Soil Spring Generator
The Soil Spring Generator is a fast and easy way to provide soil-support under a plate mesh. The generator will create compression-only springs with stiffnesses based on plate area and soil subgrade modulus. This is the typical approach to modeling support from a soil under a slab, footing, or grade beam.
To use this tool, select the plate elements to be supported and then choose thecommand.
- Speeds model creation when soil-bearing supports are modeled.
- Automatic stiffness calculation based on plate surface area.
- Spring supports are automatically applied perpendicular to selected plates.
- One-way action for springs is automatically set.
- Only generates spring supports under plate elements: not for members or even meshed-areas.
- Spring supports, once generated, are independent and do not update automatically if plate dimensions change.
- There is no way to modify the soil subgrade modulus to modify the springs, you must delete them and regenerate if soil properties change.
- Selecting Plates
- Defining Soil Stiffness
- Determine Support Direction
- Spring Superposition
- Completing the Generation
After choosing thefrom the menu, you are asked to select plate elements to be supported. Normally you will pre-select the plate elements to support before choosing this tool, but if you forget, the wizard allows you to select the plates before proceeding. Whether you select them before or after starting the tool, you may then press Next.
Note that you may select any or all of the plate elements in your model. The elements need not lie in the same plane. When spring supports are created they will be perpendicular to the plates selected. If plates connected to a common node lie in the same plane, the spring stiffness for the adjacent plates will be combined into a single spring support.
You define the subgrade modulus using a value representative of the soil. Many soil mechanics texts list typical subgrade modulus values. The spring stiffness generated is calculated using the following formula:
KSpring = Subgrade Modulus * Plate Surface Area.
The total spring stiffness, K, is distributed to the three or four plate element nodes based on a tributary area method using the plate's geometry.
The radio button for specifying the plate contact face (in local plate coordinates) is used so that the spring direction can be set up consistently and correctly with a compression-only one-way action. For more information on the plate's local coordinate system, see the VisualAnalysis User's Guide or on-line help. You may display plate local coordinates graphically in a Model Window.
Consistently oriented local z-axes are required for this tool to do a good job of spring generation.
If you already have springs attached to nodes of selected plates, two additional radio buttons are available. These allow you to either add the spring stiffness being calculated by this operation to the existing stiffness, or to replace the existing stiffness with what is being generated in this operation. Addition to existing stiffness may be useful if you have added plates to a mesh that have already had springs generated.
Before you press Finish to complete the generation, please take note of the summary near the bottom of the page. This indicates the number of plates selected and the total area of plates selected. If this information appears incorrect, you may press the Back button and return to the selection page and change the group of plates selected. Once you press Finish, springs will be generated and you should see a model similar to the one below. You may use the command to reverse the generation of springs if you make an error.
Note that your model will now require an iterative analysis due to the compression-only nature of the springs generated.
You must always have a net downward reaction against these soil springs otherwise there is a possibility they might all be removed allowing the plates to "fly off into space." The practical ramification of this instability is that analysis of certain live load cases may not be done independently. In this case, include dead loads with the live loads to ensure this overturning or uplift does not occur.
Once generated, the spring supports are independent. If you later decide to change soil properties or plate areas, you should delete these springs and regenerate them.