Spring Element Forces in Nastran

[stressebookllc]

One of the confusing aspects to me was the signs of the induced forces in various types of spring elements:

So, a positive spring force is always tension, right?

Well..... I found out not really, so i kind of summarized a few things here:

http://www.stressebook.com/spring-element-forces/

Would love to know what you guys think and if I could add some more value to the post based on your comments. You can comment here or under the post, I'd love to hear from ya..

Cheers..

Aerospace Stress Analysis and FEA Courses

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[rb1957]

CBUSH elements are awful for trying to sort out tension or compression (you have to interrogate the displacements of the two nodes to see if the element has stretched or not).

For a CELAS then +ve load = tension

another day in paradise, or is paradise one day closer ?

 

[stressebookllc]

Did some simple testing and found that CROD/CVISC (axial spring option in the CBUSH dialog box in FEMAP)is the only one that is consistent in terms of +ve tension /-ve compression.
However it is important to keep in mind it provides stiffness in all three directions with one axial stiffness value.

You can see in the table below that the element connectivity and load type (force, displacement) and direction all influence the signs in the DOF Spring and CBUSH.

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[rb1957]

so your Fx loads are always compression and the deflection loads are always tension ?

another day in paradise, or is paradise one day closer ?

 

[stressebookllc]

Right. We only need to test one load type, the other direction for force keeping everything else the same reverses the sign of the force in the element.

Same applies to "net" displacement load.

Aerospace Stress Analysis and FEA Courses

http://www.stressebook.com

Stressing Stresslessly!