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  4. TechTip: Como modelar cargas excêntricas com elos rígidos

TechTip: Como modelar cargas excêntricas com elos rígidos

Applying eccentric loads (off-center loads) to members

What is an eccentric load?

An eccentric load is any load that is not applied to the centroid of the member. In finite element analysis, cargas pontuais, cargas distribuídas, and other forces are generally applied to the centroid of the section (ou seja. the geometric center of the section). Contudo, in certain scenarios you may wish to apply a force off center – por exemplo, applying a load to the flange of an I-beam, rather than the web. Assuming this is a lateral load, this will create a torsional force and an overall different loading behavior in the member.

How to apply eccentric loads to a model

Here we’ll take you through an example of how you can use Links Rígidosto apply such an eccentric load. By using Rigid Links, we can transfer the load from the centroid of the member to a different location. Links Rígidos are extremely useful for this sort of work, as it automatically calculates the resultant moment and other forces from one location to another.

Head over to S3D and create a new model. Próximo, select the pen tool in the right panel or by tapping V. Click once to place the first node at the origin, then draw upward about 2m (6.5ft) to create a column. 

Down in the corner by the coordinate system, clique será usado: será usado, then in the left panel, clique  Construtor to open section builder. Choose a column section, for this tech-note I have selected an Australian 150UC23.4, this is about a 6×6 I section for our imperial friends. In the left panel, click supports and add fixed support at Node 1.

Now we will add another member so we can create an eccentric load that induces torsion and another that creates a buckling effect on our column.

To see the orientation of your column, click the visibility icon on the right panel and select 3Membros D. Use the pen tool to draw a 300mm (1ft) member in the X direction from the top of the column. A partir da elevação do solo gerada a partir das elevações do Google “3D memberis switched on, you will see this member uses the column section. Click the new member, then in the left panel, change the type attribute to Rígido e clique Aplicar.

Your model should look similar to the below:

Click Point Loads in the left panel. Fill out the attributes with the following values:

  1. Node ID: 3
  2. Da Magnitude: -5 (If your model uses the Y-axis as vertical, add this value to Y Magnitude instead).
  3. Grupo de carga: Buckling Load

Click apply and the point load will be added to the model. The Point Load ID will increment to 2 (Novo) so add another load to create torsion in the column:

  1. Node ID: 3
  2. E Magnitude: 2 (If your model uses the Y-axis as vertical, add this value to Z Magnitude instead).
  3. Grupo de carga: Torsion Load

Nó 3 of your model should now look something like the following.

Run the solver, then open render modethe cube icon in the right panel. Change the drop-down in the left panel to Buckling load, the deflection scale to Auto, and the Member Results dropdown to Displacement Sum. You can now see how the first eccentric load is causing a buckling effect. 

eccentric-buckling

Note that if you would like to see a finer subdivision of the member you can go to Configurações hip-delete-não usado Os métodos de combinação de carga disponíveis incluem, change the Pontos de Avaliação por Membro to a suitable number and run solve again.

Now change the dropdown from Buckling Load para Torsion Load e a Resultados de membros para Torsion Stress. Nesse caso, the member will appear grey as the torsion load is consistent throughout the member.

eccentric-torsion

Notice that the contour scale in the top left still shows the magnitude of torsion that the column is experiencing.

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