Member Supports

A member support describes the boundary conditions of all FE nodes available along a member. Displacements and rotations on these internal nodes can be prevented or limited by translational and rotational springs. This allows you to model, for example, foundation beams as elastic member supports in the model.

Dialog Box "New Member Support"

Main

The Main tab manages the basic support parameters.

Support Conditions

The support conditions are divided into 'Translational', 'Shear', and 'Rotational' degrees of freedom. To define a support or restraint, tick the check box for the respective axis. The check mark indicates that the degree of freedom is blocked and the member's displacement in or rotation about the corresponding direction is possible.

If no support or restraint is available, clear the respective check box. The constant of the translational or rotational spring is then set to zero. You can adjust the 'Spring constant' anytime in order to model an elastic support of the member. Enter the spring stiffnesses as design values.

Translational

The 'Spring constant' of a foundation beam is determined considering the cross-section width in order to obtain a member-related translational spring in [N/m²] based on the subgrade reaction modulus. It describes the member force in [N] per meter of member length and width that is required to compress the soil by one meter. Multiply the value by the actual cross-section width and enter the result as the spring constant C_u,z (the local z-axis is usually directed downwards for strip footings with members in a horizontal position).

When a member is horizontal or inclined, the support symbols are always displayed under the member (that is, on the side in the direction of gravity). Bei einem vertikalen Stab ist die Ausrichtung der Stabachse z maßgebend. Ein Ausfall hingegen wirkt unabhängig von der grafischen Darstellung: Hier entscheidet das Spannungskriterium, welches auf die Orientierung der lokalen z-Achse bezogen ist.

It is possible to specify a 'Nonlinearity' for the u_z direction (see the image New Member Support). The criterion 'Failure if negative or positive contact stress in z' controls the forces that the support is able to absorb.

Check the orientation of the local z-axes when applying failure criteria. You can display and hide the member axes using the member shortcut menu.

If the support fails in the direction of the z-axis, also the structural supports in the other directions are not effective.

Shear

Shear springs can be used to determine the shear capacity of the soil. The shear stiffness s_x describes the axial stiffness due to the strain stiffness E ⋅ A. The spring constants s_y and s_z are determined as the product of the Poisson's ratio of the soil and C_u,z.

Rotational

The constant of a rotational spring describes how the rotation of the member is restrained about its longitudinal axis.

Options

The 'Support dimensions' check box is available as soon as a structural support is available in the direction of the member axes y or z. In the Support Dimensions tab, you can then define the geometry of the member support.

Support Dimensions

The dimensions of the support are required to determine the support stresses (in preparation).

Defining Support Dimensions