Shell Analysis using Structural Mechanics Application
This tutorial explains step by step how to create a model of a simple structure with shell elements. For his purpose, a short beam simply supported in the middle and with two boundary descents is analyzed. Besides, new versions of GiD (GiD 13.0) and Kratos are used.
Once GiD 13.0 is opened, we must load the problemtype Kratos through Data > Problem type > kratos (if kratos doesn't appear, we can download it through Internet retrieve).
When Kratos is successfully loaded a new windows opens. Inside Applications market window we choose Structural and then we choose 3D dimension.
The problem to analyse is a 8.5 m beam (this beam could be, for example, part of a longer girder). The beam has a support of 1.2 m in the middle and two transversal stiffeners. Moreover, web height is 1.5 m and flange width is 0.7 m.
This geometry is composed of nodes, lines and surfaces. For details about how to create this entities we can check section 3.1 of GiD Reference Manual.
We can create groups to cluster entities with the same properties. Then it will be easier assigning data. Through Utilities > Layers and groups or Ctrl-l you can open this window.
For carry out the current analysis 6 groups are created:
- BC_Dirichlet_left_boundary contains the 5 lines of left boundary section
- BC_Dirichlet_right_boundary contains the 5 lines of righ boundary section
- BC_Dirichlet_support contains the 2 surfaces where there is the beam support
- flange contains the 16 surfaces of upper and lower flanges
- stiffener contains the 4 surfaces of stiffeners
- web contains the 3 surfaces of beam web
Set problem data
Showing how to set problem data is the main purpose of this tutorial. We can open this toolbar through Kratos > Kratos data. Problem data includes Model information, Parts, Boundary conditions, Loads, Solution and Results. It is advisable to fill these fields following the order in which they appear.
Default values are enough for a basic analysis.
In this section we define the properties and characteristics of our model. With double click on Parts (or left click on Parts and Apply to entities) we add a new part. Altogether, we have 3 parts:
These are the properties to be filled:
- Element: type of element we use in the mesh. For current model, Thin Triangular Shell works properly and has fast convergence.
- Thickness: it refers to the thickness of plate girder, i.e. Shell thickness.
- Density, Young Modulus & Poisson Ratio: it refers to material
Boundary conditions can be Displacement and/or Rotation. By clicking twice we can impose those to a group entities. In the current model only displacements are imposed.
To impose a boundary conditions: firstly, we have to set the directions with a fixed degree of freedom (with a cross in fornt of Imposed). Secondly, we have to introduce the value of the condition in that direcction. Finally, choose the type of entitie (node, line, surface or volume) and group to which we are imposing the condition.
For current model, group BC_Dirichlet_support has DISPLACEMENT_Z = 0 m. And both BC_Dirichlet_left_boundary and BC_Dirichlet_right_boundary have DISPLACEMENT_X = 0 m and DISPLACEMENT_Z = -0.005 m.
Loads are added in the same way by clicking twice on the kind of load the want to introduce and setting the corresponding values.
In our model, a self weigh load has been applyed to flanges, web and stiffeners.
SOLUTION & RESULTS
Here we set some computation parameters and outputs. Usually, default values are enough for a basic analysis.
We can choose which values (displacement, rotation, reacction, stress, strain...) we want to see during Post Process changing Yes/No on these ones on On element and On node.
When all the problem data has been set we can mesh the model. Direct meshing can be done through Mesh > Generate mesh or Ctrl-g. Then we only have to indicte the size of the elements to be generated.
Typically, as better is the mesh, better are the results. So a lot of implementations can be done using the options of Mesh menu and Utilities > Preferences > Meshing. For more details we can check section 220.127.116.11 and section 3.3 of GiD Reference Manual.
Calculate & Post Process
Before calculate is strongly advisable to save the project. We can run the calculation through Calculate > Calculate or F5. Then we acces directly to Post Process to see the results.
(there is a button at upper-right corner to switch between Pre and Post Process window)
All the Post Process capabilities of GiD 13.0 are detailed at section 4 of GiD Reference Manual.
Boundary Conditions programing in Kratos
During Pre Process some problem data can be set. For analyzing simple structural problems this data is enough. However, for solving more complex cases, programming in Kratos is necessary in order to properly reproduce the reality. Here you can find two examples of how to impose Boundary Conditions programming in Kratos with Python: Boundary Conditions programing in Kratos.