AASHTO - Vortex Shedding Analysis Method

[pamandtovi]

I am designing an aluminum truss type structure (see attachment) to resist vortex shedding in accordance with Section 11.7.2 - Vortex Shedding of AASHTO Standard Specifications for Structural Supports for Highway Signs, Luminaires, and Traffic Signals, Fifth Edition, 2009. Any suggestions on how to apply this section to a truss type structure?

 

[dik]

Is this just a single 'tower' or is it a series of similar structures? I'm not sure how AASHTO applies to a single structure for vortex shedding. The length of side I assume is approximately 3'ish.

Dik

 

[pamandtovi]

This is a single structure. Also, the distance between the 3" dia. members is 16".

 

[WARose]

I don't know that it really is applicable because vortex shedding (almost always) is a phenomenon associated with cylindrical structures like chimneys, cylindrical towers, masts, etc. I'm not sure I've heard of it with a lattice tower before. (I suppose it is possible if the solidity ratio is high enough and the overall geometry is right. But here the solidity ratio appears low.)

But if you want to check it......looking at this, it would appear you have all the variables you need (assuming you've done a dynamic analysis of the structure to get the natural frequencies) except one: the equivalent diameter of the bluff body as a whole [your "d" variable in Eq. 11-2]. Therefore, you would probably need to work up some numbers in the design wind velocity range (I assume we are talking from 0-90 mph here) to see what diameter (if any) would cause any unwanted excitation. I guess if it's between 16 and 3 inches you should be concerned.....but if you have issues, perhaps changing the stiffness (and ergo the frequency) would help. You would probably have issues with individual components (especially guy wires, if you have them) before the structure as a whole.

 

[ElliottJames]

I really don't see how vortex shedding would be a problem here. Shedding won't occur around the tower as a whole because, like WARose said, the solidity is far too low. Shedding can develop as the air flows around each leg, however. But the entire length of the leg can't develop a harmonic vibration because the W braces act as dampers. You could check the vortex shedding for the unbraced length of leg, but it's highly unlikely to be a problem. Same goes for each length of W brace. The structure is very highly damped as it sits.

All that W bracing you put in to prevent buckling in the legs also serve to mitigate vortex shedding. It's like a 2-for-1 deal




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