jimlux wrote:
> Steve Maki wrote:
>
>> To be honest, I'm not sure what the question is either :-) Folks seem to
>> be equating self support towers with crankups, and who knows what else...
>>
>> Just to clarify the question in my OWN addled brain - I'd like to see an
>> example of a normal tapered self support tower that has to be de-rated
>> after a reasonable guy system is added.
>>
>> That's all.
>>
>> Steve K8LX
>>
>>
>
> You're talking about something like a Rohn BX with added guys?
>
> Not trying to get any additional wind load capacity? (because the BX
> isn't rated for very much in the first place)
>
> I don't remember the exact numbers, but I seem to recall that there's
> not a lot of margin in the design(i.e. the verticals and diagonals are
> pretty close to their buckle limit, when the tower is loaded at rated
> wind speed). In these sorts of designs, the limit is usually in the
> short segments, which are "slender" and fail by buckling, not by simple
> compression. Part of the challenge is deciding whether the column ends
> are fixed, free, or pivots, because that changes the buckling behavior.
>
> Putting guys on will inevitably increase the compression loads on the
> downwind side a bit (from the vertical component of the tension in the
> guy, even if slack when unloaded, they will develop tension under wind
>
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. Beyond that
point I'd expect to see the load more evenly distributed (at the base)
as the upwind guy is going to add some of that tension on that side.
I'm more concerned with where the guys are attached, how they are
attached (not directly to the tower legs), and the tension.
Addressing where they are attached. Any force above a set of guys will
translate into one of the opposite direction below the attach point.
Worst case being a single set of guys some distance below the
antenna(s). In that case the compression load on the downwind leg is
going to be increased considerably just below the guys. On a BX or HBX
this is a bad place to have increased compression. A second set of guys
can be placed where the bending moment is greatest on the skinny
sections and if possible the top guys can be moved up closer to the
antennas. The important point is that second, or lower set of guys be
located at the point of maximum bending moment or close to it. Part of
this location is determined by the strength of the tower in those
locations. It might be prudent to arrange the guys so one set is
attached in the weaker area. IOW it would work out for two *relatively*
close spaced sets of guys on the top sections.
Guys should be attached with a bracket much like ROHN uses on the 25 and
45G series. The would be easy to make and no where near a beefy as the
ones used on the G series. The legs of the BX series appear to be
designed to handle compression, but would be weaker when pulled on and
particularly so when pulled on in a small area such as when a cable/guy
is wrapped around the leg.
In my opinion were I going to guy one I'd use a light weight, strong
material such as Phyllistran. No low frequency oscillation and a small
fraction of the load presented by steel guys.
> load). I'd be more interested in the change of the distribution of
> forces among all the bracing members.
As they are "X" bracing "I don't think" there would be a major change
to overall force on the braces.
Roger (K8RI)
> On some theatrical trusses, the
> main tubes are pretty fat and strong (because that's what stuff gets
> clamped to) so the overall strength limit is in the fairly small
> diagonal braces, particularly when there's a combination of bending and
> torsional loads (imagine a truss hung by cables from each end, with a
> bunch of lights hung on one pipe).
>
> You might be depending on having components and assemblies that are on
> the "good end" of the expected range of material properties.
>
> One advantage that might accrue is that you could claim the guys will
> make sure the crumpled tower debris is contained within a smaller area,
> rather than laying out flat.
>
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