Tower Anchors Revisited

[Bigwheel]

Hi again,

Here's the update on this two year project in the works.  My 80' tower is finally finished along with the gin pole.  I have installed the main pivot base and winch base/anchor in concrete.  I picked up some 5' long utility screw in anchors last year and have been debating on screwing them in verticaly verses on angle( about 60 degrees).  This is my thought and maybe someone can provide me with a better mechanical advantage over one from the other.  Vertically the anchor is 5' down and if on a 60 degree angle it would be about 4.4' below the surface.  The soil begins out loamy and becomes hard packed sand.  I would like to be in as far into the hard pack as possible.  I am also trying to avoid cementing them in if possible, for future removal if I were to move.  Another way I have looked at it is, envision the anchor rod as a knife and when heavy forces are appied it may over time cut through the soil and the tention on the guy wires will loosen up. One possible draw back for going vertical.  Hopefully in about two weeks I'll start assembly and raising into position.  Thanks again for your help.  Chad

[Ungrounded Lightning Rod]

Screw anchors should align with the pull.  Then they're loaded in tension and the anchor plate is at right angles to the force, providing maximum hold.

Screw them in vertically and you load the rod in bending.  Further, the load will attempt to rotate it into line with the pull and the only thing reisisting that will be the cross-section of the rod working against the soil.  It will turn to the right orientation, but your cable will go slack in the process.  Meanwhile the turning will loosten the dirt and once aligned the anchor will be working against loose clods rather than solid ground.  A much smaller force than its normal hold will dislodge the clods and allow the anchor to pull out, rather than the normal case of sliding at least the weight of the slug of dirt above its plate against the friction with the surrounding dirt.

[imsmooth]

I agree with the above comment.  When inline with the cable the anchor only sees tension and is the strongest.  Anything off of inline will result in a bending moment and will try to torque the anchor out of the ground.  In clay soil a 4' anchor with a 6" round auger can hold close to 5000 lbs of pull.  I put a 12" brake rotor over my 48" anchors and buried them each with 200lbs of crack resistant 4000psi cement.

[Ungrounded Lightning Rod]

Also:  IMHO You want to point it a tad below the top guy attachment.  (Maybe somebody can correct me if that's wrong or define "tad".)

The top guys hold the mill's wind load while the lower guys mostly brace the tower against folding up, so they're mostly loaded with just the tower's and their own much smaller wind load and their own tension (which can be non-trivial but probably pales compared to the storm load on the mill).

[SparWeb]

The "theoretical optimum" angle would be lined up with the sum of the force vectors.

In practice, pointing the anchor just a "tad" below the angle of the top guy is close enough.  The load on the top guy wire in a storm is much higher than the sum of the pre-tension and wind loads on the lower guy wires.  If you went to the trouble of measuring all of the loads, or at least calculating them with a finite-element model, you could find out how big the "tad" would be for that tower.  

As a guess, a 4" pipe tower is so flexible that the top guy rules (line up the anchor with it and you're done), while a heavy truss tower has so much cross-section drag and stiffness, that the intermediate guy wires would have more effect (so tilt the anchor down some).

Don't forget that you will put pre-tension on the cables (they should never go slack, even the back cables in a strong wind) so your pre-tension will straighten the anchor some, too, and give you a hint about the angle they really want to take up.  

Information that would only be useful if you were to build that tower again, mind you...

[Bigwheel]

Hi again and thanks for the input.  Well all three of the anchors are in (what a Job by yourself).  Needless to say it took a solid piece of 1 3/4 12' long to get enough leverage to spin them in the last foot.  It's whild to see the how much flex I was able to get out of the round stock with 200 pounds of force behind it.  What I also found interesting is just how much resistance the ground has at that depth when your trying to turn something in undisturbed soil.  I'm confident they should hold quite well, but for future reference I will be jotting down how far there sticking out of the ground to see if I do infact get movement in the future.  The angle of entry was between 50-60 degrees.  The hardware came in yesterday and all that is left is to get my winch up and running.  Is there a rule of thumb on how much cable should be allowed to be exposed past the last u-clamp?  Thanks again for the help.  Chad

[SparWeb]

Good quality saddle clamps come with instructions.

Cheap saddles in a surplus store bin don't.

Buy the good ones, and you won't have to ask the question.

Ask for "CROSBY", at an industrial supply store like Acklands Grainger, etc.