Designing a Tower for a Homemade Wind Turbine: Considerations and Requirements

[fabricator01]

Hello all, I am not an engineer, but I am a 32 year steel fabricator, I have recently built a 17 foot wind turbine with an axial flux alternator, I would like to build a tower for it but would like to get a general idea before I go to a local engineer for actual stamped drawings which are required for building inspector approval.
The turbine weighs 350 pounds and has a 17 foot diameter swept area, my general idea right now is to start with a bottom section of 16 inch 3/8 wall schedual 40 pipe 20 feet long and then go to a 20 foot section of 14 inch and then a 20 foot section of 12 inch and then a 20 foot section of 10 inch pipe for an eighty foot tower.
Each section would have a flange with probably 12 holes and gussets going from the pipe to the flanges.
The bottom flange will be 24 inch diameter with longer gussets from flange to pipe.
The foundation would be long J bolts into a large concrete foundation, the J bolts would probably be in the 1.25 inch neighborhood, probably 12 of them or so.
You can see a video of the turbine flying on a test tower here.

 

[Unrest]

Some things to consider:

Wind load
http://en.wikipedia.org/wiki/Drag_coefficient" The first formula on that page tells you the force applied by the wind. Find a worst-case Drag coefficient for a cylinder.
Calculate that force for the full length of the tower, and treat it as a concentrated load in the middle. Then find the moment it applies at the base. (M=force * distance) Add to that the moment applied at the base due to wind load on the turbine itself. The moment at the base can be compared to the strength of the foundation bolts and the bending strength of the pipe. You may also want to check the deflection caused by this load. Even if it doesn't fail, it can scare people to see things swaying about.

Buckling under self-weight
http://en.wikipedia.org/wiki/Buckling"
For a conservative estimate use the case of "one end fixed and the other end free to move laterally, K = 2.0", and the force is the total weight of the structure.
Or use the "Self-buckling" formula which should be more accurate but less conservative.

Resonance
Find the natural frequencies of the structure and compare to imposed frequencies from eg. turbine rotation, changes in wind direction, vortex shedding.

Earthquakes
Maybe doesn't matter?

Fatigue
Bolted joints can be much better than welds. Maybe keep open the option of bolting the sections together, perhaps with some adapter plates/pipes to make them fit. This might also give you an advantage erecting/lowering it if you can telescope it section-by-section.I'd let the engineer decide about adding gussets. If they're not needed then it makes his job easier.

There might also be codes for this sort of thing. The could make it a lot easier.

 

[fabricator01]

Thank you for the reply, it gives me some insight into what they will be looking at.