I?m having a solar contractor install a bank of solar panels at my
house and was wondering if anyone here could take a look at this
design and see if it is adequate or not.  The contractor claims he
installs then like this all the time but I don?t think that there is
any engineering behind is design.  Just a quick look is all that I?m
asking.
The design :

Post: 21 foot 6? sch 40 steel pipe imbedded 7 feet in the ground.
Footing: 7 feet deep, 24? diameter filled with 2,500 psi concrete.
Solar array: 350 lbs at top of post.
Wind: 90mph max wind.
Soil: stiff clay, frost depth 6?

I?m mostly worried about the 6? sch40 supporting the solar panel under
extreme winds.  Would filling the post with concrete help?  I have
been trying to do the math from some old books but keep getting stuck
trying to figure out the allowable bending stress in the post.  Thanks
for any help.

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Request for Question Clarification by redhoss-ga on 14 Sep 2004 07:34 PDT

I can calculate bending moments due to wind loading, but you left out
the most important info. I need to know the dimensions of the solar
panel bank. If you can provide that info, I will do the calcs.

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Clarification of Question by gvjeep-ga on 14 Sep 2004 11:38 PDT

The solar array is approx. 10? x 10? and is tilted at 55 degrees from horizontal.

Hello again gvjeep, I don't know what made you question the use of the
6 inch sch. 40 pipe, but I am glad that you did. Here is a simple
analysis of the wind load on the post:

Wind PSF @ 90 mph = 90^2 x .0027 = 21.87 PSF

Projected height of solar panel = 10 ft. x sin 55 deg. = 8.19 ft.

Height to center of panel = 21.5 ft - 7 ft. + (8.19/2) = 18.595 ft.

Bending moment at ground level = (8.19 x 10) x 21.87 x 18.595 x 12 in./ft.
                               = 399,678 in.lb.

Section modulus 6 in. pipe = 8.50 in.^3

Bending stress in pipe = 399,678/8.5 = 47,021 psi

This number (47,021 psi) is very close to the ultimate strength of the
pipe which is probably around 50,000 psi. In this application you
would like to see at least a Safety Factor of 2 based on "yield
strength" (which is probably around 33,000 psi). Also, there are other
bad things that happen with a high wind on this type of structure. I
am sure you have seen stop signs in high winds shake back and forth
side to side. This is a natural phenomenon that occurs under these
conditions. It will also happen with your design. Your idea of filling
the pipe with concrete is good, but you would have to also put steel
in the concrete. You would need several pieces of rebar wired or
welded into a cage. If there is any way you can use guy wires, that
would be my preference. I didn't do any footing calcs because without
a soil test it would just be a guess. Based on my experience I agree
with your assessment that the footing is adequate. I hope that this is
what you are looking for and please ask for a clarification if I can
help you further.

Good luck, Redhoss

Thanks for the answer.  Just what I was looking for!