Wind-induced load on wire

[hpon]

Hi,

Let's say we have a long steel wire in tension that is rigidly supported by its endpoints. The wire is slightly curved due to a transversal gravity load.

Will the longitudinal load in the wire be significantly increased by a wind load?

I understand that this information is very limited, but I need to know whether wind-induced loads of this type are normally considered when designing large bridge-like structures.

/hpon

Regards,
hpon

 

[hpon]

I must clarify myself..

The question should be:

Could the longitudinal load in the wire be significantly increased by a transversal wind load on the wire itself?

/hpon

Regards,
hpon

 

[rb1957]

i'd say "yes" ... and you should be able to do the calcs for yourself ... a cable loaded by weight (uniformly distributed load), add a force at 90deg to the wieght (something like windage), cable will now deflection in the plane of the resultant, which is "just" a factor times the weight and i think the loading's linear (but i haven't invested much thought into that point, and it's easy enough for you to prove it to yourself).

 

[hpon]

Thank you rb1957!

That is what I thought (so long as the geometry is approximately identical). I appreciate the reassurance!

/hpon

Regards,
hpon

 

[btrueblood]

The bigger problem with wind loading is/can be the transient nature of the loading, and the possibility of exciting a resonance of the wire or attached structure, e.g. Tacoma Narrows Bridge.

 

[gwolf2]

Following on from btrueblood.

Calculate the natural frequency of your cable in it's loaded state. Then calculate the eddy shedding frequency of the von karman vortex street for a range of credible wind speeds. If the frequencies are similar, you MAY have a problem with resonance.

I say MAY rather than WILL because there can be a lot of aerodynamic damping in a cable which is vibrating a lot and it is often not a big deal - power and phone lines don't often seem to come down because of resonance. (someone will say that they do)

 

[btrueblood]

"(someone will say that they do)"

Ok, I will

Actually, I will just comment that we live in an area subject to occasional high wind storms. The power company presumably makes a calculation on how much they should spend to mitigate wind damage to power lines, and I think they err too much on the side of not spending for my taste (it sucks to be w/o power).

Gwolf mentions eddy vortex shedding, but even just the transients due to gusting of winds can cause trouble if not designed for. Similarly, icing during cold weather (and ice shedding as the weather warms) cause transient effects.

Some more info on how power lines can/are designed for wind loading:

http://en.wikipedia.org/wiki/Stockbridge_damper

http://en.wikipedia.org/wiki/Aeolian_vibration

http://en.wikipedia.org/wiki/Conductor_gallop

I've seen lines with these installed, I'll bet others have too, whether they knew what they were for or not:

http://www.arproducts.org/05-2005 Utility Applications.htm

The above should give you a veritable plethora of search terms you can use on Google.

 

[gwolf2]

Hey, I never knew that those things were stockbridge dampers! but occasionally wondered at them.