Ken,
Depending on where you live (how dry the soil is, what type of soil),
and how much you expect the new foundation to cost you may save money
hiring an engineer to design your new foundation. If you lived where
I do your existing foundation would be more than adequate for a 60'
tower. My freestanding is on a "smaller" foundation than you
currently have. Rohn's recommendations basically create a ballast
foundation where the weight of the concrete prevents movement of the
tower. As a soil engineer I see many problems with this ... we can
get into that another time.
I personally wouldn't remove it unless there are some structural
issues with the existing foundation, ie. it's cracked, there is no
steel reinforcing in it, it has settled quite a bit, or is leaning bad
on one side. For the last two recommendations I not only would remove
it, I'd get an engineer to review the design for the new installation
to ensure it won't have similar problems.
If the existing foundation is sound, I would recommend doweling into
the existing foundation and tying the steel cage for the additional
concrete to the dowels. Remember that for a ballasted foundation the
big issue is counterweight. As long as you use the save volume of
concrete in it doesn't matter if you go vertically or laterally with
the additional concrete. After about 36" thick you are not adding any
structural strength. (I have designed sky scrapers with 24" thick
footings). I would also argue that as you go wider you are gaining an
advantage in overturning and bearing capacity and thus will end up
using much less concrete.
I am not familiar with all of the rohn towers, if this is a guyed
tower then all you have to do is support the weight of the tower, as
long as you are below frost depth you can assume that every square
foot of foundation placed on dry soil will support 1,500 lbs (this is
published in the international building code that every U.S.
municipality follows). You just have to make sure you can keep the
soil dry.
As for the ground system... I'm not an electrical engineer, I did weld
my tower to the foundation rebar, that was a structural decision...
but i do maintain a sufficient ground system with copper ground rods
due to my high risk of lightening strike, although it hasn't happened
yet and I keep my fingers crossed it wont. I don't rely on current
passing through the concrete and rebar, hopefully any strike will go
through the thick copper wires into the ground rods.
Brian
KF7OVD
On Sun, Aug 17, 2014 at 4:06 PM, Matt <maflukey@gmail.com> wrote:
>>> Any reason not to consider this as a possible plan for the new tower
> foundation?
>
> This can be done with a relatively small piece of equipment if you can get
> close enough to the foundation. I have seen underground concrete thrust
> blocks used on large piping systems successfully removed in this manner.
> The thrust blocks were substantially larger than your tower foundation. It
> did require some pre-excavation which could be an issue if you need to place
> concrete back against un-disturbed soil.
>
>>> Do you tie the tower and the rebar into a ground connection at the
> bottom of the foundation?
>
> At risk of restarting an ongoing debate... my understand is do not tie the
> your rebar into the ground system unless you are intending to build a UFER
> grounded foundation. At risk of restarting another debate... do maintain
> at least 3" concrete cover over your rebar unless you are using non-metallic
> rebar.
>
> Good luck on your project.
>
> Matt
> KM5VI
>
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