Any urbanite knows the feeling: you’re walking down a street with calm climatic conditions, but, after turning onto another block, you feel a massive gust of wind travelling at momentous forces. It’s called the wind tunnel effect, or the downdraught effect, and it results from the many tall buildings in a city, which, due to their narrow proximity from one another, create a low pressure region causing winds at ground level to move faster. This same phenomenon is harnessed in industry, where artificially-constructed wind tunnels are utilized for aerodynamic testing of vehicles. However, to the common pedestrian, wind tunnels remain a nuisance.
The effects of wind tunnels retain strong interest to those involved with the design and construction of tall buildings, who simulate the expected wind forces to recreate the phenomena. As detailed in ASCE/SEI 49-2012 – Wind Tunnel Testing for Buildings and Other Structures, wind tunnel tests are to be used to determine wind loads, including wind-force-resisting systems (MWFRSs) and individual structural components and cladding (C&C), on buildings and other structures, as well as their responses.
Furthermore, while the provisions of the ASCE/SEI 49-2012 standard are meant to satisfy guidelines for the minimum design loads for buildings and other structures, it is acknowledged in the document that wind tunnel testing on buildings can perform measurements beyond those specifically addressed in the standard, such as pedestrian wind forces. As stated in the standard, “these studies are permitted to be included within the test report addressing wind loads.”
The impact of wind tunnels can vary, depending on the speed of the wind and the individuals affected. Generally, the result of these climatic loads is discomfort, as they can place pedestrians in disarray. This burden was present as far back as the early 1900s in New York City, when the wind tunnels around the Flatiron Building inconvenienced residents by pulling on their clothing. Wind tunnels can also contribute to detrimental economic and financial effects. In fact, studies have shown that shops may be left untenanted due to discouraging windy environments.
However, some downdraughts reach speeds great enough to knock over signposts and topple pedestrians. Lawson and Penwarden (1975) reported the death of two elderly women after they fell in response to the forces of high-speed winds at the base of a building. More recently, in 2011, a 35-year-old man was crushed by a truck that was toppled by the effects of a wind tunnel in Leeds, England. These phenomena can truly lead to disaster.
Mitigation considerations for the effects of wind tunnels have at least been considered throughout the past several decades, as the topic was discussed in newspapers in the early 1980s. However, much more focus has been placed on the issue in recent building design, as skyscrapers are being installed rapidly, generally in urban settings that follow pre-Modern grid designs, placing them in prime distribution for wind tunnels.
For example, Dubai’s Burj Khalifa, the tallest building in the world, underwent micro-climate analysis of its potential effects around the tower base. Buildings that are constructed without this interest in mind are susceptible to public outrage. This is exemplified by the “Walkie Talkie” skyscraper in London, which caused a downdraught that has knocked over street signs and pedestrians, and it is even responsible for overturning trolleys.
To respond to the wind tunnel issue, some cities, such as Toronto, require pedestrian wind studies prior to new building developments. Ultimately, as reflected in the primary focus of ASCE/SEI 49-2012, the impact of any potential wind tunnels is viewed through a building structural integrity perspective, but the comfort and safety of the pedestrians below is still important. It is for this reason that the standard also permits this interest.