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VisualAnalysis Tutorials: Elastic Foundation

Project Description:

This example shows how you might model an elastic foundation or a "grade beam" Foundations can be modeled using spring supports to represent a soil's subgrade coefficient. Springs may be added to VA models that provide resistance in compression, tension, or both. This tutorial will explore the differences of modeling a foundation as springs providing "compression only" support and a conventional "two-way spring".

Setting Up the Project

Start a new model using File | New.You should have a Space Frame (Modify Tab), set the unit style to Kips & Feet (Edit | Project Units), and name the project "Elastic Foundation".

Modeling the Structure

Draw an 11 ft beam and specify it as a standard parametric rectangle shape 12 inches wide and 18 inches deep. Choose a material of 4 ksi concrete. Use the Model | Split Member command to split the beam into 11 equal sections. The more sections you use, the more accurate the results of the model will be. The trade off is a more complex model, more results, and potentially slower analysis time.

3D Stability

Now we will set the boundary conditions of the model. Fix the left most node of the model for both "X Displacement" and "X Rotation". Fix all the nodes for "Z Displacement", as we are not loading in the Z direction and need global stability. These supports ensure the model will be stable in 3D space.

For the subgrade conditions select all the nodes, right-click, and choose Create Spring Support. Under the Properties tab, select the radio button for Displacement spring and enter 150 K/ft for stiffness. Under the Location tab, specify orientation in the -Y direction. Under the Options tab, select Linear & elastic. Click Ok when finished.


Picture View of model

Loading the Structure


Apply a 25 K/ft dead load to the first five feet of the member, starting from the left. Then use the Model | Generate Copies command to create a copy of the entire model and loads 5 ft above the original. Change the springs on the upper model to compression-only. (Project Manager, Modify, Action)

Top model has compression-only spring supports, bottom model has normal two-way.

Analysis and Results

Now that we have our two models created and loaded, we should check them for errors before setting the analysis options and actually analyzing the structure. Select Analyze | Check Model for Errors to make sure there are no problems. Now select Analyze | Analyze Now.


Top model is compression-only springs; the bottom model is two-way springs.

The figure above shows y-displacement, along with spring reactions. The conventional two-way spring model would have to be altered by deleting the springs in tension and re-analyzing (Because soil has no tensile capacity). This process would then be repeated until you only have results with springs in compression. VisualAnalysis automates the iterative process with the compression-only spring models.

This marks the end of this tutorial. You now can model a beam on an elastic foundation. Congratulations!