Some thoughts at the end about what I'd do different. I know
this is all negated by the US Towers approach, but this might
be interesting for people re-using old towers.
Just poured 8 3/4 yds this weekend for a refurbished HG-70HD
I'm putting up. Tower bases are still sold for this crankup...
didn't have to make one.
This is the old style triangle with #8 rebar and steel bar "ears"
I had previously put up a Tri-Ex WT-51 (much smaller base) and
thought I had this base pouring/alignment/stabilization all figured out.
In the end, one of the three ears is a little more out of level than I should
have been able to get, but good enough that I'll be able get the tower
plumb given slop in the base mounting holes.
So it's fine. But it bugs me that I didn't get it perfect.
In contemplating what I should have done afterwards, I realized
that most of the info I've read really isn't optimal for the job.
Here are my thoughts about what I'd do next time.
- Although everyone says "check for level" while pouring, I didn't
check a lot..since I had everything screwed down, figured it wasn't going to
matter. Couldn't imagine what you're adjusting if everything is
screwed down. Thought I could hammer down the offending points
at the end, to level.
-But with a stiff concrete mix, the triangular base rebar plus extensions is so
long and massive, (think of the wet concrete yield pressure against
the sq in cross section of the rebar, down in the hole, creating a lever moment)
that hammering it really doesn't make it move. (was using a sledge).
I had removed the supporting bolts at this time.
-At the end of the pour, I should have left the bolts screwed to the 2x4 cross
supports longer, and used shims/levers under them at the form to adjust the
height/level (more steady force over time). but I might still have had the
problem of not being able to move the base in the stiff mud at the end.
But I wouldn't want to unscrew the wooden support ends from the forms
while I was pouring...so the idea of adjusting while pouring seems kinda
bogus. What you really want is to be able to set it up to guarantee
no movement while pouring.
-Made me realize there were a couple of key issues:
The triangular rebar extending into the hole,
cause a huge lever moment at the bolts in the base holes,
attached to the 2x4 cross supports.
When the concrete at the start of the pour pushes
sideways in the hole, there can be a huge amount of force at
the bolts between the base and the wooden supports at the top.
Even if you're trying to pour evenly, there's varying pressure while
you're pouring...and this can cause the level to change because:
With a 5' hole, there's enough wood, and enough force,
that you get deflection in the 2x4 cross supports
(this causes variance in the level)
Also turns out the tower base had 7/8" holes for 3/4" bolts. I used 3/4"
bolts for the wooden support, so there's 1/8" of play that I didn't
want.
I don't remember tightening the bolts "hard" so I suspect there
was some play there..because the concrete weight plus lever moment
can create so much force. And even if I did, you can get some variance
from the bolt threads being forced into the wood....
-I might have unbolted the wood supports too soon after the pour, so
I may have had some sinkage (but then why couldn't I make it move
with my sledge hammer). The only reason to remove, was to finish the
concrete. A better solution would have the supports above the final
concrete surface, so no need to remove till dry.
-Rather than constantly "adjusting", a better solution would be to
stiffen up the whole support system. i.e. use angle iron instead of
wood for the base cross support over the hole. and space these up off
the form, so you can get a float underneath...so you can leave
them on while finishing.
-The Tri-ex base was supported by mounting fixture holes, so the
actual tower leg holes are much higher off the concrete. On HG-70HD base,
the only holes are the tower holes. So when I crowned the concrete,
(adding concrete at the middle), I came pretty close to having the holes
too close to the final concrete surface..(the tower needs a minimum, plus
you want a gap to make sure water can get out of the tower tubes).
I can always grind the concrete if necessary. But should have thought that
through a little more.
I should have started with the base supported higher than the 2 1/2"
I used, off the form base.
So all in all, next time I'm thinking I might do this:
1) Use angle iron cross supports to support the tower base, not 2x4's
2) Support the tower base so the holes will be 2 1/2" higher than
the crowned middle of the base. Probably means 3 1/2" higher than
the form top.
3) Use some wood at the form sides to raise the angle iron to achieve 2),
and still have room to get a float under the angle iron..so I won't
have to remove the supports to finish the concrete.
4) Make sure the bolts attaching the base to the angle iron, are
the same diameter. And tighten them down hard (no movement)
5)If I still need to adjust, check while pouring (like everyone says)
but adjust by loosening the angle iron attachment to the form base
(wood screws) and driving shims underneath.
(the sexiest solution would have adjusting bolts so I could turn
bolts to adjust the height at the form!)
other notes about the pour:
My driveway is 16% at one point. There are lots of curves. So
the plan was to back the truck up from the cul-de-sac. (they can't
steer if the front wheels get unweighted)..... Well, didn't
even get that far before the power steering went. Turns out
the steering fluid had gone dry (driver didn't check).
Had to wait an hour to get new fluid. Luckily that was the problem, and
steering worked after that. Started worrying about the concrete.
So we were pumping fast when we finally got going. I remember
standing there as the concrete's going in "Hmm...now's when
you start thinking about what you should have done".
8 3/4 yards pumped in 20 minutes! Nice stiff mix. No added water.
-kevin
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