Is there an equation/calculator for how much side load is being put on a tower?

[makenzie71]

Title says it.  We had 60mph gusts the other day, and speeds regularly exceed that.  Trying to calculate how much stress that puts on a tower.

[makenzie71]

What ikm trying to sort out is if i should consider the entire swept area as surface area, just the surface area of the blades, or somewhere in between?

[Adriaan Kragten]

The tower is loaded at the top by the rotor thrust but there is also a load acting on the tower itself. If the wind turbine has a safety system which turns the rotor out of the wind at high wind speeds, the rotor thrust is limited. Formula 7.4 from my public report KD 35 gives the rotor thrust for a yawing rotor. The thrust coefficient Ct is about 0.7 for a rotor with blades with a normal airfoil.

The load on the tower depends on the tower geometry. This load isn't reduced at high wind speeds. So at low and moderate wind speeds, the load on the rotor gives the highest load on the tower but at very high wind speeds the load on the tower itself becomes significant. An example of tower calculations for a free standing tubular tower is given in my public report KD 582.

[makenzie71]

Thanks Adriaan I'll look through your reports and see what i can figure out...already tells me enough to know i don't need to factor in the whole swept area like it would a sail or something.  I'm trying to calculate how “ballast” i need at the bottom of a 32ft pole (24 above the ground and 8 under) with a 6ft rotor at the top.

[Mary B]

Are you guying or freestanding? Tower style? The 4 legged Wind Charger towers don't take that much concrete, a free standing lattice style tower can take a huge amount... my 32 foot tower has a 6 foot x 6 foot x 8 foot deep concrete cube under it! Tower is rated at 32 square feet of antenna load...

The calculations for some tower styles are in here https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&ved=2ahUKEwjHi8-EkZjmAhVjFTQIHUzPCQMQFjAAegQIAhAC&url=http%3A%2F%2Fwww.va3cco.com%2Ftowersheight.pdf&usg=AOvVaw1T1puW9IKam_L2n2qlQSgs to simplify and give you a failure margin of error treat round members as flat for the calculations.

A guyed tower doesn't need much, a 100' rohn 25G guyed uses a 3x3x3' concrete base because it just has to support the down force of the tower, the guys take the side loading force.

[makenzie71]

Freestanding heavy 4” od oilfield pipe .  I have my little 4ft turbine 15ft up on the same stuff and it's doing okay....i have nothing invested in them, though.  My 6ft turbine i want higher up and i have a little more money invest in it so i'm trying to make sure theres some match involved.

[Mary B]

I do believe that article covers the calcs for the tower base once you calculate the overturning momentum... call it 4" wide flat plate for the calcs... but you may exceed the weight it can carry before it collapses. I used to have a calculator for that but lost it... look up the strength of the steel, thickness of the pipe to see how many PSI it can handle before breaking. Some data on it here https://www.globalspec.com/learnmore/specialized_industrial_products/mining_equipment/drill_pipe This page calculates for an antenna mast they sell, 100k psi steel. Only up to 3" though but you can play with it to get an idea... https://www.dxengineering.com/mastloadestimator

and an excel spreadsheet you can use to calculate the forces is here http://thebont.com/spreadsheets/AnalysisOfAntennaMastStrength.htm I would go with low yield steel strength of 77,000 psi unless you KNOW what you have.

30' mast, 4" OD .5" wall with a 30 square foot wind load on top has an over turning force of 313528.32 inch pounds or 26k foot pounds so you need approx 7 cu yard base of concrete based on concrete weight. that is not accounting for soil pressure that would lower that some. That is a hairy calculation I don't want to get into! 6'x6'x6' base would do it with some extra safety factored in...

concrete weight calculator https://www.inchcalculator.com/concrete-weight-calculator/

Hope this isn't to much info all in one post LOL

[makenzie71]

It's actually only 24-25 ft...the stick is 32ft, about 8 of it has to go in the ground

[SparWeb]

This is the drill-stem pipe you mentioned before, right?
So I don't remember seeing if you answered my question about how the pieces are put together.
Are they acme-threaded?  Flared?  Are you welding them together, or clamshell clamping?
Something has to take the bending moment, especially if you aren't going to use guy wires.
It's a LOT of bending, if you don't have guys.

[Adriaan Kragten]

Thanks Adriaan I'll look through your reports and see what i can figure out...already tells me enough to know i don't need to factor in the whole swept area like it would a sail or something.  I'm trying to calculate how “ballast” i need at the bottom of a 32ft pole (24 above the ground and 8 under) with a 6ft rotor at the top.

The thrust coefficient Ct (or Cw in German) of a real rotor is much lower than for a flat closed disk for which it is 1.16 (see report R 443 D page 3-4) if the disk is perpendicular to the wind. Betz has found that Ct = 8/9 = 0.89 for the ideal condition without any losses. But in reality the blade isn't effective up to the center of the rotor and some thrust is lost at the tip of the blades because of tip losses. This finally results in a Ct value of about 0.7 if the rotor is turning close to its optimum tip speed ratio. The Ct value for a non rotating rotor is much lower and is determined by the area of the individual blades and the drag coefficient of the airfoil for large angles of attack. The Ct value for an unloaded rotor isn't much lower than for a rotor which is turning at the optimum tip speed ratio. Although no nett power is produced, a lot of power is produced to overcome the airfoil drag and the wind speed is slowed down because of this power. This slowing down of the wind speed is only possible if there is a pressure difference over the rotor and this pressure difference causes the thrust.

[makenzie71]

This is the drill-stem pipe you mentioned before, right?
So I don't remember seeing if you answered my question about how the pieces are put together.
Are they acme-threaded?  Flared?  Are you welding them together, or clamshell clamping?
Something has to take the bending moment, especially if you aren't going to use guy wires.
It's a LOT of bending, if you don't have guys.

Yeah same stuff...that thread got deleted for some reason.  They are flared and threaded (couplings) but I'm not sticking them together.  I wasn't clear I guess when I first started that...I know higher is better but I don't have the equipment to go that high even if I wanted to, and this stuff couldn't handle that height without guys.  I've already got one in the ground at 15~16ft and it's holding pretty stable with the little 48" turbine.  Highest I want to go is 24~25ft.

The thrust coefficient Ct (or Cw in German) of a real rotor is much lower than for a flat closed disk for which it is 1.16 (see report R 443 D page 3-4) if the disk is perpendicular to the wind. Betz has found that Ct = 8/9 = 0.89 for the ideal condition without any losses. But in reality the blade isn't effective up to the center of the rotor and some thrust is lost at the tip of the blades because of tip losses. This finally results in a Ct value of about 0.7 if the rotor is turning close to its optimum tip speed ratio. The Ct value for a non rotating rotor is much lower and is determined by the area of the individual blades and the drag coefficient of the airfoil for large angles of attack. The Ct value for an unloaded rotor isn't much lower than for a rotor which is turning at the optimum tip speed ratio. Although no nett power is produced, a lot of power is produced to overcome the airfoil drag and the wind speed is slowed down because of this power. This slowing down of the wind speed is only possible if there is a pressure difference over the rotor and this pressure difference causes the thrust.

Thank you, sir!  I haven't had time yet to go through your site...going to try to tonight.  I want to get the taller one stabbed in this weekend.

[bigrockcandymountain]

Your question very much depends on soils. 

If it was a 4" hole drilled in solid rock, then 0 pounds concrete.  If it is in swampy muck then mary b has the right answer. If it is somewhere in between then i would go with less concrete, wide at the ground level and smaller like 12" hole down to 8' with a pad at ground level.

I am pretty sure a 4' x 4' x 12" pad at the top would suffice in most soils (gut feeling, no math involved). Concrete is $200 a yard here so i bet you don't want 7 yards of it. 

Guy wires would be my choice.  Even just light wire ones.  I think you will find your pipe plenty strong, but not stiff and it will wave around more than you like.

You are doing some valuable evaluation on these cheap turbines.  Keep the posts coming.

[makenzie71]

Someone on reddit forwarded me this calculator.  Pretty neat I think.  Of course it's only gives you a rough idea and you're limited on height to 22ft and on pipe sizing to their 3" chromoly stuff...but you can compare whatever you're using to their specs and get an idea.

https://www.dxengineering.com/mastloadestimator

Your question very much depends on soils. 

If it was a 4" hole drilled in solid rock, then 0 pounds concrete.  If it is in swampy muck then mary b has the right answer. If it is somewhere in between then i would go with less concrete, wide at the ground level and smaller like 12" hole down to 8' with a pad at ground level.

I am pretty sure a 4' x 4' x 12" pad at the top would suffice in most soils (gut feeling, no math involved). Concrete is $200 a yard here so i bet you don't want 7 yards of it. 

Guy wires would be my choice.  Even just light wire ones.  I think you will find your pipe plenty strong, but not stiff and it will wave around more than you like.

You are doing some valuable evaluation on these cheap turbines.  Keep the posts coming.

If I do guy wires, the cost will be way more than just a heavy concrete base...I can't have them anchored at ground level, I'll have to set them all 6~8ft above the ground, which is then more posts and concrete.  And I can't go perpendicular to my shop, meaning I'd have to have a minimum of two sunk with one one anchored to the building.  It's simpler in one way but more complicated in others.  I'm putting this one right next to the building, though, and I think I'm going to cut it down closer to 20ft than 25ft above the ground.  If it's too noodly I'll be able to brace it against the building at 11ft up.

[Bruce S]

If it's too noodly I'll be able to brace it against the building at 11ft up.

Just remember, if you brace it . You could have issues with vibration transmitting to the building.
So if you brace it to the building, you may need to put in something to negate any vibrations being transmitted via the bracing.

Bruce S

[makenzie71]

Rubber bushings will be involved if it gets that far.

[makenzie71]

Formula 7.4 from my public report KD 35 gives the rotor thrust for a yawing rotor.

I had time this morning to actually sit down and start reading KD 35 and I think everything in that report is so far above me that I'm going just going to wing it and hope for the best

[DanG]

Rubber hardens & shrinks with cold temperatures - think repurposed automotive style w/ some hybrid polymer, something like a pair of sway bar bushing keepers or transmission mounts, where in worst case most still have mechanical catchment guard designed in if/when the bushings tear...

[Bruce S]

Rubber hardens & shrinks with cold temperatures - think repurposed automotive style w/ some hybrid polymer, something like a pair of sway bar bushing keepers or transmission mounts, where in worst case most still have mechanical catchment guard designed in if/when the bushings tear...

I was going to say try engine motor mounts ,,but your suggestions are easier to be found.

Bruce S

[makenzie71]

I actually have a 1" thick rubber material that's intended to be used as a bushing for medical equipment when it has to be bolted to either a solid floor or wall to prevent the stuff shaking the buildings apart.  We do shouldered lags through a 2X12, then the rubber, then screwed into either a 2x12 mounted on the wall or into backing inside the wall.  It should suffice.  I really don't think this stuff will get too crazy in the wind.

Standing the damn things up is turning into the real problem haha

[Mary B]

I would do a hinged base made of a U shaped piece of heavy steel(1/4 inch plate) that is then bolted to the concrete, drill the pipe for a sleeve of steel that is welded in then run a bolt through it and the base bracket. Then house bracket it after using a winch attached to the house to lift it.

I have a tower bracketed to the house, yes it makes a racket on windy days!