Member Rotational Restraints

Member rotational restraints allow you to determine the torsional restraint around the longitudinal axis of the member or member set using components such as trapezoidal sheets or purlins. A member rotational restraint is taken into account when determining the elastic critical moment or the critical load factor for lateral-torsional buckling if this is determined by the internal eigenvalue solver. A rotational spring that is constant over the entire member or member set applies.

Rotational Restraint Type

In the list, you can select various types of rotational restraints:

• Continuous
• Discrete
• Manually

Dialog Box "Member Rotational Restraint", Tab "General"

Continuous

The following information is required to determine the shear panel stiffness from trapezoidal sheeting and the connection deformation:

• Sheeting material name
• Sheeting name
• Spring stiffness C₁₀₀
• Load introduced from sheeting to beam
• Continuity effect
• Beam spacing
• Section deformation C_D,B

Dialog Box "Member Rotational Restraint", Tab "Continuous"

You can select the material of the trapezoidal sheeting from the library by clicking the button (see the image Material Library). After selecting the material, the modulus of elasticity is transferred automatically. However, you can also define it manually.

Dialog Box "Select Material from Library"

You can select the trapezoidal sheeting from the library using the button (see the image Cross-Section Library – Trapezoidal Sheeting). After selecting the trapezoidal sheeting, the position, thickness, moment of inertia, rib spacing, and the flange width of the sheeting are transferred automatically. However, you can also change them.

Cross-Section Library - Trapezoidal Sheeting

The rotational stiffness of the connection between the sheeting and the purlin C_D,A is determined according to EN 1993‑1‑3. You can transfer the rotational spring stiffness C₁₀₀ from Table 10.3 of EN 1993‑1‑3 by using the button (see the image Dialog Box "Import of Coefficient C-100 from Table 10.3, EN 1993-1-3"), or enter the value manually.

Furthermore, you have to define Load A, which acts between the sheeting and the purlin.

Dialog Box "Import of Coefficient C-100 from Table 10.3, EN 1993-1-3"

The spring stiffness C_D,A as well as the Load A is assigned to all load or result combinations. Activate the "Differently depending on loading" check box to assign them by load case. Then, you can make the assignment in the Load-Dependent Properties tab (see the image Tab "Load-Dependent Properties").

Dialog Box "Member Rotational Restraint", Tab "Load-Dependent Properties"

The continuity effect has an impact on the coefficient k of the rotational restraint from the cross-section deformation of the trapezoidal sheeting C_D,C. For the exterior span, k = 2, and for the inner span, k = 4.

You can enter the beam spacing manually. As an alternative, you can also define this graphically by selecting the "Measure" function using the button. Finally, you can select two snap points in the work window to adopt the distance between them.

Activate the "Cross-section deformation C_D,B" check box if you want to consider the rotational stiffness due to the cross-section deformation of the beam. The cross-section deformation component can only be calculated for constant I-section members, but not for tapered members or non-I-sections.

Discrete

To determine the stiffness component from single supports, for example purlins, the following specifications
are required:

• Section material name
• Section name
• Rotational stiffness C_D,A
• Continuity effect
• Section deformation C_D,B
• Purlin spacing
• Beam spacing

Dialog Box "Member Rotational Restraint", Tab "Discrete"

You can select the material from the library by clicking the button (see the image Material Library). After selecting the material, the modulus of elasticity is transferred automatically. However, you can also define it manually.

You can select the cross-section from the library by clicking the button. After selecting the cross-section, the moment of inertia is transferred automatically. However, you can also change it.

You can apply the connection stiffness C_D,A as an infinite value or define it manually. For the manual definition, enter the rotational stiffness C_D,A directly in the "Parameters" section.

The continuity effect has an impact on the coefficient k of the rotational restraint C_D,C. For the exterior span, k = 2, and for the inner span, k = 4.

Activate the "Cross-section deformation C_D,B" check box if you want to consider the rotational stiffness due to the cross-section deformation of the purlin. The cross-section deformation component can only be calculated for the purlins with I-sections, but not for the purlins with non-I-sections.

You can specify the spacing of the purlins and the beam manually. As an alternative, you can also define this graphically by selecting the "Measure" function using the button. Finally, you can select two snap points in the work window to adopt the distance between them.

Manually

You can also manually specify the rotational spring stiffness C_D.

Dialog Box "Member Rotational Restraint", Tab "Manually"

Assignment to Member or Member Set

Member rotational restraints have to be assigned to member supports. You can assign member rotational restraints to one or more member supports using the editing dialog box of the support. To do this, select the fictitious stiffness "Rotational restraint about x" in the "Nonlinearity / Fictitious Stiffnesses" list of the "Main" tab.

Dialog Box "New Member Support", Tab "Main"

You can then assign the desired rotational restraint in the "Rotational Restraint" tab.

Dialog Box "New Member Support", Tab "Rotational Restraint"

You can also graphically assign the member rotational restraint to a member support in the editing dialog box of the member rotational restraint by clicking the button. However, the rotational restraint needs to be already assigned to a member support.

Dialog Box "New Member Rotational Restraint", Assigning Member Support

The member supports with the member rotational restraints defined therein are then assigned either to the members or to the higher-level member sets. Assign the member support by using the editing dialog box of the member or member set, the selection from the Member Support dialog box, or via the input table.