Dynamic Combinations (SRSS as 'Equivalent Linear Combination')

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Result combinations exported from RF‑/DYNAM Pro - Equivalent Loads are generated by superimposing the results from the individual modal responses. For this, the SRSS rule can be used as ‘equivalent linear combination’. When result combinations are used in RF‑/STEEL there are two options available for the calculation of stresses. Either the results from the result combinations are used directly. This is done line by line for each maximum and minimum controlling internal force. Or stresses are determined from the individual load cases. The quadratic superposition rule is then performed again in RF-/STEEL.

The ‘equivalent linear combination’, as it is used by RF‑/DYNAM Pro, is also considered by RF‑/STEEL. RF‑/STEEL calculates the governing internal forces. Those are sometimes difficult to understand and not directly comparable with the result combinations itself, for example when the equivalent stresses are used for the calculation of the governing internal forces, which is the default in RF‑/STEEL.

When you use the pure normal stresses or pure bending stresses for the calculation of governing internal forces in RF‑/STEEL, then you can easily reproduce the resulting internal forces. You will see that the option of ‘equivalent linear combination’ is used by RF‑/STEEL.

The formulas and an example are provided in the figure.


Gerlind Schubert

Dr. Dipl.-Ing. (FH) Gerlind Schubert, M.Sc.

Product Engineering & Customer Support

Ms. Schubert is responsible for the development of the DYNAM Pro add‑on modules.


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  • Updated 10/26/2020

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RFEM Steel and Aluminum Structures

Add-on Module

Stress analysis of steel surfaces and members

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