Deflection of fiberglass rods

[ErwinRedl]

Dear All,

I am involved in a rather unusual outdoor sculpture project.

The concept is to create a large assembly of 20+ feet high vertical fiberglass rods that sway with the wind but always return back to their (perfectly) vertical resting position if there is no wind.

What’s the best way to predict their deflection other than trying out different rods from let’s say 1-2 inches diameter? Is there a (not too complicated) way to calculate their deflection? Any guestimates of applicable rod diameters? Any application where I could plug in different diameters and lengths that would visualize the swaying of the rods?

Thank you for your help!

Erwin

 

[dicksewerrat]

Get 5 of these rods in diameters from 1 inch to 2 inches by quarter inch increments. Mount them the way the 'plan' calls for. watch them in different winds. /See if they go back to 'perfectly' straight.

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com

 

[jayrod12]

The fiberglass rods will have different properties and reactions to load based on the method of construction and the materials used for the fiberglassing. The rod manufacturer should be able to provide some insight into the proper modulus value to use as well as rupture stress values. Fiberglass should act on an elastic straight line response to stress just like FRP, straight line stress to strain diagram until rupture.

 

[TehMightyEngineer]

This would be HIGHLY dependent on the modulus of elasticity of the rods. I'd check with the rod manufacturer to find out if they have data on their rods. Also, the modulus will likely change depending on ambient temperatures, thus they will move differently in winter vs summer. At 20 feet long you're going to be REALLY slender which will probably require some interesting math to make sure you don't buckle the fibers in the rods.

Probably your best bet is to come up with a calculation for deflection and strength required then mock up one of the rods. Put a measured load on the end and compare the results to your calculations. From there you should be able to fine tune your design. You could also then "load test" the rod to failure and make sure it can withstand a 90+ MPH wind storm.

For a uniform wind load on the rod "w" the deflection at the free end will be as follows: def = (w * (L^2)) / (8 * E * I)
For a point load "P" on the end the deflection will be: def = (P * (L^3)) / (3 * E * I)

Maine EIT, Civil/Structural.

 

[JStephen]

Note that the deflection above is for a static deflection, whereas you're dealing with a vibration problem.

There is some literature on stack and vessel vibration in winds due presumably to vortex shedding, which may or may not be applicable to your situation.

The loading may not be that different from that of radio antennas on vehicles- wonder if there's any load information for that?

 

[dhengr]

ErwinRedl:
You want the rods to be dancing in the wind, so you want them to be fairly flexible. I don’t know that you really mean that they have to go back to perfectly straight bundle with no wind. In fact, they might splay a bit, such that the bundle has a dia. of D at the base and 2 or 3 or 4D up at the top. They just shouldn’t yield or creep so that they droop like a piece of wet spaghetti over time. You have to go to the supplier of the rods for their mechanical properties. Then it’s a uniformly loaded cantilever beam, as a first step, you pick the max. wind velocity and pressure/force. Then, you can get vortices forming on the back side of the rods, along their lengths, which make them dance and vibrate along their length. This isn’t a problem for the faint of hearth or mathematical abilities. I think you would do better just testing a few different dia. rods to get a feel for how they act in a wind. Then grow the bundle. I’d take a bunch of steel pipes, located as you wish at the base, with an i.d. larger than the glass rods; install the glass rods and use a pourable epoxy to fill the void btwn. the rod and the pipe.

 

[TehMightyEngineer]

Very true Jstephen, something this slender could definitely pick up weird modes of vibration.

Maine EIT, Civil/Structural.

 

[msquared48]

Not to mention ultraviolet degradation of the fiberglass resin over time, altering not only the dynamic response, but also weakening the rods on the side most exposed to the sun, causing the rods to permanently bend due to their own weight, more and more with time - another form of creep.

Mike McCann
MMC Engineering

 

[MikeHalloran]

How did the stainless rods work out?

Mike Halloran
Pembroke Pines, FL, USA

 

[LittleInch]

I hesitate to butt in on something here, but I think a single diameter rod just won't work for the huge variation in wind speed and load which these rods will see, not to mention the vortex issues and variable properties over time / temperature

I think you're going to have to go down the much more complex road of a gradually reducing diameter, more like a long fishing rod, which may not be a bad place to start and do some tests / experiments. Hence the tops will "dance" in a small breeze, but when you get a gale they will not bend to the point of snapping and becoming potential projectiles.

After all there is a reason why trees are broader at the base than the top (same with aerials, transmission line towers etc etc).

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way