One factor not mentioned is that it is unlikely a significant moment can
be transmitted by the two plates in contact. One side of the plate will
lift as soon as the base moment exceeds the downforce in a particular
leg. That might not be obvious on a 100' tower with a pier pin base
plate that is about 1.3 ft edge to point. A 5' sway at the top or 2.6
degrees would cause one edge or leg to rise 0.8" if the tower stays
perfectly in column (which it doesn't with all guy levels having the
same baseline).
Sometimes a simple calculation can show some insight, although for
anything serious using finite element analysis as N7NV did is the best
way to go. For a 100' tower with 70' guy baselines, the simple (aren't
simple for a lattice tower but) statics are in a very rough
approximation (assuming no bending resistance by the tower) -
every lb of side thrust (wind load) on the top guy generates 0.1.43lb of
downforce and 1.75lb of additional guy load
for the 67' guy height a lb of side force generates .96lb of downforce
and 1.39lb of additional guy load
and the 33' guy height a lb of side force generates .46lb of downforce
and 1.10lb of guy load
so 15 sq ft of antenna + rotator + mast at the top at 70 mph is
20lb/ft^2 * 15ft^2 = 300lb or 428lb of downforce spread among three legs.
How to estimate the rest is a little bit problematical since I don't
know the wind surface for Rohn towers. So let's make an assumption that
each 16.67' of tower is 3 sq- ft of surface. Breaking the tower into
"wet noodle like" sections, and assuming the wind load averages, the
top guy creates an additional 1* 3 * 20 * 1.43 or 86lb downforce, the
middle 2 * 3 * 20 * .98 or 117lb downforce and 55lb for the lower guy,
(ignoring the wind load below 16' elevation), or a total downforce plus
the weight of the tower, beam, mast, rotator, coax and dead weight load
from the guys. Let's assume 1000lbs. So the total column load at the
base is 428 + 86 + 117 + 55 + 1000 = 1686lb spread among three legs.
Meanwhile, at 70mph wind speed, the top guy is loaded to 400lb preload
plus 1.75 * 300, or 925lb, about 1/4 the breaking strength of 3/16 EHS.
Since SWL for such applications is 1/4 or 1/3 of breaking strength, why
3/16 EHS is the proper size.
OTOH, an unguyed tower of 100' height will experience, just from the
antenna etc 15 sq ft at 70mph, 300lb * 100 ft or 300,000 ft-lbs of
bending moment. That relative to the downforce of 582lb per leg for the
guyed tower is a VERY big number.
So above is a rough approximation of why guyed towers fail in bending
rather than column buckling. The tower bends more if top guys are more
elastic Phyllistran than lower EHS guys. The tower bends more if the
base is a tower section in concrete and trying to resist the bending
moment. And why pier pins are best and why keeping a tower in column,
even if it is leaning a lot, helps it survive.
Grant KZ1W
On 5/11/2011 12:50 PM, Steve Maki wrote:
> Commercial towers have no fancy thrust bearing. It's usually just a very
> heavy but small diameter plate that is just large enough to accept the
> angled in tower legs, which are welded to the plate in the most amazing
> mass of welding you ever saw. And this small plate just sits on a larger
> plate on the concrete base centered by the pier pin.
>
> -Steve K8LX
>
> On 5/11/2011 10:45 AM, Dick Green WC1M wrote:
>
>> Regarding the pier pin base, I agree that in theory it's the way to go, and
>> that's what I have on my 110' Rohn 55. However, I've often wondered if the
>> Rohn flat-plate pier pin base used by most hams who install pier pin bases
>> really does what a pier pin base is supposed to do, namely provide some
>> "give" in response to torquing forces on the tower. Seems to me that the
>> friction between the base plate and concrete is substantial, especially
>> given the weight of the tower/antennas/rotors/cables and the downward force
>> of the guys. Does the plate really move? How much wind would it take to move
>> it? I've never detected any evidence that mine has moved. Commercial pier
>> pin bases are tapered and, if I'm not mistaken, the pin fits into a bearing
>> of some sort (probably a thrust bearing) attached to the concrete.
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