Cables transferring compression forces

[curly.hart]

Hi Friends,

I'm very new to Robot and trying to understand how to use cables. The results I was getting didn't seem quite right. So, I modelled 2 nodes vertically aligned, pinned supports, connected with a cable. I applied a point load at midspan. The horizontal reaction was zero, good. The vertical reaction was the same (half the applied load) at both supports. I would expect the reaction at the top to equal the total applied load and the reaction at the bottom to be zero. The results mean that the cable must be transferring half of the applied load in compression.

What I really want to model is cables at other angles, 15, 30, 60 degrees etc., but when doing that the results seemed odd. The above was my way of testing, I showed what I suspected.

I would really appreciate any help with this one. As it is, I have no value for this part of the program.

Thanks,

Curly.

[Simau]

@curly.hart 

In this test with a wind load at the center, there is no compression on both supports

Test model  attached

[Rafacascudo]

When defining a cable  you have to input the initial Force F0 or an elongation or the initial lenght or a stress . All of them means that you are pre tensioning your cable. This pre tensioning + the 1st load case will be your " assembling " conditions and will be considered/added on ALL your following load cases .

What you are describing is a loose cable with no pre-tension at all. It will not work.

So if your midspan load is 10kn ,  your  initial force F0 will have to be greater than 5kn , so the cable will be still all in tension.

[Simau]

@curly.hart @Rafacascudo 

It seems that for SW, there is compression on both support (vertical and horizontal postion).

Initial length was taken as distance between support (no elongation)

[Simau]

No need to input an initial force or an elongation.

In this test all works ok

[curly.hart]

Hi @Rafacascudo @Simau,

Thanks for your replies.

I'm using steel core ropes in playground structures, so there will be zero pre-tension.

Rafa, in your first example here, 100 kN pre-tension, 10 kN point load, you get reactions of 105 and 95. Wouldn't you expect that all of the point load would transfer to the top support, with reactions of 110 and 100?

What I was trying to describe originally was similar to this model, but without the pre-tension.

[Rafacascudo]

Wouldn't you expect that all of the point load would transfer to the top support, with reactions of 110 and 100?

No , because it is not a linear problem. You cannot add results from 2 different nonlinear load cases. The combination of the 2 is a new nonlinear case

Using "tension only" bars(Geometry/additional atributes/Advanced bar properties) , You get the 10Kn reaction on the upper node on the model to the left.

On the middle model , you have a temperature load that gives 100Kn tension force on the bar.

On the model to the right ,you have both previous loads acting together .

See how the results are different and agrre with the cable model.

Test model attached , done in RSA2021

[curly.hart]

Hey @Rafacascudo , thanks again for the reply.

I thought cables were, by definition, tension only members? So I would expect for the single point load to get the same result as you here. I have limited knowledge of the behaviour of cables, but can see how a combination of loads would produce the result you have shown. I still can't understand how a single 10 kN point load would cause reactions of 5 kN at each support. Screenshot below. Getting an error message when I try to attach the model.

Curly.

[benoit.thiry]

when you apply the 10kN load downward, the point of application tends to go downward, the upper part of the cable is extended, increasing the tension, and the lower part is shortened decreasing the tension.
It will behave like that until the moment where no more tension will exist in the lower part of the cable..

[Rafacascudo]

For cable element, in RSA, You got to have the initial tension load. This is from Robot manual on

https://help.autodesk.com/view/RSAPRO/2021/ENU/?guid=GUID-DCDD6D6E-D1BA-4535-9A21-AA28C9277CF7

"I still can't understand how a single 10 kN point load would cause reactions of 5 kN at each support"

For this example( point applied at MIDSPAN on a vertical cable), If the cable is in tension , yes , it will work as a standard bar , as long as the point load is less than twice the cable tension load . The moment the point load is 2 times the tension load , the bottom part of the cable will be distensioned and the load will go entirely to the upper support.

This is proved using RSA "Tension  only" bars .

[curly.hart]

Thankyou @benoit.thiry   and @Rafacascudo  for your replies. Both good explanations and the diagrams really help. The only problem I'm having is that the cable is not becoming distensioned in the lower half, as it does in your example. Is there a minimum pre-tension that can be applied? As mentioned, these are ropes that really will have zero pre-tension.

Curly.

[Rafacascudo]

It will work for the vertical cable , dividing it in 2 cables. With pre-tension ,it will also work if you apply the pre-tension only to one of the cables (don´t ask me why).

[Rafacascudo]

With pre-tension ,it will also work if you apply the pre-tension only to one of the cables

Sorry , this is incorrect. Result is not right although analysis gives no errors. With pre tension it is better to not divide the bar.

But what is your real  problem?? Can you provide a sketch?