[TowerTalk] FW: FW: FW: FW: Crank up questions.

Roger (K8RI) on TT K8RI-on-TowerTalk at tm.net
Sat Nov 30 20:21:29 EST 2013

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 
"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.


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
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