Hi everyone,
I am doing dynamic analysis design for an equipment foundation to check the natural frequency and vibration and I use solid elements to model the footing. The force I applied is selfweight, Fx, My (Fx and Mz were applied as time force). I calculated the soil spring in Fx, Fy, Fz, My, Mz, Mx direction and created a support (S2) assigned to the bottom nodes. Everything worked well so far. The modes were reasonable and no errors occured.
(S2)
Then I realize that the soil should not provide tensile resistance in vertical direction, which means the spring in Fy should be compression spring only. To achieve this, I added a compression only spring (S3) in Fy direction and applied it to all the nodes that I have assigned to S2 before. However, this time I got the error saying that " Erro-zero on diagonal in jacobi iteration ". I also got a warning saying instability at joint in Fy direction and I didn't get any elgensolution result.
(S3)
It seems like the structure became unstable. The foundation was very heavy and the Mz I applied was small. I think the weight of the foundation should be able to make the structure stable without tension resistance. Can anyone be so kind to explain to me why I have this error and how can I fixed it?
I search on the staadpro help and it says "This method does not always converge and may become unstable. Check the output for instability messages. Do not use results if the last iteration was unstable. You may need to include some support in each global direction that is not tension (or compression) only to be stable on every iteration." However, in reality the soil should provide any tensile resistance. If I only assign some of the nodes as compression, will this impact the accuracy? If I need to do this, how should I pick which nodes should be compression only?
Could anyone give me some clues? I need to complete this model ASAP. Thank you very much for any help.
Please attach that STAAD.Pro model here if possible.
INPUT (2).txt
PLEASE SEE THE STAADPRO INPUT. THANK YOU VERY MUCH
Couple of observations.
Compression/tension springs cannot be used in any kind of dynamic analysis. So you need to take those out if you are doing a time history analysis.
You have a redundant PERFORM ANALYSIS which may be taken out.
In the time history definition, you have used a very small value for STEP parameter. You may simply use the SubDiv as 3 instead and that would set up the STEP appropriately
Thank you very much for your reply, Sye. If the compression only spring can not be used in time history analysis, how should I simulate this " compression only retrain" provided by the soil to my equipment foundation?
I need to know the maximum vibration of my foundation induced by the dynamic force due to equipment operating. If the supports I used to simulate the restrain provided by soil have tension stiffness, then they would prevent my foundation from moving up, which could result in a smaller vibration compared with the actual vibration. How should I simulate this situation? I am really appreciated if you could give me any suggestions regarding this.
Modal analysis is a linear analysis. From my understanding, Staad pro can only do linear type of dynamic analysis. If you want to consider nonlinear effect, you need to use some other more comprehensive finite element program.
As you said, your equipment is very heavy. No tension will be in the spring. Then why you bother to use compression only spring? It will not happen in your case.