zerolength element

[kyungtae79]

Hi, all

I am modeling a simple bridge with a intermediate hinge.

The hinge skews with a 45 degree with respect to a global coordinate.

To connect node 1 and node 2 at the hinge, I used the zerolength element.

< Case 1 >

uniaxialMaterial ElasticPPGap 302 4.529e+004 [expr 3478.5/3.] 0.0127
uniaxialMaterial ElasticPPGap 304 4.529e+006 [expr -3478.5/3.] 0.
uniaxialMaterial Parallel 307 302 304
uniaxialMaterial Elastic 308 4.529e+010
element zeroLength 1 1 2 -mat 307 308 -dir 1 2 -orient 0.707107 0.707107 0 -0.707107 0.707107 0
equalDOF 12 19 3 4 5 6
...................................................
It's because the vector of local x of zerolength element is defined with a 45 degree.

Moreover, I tried a different way where

< Case 2 >

element zeroLength 1 1 2 -mat 307 -dir 1
equalDOF 1 2 2 3 4 5 6

.............................................
Compared with the case 1, case 2 had the same results.

Here is my question.

If I did not assign the local x coordinate of the zerolength element in Case 2,
what is the vector of the local x of the element?

I think it might be the same as a vector of global X because two nodes locate at the same coordinates and the material direction is defined with 1st degree of freedom.

However, responses of the element are same...That's not my expect..

Could you clarify me?

[vesna]

What you have specified in the first case are two springs that coincide with local x and y axes that are rotated 45 degrees from the global axes. In the second case you have one spring in global x direction. If you test your model for force in global x direction such that your springs are still in the elastic range you will get the same answer for both cases. Here is the reason: if you decompose the forces in the springs of case 1 (for this type of load) the resultant force will only have a horizontal component and will be the same as the force in the spring of case 2 as all springs have the same stiffness.

Here is how you can define one spring at 45 degrees:
element zeroLength 1 1 2 -mat 307 -dir 1 -orient 1 1 0 -1 1 0