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Cable elements

I´m trying to model structural elements which have zero stiffness in axial-compression and non-zero stiffness in axial-tension, in other words I want to model a cable like the cables in a bridge.

How can I do it with 1D elements?

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Altair Forum User

Hi AsahelM.

You can use GAP elements to model cable(s).

In Optistruct you can make use of CGAP elements to capture this behavior.

Please refer to help for more information.

<?xml version="1.0" encoding="UTF-8"?>

Altair Forum User

thank you for your reply

Altair Forum User

Hello together,

I have the same problem and played a little bit with this function. The problem is that it doesn´t become clear to me.

My procedure is the following:

1. I create two nodes

2. I create a Material Type: isotropic, card image: MAT1, named 'Stahl', E: 2.1e+05 NU: 0.3

3. Create property, type: 1D, name: 'Kette' Card Image: PGAP, assign material 'Stahl'

But then there is the card image and I already read the help but I don´t unterstand it. Could be that my english is too bad for that. When I leave all options the standard ones I only get errors at the end. I would like to model a chain which is a little bit elastic.

Is there anyone who can help me?

Thank you so much!

Sebastian

<?xml version="1.0" encoding="UTF-8"?>

Altair Forum User

Hi Sebastian,

1) First create a CGAP (1D>> gaps) element

2) Apply PGAP property to the CGAP elements.

3) Card edit the CGAP element and change the CID option to FLIP as shown in the image attached.

<?xml version="1.0" encoding="UTF-8"?>

Altair Forum User

Hi Sebastian,

1) First create a CGAP (1D>> gaps) element

2) Apply PGAP property to the CGAP elements.

3) Card edit the CGAP element and change the CID option to FLIP as shown in the image attached.

Thank you for your reply, I have done this steps. When I run OptiStruct in this configuration I get an error:

'*** ERROR # 923 ***

Impossible to calculate automatic KA for gap elements. Global stiffness

has an all-zero diagonal.'

Because I don´t know the meaning of 'KA' I replaced 'auto KA'. I filled in '5000'. Then the error doesn´t occurs. This sounds a little bit crazy I think. What is the real meaning of KA?

There is another problem when I want to show the 'Von Mises Stress' because there isn´t.

Thanks for your sympathy!

<?xml version="1.0" encoding="UTF-8"?>

<?xml version="1.0" encoding="UTF-8"?>

Altair Forum User

Hi Sebastian,

can you give a value instead of AUTO, say 1e5?

If you keep AUTO KA is determined by stiffness of surrounding elements.

Altair Forum User

Yes, when I replace AUTO by '10000' the error doesn´t occur.

But how is it possible to calculate the force in the cable? I can only show the displacement.

Altair Forum User

Sebastian,

If you know the cable material properties, then you can calculate the KA as recommended in PGAP card

<?xml version="1.0" encoding="UTF-8"?>

Altair Forum User

Hello,

I was wondering how you would model a cable for nonlinear geometrical analysis. Optistruct would give an error if i were to use CGAP with Large displacement nonlinear analysis.

thanks

Rahul Rajan_21763

CGAP, CGAPG, CWELD, CSEAM, CFAST, RBE1, and CONM are not compatible with LGDISP.