[TowerTalk] Tower base over ROCK (shale) experience??

[Bob Shohet, KQ2M]

Yes, A LOT of experience at my qth!

My 130’ Rohn 45G tower rests on a pier pin on a Rohn 45 flat base on a very wide and long concrete base ranging from only 9” – 18” deep MAXIMUM!  To spread out the stresses, I made the concrete base overly large ~ 6’ long by 4’ wide and I used LOTS of rebar in the concrete in the base which was in a short wire cage also constructed of rebar, and all inside a wooden form that we constructed.  We could not go down any deeper because of all the ledge on the top of my hill.  Blasting would have been the only other option and it was not an option for me.

So, to minimize the tower rotation stresses of the extremely high winds at my qth, I decided to use a flat base and pier pin so that the tower guys could “self-equalize” the torque as necessary.  The first day of tower building we were quickly alerted to just how much stress can quickly build up when, shortly after the first 40’ was put up and the tower guys were being tightened, we heard a ear-splitting incredibly loud metallic BANG!!, which apparently was just the tower “equalizing” the guy torque by rotating about 1inch  !!!  And that was just from the first 40’ of tower with nothing on it!

If we ever had any questions about how valuable a flat base and pier pin installation is, they were immediately and emphatically answered!

Now to answer your question, my 130’ tower holds 3  5L Hygain HG205CA’s (top rotatable with Orion 2800PX and 14’ chrommoly mast) and an 40-2CD fixed SSE.  These antennas and tower has been up through Hurricane Sandy (110+ mph wind gusts), Hurricane Irene 85 mph wind gusts, an F0 and F1 tornado (direct hits) and more Nor-easters and T-storms with 70 – 90+ mph wind gusts than I can count.   Not to mention the stresses from 100+ icing events since 1998 when I put up the tower.  No problems with the tower ever.   I must add though that in 2009, I made the decision to switch from regular guys to star guys at the 80’ and 120’ levels.  It was after the star guys were put up that we had the two hurricanes and the F1 tornado.  I can’t say for sure whether the tower would have survived them with just regular guys.  But I do know that the tower base has never cracked despite all violence.  I would suggest that you use oversize guy anchors and equalizer plates as I did and make your guy anchor holes longer and deeper as well and use a high grade concrete from a concrete company rather than making your own in a mixer.  The higher the quality and smoother the concrete the better.  And, the bigger and stronger the guys, guy anchors, equalizer plates and turnbuckles, the better; especially if you are concerned about the tower base.

73

Bob  KQ2M


From: bear 
Sent: Friday, October 13, 2017 9:58 AM
To: towertalk at contesting.com 
Subject: [TowerTalk] Tower base over ROCK (shale) experience??

Anyone have advice/experience with placing a tower over/on a rock surface?
Short of blasting a hole, that is...

My QTH is very thin soil over solid shale surface, it's a hill mostly.

                        _-_-WBear2GCR
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