Is cable always tension only object? 1

[jowook]

Dear Engineers,

I have little experience designing cable or presstress strucures and need some advice. Please look at the below linked picture. It seems like a prestressed cable object can take compression as long as the cable are still in tension due to prestress. The left guyed post is supposed to show same relative displacement as the right cantilever does. Thanks in advance.

 

[dik]

Only if you remove some of the effective prestress... <G>

It's like pushing on a rope...

Dik

 

[jowook]

Thanks for reply,
You mean that the prestressed cable or rope can resist compression?

 

[dik]

Prestressed cable is a little stronger...

Dik

 

[MikeHalloran]

I'm not sure what your diagram is supposed to mean in the real world.

AFAIK, in the world of wire rope, the word 'prestress' has a special meaning, i.e. somebody paid the manufacturer a premium price to set up a given length of rope and pull on it, hard, before shipping it. I'm not sure exactly how hard, but I think they go a little past the normal yield point, which work hardens the wire a bit and removes a lot of the internal slack, so that the prestressed wire rope will stretch a bit less than common wire rope when load is first applied.

It's normally considered extremely bad practice to apply axial compression to wire rope; the usual result is a 'birdcage', which is considered a failure mode, and justification to remove the rope from service and discard it.

<tangent>
My first car, a 1958 Hillman Minx, had a column shift mechanism that pushed on a cable.
The transmission had a top-mounted selector, with the selector rod extending forward instead of rearward. One of the motions of the column shaft was transferred to the selector rod by bellcranks and a rod; the other motion was transferred by a stout cable that pushed or pulled. I can't remember which motion used the cable. The cable looked sort of like a speedometer cable, but the inner cable was maybe 8mm diameter overall, and wrapped with flat wire. The stroke was maybe an inch and a half, and it worked fine, once you got the adjustments right, and kept it well greased.
</tangent>


Mike Halloran
Pembroke Pines, FL, USA

 

[rapt]

Cable Structures and prestressed Structures are 2 different things.

In a prestressed structure, the "cable" or tendon is enclosed in concrete, and if bonded, experiences the same changes in strain as the surrounding concrete. So if the concrete is subjected to compressive strain, the tendon/cable stress will reduce. If it were technically possible to apply sufficient compression strain to place the tendon into compression this would be realistic, but it is not normally possible unless the tendon started with a very low level of prestress.

In a cable structure, if the cable were to go into compression, it would buckle immediately as it has basically zero resistance to buckling, so no it could not act in compression.

 

[ACtrafficengr]

MikeHalloran, was it really being pushed, or were you reducing tension as in a bike derailer cable? Just curiouos.

 

[MikeHalloran]

Nope, not like a derailleur; no springs. Fat cable, push and pull.

Mike Halloran
Pembroke Pines, FL, USA