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# How do you input the boundary conditions for underground structures?

## Question

How do you input the boundary conditions for underground structures?

Underground structures are put on the ground, and the soil underneath can both support and be sinking under the loads of the structure.

When inputting boundary conditions in CIVIL, which take into account the stiffness of the soil, the most commonly used functions are 'Point Spring Supports' and 'Elastic Link'.

First, the Elastic Link is similar to a beam element. While a beam element calculates axial stiffness (EA), flexural stiffness (EI), etc. using material properties and cross-section, the Elastic Link directly inputs the calculated stiffness. By using elements with directly inputted stiffness to represent the ground, fixed ends are placed underneath. Elastic Links are mainly used in underground structure due to the advantage of being able to consider characteristics such as "compression-only" behavior, which is similar to the behavior of soil.

Point Spring Support is similar to the combination of Elastic Links and fixed points. The stiffness is inputted in six directions, the same as the Elastic Link.

In addition, there is a function called 'Surface Spring Supports'.

The Surface Spring Supports function is a tool that automatically calculates length or area to conveniently input Point Spring Supports or Elastic Link. (It is not a type of boundary condition used to represent an elastic boundary.)

The left image shows the three-dimensional shape of a water tank, while the right image shows a plan view of the tank's bottom surface. When entering ground springs on the bottom surface of the tank, point 17 should be given a ground spring value that corresponds to an area of 1.0 square meters, while points 20 and 5 should be given ground spring values corresponding to areas of 0.5 and 0.25 square meters, respectively. In the above example, only three values for three different areas of 1.0, 0.5, and 0.25 square meters need to be calculated. However, in actual projects, the model is much more complex, making it a tedious task to calculate the covering area for each point.

Soil spring coefficient = Covered area per node X Coefficient of subgrade reaction per unit area

(Automatically calculated)                (Input)

Since Surface Spring Supports automatically calculates the covering area for each point, it is easy to input numerous soil springs by simply inputting the coefficient of subgrade reaction per unit area.

This function can be used in both 2D and 3D models.

When modeling with Plates or Solids, the covering area is calculated in the same way as described above. In 2D models where only the representative cross-section is expressed using Frame elements, the ground spring values are calculated as [Width X covering length per point X Coefficient of subgrade reaction per unit area] by inputting the Width.

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