Hi Bud:
You said:
"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. "
Not quite true. I will depend on how the overturning moment of the wind on
the antenna and mast are resolved. If the first radial bearing is located
at the place where the guys are attached, yes the direct side load is
resolved in the guys at that point. If not then the side loads create a
moment. If the bearing at that location is not able the carry moment then
the moment must be resolved somewhere else, usually at the rotor, resulting
in a bending moment in the tower relating to the wind load on the antenna
system. If the radial bearing can carry moment then a moment is induced
into the tower at that location and carried throughout the tower. If the
moment is resolved at the first radial bearing then there will be an uneven
distribution of the moment which is shared by the tower as a moment and the
guys if the guys are not horizontal. (If they are the entire moment is
transferred to the tower). That is where it gets very complex. ;-)
Any bending moment in the tower will result in some deflection. It is that
deflection that creates the susceptibility to buckling since the downward
forces of the weight and the guys now have a moment arm to work against to
create additional bending stersses The actual deflection characteristics of
a uniform tower in a strong wind may involve bending toward the wind in one
section and bending away for the wind in another depending on the induced
moments in the tower are resolved at the guy locations. These bending
moments contribute to the stresses in the tower and when combined with the
other sources of stress (including the downward load of the guys) may
create principle stresses that are not perfectly collinear with the tower
legs and yet exceed the yield strength of the tower material, usually in
compression.
I would expect these characteristics to be less significant in a tapered
tower of course, since the section moduli get progressively greater nearer
the ground. However, in the end the horizontal guy scenario is terrbily
oversimplified and untenable.
And all of this is in the static mode only and does not take into
consideration the dynamic stresses.
73
Bill
W7VP
----- Original Message -----
From: "W2RU - Bud Hippisley" <W2RU@frontiernet.net>
To: <towertalk@contesting.com>
Sent: Sunday, November 09, 2008 7:02 PM
Subject: Re: [TowerTalk] Guying a self-supporter
>
> 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
>>> wind
>>> leg loading increase to a point, depending on guy tension.
>>
>> Roger,
>>
>> 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
>> opposite...
>>
>> In low winds, yes the static compression from the guys will add
>> moderate
>> 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),
>> but
>> I'm betting the results are similar in most reasonable real life
>> scenarios.
>>
>
>
> 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
> two things:
>
> 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
> bracket.
>
> 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......
>
> Bud, W2RU
> _______________________________________________
>
>
>
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