces of the wind and allowed for a thinner exterior, while hollow steel tubes filled with high-strength concrete allowed for thinner interior columns. Both elements allowed for larger floor plates while using fewer materials.

Compared to a traditional skyscraper layout, the final Tianjin design lead to “an extraordinarily efficient, high-performing structure and conservative use of resources,” the study notes, saving an estimated 19,000 metric tons of steel, 10,500 cubic meters of concrete, and 3,900 metric tons of rebar. The unique shape and structural design also resulted in lower fabrication and erection costs and a shorter construction time.

CTBUH

But for most buildings, Safarik says, this information isn’t easy to get. There are few agreed-upon data standards or required disclosure.

“I think we can do a lot better in this industry and field,” he says. “We want to be involved in environmental and performance advocacy, as well, but it’s just not as easy to make, say, a list of the best performing supertall buildings of 2020.”

While there’s more to learn, many existing studies supported by the CTBUH point toward solutions. A paper released last year that analyzed energy use and building structures in Western Europe found that structural design can make a significant difference in material usage and environmental impact. Using a diagrid design—a latticework of intersecting diagonal beams, as opposed to so-called tube structures, which rely on a central concrete cylinder for support—for a tower between 150-250 meters can reduce the need for concrete by 17 to 33 percent and steel by 28 to 41 percent.

A research paper by Christopher Drew, director of sustainability for Adrian Smith + Gordon Gill, a pre-eminent firm for skyscraper design, suggests that achieving a carbon neutral building is indeed a possibility. But buildings will likely only reduce their life cycle carbon emissions if regulations encourage them to do so. They suggest cities and countries begin to adopt new regulations, including: mandating Environmental Product Declarations, which establish embodied carbon value for building materials and make it easier to track and reduce embodied carbon emissions in construction; new building standards for sustainability that give owners marketing and bragging rights for greener construction; and zoning incentives from local planners that let more sustainable buildings add more floor space, which provides an economic incentive to cut embodied carbon.

Context is key

Materials and methods aside, the location of these buildings can play a great role in their overall environmental performance. A glass-clad supertall in a desert or tropical environment may require extensive energy to cool. The placement of such a tower within the urban landscape can make a massive difference: a stand-alone tower cut off from the grid doesn’t offer the advantage of density; a supertall built atop a transit stop in the center of a walkable downtown does.

Safarik points to the Willis Tower in downtown Chicago, near the CTBUH office, as an example. The tower, initially known as the Sears Tower, counted the famed retailer among its early tenants, and served as the company’s downtown office. Packing all of those workers in a dense office tower, one located close to rail lines and public transit, was more sustainable, he’d argue, than the corporate campus that Sears would later open in a nearby suburb.

Beyond cutting down on operational energy usage, a building’s overall energy usage and sustainability also depends on the lifestyle of those living inside. A 2017 CTBUH study that compared high-rise living in Chicago to suburban life in Oak Park, a nearby suburb that’s linked closely to commuter rail lines and the city’s subway system, found that residents of downtown residential towers consumed 27 percent more energy per-person than suburban residents; the results, which compared a single, relatively transit-dense suburb with very high-end luxury condos and apartments, was admittedly looking at very distinct and small demographic groups. But it does point to the value in continued research into the ways design can impact lifestyle and energy consumption.

In the next few years, a number of new high-rise designs and building concepts will be tested at ever-increasing heights. The House, the world’s first passive house high-rise—a type of green design and construction that creates an airtight building envelope to reduce energy costs—opened on New York City’s Roosevelt Island last year, and a 60-story design has been proposed for Vancouver. New facades meant to save energy, including a plant-covered high-rise in Singapore and a concrete-clad tower in Mexico City, have become more prevalent around the world. A 38-story all-electric tower has been proposed for Brooklyn, and increasingly tall wooden towers are being proposed in Tokyo and elsewhere. While ever-taller towers will cast longer and longer shadows, hopefully they’ll begin to reduce their environmental footprints.