[TowerTalk] Calculating forces in guy wires - How

[Jim Lux]

At 12:06 PM 11/25/2003 -0800, Jim Lux wrote:
>At 02:31 PM 11/25/2003 -0500, va3pl at cuic.ca wrote:
>>How do you calculate forces in guy wires?
>>Here is scenario:
>>
>>Tower 80 feet high
>>Two Antennas spaced 10 feet totaling 25 sq feet (bottom 15 top 10 sq feet)
>>on 22 feet, 2.5" dia. mast
>>Rotor is Orion 6 feet below top plate
>>Wind 80 M/h
>>
>>What are the forces in guy wires then?
>>How strong the guy wires I need?
>>
>>73 de Andy - VA3PL
>
><big snip here>



>Bottom line here:
>
>Consult someone who KNOWS (or can calculate) the answers!  The mfr of the 
>tower may have cookbook numbers for standard installations, with suitable 
>margins and allowances for installation variations  and materials.
>
>Jim, W6RMK

While going through an analysis for something else, a couple loads that 
need to be taken into account cropped up (more traps for the unwary, and 
why you really, really need to have someone competent engineer these things)...

1) Off plumb gravity load.  If the tower is, say, 0.5 degree out of plumb, 
an amount you probably couldn't see (less than a foot at 100 ft), there is 
a small horizontal component that needs to be taken up by the guy system. 
(For a 100 ft tower of Rohn 25, each section is 40 pounds, and would take 
roughly 10 sections, so the tower would weigh about 400 pounds (plus the 
antenna on the top)).  Yeah, the 4 or 5 pounds isn't huge, but, it's 
something you need to bear in mind, and if you start going to bigger, 
burlier tower sections (or steel pipe, etc.) and particularly if your 
column gets real "fine" (i.e. the length/diameter ratio gets over 80-100), 
it's important.

2) Seismic loads.  This is a real important one.  If you use .66g as a 
design criteria, that 400 pound tower is going to put a 264 pound 
horizontal load, assuming it's perfectly rigid.  Since it's not perfectly 
rigid, and the natural resonant frequency is probably fairly low (i.e. 
comparable to the earthquake excitation), the loads could get quite high, 
particularly if there's very much displacement.

Frankly, I wouldn't want to have to do the design calcs for something like 
this, but I'll bet that reputable tower mfrs have done some of the 
analysis, or failing that, there's probably guidelines from the state or 
county in areas where seismic loads are important (like California...)  One 
might be able to sufficiently overdesign to the point where you could 
consider it a rigid body. (http://www.heightstowers.com/seismic.htm )

This would be a real issue for a free standing tower with a significant 
weight on top(like all those rotators and that moonbounce array).  The 
bending moment could get pretty impressive.

I imagine that ANSI/TIA/EIA-222, rev G, will cover seismic stuff in a lot 
of detail. I don't know if it's out yet.  I know they changed a bunch of 
stuff about soils in rev G.

The Building Seismic Safety Council (BSSC) has a web site with recommended 
provisions, etc. which may be relevant.
http://www.bssconline.org/NEHRP2000/comments/provisions/P14.pdf

While a lot of this might seem to be cannonball polishing or gilding 
refined gold, a bit of basic familiarity might be wise when standing at the 
counter trying to get your plans approved.  Much better to say, sure, we've 
looked at the seismic issue and the manufacturer's engineer has determined 
it's not significant for this structure, rather than saying "huh, 
earthquake? never thought of that".  Increased regulatory oversight is a 
fact of life, and your familiarity with how it all works will certainly 
grease the skids on approvals and enable quickly shutting down spurious and 
specious complaints from others.  The worst thing to have happen in a 
public review is to say "I don't know" in response to a potential issue 
raised by someone who has no interest in your plans going forward.