'Positive curve' when lifting tilt up tower

[pellets4fuel]

There were a couple different tower topics started already, but I thought it more fitting to start a separate one.  I have built a 120ft tilt up guyed tower that I am very comfortable with.  I am comfortable with it because I have worked along side Dan B for a day seeing him raise a tower "Met tower" at 140 ft used for a larger turbine than mine, and I know others doing the same (using Met towers)  The Met towers are "interesting" to me to use for a wind tower mostly because all the segmented sections of pipe just slip fit together every 9ft or so.  In building my own tower because I have access to galvanize round tubing at a decent price I went with 40ft long sections of 8in galvanized 10ga 'pipe'  This gives me just 3 joint sleeves in my tower made just like the ones shown in Homebrew Wind Power though mine are maybe a little longer at 24in. 
I have my tower planned to be guyed a 5 levels about every 22.5ft.  Anyway, I have been using the NRGSystems tower installation manual as a guide to a few different things though not all of it applies.  One thing though is the discussion of lifting the tower just a few feet and observing a positive curve to the tower, and maintaining it all the way up to avoid tower collapse.  They define positive curve as the top of the tower being 1 to 2ft higher at the top than the base when lifted that few feet.  I used the offset hinge/gin pole pivots in a base just like NRG makes.  I took my dimensions of one of their bases so I now I am dimensionally right.  I have done a lot of drawing on cad, and everything works out good tilting the tower up.  So the way they get positive curve is they have designated lifters to the gin pole, and then once they are up they are transferring those lifters to actual locations.  Then to straighten the tower your adjusting (putting tension back on back cables) that you had to leave to allow for the positive curve.  I get all this and it works, and is fine for a tower getting raised to stay up and not being lowered every few months for routine maintenance to a turbine.  I was planning on transferring the top two lifters on my cable anyway just because my gin pole isn't as long as my guy radius is.  Are others here using Met type towers, and are you aware of the positive curve concept (even if your not using Met tower) and the importance of it to prevent tower buckling?
David

[JW]

I am comfortable with it because I have worked along side Dan B for a day seeing him raise a tower "Met tower" at 140 ft used for a larger turbine than mine, and I know others doing the same (using Met towers)  The Met towers are "interesting" to me to use for a wind tower mostly because all the segmented sections

I have added DanB and DanF to the forum as GM's This gives them some flexibility

[pellets4fuel]

Not sure what you mean by that post JW.  Dan would talk to me about it if I emailed him or called.  Just feel I've bugged him enough about stuff with my project.  Thought someone else may have some input with building a "positive curve" into a tower for lifting.  I know right now if you take the erect cable length from lifting side anchor points to tower when the tower  is erect, and use those lengths to "model" on the cad the tower laying down at about 15deg increments, by the time you get the tower almost laying down your actually dealing with a negative curve with the middle being higher off grade than the end (top) of tower.  Again this is using the offset hinge design like the Met Towers use.
It wouldn't be a problem with keeping the turbine on tower stub, because with my site I am laying down up hill and my tower will never get flat flat but rather be at appr. 8.5 deg slope when down.  I should have mentioned also that the positive curve concept is mentioned in the 60meter manual, so 180ish foot tower.  My tower is only 120 and is made out of stiffer pipe being 40ft lengths.  Still though, a negative curve with the middle higher then the end when starting lifting, and then the weight of turbine dangling up on the end yet of a giant lever.  Seems like what they call a positive curve would be more desirable. 
David 

[JW]

Don't worry to much about it I like to see and hear about the Dans are involved with projects. They are the founding fathers of Fieldlines. The mention of the Dans as GM's is something I did because after we made the transition they were just regular user's

I can understand what you mean by not wanting to bother them. About a week ago a user wanted to contact them but I would only give there email via PM.

I have known them for years good people

   

[pellets4fuel]

I have never met Dan F.  Have PMd with him on a couple occasions.  Dan B is very nice guy.  Spent 6 days with him building my turbine one on one, and helping raise a tower.  Yes great guys. 

I did happen to read through the Bergy tilt up tower manual last night.  They have a 100 footer that I think is made out of 6in tube with belled sections just like the met towers.  They make no mention of positive curve in fact they mention a slight downward bow being normal, and if it is more than 1ft your suppose to adjust your lifter cable lengths.  They also are transferring all their lifters from gin pole to anchor point. 
David

[SparWeb]

P4F,

Firstly, thanks for letting me know about the new NRG Systems towers manual.  I've been reading it through this morning and found a few gems.  I was not previously aware of "snap through" so this alone makes it worthwhile.

Your question about the "positive curve":
So I never really gave it that name, but I was certainly very cautious about not applying too much compression to the column.  Before building my tower, I worked out the compression force in the lift process, and compared it to the compression in the upright position (with/without ice) and the lifting condition is pretty severe.

It's not easy to do general theoretical calculations when working with column compression on an assembly like a tower.
One reason for this is that the restraint given by the guy wire stations are "pin joints" and the sleeves are neither fixed nor pinned.  They can transfer some bending from section to section of the tower, which combines a bending moment with compression.  That's a tricky set of problems.  To solve it with mathematics and not make wrong assumptions or over-simplifications took a fair bit of research on my part.  I'm still not sure I did it right, but, hey, my tower goes up and down.

The lift cable running from winch to gin to tower is pulling at an angle, so it applies both the lift force and a compression force at the same time.  Each of the successive guy stations on the tower that you have also connected to the gin add even more compression if they are tight.  So if you had guy wires from the gin pole pulling tightly in the middle of the tower, they would be adding 1000's of pounds of compression to the tower.  It would all add up to a very large compression load at the base.
That's my take on why you want a "positive" curve.  If the middle of the tower "smiles" a little as it goes up, then that means the guy wires in the middle are holding it up somewhat, but not applying a strong lift load at the middle.  They will add much less compression load to the base.

Oh, and I also just realized one more thing.  If your gin pole is supporting the tow cable not just at the end but also cables to the middle guy stations on the tower, consider the angles THEY make to the gin pole.  A guy wire going to the gin pole from the middle of the tower reaches the gin pole at a steep angle.  So it puts a lot of compression on the gin pole, too.  Wow, that's bad for the gin pole AND the tower.  No wonder NRG warns people about that.

A caveat: My tower has only 3 guy wire stations and the upper 2 are quite close together so I've never been confronted with the problem of middle guy-wires doing to much lifting.

I'm not sure I could follow all that explanation myself without a diagram.  Hope it makes sense to you.

[SparWeb]

In related news, yesterday I lowered my 30' met tower to replace the anemometer cups and lubricate the spindle.
Since it's a little TV tower that can be picked up by hand, fully assembled, lowering it with a gin pole is overkill, no winch needed.

[pellets4fuel]

Steven what your saying makes total sense. Compression has to come into play in both tubes as your lifting as well.  That's when I first read about having any kind of curve I initially though "you kidding"  I first thought as straight as you could possibly get it.  Then I visualized it, and it made sense why a positive curve is best.  As you apply force and it goes to compressive you don't what it going side to side so tending those cables is crucial.  it is unlikely to go earthward and follow the "smile" because your holding it with the lift cables.  Up it can go, and you would have a negative curve to start it is more than 1/2way there in my mind and all you have left is how much strength is in your tube.  I am just trying to come up with a way now though to make up and down movements for service easy without having to readjust back cables every time to put positive curve back in when lowering.  Thought turnbuckles with recorded lengths for erect vrs lowering.  I wanted to stay away from turn buckles though.  They would need to be wired every time for safety, and it probably wouldn't be redundant to put a safety chain across them so if they did fail you wouldn't loose a cable completely.

Your welcome on the NRG find.  I have been using a lot of their info for design even though it could be somewhat un-applicable due to putting a turbine on top.  I will have to go back a research again, but I am pretty sure they only mention positive curve in the taller towers manual.  I don't recall seeing it in the 34 meter manual.  So with that in mind I wonder to what degree height plays in.  Also like I said Bergy towers makes no mention of it at all and in fact the opposite.  Who else may have a tilt up tower manual out there to research this?
David

[Mary B]

Height plays a huge role in the forces at the base of a tower as it goes vertical... while lifting you add the compressive forces of each guy station so the more guys the greater the force at the base to an extent... think of the lifting guys as pulling the tower towards the gin pole but the base is fixed so it can't move... I have those calculations around here somewhere... ham radio tower guying article...

[SparWeb]

Here's an installation manual for a Whisper:

 SWWP_80_foot_tower_manual.pdf (1429.79 kB - downloaded 192 times.)

You might be planning a turbine bigger than a Whisper 200, but, it may help anyway

[pellets4fuel]

Thanks for the comment and link.  The Whisper manual reveals little for new info.  One thing it possibly confirms though is the even placement of guide cables along the mast (for their height and turbine size at least)  I believe Bergy is the same as well as Met towers though they are not calculating a turbine at all.  I contemplated closer spacing at the top of mine due to the snap through concept and because my anchor points are at about 57% (65ft out for 113ft of height)  My mast is tower is actually 120ft total, but top cables are down 7 to allow for blade sweep.  This puts my top most cables right at the 60deg mark (except for back ones as I am falling up hill at 8.75deg)  I have 5 anchor points so I am supporting every 22ft-8in up the length of an 8in 10ga tube with only 2splices at 40ft increments. 
It makes no mention though of positive curve at lift-off and no mention of back haul on back cables after about 70deg up. 
David

[SparWeb]

The Bergey and Whisper towers are 1/2 the height of the tower you're planning.
It may not be friendly to pry into this, but have you evaluated the column stability of yours?
You've given enough information that I can guess the rest, but depending on what I guess, I don't like the answer.
Is the pipe at least 8 inch diameter? 
In the scenarios I've worked out for my tower and others, with guyed-towers being lifted, I get a towing cable tension about 2.5x the tower weight.
That scales up with the weight of the tower, but its column stability scales up as a square of the diameter, so every inch of diameter helps a lot.

[pellets4fuel]

Yes my pipe is a full 8in in diameter, 10ga, and like I mentioned 40ft lengths.  The base 40ft has been reinforced in the pivot.  Not dealing with all the joints every 20ft, or even every 8ft or less.  My guy cables are all heavier at 3/8, (others only using 3/16 or 1/4 even on winch lifter) My winch cable is also 3/8, and it is a large Braden 20,000lb recovery winch with dual drums.  More than a few yards of concrete between base, 4-anchor points, winch mount, and redirect anchor for gin pole. 
The met towers at 60m go with a combo of 10in in the base sections and convert to 8in higher up (about the top 108ft).  34m towers are 6in the whole way.  Bergey's manual is for a 60,80,100ft tower, and makes no distinction that they are going any larger than "4.5in thin wall tube" with swaged  ends in 10ft lengths.  The swagging actually stretches and thins the material.  That concept works good for auto exhaust pipes IMO
I believe the positive curve for lifting is crucial.  I just need to make some time to sit down at the cad and figure out how to make it happen with the offset hinge design.  The lifter cables in my mind need to be as if the tower were straight when erect, but when laying down or near down holding the positive curve.  It has to have something to do with the mast and gin pole not sharing the same pivot point, and/or the possible cable spacing along the mast.  A few inches up or down the length of the mast changes it's curve.
David

[SparWeb]

Here's another train of thought.
This carries over from a previous discussion I had a number of years ago.  It involved tricky details in the attachment of the gin pole at the base, which seemed to be causing strange effects on the tower when the guy started to lift the tower.  It's a good thing he noticed.  This condition could have run away before he stopped and asked questions.

The hinge point of the gin pole, if it's attached to the tower directly, can interact with the flexibility of the tower to create an offset.  When the gin pole is under high compression load, then the offset distance X compression force = moment that bends the tower.



Here's a case where the "positive" curve definitely does not make you smile.