Structural Harmonics

[msquared48]

Does anyone know if there is a feature within RISA that would allow you to load a structure with sustained wind forces that would induce various modes of torsional harmonics through vortex shedding.

I have a pole structure with a large "sail" if you over about 2/3 of the length that has to be restricted in movement for the asssembly to serve its purpose. I am worried about harmonically induced torsional deflections and am considering going to two poles instead of one to eliminate this problem. The client may not buy the idea, but that's another discussion for another day.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[JoshPlumSE]

Vortex shedding.... I've always wanted to research this portion of the code more. Ever since I took an aerospace structures course in college, I've been interested in the interaction of the aerodynamic and dynamic properties of structures. You don't often look at "Flutter" of airplane wings in civil structures. Vortex shedding is as close as you get.

RISA does not currently have the ability to do a harmonic loading or time history analysis. The closest that we can get is by running a response spectra analysis / eigen solution. The good news is that running an eigensolution / response spectra (while geared more towards earthquakes) will give you a ton of useful information about the dynamic properties and natural frequencies of your structure. You can use that to determine if you are indeed in the range of frequencies where vortex shedding will be a potential problem.

On another note, I always thought there were some simple modifications that you could make to a pole to reduce it's susceptibility to vortext shedding. Akin to adding fins to the structure. I'm no expert and am certainly over simplifying. But, I do think I've read about known ways to effectively control that behavior.