Ted:
See my comments below.
-----Original Message-----
From: Ted Huf [mailto:w4ze@arrl.net]
Sent: Tuesday, March 20, 2001 05:45
To: TOWERTALK@contesting.com
Subject: [TowerTalk] Tower Base Construction
There has been a lot of discussion lately on the strength of concrete
When you order your concrete from the redi-mix plant just tell them you want
3000 psi or 4000 psi concrete or whatever strength you want. When it is
delivered ask for the batch slip for the load which should have the mix type
and strength information on it. Typically, tower manufacturers designs are
for 3000 psi. Most redi-mix plants as a standard mix almost 4000 psi with
lower strengths being the exception. Be careful though because if you have
to get a permit, the building department can ask for special inspection of
the concrete (per UBC) if the design strength for the foundation exceeds
2,500 psi, it all depends on the building department.
. It
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 is
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. Then
after a few days, remove the form, and back fill the hole with compacted
earth. Another way is to have the hole drilled which might make my question
mote.
This may be true for your particular area if the soil is very loose or fine
grained like sand. Actually last summer I was involved in a client
installing exactly the tower you mention and he dug the hole 3'-6" by 3'6"
by 7'-6" without it collapsing. It all depends on the specific soil
conditions at each site. You cannot make a blanket statement concerning
soils.
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 requires
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 required
for my Tri-Ex.
Could have been terrific soil and most likely had a full soils program done
for the site. The concept of undisturbed soil is a little misunderstood. It
is used to describe a soil condition that indicates that the in-situ soil at
contact with the foundation is about the same as the surrounding soil that
has not been loosened. It is entirely possible to over-excavate a
foundation, backfill and properly compact to line and grade with a clean
granular soil. As a matter of fact it is done a lot. The trick is to compact
the fill to optimum density which is normally greater than the surrounding
soil density. The reason foundations are formed in the first place is to use
the minimum amount of material.
Another point, soil density is not the same everywhere. The coastal Florida
locations where I have experience had very sandy soil with a high water
table.
This is the real issue, every site is different and even within the site the
conditions are extremely variable.
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 called drilled pier, cassion or auger cast foundation construction.
This is not something the average
Ham would undertake. I believe that Tri-Ex and the other tower manufactures
of Ham towers design a base that will work anywhere.
This is correct
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 or
3.7 cubic yards it would weight 14,400 pounds. What are the opinions about
this modified base being equal to the specified one?
The first thing is that you can't do a cubic yard for cubic yard comparison.
The design concept of a pier/post foundation are entirely different from a
spread footing foundation. The post type uses the embedment vertically to
withstand the overturning moment created by the free standing or cantilever
tower. A resisting force is created by the segment in the ground that is
proportional to the depth of embedment and face width times the horizonal
soil bearing capacity.
The Spread footing type of foundation uses the geometric properties of the
shape, usually a square or rectangle, to distribute the loads , vertical and
overturning moment to the soil., in this case the maximum force delivered
needs to equal or less than the vertical bearing capacity of the soil.
As an example I reviewed an used 67' tri-ex tower for an 85 mph wind load
for a client and developed 2 different foundations for this tower. One a
post type, and one a spread footing type. By the way the tower itself did
not calculate for the 85 mph wind since it was originally designed for 50
mph.
The overturning moment was 28,000 ft-lbs, the horizontal shear was 762 lbs
and the vertical load was 870 lbs. With this loading a post-type of
foundation of 3' by 3' by 7 ' deep was adequate, given the worst case soil
in the UBC. (table 18-I-A, 97 ed., class 5 soil)
The spread footing was 7' by 7' by 2' thick. This is 3.63 cy and the post
type is 2.33 cy. The spread footing is more concrete but is easier to dig
and place. There is a trade off but concrete is relatively cheap. You should
have the spread footing designed by someone who knows how to do it or go
with the manufacturer's recommendation.
Sorry to be so long winded.
73
Hank Lonberg P.E. / KR7X
73
Ted W4ZE
w4ze@arrl.net
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