Problem cable beam

[Anonymous]

Hello,

I'm trying to estimate the displacement of my prestressed cable beam and i have a problem: my cables are pretensioned to 20kN and when i load the beam with 1kN point load of wind to the beam, i found 11.71mm displacement max on Y that i think to much. I try to compare it to another beam in replacing cable elements by truss bar and found that the displacement of the truss beam is smaller (around 5mm).

i just think that with the pretension, the cable beam must be more rigid than the truss beam (is this right or wrong?), so why the displacement of the cable beam in my model is so much bigger than the truss beam ?

I joined the file below, anyone can help? Thanks a lot.

[Artur.Kosakowski]

I think that the fact that you defined both models in one file makes the investigation a bit harder due to the fact that you have the starting load case used for the tension only model as well. Try to move it to a separate file and check the results for the wind load case. You will see that the structure acts as a truss with one chord under compression which is more effective for reducing displacements than applying tension force in the perpendicular direction. In other words the tension only model is sort of big beam with large stiffness in the direction of the applied wind load.

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[Anonymous]

Hi Atur,

I separated the files to 2 files: "tension only" with cables element and "truss" with axial force only bars. After running, i found that the displacements are the same in comparison to the original file with 2 models. I have no doubt of the "truss" model, but for the "tension only" model i don't know how to verify the result. I tried to use another software (SAP2000) and i found just 3.28mm of displacement under wind load, that confused me alot. Can you take a look at my "tension only" model and tell me what do you think?

Thanks a lot 

[Artur.Kosakowski · Accepted solution]

I think that actually you should look at the truss model. You allowed for compression in some elements so this reduces the horizontal displacement comparing to the cable model. I'm not able to make any comparison with SAP but out of curiosity: what are the forces in 'cables' on each of the sides there?

[Anonymous]

I think that actually you should look at the truss model

Yes, i just want to know if there isn't compression in my cables under wind load, can i consider my cable truss like a truss with compression and tension bars with the same cross section of my cable, so i tried to make a truss model and compare the displacements of 2 models. That's why you see compression in the truss model.

But i think this assumption is wrong, because the stiffness of cable truss depend a lot on the pretension in the cables while stiffness of truss depend only on the geometry and cross section of the elements. What do you think of it?

My worry now is about the displacement of the cable truss that i think too big, i don't know if my model is correct or not.

[Artur.Kosakowski]

IMHO you can't compare these two models as they behave in entirely different way. It seems that in order to reduce the displacement you need to increase the forces in the cables.

If you find your post answered press the Accept as Solution button please. This will help other users to find solutions much faster. Thank you.