[TowerTalk] Adding guys to self supporting towers

[Jim Lux]

On 8/13/13 11:02 PM, Steve Maki wrote:
> On 8/14/2013 12:27 AM, Jim Lux wrote:
>
>
>>>>> ##  The stress on the cross bracing is actually reduced when guys are
>>>>> added to a
>>>>> free standing tower, not an issue.
>
>>>> How so?
>>>>
>>>> In a no wind static case, the guys add down force, some of which will
>>>> appear on the diagonal cross braces.  How much depends on the relative
>>>> compression stiffness of the brace vs the verticals.
>>>>
>>>> In the wind loaded case, the guys transform some of the horizontal
>>>> bending load into down force.  That could be a wash in terms of loads
>>>> but it's hard to tell, depending again on the relative stiffnesses.
>>>> E.g.
>>>> downwind side has less load due to bending, but more load due to guy
>>>> induced downforce, but the guy downforce is distributed among all
>>>> three/four vertical legs.
>
>>> I'm having a hard time with this. It seems to me that in the guyed
>>> no-wind state, the added force to diagonals from guy tension will be
>>> almost nonexistent as long as the guys are tensioned equally. Isn't all
>>> of the additional vertical force, well, vertical?
>
>> No.. think about two angled elements forming a triangle, and loading the
>> apex of the triangle.  Both elements are in compression.
>
> Sure.
>
>> Same thing if you had 3 elements, one vertical, and two angled on either
>> side.  The forces will distribute according to the compressive stiffness.
>
> Sure.
>
>> The whole stiffness thing is a real challenge..
>>
>> Think of this.. say you have a 10 foot length of piano wire which can
>> support 1000 lbs.  You also have a 10 foot length of nylon rope that can
>> support 1000 lbs before it breaks.
>>
>> Two nylon ropes in parallel support 2000 lb.
>> Two piano wires support 2000 lb
>> A piano wire and a rope in parallel support just over 1000 lb.  The
>> piano wire is a lot stiffer, so it takes all the load
>
> Again easy to see.
>
> My gut feeling on this is mostly the observation that a self supporter
> with roughly the same loading capacity as a guyed tower is built with
> MUCH more material, which tells me that horizontal wind force dwarfs
> dead weight in terms of member stress and requires heroic methods to
> withstand. These heroic assemblies if not actually helped by them,
> should be able to laugh at a few puny wire restraints.
>
> I'll have to look up some actual structural's I have. We used to do a
> fair number of beef-ups. On self supporters it almost always involved
> beefing up diagonals and horizontals, but the failure was wind force
> related. Of course the SA never separates out what we would like it to
> for education, i.e. the contribution of dead weight vs wind force.
>

on the BX, the weight of the tower is a pretty big contributor to the 
loads, at least at the bottom.

I think there's also the problem that the bigger structure required to 
deal with the "self support" aspect means it has more wind area, which 
means that the wind loads on the tower (independent of any antenna) are 
bigger.

OTOH, you look at something like the HV transmission towers, which are 
self supporting, tolerate huge ice loads, as well as hundreds if not 
thousands of pounds of power line, and they're not that beefy.  They're 
relatively few triangles, with pretty long members. Yes, the members are 
big, but I'd guess that the "strength/weight" for those 500kV towers is 
much higher than for the usual ham self supporter.  Maybe it's a scale 
thing.  An 8" diameter tube with 1/4" walls is a lot stronger than a 4" 
diameter tube with 1/2" walls (using the same amount of metal).  The HV 
transmission tower has a lot bigger foot print for the height than the 
typical ham self supporter.

Ultimately that's the problem.. everyone wants a nice slender sky needle...