Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Lifting a 70' tower

  1. Feb 21, 2014 #1
    This seems simple but I am having trouble with this. I have 70' self supporting tower that I want to be able to raise and lower with a winch. The tower is hinged at the base.
    I want to use rectangular steel tubing 7' high with a pulley at the top.
    See PDF attachment. I need to know the force applied to the cable in order to get the correct size tubing.


    View attachment lift force Model (1).pdf
     
  2. jcsd
  3. Feb 21, 2014 #2

    SteamKing

    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper

    That's a tricky way to lift such a long tower. The structure of the tower should be checked to see if it can withstand not only the loads imposed by the lifting cable, but also bending due to the self-weight of the tower as it is being raised. Towers may be self-supporting in the vertical position, but raising them from the horizontal is a different story.
     
  4. Feb 21, 2014 #3
    This is a heavy duty all aluminum and is designed to be raise by hand walking it up. I don't have any help so I need to power it down and up. If it doesn't work i'll have scrap aluminum to sell.
     
  5. Feb 21, 2014 #4

    berkeman

    User Avatar

    Staff: Mentor

    Do you have any pics? Are there any nearby tall structures (buildings, trees) that you could use to raise the winch/pulley point?
     
  6. Feb 21, 2014 #5
    No on the trees. Just need to the force applied to the cable.
     
  7. Feb 21, 2014 #6

    berkeman

    User Avatar

    Staff: Mentor

    Why is the cable tie point so low? It would be better if you could tie it more like 2/3 of the way up the tower. Or connect multiple cables spaced along the length of the tower, and vary the speed of the multiple winches to work in concert during the lift.

    I've helped lift up some pretty heavy and tall HAM radio towers, and it's darned dangerous...
     
  8. Feb 22, 2014 #7

    Baluncore

    User Avatar
    Science Advisor

    Tapered or parallel tower, triangular or square section.
    Lump the 400 pounds at the centre of the 70 foot tower.
    That makes 400 pounds at 35 foot. I assume the base is 7 feet.
    400 * (35 / 7) = 2000 pounds, or about one tonne.
    With a pulley on the tower the cable tension will be halved to 1000 pounds.

    That all assumes a very slow smooth movement. Any jerky movement will significantly increase the winch and wire specification required. I would specify at least a two tonne winch, cable and four tonne pulley.


    Here are a few obvious points.

    Firstly, as it is self supporting it will not bend like a mast or pole while being raised, which probably eliminates folding the mast, which is the most difficult situation normally encountered during a tilt up or down.

    Secondly, as it is self supporting, it is able to survive higher side forces than gravity because the wind drag on a tower or wire is very often greater than gravity. Look along a catenary wire during a strong steady cross wind and you will see the wire hanging much closer to horizontal than vertical.

    Thirdly, side forces on the tower legs during erection with a winch or long jack, can easily exceed the forces encountered while standing, as designed self supported. You should consider building a strong hinged base for the tower. That base would be attached to the lattice points closest to the base of the tower and to the tie point for the winch. The forces during erection are handled by that base assembly. It should be designed to remain once the tower is erected. I prefer to add redundant diagonal braces to the lowest module of a tower to handle the side forces expected during erection.

    Take care.
     
    Last edited: Feb 22, 2014
  9. Feb 22, 2014 #8
    Finely an answer to my question. Very good. Now I can proceed. The tower is up now so lowering it will be next.
     
  10. Feb 22, 2014 #9

    Baluncore

    User Avatar
    Science Advisor

    There are a couple of factors hidden in my estimate.

    If a triangular tower, then base has hinges on two feet and pulley on third. Base dimension is then only Sqrt(3/4) = 0.8660 of the distance between the tower feet. That increases the cable tension by Sqrt(4/3) = 1.1547

    At the start of the lift, the 45° diagonal angle of the wire will require a Sqrt(2) = 1.4142 increase in cable tension to lift the tower.

    So the effect of those two cable tension requirements comes out at 1.633 which is allowed for in my original four tonne estimate.


    One question you have not asked is how much concrete you need under each foot of the tower to withstand a strong wind. A quick estimate is that it will need to be similar to the strength of the pulley block you need to use. That is four tonnes, or 1.6 cubic metres per leg for a triangular tower, maybe slightly less for a square base tower, say 1.2 cubic metres.

    Footings are always a special challenge because your maximum wind speed, soil/rock type, tower section and antenna mass/windage all have an effect on design.

    This has not been an engineering analysis so much as a quick estimate of the magnitude of the problem. You neglected any antenna load in your question.
     
  11. Feb 22, 2014 #10
  12. Feb 22, 2014 #11

    AlephZero

    User Avatar
    Science Advisor
    Homework Helper

    One thing that Baluncore didn't mention: at the start of the lift, the force on the base of the tower is large (about the same as the cable tension, not the weight of the tower!), and mainly upwards not downwards. In an extreme situation, you might pull the concrete footing loose from the ground - but you might not discover that fact for a long time afterwards when the tower collapses because the concrete had cracked from the bottom up, where you couldn't see it....
     
  13. Feb 22, 2014 #12
    The base is 6' deep and 5' square. The anchoring legs go to the bottom.
     
  14. Feb 22, 2014 #13

    AlephZero

    User Avatar
    Science Advisor
    Homework Helper

    I'm not sure that is very relevant. If the legs bend and/or break through over-stress at the point where they meet the base, if doesn't matter if the base is 5 feet or 50 feet deep.
     
  15. Feb 22, 2014 #14

    Baluncore

    User Avatar
    Science Advisor

    During raise and lower operations you will need an independent anchor that weighs about 5 tonne.

    The triangular tower base is only 24.25”, but your gin pole is 7 foot. I would use an A frame gin pole.
    The manufacturer should recommend an erection procedure.

    See; http://en.wikipedia.org/wiki/Jin-pole
    Also Google; Gin Pole

    I would have expected three guy wires on a tower like that.
    No lightning conductor? Will it crack the concrete at one leg then blow over in the wind?
     
  16. Feb 22, 2014 #15
    A segmented antenna ...
    Use a crane, helicoptor, or the gin pole technique to remove each segment.

    With the technique of lowering as a unit, the lateral stress at the cable attachement and the swage joints will pull the thing apart.

    One other method is to attach a stiff enough vertical beam to the unit, securely at each segment, and base of the beam . Detach the unit from the concrete base so the beam takes all the load. Do the pulling on the beam.
     
    Last edited: Feb 22, 2014
  17. Feb 22, 2014 #16

    nvn

    User Avatar
    Science Advisor
    Homework Helper

    aknisley: The theoretical, static tensile force applied to your cable shown in post 1 would be Ts = 12 580 N. But that assumes theoretical, very slow, perfectly smooth movement. No matter how carefully, and slowly, you move the system, realistically your cable tensile force will fluctuate. Therefore, the actual cable tensile force probably will be closer to T = 15 700 N. Therefore, use T, not Ts.

    How is your vertical rectangular steel tube, shown on the right-hand side of your diagram in post 1, anchored? The cross-sectional size required for this tube would depend on how, and where, you anchor (or support) this tube.
     
    Last edited: Feb 22, 2014
  18. Feb 22, 2014 #17
    T=12,580 N = 5800 pounds.
    Does the antenna have the structural integrity at one spot to support that load?
     
  19. Feb 23, 2014 #18
    Manufacturer erection procedure:
    http://www.universaltowers.com/I-T.pdf

    What I am getting at is that by having the cable attached to one specific spot on the tower, with lowering as the tower becomes more vertical, the antenna may not be able to withstand the cable stress at one point.

    With the walk up/down method, in horizontal position, the person at the end(top section ) and the base each carry only a vertical load of 1/2 the tower weight, and no horizontal load on the tower.
     
    Last edited: Feb 23, 2014
  20. Feb 23, 2014 #19

    AlephZero

    User Avatar
    Science Advisor
    Homework Helper

    Yep, very clever: so how you balance the tower on two bolts without holding it, while you fix the other three bolts? :biggrin:

    More seriously, that''s a good way to raise the tower (if something goes wrong, you can back out and try again), but lowering it could be a bit more hair-raising, when the bolts etc are nicely corroded after a few years outside in the rain!
     
  21. Feb 23, 2014 #20

    nvn

    User Avatar
    Science Advisor
    Homework Helper

    aknisley: Upon closer inspection, the tower CG is located at 40 % of the tower height, instead of 50 %. And if you move the tower very slowly, carefully, and very smoothly, then multiply the tower weight by a dynamic amplification factor of 1.25. Therefore, working this out gives an actual cable tensile force of T = 12 580 N (2830 lbf), instead of what I posted in post 16. Therefore, use T = 12 580 N.

    And see my last paragraph in post 16.

    That is the important question. aknisley would need to post a detail drawing of how he plans to connect the cable to the tower, with dimensions. Then we could obtain the tower member forces using, e.g., the finite element method. Then we could see if the tower would collapse.
     
    Last edited: Feb 23, 2014
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Lifting a 70' tower
  1. Cooling Tower (Replies: 7)

  2. Scissor Lift (Replies: 10)

Loading...