Calculation of Guy wire length for Tall guyed mast 1

[am]

I am looking for some analysis tool to calculate the exact [as close as possible] length of guy wires for a very tall guyed mast. I believe guy attachment location on the mast will at about 250m, anchor radius will be about 180m. I anticipate guy wire diameter will be in the range of 75mm or more. The guy will have high initial tension once installed. The straight length is 308m with above parameters, but there will be significant sagging in the wire. The majority of sagging will occur towards the upper half of the guy wire. I have to calculate the length considering the sagged length under specified initial tension.

It will be of great help if any of you can give me some guidance in this regard.

AM

 

[JedClampett]

Check Gaylord and Gaylord "Structural Engineering Handbook." Chapter 24 in the first edition has the design of a guy wired tower.

 

[ERV]

Here is some helpful info on guidewires:

http://www.cam.org/~burt/circuits/rohntens.html

 

[ERV]

Sorry didin't finish my post (and mispelled guywire), check out this site for software that may help:

http://www.mecaconsulting.com/guydes.htm

 

[linslade]

Have a look at web site,

http://home.earthlink.net/~jimlux/radio/math/catenary.htm.

This gives formulae for horizontal and inclined catenaries.

Linslade

 

[am]

Thank you very much for all the suggestions. Our internet was down yesterday and could not access these leads. I will soon explore them.

I have seen the guydes site. I am a little bit skeptical about its ability for such a big system. Any comments from any of you?

Thanks again for all your input.

Regards

AM

 

[flamby]

am,
75 dia rope stretched to 308m! Looks awkwardly thin. You have reasons to be skeptical. I feel you should go for a bundle of wires instead of single wire.

The self weight of wire itself will induce stresses to the order of 24MPa in the wire so I am just reminding you to take this in account. Add to this the initial tension, and very less is left for working margin. Why dont you expand the base of the tower and avoid guy ropes altogether?

As to exact length, it will be very marginal over computed 308. Something like 309 perhaps.

 

[am]

flame

Believe me, I am skeptical; but let me get the full picture of it. I am making a ground work at this stage.

Probably I did not explain, 75mm diameter guy wire is actually comprised of large number of very high strength strands. I am not sure if they are of 1570MPa or higher grade at this stage.

I beg to differ with your perception of exact length. I have seen some of these structures. Even under substantial initial tension, due its own weight and other catenary action these wires experience huge sagging. Therefore the actual length will be much more.

Exact length is required to have swaged ends. Once ends are swaged, site personnel will not have the opportunity of altering the length at site. You will have only a fixed length in the guy tensioning device to play with. If the length calculation is incorrect, then you probably will not achieve the determined initial tension.

Regards

AM

 

[ERV]

am,

Have you selected a manufacturer for this 75mm guy? Manufacturers differ somewhat in rate of twist in their wire rope, this will affect elongation under tension. Elongation will therefore affect final computation of overall length. Suggest contacting manufacturers for specifics on their products and starting there. If percent of elongation equals (or exceeds) percent of sag, the difference in added length (due to arc) may be positive or negative.

In short, key to your question is elongation of the wire rope, added length should be easy to compute based on anticipated arc of the sag.

 

[am]

ERV

Thanks for your suggestion.

Yes, we have selected the manufacturer. We will check these points with the manufacturer.

Regards

AM

 

[wahmit]

I am a little concerned that you need to ask the question, but hopefully I have missed something. Once you start putting guys on a structure it requires non-linear analysis, and a little knowledge and experience to avoid costly mistakes.

Any suitable non-linear analysis package can be used to give you the stay length one way or another. Programs written specifically for masts and towers give this to you as a matter of course, general non-linear analysis programs need a bit of work; one way is to specify the length of the stay in the model and adjust it to give the desired behaviour of the structure.

You say you want to accurately calculate the stay length, but how accurately? The chord length is sufficient for costing. The installed stay linkage will need adjustment to allow for tolerances in the stay stiffness and length, the bedding stretch (there will be some even if the rope is pre-stretched), tolerances in the construction of the mast and anchors, and to simplify installation of a stay of this diameter.

I’m sorry if this sounds a bit negative but I would like to help you avoid some of the mistakes I have come across (and made) when engineers inexperienced in the design of masts and towers design these structures.

 

[am]

Wahmit

Thank you very much for your kind advice. I intend to check every suggestion in this matter.

We use MSTOWER, Weisman Guymast programs mainly for analysing masts and towers. I will review the output of MSTOWER again if that gives me any direction. Though I have been designing towers and masts for last ten years, I know that I still have a lot to learn.

Regards

AM

 

[wahmit]

AM

I have used MSTower for tower design but not for masts. I understand that MSTower calculates the stay length required for the specified tension (neglecting the axial shortening of the mast) so you may find the length used buried in one of the output files. If you can't find the length, you could ask the distributor how you could get at this information, and request that it is made available in the next update.

MSTower is okay but it does have some shortcomings, evidently this is one. However as far as I know it is the only commercially available software intended for the analysis and design of masts and towers, unless someone out there knows different?

Regards,

Wahmit

 

[VoyageofDiscovery]

Hi am,

Just as an aside, for this diameter, I anticipate open or closed sockets for end terminations. I would have the Contractor provide efficiency factor testing for Bridge Strand in the Contract specs as the efficiency factors provided by suppliers of these terminations are not for BS.

Regards

VOD

 

[VoyageofDiscovery]

Hi am,

I've had a little time to think about your tower, and from my experience with 300m (1000ft) towers in Canada, I have not seen, say for 5 to 7 attachment levels, guyed towers with 75mm (3 inch) Bridge Strand. The most I have encountered is 44.5mm (1-3/4 inch) Bridge Strand. I understand you may have heavy loadings but something seems a little hokey.

Regards

VOD

 

[am]

VOD

You are rght - these masts are extremely heavily loaded. They are for VLF system and loading type is quite complex.

We are trying to get the efficiency factor as per your earlier suggestion.

Regards

AM

 

[VoyageofDiscovery]

Hi am,

Not to beat your tower death, but if this is a new tower, you may want to investigate the possibility of galloping for such long and heavy guys. Dampeners can be used to limit guy frequency.

Regards

VOD

 

[am]

VOD

I thought about this. But it is about 30 year old mast [rather series of masts] and they do not have any 'damping' system to the best of my knowledge. Therefore it will be difficult to introduce this concept at this stage. Aeolian [not sure about the spelling!] vibration and Galloping is very critical for transmission line towers though.

Thanks for sharing your thought.

Regards

AM