Tower Caution!!

[Rock]

Well, had 50 mph winds with gust to 70 and my tower came down!!!   Tower was built to save stator and rotors if this happend and in down position rotors/stator cannot hit ground unless bases anchor bolts get ripped out of 2 foot thick concrete.  

I must have had the guy cables toooooo tight!!!!  One of the upwind anchor bolts sheared off clean....  and tower pivited down and busted up all my blades.  No damage to my 6 inch sch 40 tower or 3 inch sch 80 base, or generator.  Guess I am lucky there.  I just need 3 new blades.  

I cannot image the impact of a 20 foot tower in free fall but turbine and tower are still in good shape.  These turbines that dan and co build must be way strong to take an impact like that and still be in great shape!!

A word of caution is do not over tighten the guy wires, 1/2 inch cable can take more psi then a 1/2 inch anchor bolt can in shear.  I have 4 additonal anchor bolts to use and next time my 20 foot tower will be guyed in 8 places instead of 4!!! and I will not over tighten them!!!

Will try to post photos tonight!!

[simonbrookes]

So you learn something every day. I have always pulled my guy ropes up as tight as possible. I have ¾" wire rope and 1" tie rods into the ground. I have had problems with rods snapping at the concreter interface. I have now set 1" x 2" chain into the concrete and a length of 6 links above ground which the shackle and rope connects. Since then have never had a problem. Should I make my guys looser and if so what is the best tension.

[Flux]

As long as the tower doesn't rock around then the least tension possible would be my choice. Any preload other than that needed to remove most of the droop from the guys is adding unnecessary tension. To have enough preload to damage fixings is just plain crazy especially as you will have expansion and contraction with changes in temperature.

Flux

[Mary B]

There is a device called a Loos tension gauge that is used for tightening guy wires. Guys should have some droop to them also. The more you tighten the guy wires the more down force you are putting on the tower base. I have seen a tower failure from over tightened guys. This was a lattice tower and the bottom section crumpled straight down, then the wind got a hold on the tower and sheared the guys off when it twisted. Totaled the tower and antennas.

[bob golding]

i use nylon rope on my 25 foot tower with 8 guys and 4 deadmen anchors buried 3 feet in the ground on 1/2 inch threaded rod. it does work loose as the rope streches but i  just tighten it up every now and again, it has worked fine for the last 9 months or so. withstood a force 11 gale a couple of weeks ago ok winds up to 61 mph. i agree that just tight enough is fine. you want some give in the rope but not so much that it will snatch.

cheers

bob golding

[electronbaby]

You need to use equalizer plates for tower guy wires.

They will help prevent this type of failure.

[Ungrounded Lightning Rod]

I second that.

The droop acts as a non-linear spring to keep the tower centered while letting it move around a tad.  As it blows off-center the tension increases rapidly, providing a braking force that increases as needed to bring it gently to a stop (and then pull it back).

Too much pre-tension and you don't get that.  Instead the tower's motion very abruptly spikes the tension on the guys and anchors and the down-force on the tower base.  Snap/crumple!

[thirteen]

We use a belt tensioner that shows lbs of preasure at a certain piont and you can measure the side pull on each guy cable that way they are the same. I don't know how you made your anchor or how it was poured. So it might help if the bolts were set at an angle to match the angle of the pull from the guy wire. If it is directly in line with the bolt and not at an angle the bolt will stretch before it snaps.

[neilho]

 Rohn's specs for guy wires for a 20' diameter turbine on an 80' hi 45G tower, 3 anchors equally spaced at approx 70' from the tower are for 7/16" Extra hi strength cable (ultimate strength 20800lbs) on the upper guys and 3/16EHS cable (ultimate strength 3990lbs) on the lowers. Initial recommended tension is 10% of ultimate. There's an easy way to do this without gauges, etc which is to calculate the the first natural frequency of the wire, tension it, get the wire moving at one node per length and retension as needed so the natural frequency is within 10% of the calculated value. There is a different formula for each diameter of wire, but they all simplify (within 1 or 2%) to F=12,500/L where L is the length of the guy wire in feet and F is the frequency of the wire in cycles per minute. This works for EHS wire, I don't know if it is accurate for other types of cable.  

neil

 

[Rock]

Just a couple of observations I made.  Turbine tower and guy wires were installed in 70 degree weather.  Cables were OVERTIGHTENED in 70 degree weather.  Anchor bolt sheered in 5 degree weather.  

I am assuming at this point the force of the wind plus the shrinkage of the cables due to temp are both factors that caused this issue.  

To compensate for this I am going to apply correct tension on cables this winter while its cold, and I am going to put a safety cable on all cables that will be anchored to another anchor bolt.  The safety cables will be somewhat loose but will activate when/if one of the anchor bolts shears again.  

[neilho]

Another factor in Rock's anchor failure may be the stress concentration where the anchor exits the concrete. The steel goes from being very stiff where it's supported by the concrete to relatively free at the surface of the concrete. Any movement of the guy wire will tend to stress the anchor right at the surface. Over time, a fatigue failure is likely to happen.

Another factor might be the low temperature crack propagation properties of the steel. Most of the steel we use (low carbon 1018 or A36) will crack much quicker at low temps than at room temp. Something as innocuous as a scratch on the surface can begin a serious crack. And in a dynamic situation, once the crack starts it's often only a matter of days before failure.

So I would be hesitant to blame the guy wire tensioning as the sole cause of the failure.

Neil

[bob golding]

hi neil,

 do you have any options to avoid this happening? maybe wrought iron anchor bolts? seem to recall wrought iron was used a lot in victorian times for bridge construction. maybe some sort of shock absorber in the guy wire? i havent looked but i was thinking something you thread the guy wire though like a double pully block with some sort of rubber mounting. might be totally over the top considering the few failures we seem to get but thought it might be worth exploring.

cheers

bob golding

[neilho]

Hmmm, good question. Oversizing the anchors would be helpfull. A small increase in size increases fatigue resistance a lot. Using a more fatigue resistant variety of steel. Real wrought iron is hard to find, fatigue properties prob unknown. Flexible mounting might help. A spring? It'd have to be very stiff, but would certainly compensate for temp changes.

I agree that it's probably overkill to make any significant changes. Oversizing the anchors seems like the simplest solution.

neil

 

[bob golding]

i have been thinking how one could use a spring without having the danger of it breaking,and have come up with this. a compression  coil spring inside a tube. a washer welded to one end of the tube with a hole in it. a lenght of threaded rod /all thread with a lifting eye bolted to one end. this a slid though the hole in the tube with the spring in it and a washer just slightly smaller than the ID of the tube slid over the over end of the threaded rod and bolted in place. a u shaped  piece of solid bar is welded to the outside of the tube at the bottom. a shackle goes though this  and fixes to your ground anchor. your guy rope goes though the top eye bolt. this should work and there is no danger if the spring breaks of the guy coming free. you only need 2 of them per tower. it could a solution for a problem that doesnt exist but it would work. does it already exist? havent looked but not sure what it would be called? temperature compensating shock absorber maybe.

cheers

bob golding

[wdyasq]

bob,

You may be thinking about 'wrought steel'. IIRC it is a very low carbon steel that went out of favor about 1900-1920 here in the US. A couple of the noted characteristics are low rusting and almost no fatigue point. It is all but impossible to find now.

Ron

[bob golding]

yes that is the stuff. it is called wrought iron in the UK i think,well thats its common name anyway.

cheers

bob