Vortex shedding is the oscillating flow of air or water as it goes past a bluff, rather than a streamlined body. As skyscrapers are rarely streamlined, this causes excessive force that could lead to structural damage. In fact, it is a likely factor in the 1940 collapse of the Tacoma Narrows Bridge. Skyscraper vortex shedding is an issue that must be addressed during design and construction in Indiana and beyond. Here is an overview of vortex shedding and ways to overcome it.
Effect of vortices
Everyone has seen a vortex. The swirling currents in rivers and streams and the flow down a bathtub drain are common types that you likely see almost every day. These types flow without much interference and are benign—unless there is a flood or other natural disaster.
However, vortices also contain tight spaces full of strong turbulence. When there is a barrier, they can build up and affect conditions around them. When one forms against the side of a skyscraper, it creates suction. While this does not produce great force, when it continues in a focused and well-organized fashion, it can start to rock a building from side to side. While swaying is rarely dangerous, it can lead to structural damage that could eventually become serious. Also, it is unsettling to occupants on the upper floors, who will likely be less productive if they believe their home or workplace will crash down onto the street below.
Vortex shedding occurs more dramatically when buildings are space closed together. Skyscrapers near the edge of a city center where there are miles of clear air space are less likely to shake side to side than their counterparts that are more closely situated. However, as many dense cities already have neighboring skyscrapers, it is not a practical solution to tear down and rebuild just to reduce vortex shedding.
Retrofits are a better idea. Fairings installed outside a building can streamline airflow and help it pass. These are mainly used on airplane wings, but similar designs help skyscrapers. Tall narrow structures like chimney stacks contain strakes to deliberately cause turbulence and distribute pressure so it does not compromise the structure. However, as this involves spiraling the strake around the building, it is likely not a practical solution for skyscrapers.
Skyscrapers normally have two factors that reduce vortex shedding. One is dampers on the top floors. These are basically fairing for skyscrapers—they interrupt wind flow so the structure does not interfere with it and cause swaying.
The second factor is deep foundations. Taller buildings have foundations as deep as 282 feet. This is why most structural damage is alarming but rarely results in a catastrophe. The deep foundation grounds the building so it merely sways, rather than falls apart. However, no one wants to deal with constant structural repairs, so it is best to combine the foundation with dampers.
Benchmark Fabricated Steel is based in Indiana and performs steel construction throughout the United States and Canada. We can assist you starting with the design all the way through the building process to avoid skyscraper vortex shedding. Contact us today to see how we can help you with your project.
Categorised in: Steel Construction
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