Plaxis: Site response facility

Question

I have a query about Plaxis 2D (v.21 Ultimate) Site Response function that can be used for 1D Site Response Analysis (SRA). When we select &ldquo;Site Response&rdquo; from the &ldquo;Modify soil layers&rdquo; menu, Plaxis automatically converts the main geometry of the model into 1D geometry and the meshing is also done with 6-noded elements. 
 I would like to know, whether such meshing is OK for the Site Response Analysis, OR I have to modify the mesh size as suggested in Plaxis 2D (2015) Ground Response Analysis report, i.e., the suggestion of Kuhlmeyer & Lysmer (1973). 
 Secondly, I would like to know whether the following conditions are OK while selecting dynamic loading. The Static Line displacement (U_x,start,ref) in the Dynamic module should be, 1) U_x,start,ref = 0.5 m ; In case of Rock outcrop motion ? 2) U_x,start,ref = 1.0 m ; In case of Bedrock (underlying of soil layer) motion ?

Sean Johnson · Accepted Answer

Generally, whenever you generate a 1D column model using the SRA script the Lysmer criteria will be met unless you have materials that fit well outside the bounds of material stiffness. The reason being is that the Lysmer criteria is based on nodal spacing and the Finite Elements are already sub-divided among many nodes. Therefore, in nearly all practical applications you should be able to run the SRA without doing a element size calculation. However, I would still recommend doing it regardless as a habit of good engineering practice. (As a reminder:) 
 The mesh elements should be small relative to wavelength (Kuhlemeyer and Lysmer, 1973) and the shear wavelength should be coupled with element size: 
 Element Size &le; &lambda; / 8 = v s, min / (5 &divide; 10) f max 
 
 &lambda; = wavelength 
 v s, min = minimum shear wave velocity 
 f max = maximum frequency component 
 
 The answer to part 2 depends on if you are using a compliant base as your dynamic y(min) boundary. In that case, the input value is only the upward propagating motion (half the within motion). This is typically in a SRA because you are only trying to capture the upward propagation component of the motion (1/2 of total).

Plaxis: Site response facility

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