Compression only springs

[par060]

I have a circular 2-d model that I want to place tangential and radial compression only springs. I am having trouble grasping the "gap link" that I believe are what I have to use. Can someone give me alittle insight to the steps I need to take to get these springs on my model?

 

[stressed]

Ok, if I understand your problem, your biggest challenge here will be to make the local axis of your joints relate to the circular nature of your model as you want radial links(compression only springs)to act normal to element. To do this, first, use View Set Display to display the local axis arrows. Red, white, and blue = axes 1, 2, and 3.. like the American flag if that helps you remember. Select joints and Assign menu>Joint>Local axis and click Advanced button, then check Advanced again until the advanced options appear. Typically you will need to adjust the local plane and the plane coordinate directions. As usual, CSI implements a useful feature, but then offers little documentation on how exactly it works, so you will have to play around with the local plane and coordinates. The benefit of having the little colored arrows displayed, is that you can experiment with these assignments and immediately get graphical feedback as to whether you've done what you want. I should mention that the advanced local axis feature was added for versions 8 and 9, so if you have an older version, you have to select joints, then use Assign menu>Joint>local axis to manually rotate each joint local axis around your diameter, each one of course will have to be rotated at a different angle if you do it that way.

Once you get your coordinate system how you want it, you've probably figured out that you define a Gap link and draw the links (make sure you have selected the correct property when you draw) by clicking or windowing. If you are unsure of the local direction of the gap (link element) after drawing, use the Set View option to turn on the little red white and blue local axis arrows for link local axes to see what you're doing.

 

[par060]

Actually I can get the local axis correct it is the spring itself that is giving me problems...does the "gap" link have to be drawn between 2 nodes?...I have tried this in a simple beam case and tried to input a high effective stiffness but the trial beams deflection is the same with or without the spring...so obviously I am doing something wrong with the actual drawing of the link and/or identifying its properties

 

[stressed]

There are 1 point links (for gaps and other link properties) and 2 point links. Unless you want a link between 2 joints, you would typically need only a 1 point link. If you input a stiffness in the gap link and it didn't make any difference in the deflections of your frame, it sounds as if you typed the stiffness in the wrong local axis of you gap/link.

 

[par060]

thanks stressed...I think I have it figured out now...i was inputing my stiffness into the linear value rather than the non linear

 

[stressed]

Ahh, I should have thought to warn you of that, especially since I made the same mistake mistake myself when I first started with SAP. It is a bit confusing to see two sets of input, one set for linear and another for nonlinear, on a nonlinear dialogue screen