On Nov 9, 2008, at 8:49 PM, Steve Maki wrote:
> Roger (K8RI) wrote:
>> Static load would increase on all legs. I'd expect to see the down
>> leg loading increase to a point, depending on guy tension.
> I want to understand exactly what you're saying here.
> In a strong wind, would you expect there to be more total
> compression in
> the downwind leg with guys attached? I would expect just the
> In low winds, yes the static compression from the guys will add
> extra loading, but I would expect the curves to cross at some medium
> wind speed. This is for a rigid tapered tower (which doesn't exist),
> I'm betting the results are similar in most reasonable real life
I agree with Steve -- at least for reasonable guy wire angles.
Consider the case of horizontal guy wires. With horizontal guy wires,
high winds add no downward force on the tower legs; the only downward
force at *any* time is dead weight of the tower and the antenna (which
you already have in your model) and the added guy wire material
itself. So let's make the guys out of phillystran, if we must, so we
can get past all the people who think their self-supporting tower is
going to drop dead when you dangle three pieces of 3/16 EHS from it.
Now start dropping the far anchor points of the guy wires a little bit
at a time. In fact, let's drop the anchor points to where the guys
form a 45 degree angle with the (flat) ground or with the vertical
centerline of the tower. Now let the wind blow. Assume the pressure
of the wind times the antenna + mast surface area = 400 pounds
horizontal force. (For instance, 10 square feet of antenna and 40 psf
wind.) *If* the guy wire has been properly pretensioned such that
its tension increases in response to the slightest wind, we can note
1. At 45 degrees, the added downward force of the guy wire on the
tower will equal the horizontal force from the wind; each component is
400 pounds. The added tension in the guy wire is 560 pounds
(roughly), but that number is germane only to the selection of the guy
wire, not to the tower loadings.
2. *If* the loads calculated in #1 above are true, it is the guy
wire, not the "self-supporting" tower structure, that is taking the
brunt of the horizontal wind load. That is to say, the only reason
the tension in the guy wire increases and the downward loading on the
tower leg increases is because the guy wire is resisting the force of
the wind on the antenna. Thus, the total static load on the tower in
this high wind situation is an added 400 pounds straight down -- on
the *upwind* leg if you loop each guy wire around the nearest leg, or
spread across all three legs if you use an appropriate load-spreading
But suppose the wind had come up before we managed to attach the guy
wires to the tower. In other words, a true self-supporting tower.
What would be the situation then? One of the main limits to the load
this tower can handle is buckling of the *downwind* leg from the
Moment created by the wind blowing horizontally on the antenna at the
top. That Moment is (dimensionally) force times distance, where the
distance is the height of the tower from the ground to the antenna
height (since in this case there are no guys anywhere up the tower to
break up the Moment arm). Let's say the total height to the antenna
is 100 feet. So the Moment is 100 X 400 or 40,000 pound-feet. At the
base of the tower, this Moment becomes transformed into a buckling
force on the downwind leg; the size of the force is roughly the total
Moment divided by the distance from the centerline of the tower to the
downwind leg. On an SSV tower I used to own, that distance was,
perhaps, 4 feet. So the buckling force in the downwind leg might have
been 40,000 / 4, or 10,000 pounds. If you had a 48-foot Spaulding BX
or HDX or ????, you'd have 400 x 48 = 19,200, divided by, say, 1.5
feet (I'm guessing as to the size of a Spaulding HDX base), or nearly
13,000 pounds compressive force.
So....given the choice of having a 400 pounds compressive force in my
upwind tower leg, or 10,000 pounds or more of compressive force in my
downwind leg, which do you think I'll choose?
Clearly, there are pathological values of guy wire dead weight
(consider 1-mile long guy wires because your tower is on the edge of a
canyon) or angle (guy wires anchored five feet out from your tower
base) that are not going to serve you well. But, for most common
guying scenarios, I submit that judiciously adding guys to a "self-
supporting" tower is helpful.
And yes, I've used "idealized" towers and wind and......
TowerTalk mailing list