4.19 Member Elastic Foundations

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.

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.

You have to specify the parameters of translational springs in direction of the local member axes x, y, and z. The spring stiffnesses are considered design values.

The stiffness moduli E_S of Table 4.8 serve as reference values. Please note that the input in RFEM refers to the subgrade reaction modulus!

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

Soil Type	ES static loading	ES dynamic loading
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

Table 4.8 shows the stiffness modulus E_S of different soils. In the case of surface foundations, the subgrade reaction modulus k_s must be used for the spring properties. As a first approximation, the subgrade reaction modulus can be calculated from the stiffness modulus while taking into account the form factor f and the foundation width b according to the following equation :

$k_s\approx \frac{E_S}{f·b}$

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 C_1,z: For strip foundations (members in horizontal position), the local z-axis is usually directed downwards.

Use the Display navigator or the shortcut menu of the member to display the local member axes (see Figure 4.169).

Shear springs can be used to determine the shear capacity of the soil. The spring constants C₂ are determined with the product ν⋅C_1,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.

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.

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

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 4.169). 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).