Trussed Structure - Vortex Shedding
Trussed Structure - Vortex Shedding
(OP)
Is a three chord truss type structure susceptible to vortex shedding? The structure is approximately 54'-0" tall.
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Trussed Structure - Vortex Shedding
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RE: Trussed Structure - Vortex Shedding
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.
RE: Trussed Structure - Vortex Shedding
RE: Trussed Structure - Vortex Shedding
The 'Shock and Vibration Handbook' has a formula for a simple [cantilever] cylinder (i.e. critical wind velocity for vortex shedding).
RE: Trussed Structure - Vortex Shedding
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.
RE: Trussed Structure - Vortex Shedding
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....
RE: Trussed Structure - Vortex Shedding
RE: Trussed Structure - Vortex Shedding
"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.
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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.