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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