Now that everyone is getting back into the swing of work for 2018, we’ll be returning to our serices taking a look at the extra knowledge our Engineers have gained through working on some of our high profile projects. This month Ian Hylands talks about vortex shedding.
“Over the last few weeks we have been carrying out some analysis work on a chimney. It is supported from a braced steel tower, but the top half is free-standing, cantilevering above the structure. Cylindrical structures like this can be prone to damage from vortex shedding, a phenomenon caused by wind being split into eddies passing around one side of the tower and then the other. This causes a small push in the cross-wind direction that moves the structure from side to side. If the frequency of the vortex shedding coincides with a natural frequency of the structure, repeated large deflections can result that cause fatigue in the chimney and ultimately collapse.”
“Calculating the magnitude of this effect is possible, but involves some mathematics that is not straightforward, particularly if the chimney is supported by another structure. Happily, it is easy to screen for the effect. The Eurocode for wind (BS EN 1991-1-4 ) gives two criteria that determine whether vortex shedding must be considered or can safely be ignored. The first is a simple geometrical check, and the second involves a simple dynamic analysis to determine the modal frequencies of the structure. Our chimney also needed analysis for seismic loading, and we had therefore already carried out a modal analysis. We were able to quickly rule out vortex shedding in this instance, and save a significant amount of time; not only in calculating the loading that would be induced, but also in carrying out the fatigue check that would then be required.”