whisper 200 tower construction height vs. material strenght

dugo1234dugo1234 Registered Users Posts: 2
hello Everyone, new guy here with basic questions.
i would like to build my whisper 200 tower, but i have 2 problems. i live in a hurricane zone and we have limited supply of materials available.
we only have schedule 20 pipe thickness available in the required 2.5" size. the schedule 20 pipe seems too weak to even raise the generator.
the question is: would it make sense to install a 2" pipe (ss40) in addition, inside the 2.5" schedule 20 tower pipe to gain strength?
i would hope that the smaller size pipe in the inside would give extra strength when we have strong winds.
the schedule 20 pipe available seems rather to bend on its own.
i am planning on using two lengths of the standard 20' stock, so my tower would be roughly 40' high, with two sets of guy wires.

any input i would appreciate very much.:D
pete

Comments

  • BB.BB. Super Moderators, Administrators Posts: 29,988 admin
    Re: whisper 200 tower construction height vs. material strenght

    From an engineering point of view (I am going to get into trouble here--it has been so long since my structural classes)--Basically, a couple of things to think about here.

    First, the material that is farther from the "center" of a structure element is, the more it contributes to stiffness/strength.

    For example and "I" beam has the material at the extreme top/bottom of the beam vs simply a rectangular solid cross section... Similar with a pipe. A larger thick(er) wall pipe going to be stronger than a solid bar (same lbs of steel per foot of round section). And, as you add more material to the center at a smaller radius from the center, the less it will contribute to the overall strength. I forget, but the strength of a cross section goes with the square of the distance from the center (hey--it has been a really long time).

    Next, look at an I beam. There are several different ways of modeling/understanding its strength depending on how it is attached (fixed to wall vs a pivot point)...

    But more or less, for an ibeam, the point of maximum linear stress is at the center of the beam for the "flanges" and the maximum shear stress for the center plate of the "I" is at the ends (if the ends are pivoting).

    Notice that for these structural shapes, there is what is called "shear" stress. Think of a piece of paper, you take the left side in one hand and the right edge in the other.. Now move one had "up" and the other hand down. That will tend to tear paper in the middle (somewhere).

    What is important is you need to have the cross section of the material so that "layers" cannot slide relative to each other.

    A simple example, Look at a phone book (anybody still have those?). Notice a 100-1,000 pages that add up to several inches of thickness, but there is no bending resistance... As you bend the book, the various layers "slip" with respect to each other and there is very little resistance to bending.

    Compare that to a block of wood the same thickness--Those "layers" (wood grain) are connected to each other and will "develop shear" stress and keep the wood beam from bending.

    So--If you were to put a pipe inside another pipe... Using a larger diameter pile inside your existing pipe will be stronger than inserting a smaller diameter pipe in your existing pipe. (all things being equal).

    Next, you need to prevent the two pipes from "sliding" relative to each other to develop shear. In aircraft, we would drill multiple holes in the outer tubing and weld those holes to the inner structure (i.e., spice in a steel tubing/multiple joint point).

    I have no clue what would be best--But my guess is that you should do those sorts of welds near the ends of the pipe. At the "center" of the pipe span, there is very little shear (remembering IBeam example).

    If you have the option--you really should be finding thicker pipe (higher schedule number).

    In any case, I don't have near enough information or knowledge to tell you that a Schedule 40 or Schedule 80 pipe would work "safely" for your needs.

    One other suggestion--Use, at least, 4 guy wires (spread 90 degrees from each other) per section. I believe that 3 is not enough. And make sure that your guys are properly terminated (thimbles, crimps, etc.) so they don't pull/fail... There is no redundancy with 4 or less distributed guys (and wind turbine/tower must not be next to home/where kids play/etc. so that if it does fall, it does so "safely".

    Similar issue with the turbine body/blades/etc... All have been known to "take a leap" from the tower for various reasons.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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