Soil Structure interaction using Springs on SAP2000

[neham]

Hello,

I am trying to model buried pipelines on SAP2000. We use a beam element to model the pipe. I am trying to show the soil structure interaction using line springs, link property.
(1) I want to know if I can access the ends of the spring?
(2) Can I apply ground Motion behaviour at the ends of the spring?
(3) When I apply a ground motion in say, z direction, where does it act? On the spring ends or the pipe?

I have attached the SAP model with GM applied and springs in X and Z-direction.

 

[stressed]

neham, I took a quick look at your model and there are a few problems:

A. You have fixed restraints on both ends which stops axial displacement. Why would you use an axial line support link on a line which is already axially restrained at both ends?

B. You didn't use a linear line spring, you used a multi-linear elastic link which may require nonlinear load cases for analysis.

C. I think your local link axes are different than what you think they are. Please verify yourself, but in this particular model, link local axis 1 = Global +Z, local axis 2 = +X, local axis 3 = +Y. So it appears that your "axial" link is acting in the vertical direction. Again, please double-check local link axes yourself.

In answer to your questions:

1) What do you mean "access" both ends of the spring? Line springs are assigned automatically for internal analysis. In order to review and select them individually you would need to instead draw multiple joint links along the length of pipeline instead of using a line assignment for the link. You can draw joint links as 1 point or 2 point links. The edit menu replicate feature will facilitate copying multiple joints and links

2) You can assign joint imposed displacements only using joints which are modeled with link supports

3) Using a 1 point spring, your spring will not have a length. It will be the same location as pipe centerline joint where the link was drawn. With a 2 point link you can draw a link to have a length, perhaps assigning an anchor to one end of the link and pipe joint on the other end. In this 2-point link example, the displacement would be assigned to the anchor end of the link. Imposed displacements can only be assigned to joints which have assigned restraints, links or springs.

 

[neham]

Hello stressed,

Thank you for your quick response.

A. Yes, we will change the constraints to free end.

B. We used a multi linear spring to account for the bi-linear behaviour of the springs.

C. Yes, we checked up on the axes.

We used a 2 Joint Link Spring to assign springs along x and x directions.

We need to input a time history load, a ground motion, at the end of this joint spring. Is that possible? It is possible to select the ends of the spring now. Is it possible to apply a time history load at the ends of all the springs? If so how?

 

[stressed]

Since you've checked your model, please attach your modified .sdb file. It's late here, so I may not check until tomorrow. You can assign a joint displacement at support locations and then use that displacement in your time history case. Read my previous email regarding modeling of 2-point spring.

 

[neham]

Hello,

I have attached the file. I have used 1 point springs to draw the spring.

I am sorry to be bothering you about this. But I did not quite understand how to put in the displacement at supports and use this for time history. I did not understand the concept.
I would be very grateful if you could explain it to me when you have the time. Or any other document on it showing the steps would be good enough too.

Thank you.

 

[stressed]

Your link local axes look OK now, but I don't understand why you have defined multi-linear links which have different displacements for the same identical force. Also, you have nonlinear links but you still only have linear analyses. Either change links to linear springs or run nonlinear analyses.

Regarding joint displacements for settlement, you must assign joint load> displacement. If you want to include these imposed displacements in a time history analysis, you'll need to add the imposed displacements joint pattern to a time history case and apply a time history function. Currently you only have acceleration loads in your time history cases and your time history cases are linear.