Guyed mast (thread507-2282)

[Hsang]

Hello all

I am asked to review a 50 m high guyed lattice mast with 3 vertical chords (ie equilateral triangle) with 3 levels of guys at 120 degrees. I intend to review it to AS 3995-1994 using either SpaceGass or MicroStran which has cable and second-order analysis modules. Gravitational and wind actions and the design can be carried out in accordance with the relevant codes.

My query is how do I start apportioning the pretensions in the guys at the various levels? Any guidance or reference is appreciated. Thanks.

Rgds

 

[thaidavid40]

Does your software allow you to program in some settlement of the guy wire anchorage points at ground level? If it does, you can program into that a settlement which stretches the guy cable enough to equal the pretension load.

Thaidavid

 

[charliealphabravo]

There is a rule of thumb for pretension that involves sighting up the guy line toward the mast. That is not very helpful for a computational analysis but it tells you that the tension is not very precisely controlled in the field. I do know that the tension in these lines is typically pretty low and mainly provided to ensure alignment of the mast. High tension loads are counter productive since they add to the compression in the mast and contribute to the buckling failure modes.

Having said all that, there are some highly qualified guidelines on initial tension in EIA/TIA 222F. It says that initial design tension is normally 10% of published breaking strength of the strand and is normally kept in the 8 to 15% range.

Here is a link to a sample analysis by a company that does this type of thing. I'm not sure how helpful it will be.

http://www.armortower.com/services/engineering/structural-analysis/

 

[Hsang]

Thanks thaidavid40 and CAB for your comments. The softwares I will be using are capable of lengthening/reducing or keeping the cables original lengths (ie straight line between the anchor points). However the cantenary of the cables will induce forces.

I managed to download a couple of papers which I will be perusing. I will be back in the office on Monday (today is Saturday in Australia) and provide the download details in this thread.

I am still interested in how others systematically apportion the pretension in the cables in a general structural program that I am using cf dedicated tower type programs. The papers I downloaded discuss the use of springs at the nodal points of the verticals or the use of equivalent beam to determine the maximum forces at the ultimate limit state (ie only one group of cables is effective as the others approaching near slack condition, ie nominal tensions). I will try out the methodology and will report on it.

I look forward to other comments. Thanks

Rgds