# 4.19 Member Elastic Foundations

General description

While nodal supports provide a support on both member ends, member elastic foundations provide an elastic support of the member along its entire length. You can use elastic member foundations to model foundation beams while considering soil properties, for example. If the elastic foundation is not effective in case of tensile or compressive stresses, it is possible to consider nonlinear effects in the calculation.

On Members No.

Member elastic foundations can only be defined for the member type Beam. Enter the number of the member into the table column or text box, or select it graphically.

Spring Constants
Translational spring constant

You have to specify the parameters of translational springs in direction of the local member axes x, y, and z.

The stiffness moduli ES of Table 4.8 serve as reference values. Please note that input in RFEM refers to the bedding modulus, which must be determined while taking the form factor into account.

Table 4.8 Stiffness moduli of selected soil types in [N/mm2]

Sand, compact

40 - 100

200 - 500

Gravel sand, compact

80 - 150

300 - 800

Clay, semi-solid to solid

8 - 30

120 - 250

Clay, stiff-plastic

5 - 20

70 - 150

Mixed soil, semi-solid to solid

20 - 100

200 - 600

The values of Table 4.8 represent area-related characteristic values: They describe the area force in [N/mm2] that is required to compress the soil by 1 mm. Thus, the unit would be interpreted in a solid-related way as [N/mm3].

For foundation beams used to model strip foundations, for example, you have to determine the spring coefficient while taking the cross-section width into account. Thus you obtain a translational spring related to the member in [N/mm2]. The spring indicates the member force in [N/mm] required to compress the soil by 1 mm, hence the unit [N/mm2] for the input. The result must be entered as the translational spring C1,z: For strip foundations (members in horizontal position), the local z-axis is usually directed downwards.

The spring stiffnesses are considered design values.

Use the Display navigator or the shortcut menu of the member to display the local member axes (see Figure 4.165).

Shear spring constant

Shear springs can be used to determine the shear capacity of the soil. The spring constants C2 are determined with the product of ν⋅C1,z, with Poisson’s ratio ν being between 0.125 and 0.5 for sand and gravelly soil, and between 0.2 and 0.4 for clay soil.

Rotational spring constant

Enter the constant of a rotational spring into the text box or table column. The constant hinders the member's rotation about its longitudinal axis.

Foundation ineffective

If the elastic foundation is not effective in case of tensile or compressive stresses, assign the nonlinear property Failure to the foundation type.

Please note that the failure criterion Failure if negative or positive only refers to the local member axis z. The nonlinearity does not apply to the translational springs in direction of the local axes x or y! Thus, a biaxially effective failure of foundation members is not possible.

Failure in case of a negative contact stress has the following meaning: The foundation is without effect if a member element moves in the opposite direction of the local axis z.

If failure criteria are applied, it is recommended to check position and orientation of the local z-axes (see Figure 4.165). It might be necessary to rotate members.

The division of members with elastic foundations can be adjusted in the Global Calculation Parameters dialog tab of the Calculation Parameters dialog box (see Chapter 7.3).