How to calculate the total shear force of an interface element

Question

Hi I do have the attachaed interface element of an slab. I want to calculate the total shear force [kN] by "Area [m2] * Interface shear stress t2 [kN/m2]".

Thanks

Vasileios Basas · Answer

Hi Manuel, A crude way to calculate that is by getting an average shear stress value for each element and multiplying that value by the area of the corresponding element. Then, sum up the products of shear stress and element area for all the elements of interest. This will give you the total shear force acting on the specified region. A more precise way requires numerical integration methods, which go beyond the purpose of the forum.

Vasileios Basas · Answer

Hi Manuel, 
 The area of a surface element is not directly visible in Output. However, there are two ways you can calculate it. 
 The first way is more mathematical and can give you reliable results when you don't have curved surfaces. The idea is to view the Output results in a table format and check the coordinates for the nodes in each element. PLAXIS 3D uses 6-noded elements for interfaces, so you can calculate the area of a triangle using the coordinates of the vertices: A = (1/2) |x 1 (y 2 &minus; y 3 ) + x 2 (y 3 &minus; y 1 ) + x 3 (y 1 &minus; y 2 )|. Please check the local numbers of the vertices and adjust the formula accordingly. 
 The second way requires using a command to retrieve the area of an interface element. For example, the following set of commands will echo the surface area for the elements corresponding to each node on the interface (PositiveInterface_1) 
 getresults PositiveInterface_1 Phases[-1] ResultTypes.Interface.Area "node" 
 echo Results[-1] 
 
 Optionally, you can cross-check those nodes by running the commands: 
 getresults PositiveInterface_1 Phases[-1] ResultTypes.Interface.ElementID "node" 
 echo Results[-1]

You can find more information in the Output objects reference found in the Help menu > Command reference > Output Program > Output objects reference > Interface

Stefanos Papavasileiou · Answer

Hello Manuel, 
 The values of the area come from the element itself. But the ResultTypes object is available per node (or stress point). That is why you will see duplicate values per 6 nodes, as the same area corresponds to a single element. 
 Here's an article in which we explain how to get a table on Notepad based on results from Output: How to get a load - displacement curve using Scripting interface