Natural frequency, Mesh element size, Rayleigh damping, vertical force on retaining wall model

Question

Q1: Is there any way to determine natural frequency of Retaining wall soil numerical model from PLAXIS? 
 
 Q2: How can we see the exact element mesh size in PLAXIS? 
 
 Q3: I am using Mohr-Coulomb soil model and want to introduce constant Rayleigh damping of 3%. 
 Natural frequency of soil is 16.53 Hz, frequency of input motion is 5.0 Hz 
 I used &xi; = 3 % for both Target 1 and Target 2 
 f = 16.53 Hz for Target 1 and f = 5/16.53 = 1.0 (closest odd multiple integer) Hz Target 2 
 w, &alpha;, &beta; are calculated automatically but when I close Material window and edit material, all Rayleigh damping parameters &xi; , f, w, &alpha;, &beta; are reset to different values. How can I solve this issue?

Q4: How can we plot total vertical wall force on the wall vs time graph in PLAXIS? (Rigid retaining wall) See Pdf file attached 
 
 PDF

Stefanos Papavasileiou · Answer

Dear Muhammad, 
 
 By checking our tutorial example and verification cases, you can find quite a few examples explaining this. In principle, n atural frequencies can be estimated by means of a power spectral density plot generated through output. Here are some useful links: https://communities.bentley.com/products/geotech-analysis/w/plaxis-soilvision-wiki/45568/plaxis-2d-tutorial-15-free-vibration-and-earthquake-analysis-of-a-building https://communities.bentley.com/products/geotech-analysis/w/plaxis-soilvision-wiki/45510/simply-supported-beam https://communities.bentley.com/products/geotech-analysis/w/plaxis-soilvision-wiki/45532/ground-response-analysis-in-case-of-linear-soil https://communities.bentley.com/products/geotech-analysis/w/plaxis-soilvision-wiki/45533/free-vibration-analysis-of-a-dam 
 The element size can be found in PLAXIS Output program by inspecting the mesh (e.g. connectivity plot). Note that the size of the generated elements will be according to the geometry and local refinements defined. The General information window, under Project menu, will give you some more details. 
 The damping parameters ( &alpha; and &beta; ) can be automatically calculated by the program when the target damping ratio ( &xi; ) and the target frequencies ( f ) are specified in the panel displayed in the General tabsheet when one of the cells corresponding to the damping parameters is clicked. Note that the exact values of the &alpha; and &beta; are stored directly in the material window while, the values for &xi;, f and &omega; are only used to derive those (so not stored). When you reopen the material data set the shown values for &xi;, f and &omega; as just another set of two points that computes the same &alpha; and &beta; values. As long as the Rayleigh &alpha; and &beta; are correct, there is nothing to worry. 
 If by rigid you mean that the wall is modelled by means of a volume element, then the results for "soil" (or better, concrete) do not include any forces as results. If by total vertical force you mean the axial force of the wall, you can retrieve that using the feature Structural forces in volume plates. For more information on this PLAXIS Output feature, please check our reference manual. Practically, with this tool, the structural forces are calculated by integrating the results in the stress points along the region perpendicular to the cross-section line.