plate properties (PLAXIS 2d vs PLAXIS 3D)

Question

Hello, 
 
 I have a question about the plate properties in PLAXIS. In 2D, there is a specific input as "unit weight (w)" and in 3D, there is another specific unit weight as " unit weight (gamma)". I know how to get the unit weight for PLAXIS 3D but for 2D, I came to know the unit weight of the soil has to be deducted from the unit weight of the plate material. I could not find any good reference about why it has to be done like this in PLAXIS 2D and so I seek your help to help me resolve this confusion. Please help. 
 
 Thank you

Vasileios Basas · Answer

Hi Faisal, 
 In PLAXIS 3D, all necessary parameters (dimensions and unit weight) are input parameters: we define a plate material with a specific height (d) and unit weight (&gamma;), and we assign that material to a polygon with known width and length (b x L). 
 In PLAXIS 2D, the specific weight is the force per unit length and per unit width in the out-of-plane direction (expressed in kN/m/m units). It represents the product of the material&rsquo;s unit weight (&gamma;) over the area of the cross-section: w = &gamma; &times; b &times; d (kN/m). In a plane strain analysis, we consider 1m in the out-of-plane direction; therefore, b = 1m, and so w = &gamma; &times; d (kN/m/m). 
 This is also what the manual explains by saying that the specific weight is obtained by multiplying the unit weight of the plate material (&gamma;) by the thickness of the plate (d). Therefore, the first &ldquo;/m&rdquo; corresponds to the length (distributed load), and the second &ldquo;/m&rdquo; is assumed to be the out-of-plane width of 1m. 
 Let us take the following example: suppose you have a steel plate (b x d x L) = (b x 1.5m x 40m). Let us assume that the density of steel is approximately &rho; = 7850 kg/m3 &asymp; 78.5 kN/m3. Therefore, the total weight of the plate is: W = &rho; * V = &rho; * (b * d * L) = 78.5 * 1.5 * 40 = 4710 (kN/m). Here the &ldquo;/m&rdquo; implies that this is the total weight of the plate is for 1m in the out-of-plane direction. If we divide W by its length, we get the unit weight (load per unit length per unit width in the out-of-plane direction), which is essentially the unit weight in PLAXIS 2D: w = W/L = 117.75 (kN/m/m).

Vasileios Basas · Answer

Hi Sasa, Indeed, the weight of the plate element is not the full weight of the structures, but it is the weight of the structures minus out the weight of the soil removed. See also: https://communities.bentley.com/products/geotech-analysis/w/wiki/58294/considerations-for-modelling-a-thick-slab-as-a-plate

Berk Demir · Answer

You just need to think what you are aiming. The link Vasileios is really good. For some cases where the bending moments are more important, I use concrete unit weight x plate thickness for 2D. However, this creates higher pressure in the soil, so settlements are higher. If settlements are important, you go for correct stress in soil, so you need to reduce the soil unit weight. 
 Not an easy task always.