The future of urban high-rise architecture has never looked rosier. Or in this case, greener.
We generally think of large urban centers as being incompatible with nature, fresh air, and a lush green environment. But increasingly, this is changing as high-rise buildings are more and more being designed with their environmental impact in mind.
Structural engineers are now being challenged to find ways to help design buildings that make a positive contribution to the urban environment. And this has led some to see building design as an opportunity to literally add “life” to the cities of today and tomorrow.
The objective of creating a greener urban future received a significant boost at COP26, the United Nations conference on global climate change that took place in 2021 in Glasgow, Scotland. At the conference, one company offered a vision for the livable cities of the future when it unveiled its concept for green towers that it has dubbed “Urban Sequoias.”
Skidmore, Owings, and Merrill (SOM) — which made a presentation on the concept to COP26 — is one of the world’s largest architecture, interior design, engineering, and planning firms. Headquartered in Chicago, SOM has taken a global leadership role in designing urban projects that put a premium on being environmentally friendly.
The Urban Sequoia is conceived to behave the way a tree does in nature. It absorbs carbon and purifies the air, just as trees do. In fact, SOM claims that one of these towers is the equivalent of a staggering 48,500 trees, removing some 1,000 tons of carbon from the atmosphere each year. SOM envisions “forests” of such towers that would not just clean the urban air but would generally add to the quality of life of residents living in the urban environment. While we believe that these figures are somewhat ambitious, and may be estimating significantly on the high side, we believe that is a noble cause nevertheless.
According to this innovative concept, the amount of carbon being absorbed by the buildings increases over time as the amount of vegetation increases. Going beyond “net zero,” the prototype of the tower after 60 years would absorb 400 percent more carbon than was emitted during the construction of the building. It wouldn’t just not worsen the urban environment; it would actually improve it. And the captured carbon could then be put to use in a variety of industrial applications.
The concept is described on SOM’s website (www.som.com) as “a buildable prototype for a carbon-free future.” It is explained this way:
“What if buildings could act like trees—capturing carbon, purifying the air, and regenerating the environment? Taking inspiration from natural processes and ecosystems, Urban Sequoia envisions “forests” of buildings that create a new carbon-removal economy and a resilient future for cities.”
An innovative structural design
Structural engineers were called upon to consider the unique requirements of these green towers when creating the design. They had to factor in the added loads of the gardens as well as of the saturated soil. And one of the most important ways these added loads have been accommodated is with the use of new and emerging technology, including new and lighter carbon-absorbing materials.
One usually associates materials like concrete, steel, and glass with the construction of skyscrapers — and those elements remain a part of the picture. But the carbon emission of buildings, as they are being constructed, is being reduced by up to 50 percent with the advent of materials like bio-brick, hempcrete, timber, and biocrete. And SOM is at the forefront of designing structures that take advantage of these more environmentally sustainable materials.
In addition to the choice of materials for the Urban Sequoias, added loads are being accommodated through the innovative design that structural engineers have used to make these urban high-rise gardens possible. Like a flower, the “petal” design of the floors offers innovative opportunities to reduce the building’s loads.
The design features a central structural core, perimeter gravity columns, and a floor-framing system that spans the area between the core and the perimeter columns. The floor design is like a circular tube that is divided into four quadrants. Each of the quadrants encompasses two circular shafts. A shaft might be taken up by an elevator or two shafts by a stairwell. The precast units are tied together with vertical, post-tensioned recycled steel strands. They are also linked horizontally with a link beam element. An article in Structure magazine (https://www.structuremag.org/?p=20638) further explains details of the design:
“Perimeter precast columns with low-carbon concrete are located in the inner concave portions of the petals’ geometry. The horizontal framing system is similarly organized around the eight programmatic floor spaces directly connecting the perimeter columns to the center core. The low-carbon precast beam framing is curved in plan around each petal, leaving an open circular floor space in the center of each petal. The precast beams incorporate prefabricated ducts for post-tensioned strands and large diameter air ducts to directly capture outside air from the perimeter and channel it to the central tubes inside the core using the naturally occurring positive and negative air pressures from wind. The precast columns are also laced together with post-tensioned strands made with recycled steel.”
The article cited above also has this to say about the strength of materials used, which is another essential element in dealing with the added vegetation loads:
“The construction process could involve precast to allow the material to reach necessary strength off-site and be quickly erected on-site, still achieving fast floor-to-floor cycles.”
It is through these new, environmentally sustainable materials — and the petal design that permits vertical air circulation through the building’s core — that the impressive carbon absorption can be achieved.
As SOM explains on its website, 40 percent of all global carbon emissions are generated by the building sector. And with the growth expected of urban populations in the coming decades, studies have predicted that another 230 billion square meters of new building stock will be needed by 2060. So a building concept that not only reduces carbon emissions but actually absorbs carbon throughout the lifetime of the building could not be more timely or more needed.