When guy wires are present, this thermal expansion is resisted by the guy wires. Stacks are often designed to relieve hot gases, and these hot gases cause the stack to thermally expand. In the case of a guyed stack, attempting to use a low deflection limit can be very problematic. Guy wire supported stacks are very flexible, which is typically why there are guy wires in the first place. Steel stack deflection due to across wind loading (vortex shedding) has the greatest potential for being noticed and causing concern with observers.
If an observer sees a stack deflection, which is large enough that they can see it occuring at a moderate wind load, then they often become alarmed and perceive the stack as being structurally unstable. In this case we are concerned with the structural integrity, but we are also concerned with the “perception” to the public that there is a problem. Since the vibration can occur at a low wind speed then it can occur much more frequently than the hurricane winds, and so it is much more likely that it can be observed by personnel. In other words, in our example if the stack experiences sustained wind speeds at the critical wind speed of 30 mph then it is possible that vibration can occur. The critical wind speed is the wind speed at which resonance can occur for the stack. For our stack mentioned previously that has a design wind speed of 140 mph it might have a critical wind speed that is much lower, let’s say it is 30 mph. For example, if a wind is blowing North to South, generally the stack deflection is West and East, or across the direction of the wind. We refer to this as “across” wind loading, since the stack deflection occurs in a direction perpendicular to the direction of the wind. We don’t want the stack deflection to compromise the structural integrity, but we are probably less concerned with the exact amount of the deflection.Īnother form of steel stack deflection due to wind is that due to wind induced vibration, or vortex shedding. Although we don’t want the deflection to be excessive, it stands to reason that if a stack is in the middle of a hurricane that there are probably not many people paying attention to the amount of stack deflection since they have more important concerns.
For example, if a stack is in the Texas Gulf coast and is designed for a 140 mph hurricane wind load, then there will be stack deflection due to that wind that will be along the wind direction. Along wind loading is considering the stack deflection due to the maximum design wind loading, in which case the stack deflection occurs “along” the direction of the wind load. There are two types of wind loading, along and across wind loading. For the purpose of this discussion lets assume we are dealing with a typical stack that is being controlled by wind loading. Most stacks are controlled by wind loading, unless the stack is very heavy and in a high seismic zone, in which case it might be controlled by seismic loading. When we refer to steel stack deflection, we are referring to the lateral deflection of a stack due to loads (wind, seismic, vortex shedding, etc.). Steel Stack Deflection due to Along and Across Wind?
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