Non Linear Analysis

[Stazz]

Does Risa have the capability to do a nonlinear analysis using time steps? I'm curious because I'm analyzing a building for progressive collapse and there are some nonlinear analysis options in the UFC that I wanted to experiment with.

If not then is there a way to do this procedure manually? My best guess would be to do this...

1) At all suspected plastic hinge locations, place a node
2) Run the analysis for partial load step = Load/10
3) Check to see if plastic hinge develops at nodes by checking if the moment at the nodes exceeds Mp = FyZ
4) If Mp>FyZ then turn that node into a pin and add equal and opposite moments at the node = Fy*Z
5) Add Load/10 to load
6) Repeat until fully loaded

8) Then if you can make it all the way to the final load without the model becoming unstable and with the stresses on par then you win.



Is there a way to add a dynamic element to this too? If not, then what program would be good for this?

 

[JoshPlumSE]

RISA does not have the ability to do this type of analysis.

For the most part, the method you describe is a reasonable (albeit very time consuming) way to obtain non-linear analysis results when using a linear program.

Two things:
Minor change with Step 3):
I would probably not set my load to exactly 10%. Instead, I would set it to whatever % of load gets me to my next hinge. That might be 8%, or 12% or whatever. That way, my model behaves perfectly linearly between each hinge formation. Makes sense.

Possible major change with steps 4 and 5:
I'm not sure that you will get the same analysis results at the end of step 3 and 4. If you can, then great. If not, then you would have to replace step 4 and 5 with the following:

4) Copy the results from the lastest load increment into an Excel file and combine it with the summation of the results from the previous increments.

5) You wouldn't generally be increasing the load with this step. Instead, you'd be analyzing the new structure (the one with the hinge) for the NEXT 10% of the load. To get your final result you would use the principal of superposition to combine the results from increment 1 with increments 2, 3, 4, et cetera.