Previous in Forum: center od mass   Next in Forum: chaudhari
Close
Close
Close
7 comments
Active Contributor

Join Date: Sep 2008
Location: Morocco
Posts: 13

tower modelization

09/26/2008 3:42 PM

hi, how can i do a guyed tower modelization in order to calculate guy tension. is there a mathematic formula.

__________________
simo
Register to Reply
Pathfinder Tags: Modeling tower
Interested in this topic? By joining CR4 you can "subscribe" to
this discussion and receive notification when new comments are added.
Guru

Join Date: Nov 2007
Location: Sherwood Park, Alberta, Canada
Posts: 1212
Good Answers: 73
#1

Re: tower modelization

09/27/2008 12:00 AM

Tension in the cables of a guyed tower depends on initial cable tension and stress under a particular loading condition. To calculate tension in a cable as a result of wind loading requires knowledge of (a) geometry of the tower (b) magnitude of lateral wind load on the tower (c) direction of wind and (d) inclination of the cables. If you have this information, the problem may be solved using statics.

For more information on guyed towers, check out some of the leads at this site.

__________________
Bruce
Register to Reply
Active Contributor

Join Date: Sep 2008
Location: Morocco
Posts: 13
#3
In reply to #1

Re: tower modelization

10/01/2008 4:40 PM

Hi Bruce, i understand the part regarding wind load and i can work it out using formula F=qz*Gh*Cf*Af with exposure D, but i do not get it about the 'initial cable tension'. - Tower is 69m high - 1,7m width. - 3 faces lattice - the base is conical. - 6 anti-twist guys at 29m height and 3 guys at 56m height. - Distance between the three anchors and base is 35m at an angle of 120°. - Angles between 6 lower anti-twist guys and earth is 40° - angles between 3 higher guys and earth is 58°. Total area of each face is about 30m² including dish antennas and some yagis. How can i work out initial tension with these parameters? Thanks. Sidate CN8SL

__________________
simo
Register to Reply
Guru

Join Date: Nov 2007
Location: Sherwood Park, Alberta, Canada
Posts: 1212
Good Answers: 73
#4
In reply to #3

Re: tower modelization

10/01/2008 7:25 PM

Hello msidate,

Initial tension is required to prevent sag and excessive vibration of the guys. I believe the initial tension should be in the order of 10% of the ultimate strength of the cable, but I do not have experience in designing guyed towers, so would want to research it a good deal more if I were designing such a structure . It is important to know the magnitude of the initial tension because the vertical component of three cables coming together places a compression in the mast which affects its design.

Wind forces, gravity loads (including icing if applicable) and seismic forces all have to be considered in design. Forces from lateral loads will produce tension in at least one of the guys or perhaps two, depending on the wind direction. Such forces must be added to the initial tension in the guy.

If you are responsible for the design of a guyed tower, and if you have not designed one before, I would strongly recommend that you find an expert to assist you through a few of the hurdles.

All the best.

__________________
Bruce
Register to Reply
Active Contributor

Join Date: Sep 2008
Location: Morocco
Posts: 13
#5
In reply to #4

Re: tower modelization

10/02/2008 5:47 AM

Hi Bruce, the actual guyed tower i described has 1 inch (exactly 26mm) diameter galvanized monotoron steel cables with an ultimate strength of 30 tons. That means i have to apply an initial tension of 3.0 tons on each cable (most references talk about 7% to 15% of maximum strength so 10% is a good average). i have been manufacturing and installing hundreds of ROHN-like guyed towers and Self Supporting Towers without any calculation (i applied ROHN catalog notes and prescriptions though) and it worked all fine since 1980s. But for this one i would like to know the theory and physics behind it. Thanks. Sidate

__________________
simo
Register to Reply
Guru

Join Date: Nov 2007
Location: Sherwood Park, Alberta, Canada
Posts: 1212
Good Answers: 73
#6
In reply to #5

Re: tower modelization

10/02/2008 11:54 AM

But for this one i would like to know the theory and physics behind it.

Sorry, you are a little beyond me on that one. I understand that below 7% you get galloping of the cables and excessive tower movement. Above 15% you get vibration and excessive compression on the tower section. I suspect these limits were found on a trial basis, rather than a theoretical one, but perhaps other CR4 members could add something of value here.

This reference cites the same limitations as we have been discussing.

__________________
Bruce
Register to Reply
Guru

Join Date: Feb 2006
Location: Piney Flats, Tennessee
Posts: 1740
Good Answers: 23
#2

Re: tower modelization

09/27/2008 3:34 AM
__________________
If you never do anything you never have problems.
Register to Reply
Guru

Join Date: Mar 2007
Location: City of Light
Posts: 3943
Good Answers: 182
#7

Re: tower modelization

11/01/2008 8:44 AM

Your problem has 2 aspects:

- statics

- dynamics

For the 1st aspect the loads as wind have to be considered and the position of cables. It is a hyper-static problem which can be solved using the deformation equations of structure and cables.

For the 2nd aspect the mass of the tower has to be in a 1st approach concentrated in at least 2 masses and considering the cables stiffnesses the proper frequencies have to be determined.

With same model it is possible to analyse effects of earth quakes introducing accelerations at the bottom as well in horizontal as in vertical directions.

There are softs to make the model and estimate the highest possible loads.

As you see there is no one formula to solve it. the catalogue indications are based on complex computations and also experimantal values as for cables stiffnesses and damping due to internal frictions.

Register to Reply
Register to Reply 7 comments
Interested in this topic? By joining CR4 you can "subscribe" to
this discussion and receive notification when new comments are added.
Copy to Clipboard

Users who posted comments:

ba/ael (3); dadw5boys (1); msidate (2); nick name (1)

Previous in Forum: center od mass   Next in Forum: chaudhari

Advertisement