Various stiffness factors can be activated in the calculation parameters of a load case or load combination. What exactly is behind it?

Answer

The stiffness modifications can be controlled separately for the following elements:

  • Materials
  • Cross-Sections
  • Members
  • Surfaces

Materials

The first option 'Materials' is only activated for load combinations by default (see Figure 02) because the second-order analysis is preset for this. When the function is activated, the stiffness of all elements is reduced by the partial safety factor of the material (see Figure 03). This is especially important for timber construction in Europe. If the automatic load combination was selected for the standard EN 1990 + EN 1995, SIA 260 + SIA 265 or DIN 1055-100 + DIN 18008, the default settings are different. Provided that the partial safety factor of the material is defined as 1.0, it does not matter if the function is activated or not.

Figure 03 - Reduction of Stiffness by Partial Safety Factor γ_M

Cross-Sections

Use this option to control the multiplication factors of individual cross-sections. In the 'Modify' tab of the 'Edit Cross-Section' dialog box, it is possible to adjust the moments of inertia as well as cross-section surfaces. This affects the stiffness of the cross-sections.

Figure 04 - Considerating Multiplication Factors of Cross-Sections

Members

When editing a member, the 'Modify Stiffness' dialog tab is available. There are various definition types (see Figure 05). The "Multiplication Factors" option allows you to modify the stiffness of individual members in analogy to the cross-sections.

Figure 05 - Stiffness Reduction of Members

Surfaces

For surfaces referred to as 'Standard' and 'Without Tension', the stiffness of the surface can be adjusted in the 'Modify Stiffness' tab of the 'Edit Surface' dialog box. There, it is possible to modify the stiffness matrix elements by a factor (as with orthotropic surfaces).

Figure 06 - Stiffness Reduction of Surfaces

Further options for stiffness modification

Additionally, a further option can be selected in the calculation parameters to adjust specifically stiffness of other elements (see Figure 07). When selecting the 'Modify stiffness' option, a new tab opens (see Figure 08). In addition to the member and surface stiffnesses, it is also possible to adjust the stiffnesses of supports and hinges individually.

Figure 07 - Further Options of Stiffness Modification

Figure 08 - Modifications of Supports and Releases

Interactions of individual factors

If several factors have been defined for an element (e.g. cross-section and member), they are multiplied by each other. For the example shown in Figure 09:

Ely,mod=0,5 ×0,5 ×ElyEly,mod=0,25 ×Ely

Figure 09 - Interactions of Individual Factors

Global control of stiffness modification

In the global calculation parameters (see Figure 10), it is possible to deactivate all options mentioned above at once. The local settings in the calculation parameters of the load cases or the load combinations are ignored.

Figure 10 - Global Control of Stiffness Modifications

Keywords

Stiffness modification Stiffness coefficient Partial safety factor Stiffness

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