PPGap material and truss element

[clairelove]

Hi,everyone! I want to use PPGap material and truss element for the expansion joint in the bridge, but I don't know how to set the gap value and the distance between the gap nodes. Is there any relationship between this two values? Thanks in advance.
here is my code related:(m)
# nodes of the link element
node 1 0 0 0
node 2 $distance 0 0
uniaxialMaterial ElasticPPGap 1111 3E10 -2e10 -$gap
element truss 1 1 2 5.044 1111

Please help with the explanation of this two values.

[fmk]

the gap is usually in a zeroLength element where the gap given in the material properties represents the gap (the material strain that is set is actually the difference in nodal displacements when used in a zeroLength).

when using in a truss the strain that is set is the displacement/L (difference in nodal displacements/L) .. so a similar gap property for the material used in a zeroLength would be L*gap and you also would have t modify the other parameters to the material.

the distance value you are specifyig in the node would be the L.

[clairelove]

Thanks for the reply. I'm not sure if I get it correctly.

If I set distance L= the difference in nodal displacements, then the gap can be 1. Is that right?
BTW, is there any difference between the zeroLength element and truss when using the PPGap material? I try to use it for the expansion gap in the bridge.

Thanks again.

[fmk]

L is the member length .. which for a regular truss is simply the original distance between the nodes. there is no difference between the two if this length is 1.0.

[clairelove]

So I want to model a bridge with a 0.02m expansion gap , is this right to code like this?
node 1 0 0 0
node 2 0.02 0 0
uniaxialMaterial ElasticPPGap 1111 3E10 -2e10 -$gap
element truss 1 1 2 $As 1111

According to my understanding, $gap would be 1.0? thanks..

[clairelove]

I tried many times,but this doesn't work. it ends with no converging.
It seems that it needs a best value to converge.

[fmk]

the gap does work and many people have used it for bridge abudments and the like .. just remember that the system needs to remain solvable even if gap element providing 0 stiffness! .. also remember that when the gap does close, if stiffness and forces are large pounding will result .. high frequency stuff that you will need to damp out if you hope to get convergence .. look into HHT algorithm in such situations .