Trussed Structure - Vortex Shedding 1

[pamandtovi]

Is a three chord truss type structure susceptible to vortex shedding? The structure is approximately 54'-0" tall.

 

[KootK]

I'd tend to think that the trussing would be enough to amply "confuse" the wind such that VS was not a big deal. What's the size of the truss cross section? Size of the individual chords? I'd think that %open would impact whether this thing feels wind like a single unit or a collection of single units.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[pamandtovi]

Hi KootK, thanks for the response. That's my line of thinking too - the aluminum structure's construction is such that it is not conducive to vortex shedding. The three chords are spaced 16" apart and are made from 3" OD tubing. The trusses range in size from 1/2" pipe at the top to 2" OD tubing at the bottom.

 

[WARose]

You may want to get codes like ASME-STS-1. (Or transmission tower codes.) They account for vortex shedding.

The 'Shock and Vibration Handbook' has a formula for a simple [cantilever] cylinder (i.e. critical wind velocity for vortex shedding).

 

[KootK]

Yeah, I'm still in the "sufficient confusion" category with those dimensions. How tall is this thing? Any concern for it getting iced up? One really doesn't want to chance AL fatigue issues. For me, it's very much about whether the dominant wind path is through the thing or around the thing. If it's around, I guess there could still be VS issues. I'll be curious to hear what others have to say.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[SAIL3]

also not sure of the layout...a sketch would help....this is a very complex problem to analyze...what prompted this concern?....round tubing will see vortex shedding....the critical wind speed where vortex could develop is a function of the dia of the tubing....so a 3"dia tube will have a different critical wind speed than say a 2" or 1/2"....if there are enough tubing of the same dia then it is possible that the load developed by the vortex shedding from those members could cause a resonant response in the whole truss...if there is shielding involved, then the turbulance caused by the members in the face plane would reduce the
chance of vortex shedding to occur in the secondary planes provided the distance between the planes is such that the wind velocity does not recover to a steady state velocity....

 

[pamandtovi]

Please see attached picture of the trussed structure.

 

[WARose]

Some info that might be useful to you:

"A latticed tower, in general, presents a complex aerodynamic
shape to the wind such that consistent vortex shedding to cause complete
oscillation of the structure over a prolonged period is almost impossible.
Therefore, only individual member behavior of latticed steel towers to
vortex shedding and aeroelastic instability has been studied (Modi and
Slater 1983; Wardlaw 1967)..........Vibration can be initiated when the frequency of
vortex shedding corresponds to a natural frequency of the structure or
individual member. Vortex-induced vibration is more likely to occur
when tubular structures (or components such as tubular arms) are installed
without insulators and conductors.........Equation E-1 can
be used to calculate critical wind speed at which vortex-induced vibration
may be initiated.

Vcr=fs/Str

where
Vcr = critical vortex-induced wind speed, in ft/sec
f = structure or member natural frequency, in Hertz
Str = Strouhal number
s = across-wind dimension, in ft

Standard structural shapes have an average Strouhal number of 0.14.
Strouhal numbers for a variety of structural shapes can be found in Simiu
and Scanlan (1996)."

--'Guidelines for Electrical Transmission Line Structural Loading: Appendix E Supplemental Information on Structure Vibration', by: Wong & Miller.
------------------------------------

I'd like to add to that the fact that I've seen people use different values than that for the Strouhal number. (Varying between 0.14 and 0.2. The 'Pressure Vessel Design Handbook' (by: Bednar; 2nd edition, p.111) suggests using 0.2 for computations but advises that for values of the Reynold's number in excess of 100000 the Strouhal number can approach 0.35.)

So it looks like you may have to look at it as a whole and in pieces. And be advised: guy wires are especially bad with this....so if you have any....I'd look at that too.