On 8/13/13 6:29 PM, Steve Maki wrote:
On 8/13/2013 10:09 AM, Jim Lux wrote:
On 8/13/13 6:47 AM, Jim Thomson 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.
Same thing if you had 3 elements, one vertical, and two angled on either
side. The forces will distribute according to the compressive stiffness.
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
I thought that stress in the diagonals is the result of flexing of the
legs, which MUST be most pronounced in the unguyed windy condition???
-Steve K8LX
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