How Are Skyscrapers Able to Withstand High Winds?

While building codes do not require wind tunnel testing for new skyscrapers, engineers and architects conduct the testing anyway to ensure precision and efficiency during construction.

Published July 1, 2006

By Deborah Snoonian
Academy Contributor

Before glass, steel, and concrete, there were plastic, plywood, and pressure sensors. And even in this age of computer-aided design and analysis, engineers still build scale models of buildings to see if the full-sized real ones can withstand strong winds.

That explains why in 2002, researchers at the Alan G. Davenport Wind Engineering Group at the University of Western Ontario (UWO) built a 1-to-500 scale replica of 7 World Trade Center and the surrounding neighborhood, measuring about a foot and a half tall. They placed the model carefully inside a boundary-layer wind tunnel, a 128-foot long, 11-foot wide, and 8-foot high apparatus equipped with a wind machine that can simulate everything from gentle breezes to gusts of hurricane intensity. Then, as the wind blew, sensors attached to and around the model logged thousands of readings of pressures, speeds, and deflections. Later, researchers analyzed the data to spot potential wind-related problems, and compared them to computer-model predictions.

Such a study is a common practice in the design of a tall building to ensure its safety and the comfort of occupants and pedestrians. The studies guarantee that skyscrapers are flexible enough to withstand high winds without toppling over (all tall buildings are designed to sway slightly), and that strong gusts won’t rip off or break the cladding (I.M. Pei’s John Hancock Tower in Boston notoriously suffered falling and broken windows during its construction in the 1970s). As for comfort, engineers aim to prevent occupants from detecting the building’s motion by making sure it moves slowly and gently. Wind speeds at the base of the building are monitored so that pedestrians won’t have to endure strong gusts.

Wind Tunnel Testing Not Required

Although building codes don’t require wind tunnel testing, they usually permit architects and engineers to base their designs on test conclusions. This typically results in buildings that are engineered precisely and efficiently—and therefore less expensively—than what is mandated by conservative building codes.

The architects and engineers for 7 WTC, Skidmore, Owings & Merrill (SOM) and WSP Cantor Seinuk, respectively, had access to data on many similar tall, existing buildings. But the timing presented a challenge, because there was then no master plan yet in place for Ground Zero. Researchers tested three models: one of 7 WTC with no structures at Ground Zero (which is what exists today), and two that included surrounding buildings at various heights and orientations, which affect the wind speed and direction around 7 WTC. “We had to make some assumptions about what might get built there, so we made them conservatively,” says Silvian Marcus, chief executive officer of WSP Cantor Seinuk.

In the last decade or so, emerging analytical methods such as computational fluid dynamics (CFD) have allowed designers to study the complex behavior of air movement around buildings without the use of scale models or wind tunnels. But by all accounts, it will be years before computer-only wind studies become the norm.

Immensely Complicated and Computationally Intensive

One reason is that wind tunnel facilities—there are just a few in North America—have given designers the ability to look not only at the effects of wind, but also at other weather-related effects like snow and at the perfomance of other systems such as air in-takes and exhaust fans. These are “all things that are critical to building performance,” says SOM partner Carl Galioto.

More fundamentally, calculating airflow around buildings is both immensely complicated and computationally intensive. At this stage, CFD software for buildings requires a high level of expertise, produces results that are highly dependent on assumptions, and tends to be used only by wind-tunnel facilities themselves.

Change will come when the software and processing power improve. “I’d like to be able to use CFD analysis to spot check parts of buildings that tend to be problem areas for wind pressure, like corners and parapets, and then confirm the CFD predictions with a physical test prior to construction,” says Nicholas Holt, SOM’s senior technical architect for the project. “Eventually, with enough data corroborated by physical models, codes will likely begin to accept CFD analysis in lieu of wind tunnel testing.”

In the meantime, though, engineers will keep the plastic, plywood, and pressure sensors handy.

Also read: Green Buildings and Water Infrastructure