Once the material properties have been defined, the next step in the numerical analysis is to generate a finite element model – i.e. nodes and elements. Cold-form steel beam or column sections have different sectional shapes according to their purpose and position in the frame of a structure. To allow creation of these sections as 2D or 3D finite elements, the ANSYS design modeller has worthy flexible tools required for building any composite model.
When models are created in ANSYS, two important points should be taken into account. First, the model must simulate the actual shape of the sections using full capabilities of the software design modeller. Second, the model must have constricted of element types which are suitable for the analysis in question.
In ANSYS, there are two methods to create the finite element model, direct generation and solid modelling.
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5.3.1 Direct generation
In this method, all nodes and elements are input manually. This can be done by defining the location of each node and the connectivity of each element. Several convenient operations, such as copying patterns of existing nodes and elements, symmetry reflection, etc. are available. Figure (5-3) demonstrates how to create a simple model using direct generation method.
Creating the nodes Copying the nodes
Creating the element Copying the elements through cross-section
Copying the elements through the length Figure (5-3) Modelling with the direct generation method
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The advantages of direct generation are: - Expedient for small or simple models.
- Provides the user with complete control over the geometry and numbering of every node and every element.
The disadvantages of direct generation are:
- Usually too time-consuming for all but the simplest models; the volume of data the user must work with can become overwhelming.
- Cannot be used with adaptive meshing.
- Can be difficult to modify the mesh (tools such as area mesh refinement, Smart Sizing, etc. cannot be used).
- It is generally more problematical for modelling rounded corners and fillets.
5.3.2 Solid modelling
For the solid modelling method, the input of shape geometry is produced by creating keypoints, lines, areas and volumes. Subsequently operations like copy, add, subtract, intersect, glue, divide, overlap, extrude, etc. can be employed to produce the final desired geometry to simulate the actual shape. The elements chosen to represent the model will dictate the geometry to be created.
The solid modelling method has the following advantages:
- More appropriate for large or complex models, especially 3-D models of solid volumes.
- Allows to work with a relatively small number of data items.
- Allows geometric operations (such as dragging and rotations) that cannot be done with nodes and elements.
- Supports the use of "primitive" areas and volumes (such as polygonal areas and cylindrical volumes) and Boolean operations (intersections, subtractions, etc.) for "top down" construction of your model.
- Supports adaptive meshing. In order to do area, mesh refinement after loads have been applied (solid model loads are also required).
- Readily allows modifications to geometry.
- Facilitates changes to element distribution; you are not bound to one analysis model. - Improved modelling of rounded corners and fillets.
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The disadvantages of solid modelling are:
- May require a large amounts of time in both modelling and solution.
- Can (for small, simple models) sometimes be more cumbersome, requiring more data entries than direct generation.
- Can sometimes fail (the program will not be able to generate the finite element mesh) under certain circumstances.
Figure (5-4) shows how to create complex model using the solid modelling method.
5.3.3 Meshing tools
Meshing is an integral part of the computeraided engineering simulation process. The mesh influences the accuracy, convergence and swiftness of the solution. Furthermore, the time it takes to create and mesh a model is often a weighty portion of the time it takes to obtain results from the solution. Therefore, the better programmed the meshing tools, the faster the solution,
The default mesh controls that ANSYS uses may produce a mesh that is satisfactory for the analysis model. In this case, it is not necessary to specify any mesh controls. Mesh controls allow the user to establish such factors such as the element shape, midsize node placement, and element size to be used in meshing the solid model. This step is one of the most important of the entire analysis, all decisions made at this stage in the modelling process can affect the accuracy and economy of the analysis.
Once the best model is found, meshing technologies from ANSYS provide the flexibility to produce meshes that range in complexity from pure hexagon to highly detailed hybrid; a user can put the right mesh in the right place and ensure that a simulation will accurately validate the physical model. The mesh is usually refined in areas of high stress gradient, or possibly to aid output of results in particular locations. Mesh sensitivity studies are necessary to ensure convergence of results and production of a satisfactory model
For cold-form steel, shell modelling and meshing solutions from ANSYS offer several approaches in providing meshes that best meet the physics. In general, this consists of two approaches that use common tools:
1- 2-D axisymmetric or planar models can be used to simplify 3-D physics in a 2-D fashion. 2-D models can mesh with quad meshes, quaddominant meshes or all- triangle meshes.
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Creating the shapes Use ANSYS operations to get the
specific shape
Keep use ANSYS operations to get the specific shape
Use ANSYS operations to get the specific shape
Extrude the final cross section to create the final shape length
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2- Shell models can be used to simplify 3-D models to a set of laminae with a defined thickness exhibiting bending and membrane action. This is particularly useful for modelling sheet metal or thin structural parts. Shell parts can also mesh with quad meshes, quad-dominant meshes or all-triangle meshes. Rectangular element shapes are preferable to triangular element shapes as the element formulation for triangular element become less accurate. Also the aspect ratio of the element can affect the accuracy of the result obtained. Figure (5-5) shows the types of meshing that can be achieved.
Meshing quad with mapped Meshing quad with free
Meshing triangle with mapped Meshing triangle with free
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5.3.3.1 Merging of nodes
If two separate entities have the same location, then merging these entities together into a single entity is possible. For example,if two regions that have already been meshed are to be joined it may be desirable to have all the nodes move together in all degrees of freedom. ANSYS has a facility to merge and renumber the nodes according to a set tolerance, the higher numbered node will be deleted and will be replaced with the lower numbered coincident node. Two merged nodes will thus be replaced by a single node. Figure (5-6) demonstrates how to merge nodes.
a) Before merge
Figure (5-6) Merging in ANSYS FE model.
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5.3.3.2 Coupling of nodes
Coupling is a way to ensure a set of nodes have the same DOF’s, i.e. producing compatibility between nodes. For example: If nodes 1 and 2 are coupled in the UX direction, the solver will calculate UX for node 1 and simply assign the same UX value to node 2. The solver will calculate the copulated displacements based upon the coupled stiffness of the coupled nodes, figure (5-7).
If pin joints are to be created, coupling can be used to simulate pin joints such as hinges and universal joints. This is done by means of a moment release: coupling translational DOF at a joint and leaving the rotational DOF uncoupled will produce this effect.
Before coupling
After coupling the displacement in X-direction
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