Wind turbine tower foundation

[kandelabr]

Hello.
The title of this post may seem too hardcore. Actually, I am building a simple wind turbine made from wood and aluminium thin plate. it has three blades and 120 cm in diameter. It should be at least 3.5 m above ground, on a steel/wood column.

I have no idea how to install this pole. The ground is pure soil, no rocks and similar. I want it to be safe for high winds (~80 km/h) that occur a few times a year (storms).

Any suggestions for drilling, foundation, anchoring? It's quite a heavy rotor - I want no one dead.

Thanks!

 

[skeleton]

You probably won't like my answer:

Foundations are designed for vertical loads (weight) and lateral loads (wind). As a structural engineer, I would need to know the location of the site so I can look up in the local building code what the design wind speed (and pressure) is. I would need a geotechnical report to advise me of the soil type, and bearing capacities. I would need a sketch of the wind turbine and superstructure (pole, etc) so I can determine weights and surface area that is subject to wind exposure. From there, a foundation can be designed.

I would suggest (prioritized) either:
1) Solicit a professional structural engineer in your area or,
2) Speak to the local building authorities (building inspector) at city hall - they might give you a general guideline.
3) Look for similar existing structures as a design reference.

 

[edgepflow]

Consider putting the pole to a flat horizontal mounting plate with gusset supports. You will need to do calculations to figure out the size of the gussets. Often times for stuff like this, a slab of concrete is poured with anchor bolts embedded in the concrete. The anchor bolts are sized to handle all the forces from the structure. Your mounting plate would connect to the anchor bolts.

Since you only have soil, you will need a foundation pad. As I mentioned above, concrete is often used for this. However, if you can't pour concrete, perhaps you could purchase a slab. You then drill holes in the concrete and use a type of epoxy to secure the anchor bolts. Make sure you back up everything with calculations from a structural engineer as the last post mentioned.

 

[Bob S]

For the wind force on a HAWT, start by using a drag coefficient of C_d=1.0 and a frontal area of A=pi R² in the turbulent force equation

F = ½ρC_d π R²v² where ρ is the air density and v is the wind velocity.

If the the horizontal propeller axis is a distance h above the ground, then the torque at ground level is Fh.

Bob S

 

[Studiot]

This type of installation will also experience considerable induced vibration, amplified by the lever arm of the mast height.

Any foundation in soft material, especially purely granular, should take this into account.

 

[LeroyLaRey]

that's not excessively big, and shouldn't take much to build, unless it's an exercise in design. If not, check out this tower: http://dragonflypower.com/towerdemo.htm A guyed wood tower can absorb a lot of vibration, in fact vibration will keep beetles out of the wood. If it's a problem though, the blade probably needs to be balanced or the bearings will wear out.
:)

 

[skeleton]

For the wind force on a HAWT, start by using a drag coefficient of C_d=1.0 and a frontal area of A=pi R² in the turbulent force equation

F = ½ρC_d π R²v² where ρ is the air density and v is the wind velocity.

If the the horizontal propeller axis is a distance h above the ground, then the torque at ground level is Fh.

Bob S

That F gives the frontal pressure. You need to include drag behind. Typically, when using the more standardized equations in building codes, the net force can be 1.5x as much. Then you need to multiply in gusting factor; this is typically about 1.4x. Last, multiply a factor of safety, which building codes demand to be 1.5x.

 

[skeleton]

Generally, we would excavate a cylindrical pit. The pit would be filled with concrete. The post would be fastened to the foundation with a fixed end (moment resisting) connection.

Alternatively, a crude way is to embed the wooden post into the hole and fill with concrete. Note: this exposes the embedded wood to the risk of wood rot. In this case, chemical wood preservative treatment would be used (such as Chromated copper arsenate-treatment).

In the old days, the pit would instead be filled with rocks and gravel.

Very crudely: the depth of the pit might equal to its diameter. The depth of the footing should be perhaps equal to at least 1/4 of the pole's height. This might work for concrete filled; gravel filled would require a larger size.

Regardless: ENSURE YOU DO THE CALCULATIONS TO ARRIVE AT THE APPROPRIATE SIZE.

 

[LeroyLaRey]

on reconsideration, The top of a 3.5m tower can be accessed w/ a ladder. use shroud wires and just bury the bottom end like a fence post. plates & cement are nice but probably unnecessary. fungicide might be worthwhile though.