In a message dated 3/20/01 5:50:24 AM Pacific Standard Time, firstname.lastname@example.org
> was mentioned in at least one messages the problem with digging the size
> hole specified for the base. For example, the Tri-Ex LM354HD tower, that
> being held for me until I move, calls out for a 3'-6" square by 7'-6" deep
> base. There is no way to dig this hole and just pour concrete in. You
> would have to dig a much large hole and build a form for the concrete.
> after a few days, remove the form, and back fill the hole with compacted
Why wouldn't you just fill the hole with concrete and skip the form? Most
specs call for installing the base in "undisturbed soil". Backfilling
decreases the strength of the adjacent soil for a period of time until it
compacts again. Using a below grade form is more work, more expensive and not
recommended - just pay for more concrete.
> Another way is to have the hole drilled which might make my question
Yes, drilling would make it moot. I've never personally used an auger so
I'm not sure what the relative costs are or how available they are. I think
it would be easier to get your engineer to re-design the hole for more
practical installation. This is something that I'd like to get US Tower to
> I have been involved in building a number of commercial towers, the largest
> was a 450' guyed tower in Florida. While I am not a structural engineer, I
> know from this experience that a base in undisturbed earth seems to
> less concrete than one with backfilled earth. By example, a 110" three
> legged self supporting microwave tower had a 3' diameter base about 8' deep
> poured in a drilled hole for each leg. This is about the same size
> for my Tri-Ex.
While your statement about disturbed vs. undisturbed soil strength is
correct, I don't think you can make any conclusions about the relative base
designs of two completely different towers. If I remember my math correctly,
the microwave tower leg takes 2 cubic yards of concrete (pi x r squared x d)
whereas your crank-up takes 3.4 cubic yards.
> Another point, soil density is not the same everywhere. The coastal
> locations where I have experience had very sandy soil with a high water
> table. The base design was done only after soil samples and density tests
> were completed by a soil testing company. It should be noted that the
> drilled hole used a special concrete pouring and drilling method using a
> drillers mud or slurry and the concrete was pumped from the bottom up. The
> drillers mud held the wet sand in place until the concrete was poured and
> was forced out by the heavier concrete. This is not something the average
> Ham would undertake. I believe that Tri-Ex and the other tower
> of Ham towers design a base that will work anywhere.
Correcto mundo. The microwave tower was designed with a soil test having
been done so they really know the strength of the soil whereas typical ham
plans are 'one-size-fits-all'. It's all a question of overturning moment and
other forces. Since neither of us are engineers, getting an engineer in the
loop to run the calcs would be highly recommended - especially if there's
some question about the soil strength.
> So we get to my question. The 3'-6" by 7'-6" base would require about 3.5
> cubic yards of concrete which would weight 13,500 pounds or almost 7 tons.
> If the base were modified for ease of digging to say 5' square by 4' deep
> 3.7 cubic yards it would weight 14,400 pounds. What are the opinions about
> this modified base being equal to the specified one?
Modified is fine as long as it's done by an engineer or you over-engineer
it yourself. Typically if the hole isn't as deep as the spec, the footprint
needs to be larger but I'm not sure by what kind of a factor. I might make it
25% bigger or more to compensate. Don't forget that if you expand the
footprint, it has implications on the rebar and cage sizes/specs.
BTW, use of a backhoe almost always results in a bigger hole than you
want. It's not a precise excavation in most soils so if you wanted a 5'
square hole, you'll wind up with one 6' or more. Factor in sloughing of the
walls and your 7 cubic yard hole turns into a 9-11 yard version. You just
pour more concrete. You would generally only want to use a form in the hole
if you have severe sloughing or as a safety feature for working in the hole.
Cheers, Steve K7LXC
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