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# Gap-spring element?

## Gap-spring element?

(OP)
Dear Masters:

I need to constrain a few beams (or nodes) within such a domain, somehow like a well with elastic walls. I am thinking of creating 4 extra nodes and connceting them to the modelled nodes (or beams)using spring elements and with certain gap between the springs and the nodes - so they can move freely before touching on the springs. But I don't know how to do the gaps.

Or, there's any simplier way to do this?

Janna

### RE: Gap-spring element?

Hi Janna

Maybe you could use a connector element with a non-linear spring property?  Connectors are quite a nice way of prescribing interactions between nodes as you can add/remove properties like stiffness & damping to a single element.

Regards

Martin

### RE: Gap-spring element?

(OP)

I am reading the connector element manual right now but could you please specify some I could use in this case? I still can't see how I can use one element to represent both gap and spring properties. It would be best if one kind of elements could be set to represent the whole well situation.

Thanks a lot!

Janna

### RE: Gap-spring element?

Hi Janna

How about giving the spring non-linear properties?  That way, it could have a stiffness when compressed and no stiffness when in tension; i.e.

If F is the force and x is the spring displacement from rest, then;

x-------------|---x at x=1mm, F=0 N (tension)
x---------|-------x at x=0mm, F=0 N (start position)
x-----|-----------x at x=-1mm, F=10 N (compression)
-> no stiffness
<- positive stiffness

regards

Martin

### RE: Gap-spring element?

(OP)
Dear Martin,

Would it be possible to define the "start position" of the spring from,say x=-1mm? That would be a "gap"!

Cheers

Janna

### RE: Gap-spring element?

Hi Janna

You can set the reference length for a connector using the *CONNECTOR CONSTITUTIVE REFERENCE property.  Say you have two nodes, 20mm apart that are joined with a connector element.  Now, suppose you want to close up the gap to 15mm before the spring comes into play - to do this set the *CONNECTOR CONSTITUTIVE REFERENCE to 15mm.  Then, define the non-linear spring to give zero force in tension and a non-zero force in compression.

This basically sets the start position of the spring in my slighty dodgy diagram above....

Regards

Martin

### RE: Gap-spring element?

(OP)
Thank Martin!
This sounds brilliant and I will try it now!

Janna

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