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# How should I input the soil spring coefficient for an underground structure?

## Question

How should I input the soil spring coefficient for an underground structure?

'Surface Spring Supports' function in midas Civil automatically calculates the covered area per each node and inputs the ground boundary conditions. Therefore, it is possible to automatically input numerous ground boundary conditions at once.

Underground structures are placed on top of the ground, and the ground can either support the structure or cause settlement of the structure depending on the amount and the distribution of the load acting on it. Therefore, the boundary conditions of the underground structure must be implemented as spring-type boundary conditions with stiffness, rather than fixed supports.

In midas Civil, there are Point Spring Supports and Elastic Links as spring-type boundary conditions where stiffness can be inputted for each degree of freedom (d.o.f.). Point Spring Supports are boundary conditions that are directly applied at the node, and Elastic Links are boundary conditions that are applied between two nodes. Therefore, when assigning spring coefficients for each direction with Point Spring Supports and Elastic Link boundary conditions, the covered area per each node has to be considered.

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

The left side of the figure shows the 3-D shape of the reservoir, and the right side shows a view of the bottom surface of the reservoir.

If ground boundary conditions are to be inputted on the bottom surface of the reservoir, the spring coefficient that corresponds to the area of 1m² should be inputted for node 17, and the spring coefficient values that correspond to the areas of 0.5m² and 0.25m² should be inputted for nodes 20 and 5, respectively.

If ground boundary conditions are to be inputted on only a few nodes, they can be inputted by considering the covered area per node. However, if there are many nodes where ground boundary conditions need to be inputted, it can be very difficult.

'Surface Spring Supports' in midas Civil is a function that automatically calculates the covered area per each node and inputs the ground boundary conditions accordingly, allowing for the automatic input of numerous ground boundary conditions all at once.

The 'Surface Spring Supports' function can be used for both beam element models and plate / solid element models. In plate / solid element models, the covered area for the selected nodes is automatically calculated, while in beam element models, the covered area is automatically calculated as 'Selected node's covered length X Input Width'.

Therefore, when modeling ground boundary conditions using Surface Spring Supports, the only input required from the user are the Coefficient of subgrade reaction and the Width of the line element.

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