Deflection of fiberglass rods
Deflection of fiberglass rods
(OP)
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
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






RE: Deflection of fiberglass rods
Richard A. Cornelius, P.E.
WWW.amlinereast.com
RE: Deflection of fiberglass rods
RE: Deflection of fiberglass 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.
RE: Deflection of fiberglass rods
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?
RE: Deflection of fiberglass rods
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.
RE: Deflection of fiberglass rods
Maine EIT, Civil/Structural.
RE: Deflection of fiberglass rods
Mike McCann
MMC Engineering
RE: Deflection of fiberglass rods
Mike Halloran
Pembroke Pines, FL, USA
RE: Deflection of fiberglass rods
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