[TowerTalk] Guy Anchor Pullout Strength

[Gale]

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.
> 

Your guy anchors should be at least as strong as the guy wire.  Remember
that they stated pull out force.  If an anchor pulls out even part way
the tower could be loose enough to fall.  Typically a tower system
should be designed with a safety factor of 2.  This allows for some
design error/uncertenty and weakening of parts from age and corrosion. 
If the tower could fall on an inhabited structure it should have a
safety factor of at least 4.  Some designers will go with factors as
high as 10.

Gale
N8GS

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