Hi Roger
Let me summarize the loads on the tower. Remember that in the equilibrium
condition the sum of the forces and the sum of the moments have to equal
zero.
1. Compressive load from the weight of the tower itself plus the structures
attached to the tower. These create compressive stresses in the legs of the
tower and are applied at the point where the thrust bearing transfers the
load to the tower.
2. Compressive load induced by the guys. These add to the compressive
stresses in the tower legs. They are applied at each location where the
guys are attached to the tower.
3. The twisting moment due to uneven wind loading of the antenna system or
due to rotational inertia in starting and stopping the rotation applied at
the base attachment point of the rotator to the tower. These loads are
partially transferred to the guys through torsion brackets but the actual
moment is also transferred partly to the tower since there must be some
tensional deflection of the tower for the guys to pick up some of the load.
These create shear stresses in the legs of the tower.
4. The bending moment from wind on the antennas and mast. These create
bending stresses in the tower legs that depend on the section modulus of the
tower and the worst case is usually the compressive part of the bending
stresses since they add to the tower compressive stresses. These may be
reduced to some extent by the proper location of the upper guys though I
doubt the reduction is very much in most cases. This moment is applied to
the tower where the radial bearing point is located.
5. The bending moment of the wind acting on the tower itself. Again this
creates a tensile and a compressive stress with the latter probably being
more significant.
6. The bending moment from the column loading induced by (a) the weight of
the tower and everything attached to it, (b) the bending moment of the wind
on the mast and antenna, and (c) the bending moment of the wind on the tower
itself. This load is non-linear since it results from the deflection
profile of the tower and creates a moment based on the other loadings being
applied at a deviation from the centerline of the column when the deflection
occurs. It is significantly affected by the relationship between the
section modulus and the length of each section of the column, taking into
account the distance between guys, and can significantly increase the
stresses.
All the stresses must be combined together in a way that determines the
magnitude of the principle stress to assure that the principle stress does
not exceed the yield strength of the tower material.
How these are affected by wind loading, rotation and placement of the guys
is the trick and are probably not much of a challenge to a qualified
structural engineer using computerized structural analysis methods these
days.
One thing I have done with my own tower is to build a triangular bracket
which is attached at the base plate of the rotator. The bracket is attached
to the tower legs so that each leg of the bracket is attached to two legs of
the tower. This give a longer moment arm for the guys to pick up the
rotational moments of the antenna system. Even better would be to create a
"Y" in the guys near the top so that the moment from the rotator is picked
up at an angle on the base thus increasing the effectiveness of the guys in
resisting rotational moments.
73
Bill
W7VP
----- Original Message -----
From: "Roger (K8RI)" <K8RI-on-TowerTalk@tm.net>
To: "Steve Maki" <StevesLists1@gmail.com>
Cc: "towertalk" <towertalk@contesting.com>
Sent: Thursday, November 06, 2008 9:55 PM
Subject: Re: [TowerTalk] Guying a self-supporter
> As I've said before, I'm not a structural engineer but I've had enough
> engineering courses to understand the basics, not run an analysis and
> have asked for input.
>
> The question to me is why would any one guy a self supporting tower?
> The only reasonable explanation I can think of is to have it support
> more than what it's rated for at that height which may be a little or it
> may be a lot.
> If that is the case, in this instance we are not talking about
> de-rating, but trying to increase the rating to handle more than the
> tower did in the self supporting state.
>
> That leads to two situations. The tower is supporting more load that
> what it was designed for in the self supporting state. In addition to
> the added weight, and compression forces from the wind load we now have
> the forces added by the guys. We've also added more twisting moment with
> the larger antenna(s). The guys also have to be properly tensioned to
> add the desired resistance to the over turning moment and also prevent
> resonance. As was mentioned earlier there is the bending moment between
> guys as well. Also that twisting moment can translate to bending
> moment using the guys as a pivot point. You can see this in practice
> when running a shaft from the antennas down through the tower to a
> rotator at the base. If the thrust bearings are in the wrong locations
> and/or the shaft is not heavy enough, the twisting forces will cause the
> shaft to kick to the side between the bearings. I've seen overloaded,
> skinny, guyed towers do that as well. This an additional reason self
> supporting towers get bigger and heaver as you move down.
>
> So to me that begs the question, is the tower more likely to fail in
> this situation than it would be as a self supporting tower loaded within
> its ratings.
>
> Certainly we can add resistance to the over turning moment/force by guys
> and to some extent even out the compression forces at the base. That
> then leaves the question as to what it does to the failure modes as you
> move up the tower. Does it reduce the margin to failure and leave us
> skating closer to the edge or does it make the tower stronger overall.
> Those were the questions I was asking originally.
>
> Roger (K8RI)
>
> Steve Maki wrote:
>> Rick Karlquist wrote:
>>
>>> Bill wrote:
>>>
>>>> The following was Steve's question to me and my response which was snet
>>>> off the reflector.
>>>>
>>>> Steve said:"Bill,
>>>>
>>>> Are you an engineer? If so, can you devise a case, using a reasonable
>>>> tower and guy system, which contradicts my (and others) "theory"? I've
>>>> put this to a few engineers, and they haven't come up with one. I'd be
>>>> the first to shut up about it if an example could be shown...
>>>>
>>> It is easy to imagine a guyed self supporter that is unsafe.
>>> Self supporters are strong at the bottom and weak at the top.
>>> If you guy a self supporter to allow a huge antenna to be placed
>>> at the top, the twisting force of the antenna will now overstress
>>> the top sections. This can bring down the tower even if bending
>>> moment etc is otherwise OK. You might be able to get away with
>>> this configuration if you counted on the mast to slip, and you had
>>> a breakaway coax link. You are still off the air, but at least
>>> the tower doesn't fall.
>>>
>>
>> Rick,
>>
>> Another way to characterize a self supporter is to say that they are
>> strong at the top, and EXTREMELY strong at the base....
>>
>> Of course hams tend to use the very weak ones :-)
>>
>> At any rate, a "reasonable guy" system in your scenario just might
>> include a star bracket and double guys for the top set, but this is
>> getting way out of the "reasonable" range that I was thinking of.
>>
>> Steve K8LX
>>
>>
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