|Subject:||Re: [TowerTalk] Calculating forces in guy wires - How|
|From:||Jim Lux <email@example.com>|
|Date:||Wed, 26 Nov 2003 10:28:12 -0800|
At 12:06 PM 11/25/2003 -0800, Jim Lux wrote:
At 02:31 PM 11/25/2003 -0500, firstname.lastname@example.org wrote:How do you calculate forces in guy wires? Here is scenario:
Bottom line here:
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.
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.
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions and ask for Sherman, W2FLA.
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