KB 001400 | Knowledge Base | Comparison of Different Soil Models Using RFEM

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Knowledge Base | Comparison of Different Soil Models Using RFEM

Annotation:
A foundation is usually created in RFEM using the subgrade reaction modulus method. The reason for this is the relatively easy and straightforward manageability. Also, no iterative calculations are necessary and the computing time is relatively short. The subgrade reaction means that, for example, a foundation plate is loaded flat elastically.

Description:
► Image | Springs for Surface Foundation https://www.dlubal.com/en/img/013000

This support is represented by vertical springs, which are applied with constant spring stiffness and independent of each other. Therefore, it is not possible to calculate any subsidence basin close to reality. This type of foundation is also referred to as Winkler bedding. To be able to apply this method, the bedding modulus k-s (C1z in the program) is required, which is calculated on the basis of the soil pressure σ-0 and the corresponding settlement s.

Disadvantage of the subgrade reaction modulus method is, among others, that the soil modeling is insufficient and the adjacent ground areas cannot be considered. Since the soil load causes the deformation directly only under the load itself, the subsidence basin does not reflect the reality. Shear stiffness of the soil is also not taken into account.

Subgrade Reaction Modulus Method with Variable Bedding Modulus

The deficiencies of the conventional subgrade reaction modulus method can be diminished by defining a variable bedding modulus. Dörken & Dehne [2] recommend a bedding modulus directed on the edge of a narrow strip rising up to twice the value. This should simulate the soil effects outside the foundation edge. The resulting settlements are significantly improved by this method.

► Image | Distribution of Bedding Modulus https://www.dlubal.com/en/img/013001

The variable bedding course can be entered in RFEM using a stepped edge area. However, some advantages of the conventional subgrade reaction modulus method, such as clear overview and fast program input, are lost in the case of this modeling.

► Image | Distribution of Bedding Modulus in RFEM https://www.dlubal.com/en/img/013002

Consideration of Adjacent Ground Areas Using Additional Springs

This model is based on the "Effective Soil Model" method by Kolář & Němec [3]. In contrast to the variable bedding modulus method, shear resistance is also considered in addition to the bedding modulus. The adjacent ground areas are taken into account using line springs and single springs on the edges.

► Image | Applying Surface Springs, Line Springs, and Single Springs https://www.dlubal.com/en/img/013003

The springs applied in our example result from the vertical bedding parameter of 54,500 kN/m³ as follows:

s-0 represents the range of subsidence basin in which the settlements drop under 1% of the foundation edge values.

c-v,xz and c-v,yz are the shear springs for the surface elastic foundation.

0.1 ∙ c-1

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