Date: Sat, 15 Jan 2011 14:25:06 -0600
From: "Marvin Gorden"<email@example.com>
Subject: [TowerTalk] concrete base for Rohn 140 foot SSV
Looking at the Rohn base specs, I am quite surprised at the requirements,
assuming I read it right. For a base, without piers, the specs call for a
14'-3" X 14'-3" X 4' concrete base ( 30.1 cubic yards).
To me that seems like considerable over kill.
My tower has 8N, 7N, 6N, 5N, 4N, 3WN and 2W sections.
I looked at a 140' SSV installation a few miles away from here and that base
is 7'-9" square. I don't know if that sets on piers or how deep it maybe.
Any qualified comments would be appreciated, before I start digging.
I have been involved with SSV installs and have been surprised by the huge
foundation requirements myself. However, if you run the loading numbers you
will see that Rohn's recommendations are not overkill or spurious. Based on
the Rohn recommendations, what you saw had to be a pad poured on top of a pier
foundation. This is often done at cell sites to give an equipment cabinet
As one rough example, for a 120' SSV 8N>3N Rohn's engineers calculated a total
OTM of 270,300 ftlbs at 90mph. This is a huge number which is easily
understandable by thinking of the self-supporting tower as a big lever. The
overturning force is input into the end of a 120 foot lever and translated into
a uplift and download force 59" (~5') from the center of the OTM at the 8N base
(the center of a 119" diameter circle circumscribed by the legs). The uplifing
force generated by this lever is quoted by Rohn at 35,400 pounds, which has to
be resisted by mass and leverage in the soil. The mat foundation has virtually
zero leverage against the soil in uplift, so the uplift is entirely resolved by
the mass of the concrete mat.
If the mat is assumed to be rigid, the OTM is resolved by lifting the mat at
the upwind edge, so there is a 0.5 derating factor of the mass of the concrete
opposing the uplift. If you were only designing the mat to resist the wind
coming from one direction, the minimum mat would be 16 feet (Rohn spec) in the
axis of the wind direction, 188.8" wide (The width of the tower leg circle),
and weigh 70,800 pounds. Most standard (non-lightweight) concrete weighs about
4000lbs/yd., so this would be 17.7 yards of concrete. Since you don't know
from which direction the wind will attempt to overturn the tower, there must be
this equivalent mass at all radials. Making the mat square gives roughly twice
this amount (minus the 1.8 cubic yards, or 7200 pounds) under the tower
itself), so the 30.1 yards Rohn specifies looks in the ballpark to me.
The cost of this concrete is a good reason to go use a pier (9 cubic yards) or
belled pier (7.8 cubic yards) foundation. Of course, this is for a 90mph wind
zone. If you are somewhere with lower winds, you can use the Rohn 70mph spec
which calls for considerably less reaction mass.
I used many over-simplifications in this example (such as neglecting the
influence of the download on the down-wind side of the leg circle). The actual
formulas used by the structural engineers take far more factors into account,
with different overload margins for each element. However as a ballparking
general illustration, I think this one gets the point across. Allowing any
sort of margin of safety for overloading and transient wind gusts, the Rohn
spec is certainly not overkill.
I would like to comment on the statements by hams who ignore manufacturer's
recommendations, and when the towers don't immediately fall out of the sky,
conclude the recommendations were overkill. (Marvin, I'm not pointing a finger
at you, you seem to be looking for the correct solution to your tower
foundation) It is like hearing a cigarette smoker justify smoking by stating
that their grandfather smoked for 70 years and never got cancer. Tower
foundations are specified to take transient conditions (wind gusts, soil
characteristics) particular to locations into account and still stay safe and
in service. Just like the decision to smoke, if a person decides to ignore a
tower manufacturer's recommendations, they accept the risk inherent in their
decision. And seriously, if someone clearly evaluates the situation and finds
the risk acceptable,/and they alone will bear the cost of a failure/ they
should go for it. I have absolutely zero problem with that, and hate it mysel
f whey others dictate what I have to do. I do however have a problem with
dealing with the fallout associated with a failure due to incorrectly assessing
the risk. I have had my station knocked off of the air by another engineer's
failure to limit the scope of risk to their own equipment. I have also had to
take care of people who have gotten emphysema and can no longer take care of
themselves. Therefore these individuals incorrectly assessed the risks./
Just my $0.4 worth/
*Howard Hoyt - WA4PSC*
*CE - WXYC-FM 89.3*
*UNC Chapel Hill*
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