In the calculation parameters of a load case or load combination, it is possible to activate various stiffness factors. What exactly is behind this?

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) as there is the second-order analysis preset for this. If activating the function, 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 has been 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

This option allows you to control the multiplication factors of the individual cross-sections. In the "Modify"tab of the "Edit Cross-Section" dialog box, you can adjust the moments of inertia as well as the cross-sectional areas. This affects the stiffness of the cross-sections.

Figure 04 - Considering 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 the individual members in a similar way as the cross-sections.

Figure 05 - Stiffness Reduction of Members

Surfaces

For the surfaces of the "Standard" and "Without Tension" type, 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 with a factor (as in the case of orthotropic surfaces).

Figure 06 - Stiffness Reduction of Surfaces

Further Options for Stiffness Modification

Furthermore, you can select the option in the calculation parameters to specifically adjust the stiffness of other elements (see Figure 07). If you select the "Modify stiffness" option, a new tab appears (see Figure 08). In addition to the member and surface stiffnesses, it is also possible to individually adjust the stiffnesses of supports and releases.

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 (for example, 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 in this case.

Figure 10 - Global Control of Stiffness Modifications

Keywords

Stiffness modification Stiffness factor Partial safety factor Stiffness

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