On 11/30/2013 5:45 PM, Matt wrote:
The base and rebar are sure different that the factory drawing I have for
the old rebar setup. The tower base is not separate from the rebar.
What size are the holes you dug?
Roger (K8RI)
The bases for the two Triex towers are my own design as I wanted provisions
for adjustable leveling and potential for future tower exchange. They were
designed to be stronger than the towers at failure.
I thought that might be the case.
I had previously
modeled all the towers using finite element analysis to analyzed tower
strength under some different loadinf conditions and also to estimate the
maximum base loads at the point of tower's failure. I used those max base
load reactions + additional safety factor for the design of the bases.
As the LM470 is as close to the property line as I can get for the
height, designing the base to accommodate a larger tower at a later date
wan't a consideration. Were I to replace the 100' 45 G, it certainly
would be and I'd very much like to do so, but the likely hood of that
happening is pretty slim. Getting the Rotator, mast and two antennas
back up there has proved problem enough.<:-))
The excavations dimensions were approximately:
66W x 72L x 94D
66W x 72L x 72D
54W x 60L x 80D
In some areas, I ended up over-excavating in the process of dealing with the
harder than expected clay. I just formed these up flat to the ground
surface beyond the base extensions and incorporated them into the concrete
pour so as to maintain the foundation bearing on undisturbed soil.
Due to climate and precipitation, having the top of the pour well above
grade is important here
The entire base will be suspended in the hole with the bottom about 6"
above the bottom of the hole.
I plan on lifting the base with the raising fixture and wooden base
support all bolted together as one unit
so everything will set on 2 X 6s
The 6" high form will be inserted inside the support frame.After a day
or two for the concrete to set, I will use a mix to form a substantial
crown.
"I plan" on leaving the forms in place for a week, then knocking them
out. That will leave the raising fixture's 140# as the only load on the
concrete after about a good 70% cure.
I would have preferred to have done this in much warmer weather, but I'd
not be able to get near the site with a concrete truck unless the ground
is frozen. Last winter, it never did freeze deep enough to support the
truck. Typical has been around 18" with 24" required for foundations.
However it's been a long time since its frozen even 16 inches deep. I
doubt we had more than 10 inches under drive ways and roads last
winter. I really don't want to spend the money for equipment on getting
the concrete pumped, but another winter like last winter and I'll have
no choice.
Had I acquired a test boring prior to sizing the foundations I would have
probably reduced the target dimensions quite a bit to take advantage of the
high bearing strength of my soil below 18".
Soil strength varies a lot in just a few feet. Where the LM470 is
going, under the top soil is alternating layers of sand and grey clay.
http://www.rogerhalstead.com/ham_files/tower10.htm shows the "chips" of
clay drying in the sun. This was the base for the 100' 45G and the
reason I hired a back hoe to dig the holes for the guy anchors. This was
mid summer in hot, dry weather. In the spring, the hole would have
likely filled with water.
The LM470 base should be in similar soil.
Were I 20 years younger and freshly retired, I'd seriously consider one
of those heavy duty, 100 foot crank ups instead of the 100' guyed tower.
73
Roger (K8RI)
But at that time I had not dug
more than about a foot so I had not encountered the Caliche clay. In any
event, I won't be worrying about the foundation strength.... I'll try to
put up some more info on the tower bases when I get some more time.
Matt
KM5VI
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