[TowerTalk] Idea for tower base. Comments please!
bamos1 at gmail.com
Sun Jan 12 10:14:12 EST 2014
Done properly this could work well. Many a solar panel is mounted in a
similar situation. Many of them lay the beams as an H and connect the tower
to the middle of the cross beam and put a pier in each corner. A steel pier
of this type wont take much lateral or moment load so this arrangement is
necessary to create a moment couple.
By screw anchor are you referring to a solid pole with a single helix at
the bottom? Or is it more of a threaded rod?
Typically these are installed with a certain torque requirement to meet the
needed capacity as they do compact the material during installation. I
imagine to achieve that load you are in dry soils with at least 10 feet of
embedment. These have a pull out strength roughly equal to the weight of
the soil the helix is supporting. That weight depends on soil type,
density, and moisture content.
I wouldn't say that undisturbed sand is the worst case. Wet clay is always
the worst case. Sand can be quite dense and a good bearing medium for any
type of foundation.
These are very commonly used by the power company to guy their transmission
line monopoles. Since you already have some installed you could use
whatever they are currently resisting for design.
When I recommend these foundation types I always recommend pull out tests
be conducted with a factor of safety of 2. That means they try to pull it
out with twice the design load and if it moves less than a required amount
it passes. Its a bit more complicated than that but that's the general
idea. Of course you can use soil type and density and no pull out tests to
estimate the capacity but you need a larger factor of safety.
That 14.4kips may be factored. Most helical pier installers have enough
experience to know what they have to do and how deep to go to get the
capacity you need.
As for corrosion that is pretty well studied and there are different
coatings that can be used, it can also be accounted for in design of steel
thicknesses. The installer will usually guarantee them for a certain period
of time against corrosion.
I say do the math. If it works out then go for it! In the Phoenix area
these can typically be installed for 800 to 1200 bucks each depending on
how deep they have to go and the pier size.
On Jan 11, 2014 11:42 PM, "Hans Hammarquist" <hanslg at aol.com> wrote:
> I just wonder if, instead of digging big or small holes in the ground and
> filling them with concrete, it would be possible to drill down a number of
> screw anchors (minimum 4) and make a base of e few (minimum 2) I-beams
> holding on to the anchors.Then attach the tower to this I-beam construction.
> Reason: The screw anchor I used for my guy wires were rated for a pull-out
> force of more than 14,400 pounds when screwed into undisturbed sand (which
> should be the worst situation, I believe). Two screw anchors and a 4 foot
> I-beam should, what I understand, be able to take a 57,600 foot-pound
> turning momentum. Arranged as a square it should be able to support a
> self-supporting tower.
> Now, question arrives how you maintain this structure well knowing than
> screw anchors can corrode, but I believe we can coop with that one way or
> Place for comments.
> With best 73 de,
> Hans - N2JFS
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