Yikes, Mark, you better check your ROHN catalog and obtain a copy of
ANSI/TIA/EIA-222-F-1996
Per my 1991 ROHN catalog, for 110 mph basic wind (no ice) your 110 ft of 25G
isn't even on the charts! 100 ft of 25G @ 110 mph wind requires 80% guying
and will take only 7.4 sq ft load.
Even the 90 mph wind no ice 110 ft of 25G tower specs out at 80% guying.
Your 45.5% (50/110) guying design is way too low and will result in a very
unstable installation (my opinion). For example, I use 93% in my design.
The higher the % guying, the more stable the design. I've heard that the rule
of thumb is no less than 60% guying. Maybe Steve, K7LXC, would like to
comment on this aspect.
Your comment on hurricane country and soils leads me to suspect you haven't
obtained a copy of the -222- spec I referred to above. Yes it's expensive
($80 in 1996), but far cheaper than replacing 110 feet of tower w/antenna.
BTW, Montgomery County MD (where I live) requires all tower designs in
building permit applications to be certified to meet the most current
ANSI/TIA/EIA-222- spec. When I applied for my permit, I had a PE design the
guy anchor system and review my guying and other mechanical design and so
certify. There are several other reasons to obtain PE certification as well:
potential future liability protection, piece of mind, for example.
The -222- spec may be obtained from Global Engineering Documents,
1-800-854-7179 (for those who need it).
73
Bill, N3RR
Lowell, Mark wrote:
> Greetings!
>
> Every amateur tower I have seen around here so far uses screw anchors.
> Certainly they are easier to use than constructing the concrete types.
> However, being the curious, cautious type, especially after reading this
> reflector, I began to wonder about their effectiveness.
> I am planning 110' of R25 with at least 8 sq feet of antennas at the
> top, anchors spaced 50 ft from base. I have estimated the maximum anchor
> load to peak around 4,500 lb with 110 mph winds (I live in hurricane
> country). I could sure use some help confirming this. I have a Quattro Pro
> spreadsheet set up to analyze forces and would like to compare notes with
> someone more experienced with tower load analysis. The preload alone is
> about 1,800 lb per anchor (15% of breaking strength per guy).
> Well, someone mentioned that the Rohn screw anchor is good for 2,500 lb
> in 'normal soil' (a local utility company rep confirmed that the soil in
> my area is 'normal'). That makes it clearly unsuitable for my tower. I
> think it has a 4 inch diameter screw, and goes down about 3 feet. I can
> get some 6 inch diameter units having 60 inch shafts from a local utility,
> but they didn't know the rating (see below). The person I spoke with
> thought that they might be made by AB Chance.
>
> I contacted the AB Chance company, a large maker of earth anchors (see
> their website @ http://www.hubbell.com/abchance). They offer an
> "Encyclopedia of Earth Anchoring" which, I assume, features AB Chance
> products, and some software which helps you select the appropriate anchor
> for your application and soil.
> The Virginia distributor, J.A. Walder (http://www.walder.com,
> email@walder.com) was very nice and will be sending me these. They faxed
> me some screw anchor model numbers, pullout strengths, and prices, which
> follow. They don't accept credit cards, so I don't think they are used to
> selling direct to individuals.
>
> STRENGTHS ARE FOR 'NORMAL SOIL'. I don't know how to adjust the ratings
> for other types.....(* indicates galvanized)
>
> MODEL # SCREW DIA SHAFT LENGTH PULLOUT PRICE
> 315SA 3 IN 15 IN 200 LB $5.25
> 330SA 3 IN 30 IN 1400 LB $6.00
> 430SA 4 IN 30 IN 2500 LB $7.50
> 404 4 IN 40 IN 3000 LB $12.90
> 604 6 IN 48 IN 4000 LB $15.24
> *4345 4 IN 54 IN 3000 LB $26.76
> *6346 6 IN 66 IN 4500 LB $34.08
> *816 8 IN 66 IN 10000 LB $52.86
>
> Clearly there is a relationship between the screw diameter, shaft length,
> and the pullout strength. Can anyone quantify this relationship?
> Apparently, it has to do with the weight of an upside-down cone of earth
> starting at ground level and pointing down to the screw.
>
> It would appear that I should go with nothing less that a 6" screw, down 5
> feet. The 8" screw gives lots of reserve, almost to the full breaking
> strength of 3 3/16 EHS guys. Since I have trees around the tower, the
> threat of one falling on a wire is real.
>
> Here's the real kicker. The Walder salesman told me that AB Chance would
> provide free engineering to determine the type of anchor required, and
> that they have dedicated full time engineers to this. This could be a
> valuable resource if they are willing to share some other data, too,
> especially a method for rating the screw anchors in differing soils.
>
> Thoughts, comments, suggestions, please.
>
> --...MARK_N1LO...--
>
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