Dynamic Analysis Using max Step and Substep

Question

I am running a earthquake analysis. Earthquakes time is 35 sec. I consider the differences for using max step=3500, sub step=1 and using max step=1, substep=3500. What is the difference between this 2 situation. What is the effective numbers of step and sub step for analysis? Earthquake records has accelaration value for each 0.01 sec.

Dennis Waterman · Answer

Dear Vu Ngo, 
 The proposed times steps and substeps by PLAXIS are based on a couple of criteria: 
 - The time step should not be larger than the time step used in the input signal (using signal from file) - The time step should divide the smallest sine wave in at least 10 steps (using harmonic signal) - The time step should be such the wave does not pass more than half a soil element per time step If one of the fist 2 are critical there is not so much you can do because you really don't want to take time steps so large that some of the data of your signal is not being used. If the 3rd is critical then it may be worth investigating. Small elements and high stiffness (hence high wave velocity) make the minimal recommended time step very small and thus the amount of substeps very high. However, if the critical element (either in size or in stiffness) is in a part of the project area that is not so important, you could decide to use less substeps and accept the inaccuracy in those parts. Or you could generate a bit larger elements there. For instance, if you're doing a seismic analysis and at the bottom of your model you include some of the bed rock in your model &aacute;nd generate small elements there then the very stiff and small bedrock elements really blow up the amount of substeps. You could solve this by either generate somewhat larger elements in the bedrock, or maybe not include the bedrock at all and just use the top of the bedrock layer as bottom of your model thus allowing larger time steps. 
 With kind regards, 
 Dennis Waterman

Dennis Waterman · Answer

Dear Tolgahan, 
 There is no difference, a step and a substep are exactly the same in terms of calculation. The only difference is that for a step PLAXIS can save the results and for a substep PLAXIS cannot save the results. So if you use max step = 3500, substep = 1 then you can potentially save 3500 steps to file and also make a curve of 3500 steps. If you use max step = 1, substep = 3500 then you will only have 1 saved result (the last step) and your curves will also only have 1 point. But the final result of both options is the same. With kind regards, Dennis Waterman

Dynamic Analysis Using max Step and Substep

Top Replies