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# Define Equation-Based Loads in Femap

30 Sep, 2012

### Femap Tips and Tricks: Apply hydrostatic loading to a water tank model.

Editor's Note: This tutorial courtesy of Siemens PLM.

To demonstrate the creation of equation-based loading, we’ll use a water tank quarter model. Prior to load application, a local coordinate system positioned at the expected water level has already been defined, and the wall surfaces have been split at the corresponding level to allow a group of wetted surfaces to be created.

The hydrostatic pressure (P) is a function of the fluid density (p), acceleration due to gravity (g), and fluid depth (h).

For water, and using the mm/tons/s consistent set of units in this model, this simplifies to:

Note that “!z” is the Femap variable that represents the depth of water in the equation.

Create a new load definition by opening the Model section in the Model Info tree and right-clicking Loads. Select New and enter a title in the New Load Set dialog, then click OK. Expand the Loads section of the Model Info tree and right click Load Definitions and select On Surface. You should now select the surfaces that represent the wetted area of the model in the Entity Selection dialog.

In the Create Loads on Surfaces dialog, select Pressure as the load type. In the Load section of the form, enter 1. as the Pressure. In the Coord Sys box, select the local coordinate system that is positioned at the water level (this must have been defined previously). In the Method section of the form, select Variable, then click the Advanced button.

In the Advanced Load Methods dialog box, select Equation and enter the equation 9.79e-6*!z in the Equation field. Click OK, then OK again.

The surface load markers are now visible, but to see the actual element loads that have been created, select Model > Load > Expand in the menu. In the Expand Geometry Loads dialog, click the Convert to Node/Elem checkbox and click OK, then Yes in the confirmation dialog. The elemental loads are then displayed.

Watch the video of this Femap tip on YouTube.

# About the Author: Alastair Robertson

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