[TowerTalk] TowerTalk Digest, Vol 86, Issue 53

[jimlux]

Michael Goins wrote:
> Okay, just for clarity, I've had a ticket for over 30 years and this
> isn't the first tower I've ever put up or helped put up or been up.
> And it is also important to me to do it right, but I am not looking to
> do thousands in the base and do this in a realistic, practical manner.
> The circumstances are different here because of the rock, and that's
> why I asked this knowledgeable group.
> 

And the knowledgeable group has given you a qualified "it depends"


> We were in the Houston area in the past, which is mostly gumbo soil.
> Here it is rock, on the side of a hill up 1800 feet, and the hole is
> in solid rock - not shale, not limestone, a boulder. A solid rock
> shelf. The specified hole, by the manufacturer, is 42" X 42" X 5.5
> feet. We are now at about 3.5 feet, making the hole a couple of feet
> shallow at the moment. It is SOLID rock, nothing more, nothing less -
> no cracks, no splits, solid.
> 

OK.. but how are you attaching that big concrete chunk (smaller than the 
manufacturer recommends, though) to the solid rock.  You don't want the 
concrete serving as an effective "claw" on a big claw hammer pulling the 
nails out of the rock. (with your tower being the handle).

You're into an area where the manufacturer hasn't given you any 
information, so it's up to YOU to do the engineering (or to hire someone 
else to do it).

> It is going to be where we will stay, likely, forever. I am a quad and
> homebrew antenna guy, so it will never have multi-stacks or whatever.
> A quad now, a monobander (if that) in the future. A tied-in pad at the
> top which is say, 5X5 feet would have to be pushed into the soil from
> the opposite side to move, and that will not happen as the area is
> rock and less than an inch under the soil. That is why I thought a top
> pad might help, even though I know it needs to be deeper or at least
> epoxied into the stone shelf. I have to have a squared up pad at the
> top anyway, so 4-5 feet square would accomplish both tasks.
> 
Sure, but how do you know how thick to make that pad to make sure it 
doesn't just crack off when the tower starts to lean on it? The tower 
mfr has kindly done the analysis for their recommended base.  You KNOW 
that a 42x42" column 5.5 feet long won't break (because the MFR has told 
you).  But, on the other hand, you probably don't want to cast a big 
cube sitting on top of your rock ledge with full rebar cage.

So you're wondering.. .if I make it 1 foot thick, is that enough? maybe 
2 feet?  Maybe 6" is enough?

You'll get some answers on the list from others about what they've done 
in similar (but not the same) situations, and you can decide whether 
they are "Close enough" or whether they "got lucky".  Either way you 
won't KNOW (with sufficient certainty) or else you wouldn't be asking 
(and asking *is* the right thing to do, if you're not sure).


That's what you pay an engineer for.  They KNOW how to calculate the 
strength of a rebar reinforced concrete pad, and they know how to figure 
out what loads your antennas and tower will put on it.  They can work 
out whether your using small antennas helps (by the way, a significant 
fraction of the load is from the wind on the tower.. you might need 
almost as a big  a base with NO antennas as with the rated size.. after 
all, the tower has some 50 square feet of area for the wind to push on)




> The tower base doesn't have holes - it is a HG52SS and I have the
> factory heavy-duty rebar base with the ears on it that double bolt to
> the tower legs. The base sits in the hole, is cross-braced, and
> concretes in with only the ear part sticking out to attach the tower
> to.
> 
> I may cut the lowest brace off the factory base and drill 3 holes for
> the three base legs to drop into and then epoxy them in with the
> special rock epoxy. The factory base will then lack the lowest brace,
> but I'd add one up higher that would be in the concrete pour and the
> three 1" rebar factory legs would be epoxied in about 18" into the
> boulder.

Now you're really getting far from the original engineered design.  Your 
scheme might work, it might not.  You;ve already said that you don't 
have any engineering background, so you're in the "craftsman" mode of 
"just weld on another gusset, it looks a bit weak", but without the 
years of practical experience that a craftsman would have .  The 
craftsman (like the guy down at the local welding shop) is probably 
pretty good at estimating this stuff: they've had stuff work and they've 
had stuff fail and they've learned from experience.


You too can learn from experience. Most of us have.. but I gotta say, 
it's a potentially expensive education (fun, though...)


> 
> Still looking for ideas so this is a right as I can make it, and, for
> the record, I won't lower the tower all the time, but will when I know
> there is weather coming or when away for any period of time. The rest
> of the time it will have a 2-el quad on it.

The "right way" is to do one of two things:
1) do it the way the manufacturer says to do it (the LXC prime directive)
2) find someone qualified to tell you how to do it a different way. 
Nobody on this list is going to do that for free, on the basis of an 
email description.
3) Educate yourself so that you are qualified (enough).

Let's be real.. this isn't a mindbendingly difficult engineering 
problem, but it is one that requires some background knowledge and/or 
experience.  There are lots of folks on this list who aren't licensed 
engineers who would be more than comfortable doing it for themselves, 
but they probably wouldn't do it for someone else (on the principle that 
you're willing to accept the risk that you make a mistake for yourself, 
but not generally for someone else).

I *am* a licensed Engineer, but not in this branch (I'm Electrical, not 
Civil).  But heck, I've worked with civils and structurals, I've gone 
through their calculations on structures I've designed (and learned a 
lot in the process, particularly seismic loads). I've also built stage 
rigging and special effects setups with a combination of engineering and 
craft. Some stayed up, some failed (thankfully, not with bad 
consequences), and I learned from that too. I also was smart enough to 
do things like proof testing things (e.g. loading up a supporting truss 
with 3 times the weight it's going to see in normal use, and seeing if 
it stays up).


  If it were my antenna, out in a pasture on 40 acres that I owned where 
all it would do is fall in the weeds, I'd get out the spec sheets from 
the anchor companies, run the calculations, add some hefty margin for my 
potential screwups, and put the darn thing up.  But I don't live on 40 
acres, I live on a 50x100 foot lot in a subdivision, and I wouldn't do 
it myself here. I'd hire someone who knew the stuff backwards and 
forwards. And not just because the law says that I need someone with the 
right license and stamp to do it. It's because I *know* I don't know 
enough in that situation.  A few hundred bucks is cheap at the price.