Question:
I design timber components. The deformations of the load combinations deviate from the manual calculation by exactly the coefficient of the material partial safety factor. Why?
Answer:
In the calculation parameters of the load combinations, the calculation type according to the second-order analysis is preset by default. For example, the calculation is performed according to EN 1995‑1‑1, 2.2.2(1)P, using the design values of the stiffness property of the structural component; that is, the stiffnesses divided by the partial safety factor. For this reason, this stiffness modification is activated by default (see Image 01). For the load combinations in the serviceability limit state, there should be no reduction of the stiffness, of course.
Manual Creation of Load Combinations
If you create the load combinations manually, the load combination cannot "know" which limit state is involved. In this case, the user must set it manually (see Image 02). This setting must also be deactivated manually when switching to the geometrically linear analysis.
Automatic Generation of Load Combinations
If the load combinations are generated automatically by the program (see Image 03), the stiffness reduction is automatically deactivated for the SLS combinations specific to timber structures. For the ULS combinations, the reduction depending on the method of analysis is considered (second-order analysis and higher) or not (geometrically linear analysis). However, this requires the definition of the calculation type in the combination expressions (see Image 04). Changing the calculation type in the calculation parameters of the CO has no impact on the stiffness.