Tower construction calculations

[trailb4u]

Hey Guys, I'm looking at options for making a tower for a wind turbine.

Some material that I'm looking at is 31" O.D. X .425" wall thickness

steel tubing.  I would like to make a stand alone tower.  With this

material, do you think it would be sufficient (assuming properly

anchored to a suitable concrete base) to support a 28 foot dia turbine

at a 70 foot height?  

Thanks,

Clint

[Janne]

To me it sounds like it might just do.. But really, the gut feeling doesn't matter in here. What you need to do, is grab the calculator and see if it will hold up under the stress. A 28' turbine will put guite a stress to a tower, and it will need to hold up in the most severe storm you can imagine hitting the turbine.

[scottsAI]

Clint,

Back in 2006 worked on a 24 ft wind turbine design.

Found a cell tower at the local scrap yard.

Iron scrap value + 10%, the 70 ft tower cost $1000.

Tower rated to carry 10,000 lb in 140mph winds.

Planed to weld stairs wrapping around it to the top.

Searching the Internet found calculations for the wind forces on blades, tower, steps etc. Realized tower was at least 5-10x stronger than needed. 5x is about the minimum I would consider, never did this before, falling over was not an acceptable option.

More digging found equations for base. I did not like the recommendation. Basically a 30 ton blob of concrete (assuming I remember it correctly, could have been 30 yards of concrete maybe more). Pictures of the fallen Mega Watt wind turbines looks like it's only a blob of concrete. For me this did not cut it. Designed the mass as a steel reinforced buried disk, taking advantage of the weight of the dirt covering it, giving it a 4x mass factor at double the cost. Shrinking the disk would reduce the cost. Since then read the dirt is not allowed in the mass calculations... yet I feel it adds to the overkill factor.

About this time the guy wanting the wind turbine bailed for unknown reasons.

We had a target of $6k as a DIY wind system, as things stood about $7k cost.

Labor cost on this project would be huge if hired out.

Have fun,

Scott.

[bzrqmy]

It's time to consult an engineer.  Get drawings with an engineer's stamp on it.  If something really bad happens, you are covered.  28', that's huge.  Keep us up to date on your project and good luck.

[fabricator]

My question would be, are you a certified welder? Or do you know one? Welding tube tube that size and making it structurally sound is a whole different animal.

[trailb4u]

Janne, Scott, bzrqmy, fabricator,  thanks for the input.

Janne, my gut tells me it might work too.

The following is a link to a post made by AdobeJoe on this board

in July 2007.  I don't know what the wall thickness of the salvaged

oil pipe he used, but the diameter of the largest tube at the bottom

was just 22 inches and he has a 55 foot tower height.  The Diameter

of the blades is also not posted, but the turbine is described

as a 10 kw unit.

http://www.fieldlines.com/comments/2007/7/16/3450/63291/4#4

Scott, the cell tower at the salvage yard you mentioned sounds

like it was a great deal.  I wonder if it's still there?

If I were to go with the tube type tower, I'm sure I'd use

a rebar re-inforced concrete disk footing, no blobs for me.

The dirt may not be allowed in the mass calculations, but

I agree that it would apply toward it anyway.

bzrqmy, I agree that it would be wise to get an engineer to

approve the final tower design.

fabricator, I'm not certified, but I could if I wanted to be.

I do have a cousin who is Certified and know others who are

as well.

Can anyone direct me to some useful equations online that

would help me determine the feasibility of this tube for

the tower?  I do have some books which have formulas for

determining how much thrust the turbine blades can create

in high winds, but no formulas relating to free standing

tower design.

Thanks guys, if I do ever build a turbine, I'll certainly

share my experience.

Clint

[jclaudii]

If you have a college near you, see if they have an engineering program.  If they do, ask one of the professors there if they would like a real world problem for one of their lessons.  Some of them strive to look for real world problems so they can give them to there students like "client xyz want's a 75' free standing tower with a 400lb windmill head on top, how can he build this" and let them figure it out.  You'll get some great ideas and help teach the next generation.  Pus as a collge, they usually have some of the latest and greatest software for stress testing etc before it's even built.