Telephone Pole Brace

[XR250]

Has anyone ever seen one of these fail? Seems unstable. How does the pipe have enough lateral stiffness to brace the joint? I know we have discussed this previously with trusses and I think we concluded that, in part, gravity was helping keep the brace from rotating. Gravity does not help in this situation.

 

[dik]

I've not seen one fail, but that doesn't mean much... rotation about the axis of the pole is restrained by the cable fixity at grade. Rotation about the axis of the brace is restrained by both the cable and the attachent at the pole.

So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[kipfoot]

my thought:
a) the strut has some fixity at the base, but regardless,
b) lateral movement at the outer end of the strut creates elongation of the cable, and a component of the tension pulls it back to equilibrium.

 

[human909]

Good question XR250.

I don't believe that rotation about the axis of the pole can be described as "restrained". However it is currently in its lowest energy state as long as the cable is under tension. So despite being not restrained it is stable due to being already at the point of lowest energy. Given it isn't restrained though, I'd allocate it as Leff of 2.0 due to it effectively being a sway compression member. (The tension of the cable does add stiffness to sway, but in my opinion it is relatively low and hence a sway member.)

Rotation about the horizontal axis perpendicular to the pipe and the pole is achieved through clamping of wire rope. Without this clamp (or friction) the end of the pipe would move down shortening the cable length and reducing the tension in the cable. Thus moving to a lower energy state.

EDIT: Kipfoot beat me to it. His second statement is essentially a less verbose version of my explanation.

 

[XR250]

Ahh, makes sense. The upper portion of the cable does not add any side to side stability, but the lower portion would have to elongate for the brace to move laterally.
Thanks

 

[BridgeSmith]

The cable and strut restrains the pole in only one direction - in the direction of the anchors, which is the only direction where the pole needs additional restraint. By far, the largest force at the top of the pole will be in the direction the span wire, or the resultant of forces parallel to the wires, if there is more than one. The bending and torsional resistance of the pole takes care of the rest.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[RWW0002]

Even without the fixity at grade the condition may still be stable. The telephone guy-wire strut is very similar to a column-supported jib crane condition. I think I offered up the same analogy on one of the truss posts of the past.

In a jib crane with a handing load you still have the cables in tension with the lower cable completely unrestrained (truly floating) and a joint at the column and at the top cable connection that are pined in the horizontal direction, yet I know of no stability issues/failures that are common with jib cranes. I will say I have seen some "walking" of the jib boom where the jib tries to rotate laterally slightly under heavy load, but have not heard of failure or stability concerns)

Regardless of the fixity, it seems that the tension in the cable appears to restrain the system from out of plane buckling, similar to Kipfoot and human909's comments above.