By Mark Chatten, Principal
When people admire Saharan dunes, sinuous rock formations and impossibly slanted trees, they see the inexorable power of the wind to shape forms. But this phenomenon is no longer the exclusive preserve of nature. What’s more, the effects no longer take decades, centuries or millennia to become visible. Through sophisticated modeling and testing, architects can use wind to sculpt their buildings.
The results can be seen in three iconic supertall buildings: the Shanghai Tower, 432 Park Avenue in New York City, and the world’s tallest building, the Burj Khalifa in Dubai. All three owe their form—and their very existence—to the shaping power of simulated winds. In each case, the building’s final form emerged through a collaboration between RWDI and the design team.
The centerpiece of such a collaboration is a “shaping workshop.” The entire design team gathers for a day at our headquarters in Guelph. Our staff has built a precisely detailed model of the building and its environment. The model waits in our wind tunnel, instrumented with hundreds of sensors.
Working with this instrumented model, our engineers, analysts and model-makers test different configurations of critical features. These tests happen on the fly: Clients see the results within minutes.
We use each set of results to guide the next round of tests. Our goal is a form that will be safe, economical and comfortable—and still faithful to the architect’s original vision.
We recently worked on shaping the form of the Korean Cheongna International Tower. This leisure and observation tower, planned to be about 450 meters tall, will be an anchor for the Cheongna International City near Seoul, Republic of Korea. The design calls for LED digital façade technology that will make the faceted tower “disappear” against the sky. We recently held a shaping workshop for the project. Our objective was to minimize the effects of the wind while allowing the design team to achieve the effect of “invisibility.”
Our primary goal in this type of wind analysis is to control a phenomenon called vortex shedding. As wind passes around a building, the airflow curls into vortexes. These vortexes detach, or shed, from the building with a characteristic frequency. This frequency depends on the wind speed and the shape of the building. As the vortexes detach, they exert force on the building. In very high winds and at the right frequencies, these forces may cause the building to sway. And in certain conditions, the swaying can be unpleasant—or, if not properly addressed in the design, even unsafe.
To support decisions about the form of the tower, we developed a statistical analysis of the local prevailing winds. Then we tested 10 detailed scale models—a baseline design and 9 variations—to understand how the building will sway during extreme windstorms. Given the slenderness of the tower, we expected strong vortex shedding, and tests confirmed this behavior.
Often, part of the solution is to use sharp corners. However, we found that corner treatment provided only modest improvement, because of the tower’s faceted, crystal-like profile (roughly hexagonal in plan, with the profile changing with height).
Another good option is to open various sections of the building to the wind. (We used this strategy with great success in the 432 Park Avenue tower and more recently the Vista Tower in Chicago.) In the end, we found adding one or more “blow through” floors could reduce the structural wind response by more than 60% for the critical wind directions that would cause vortex shedding. Such a design not only keeps occupants comfortable but also significantly reduces structural costs.
With our approach to form optimization, architects gain an invisible but powerful design partner: the wind. As in nature, the results are stunning.
- Dong Il Architects & Engineers, Seoul
- Dongyang Structural Engineers, Seoul
- Arup, Hong Kong
RWDI works in collaboration with many of the world’s leading architects and structural engineers. We have provided wind engineering for hundreds of buildings and structures worldwide, including seven of the world’s tallest buildings. A related and integral part of our work with tall buildings is the design of the tuned mass dampers that help make them possible.
- Introduction to RWDI’s form optimization services: Aerodynamic Form Optimization - High Performance Structures
- Technical overview of building aerodynamics and shaping workshops by RWDI Principal Jon Galsworthy: "Form Follows Physics" Structure Magazine.
- Cheongna City International Tower" entry in The Skyscraper Center, the Council on Tall Buildings and Urban Habitat’s tall building database.
- Coverage of wind design for tall buildings in New York, including RWDI’s work on 432 Park Avenue: Gareth Cook, “Air Assault: The Violent Physics that Transpire Just Outside Every Skyscraper Window” New York Times Magazine, July 5, 2016, story 6.
- IMAX short film featuring RWDI’s wind tunnel modeling and tuned mass damper design for the Shanghai Tower: “Holding Sway,” Dream Big: Engineering Our World series by MacGillivray Freeman.