Signed Results Using the Dominant Mode

Tips & Tricks

In RF-/DYNAM Pro - Equivalent Loads, a signed result option in accordance to the dominant eigenmode is available since version X.06.3039. For the modal combination of results corresponding to the single eigenvalues a quadratic combination rule has to be used. In RFEM and RSTAB, the SRSS and the CQC rule are available. It is only allowed to combine results not loads directly. The reason are the mode shapes which are arbitrarly scaled and signed.

Following from the quadratic result combination is the loss of signs. Internal forces at different position in the model can only be provided as result envelope where the maxima are all positive and the minima are all negative. The result combination is not unique.

To produce a more realistic figure of internal forces the signed results combination option has been introduced. Equivalent loads corresponding to each eigenvalue and mode shape are exported to RFEM or RSTAB into Load Cases. The results in Load Cases are always unique.

If the structure that you consider has a dominant eigenvalue (large effective modal mass factor) the signs of the corresponding Load Case can be applied on the results in the Result Combination. Using this option the Result Combination gets unique, too.

RF-DYNAM Pro - Equivalent Loads can select the dominant mode automatically by means of the largest effective modal mass factor, but you can also select a mode manually. In the exported Result Combination the option ‘According to LC / CO’ is automatically selected and the correct Equivalent Load Case is set.

Independent of RF-/DYNAM Pro this option is available in RFEM/RSTAB for any Result Combination that uses the quadratic combination. The calculation parameters Maximum and Minimum are extended by the option ‘According to LC / CO’.

Further notes and an example are shown in the figure.


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  • Updated 15 November 2020

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RFEM Dynamic Analysis
RF-DYNAM Pro - Equivalent Loads 5.xx

Add-on Module

Seismic and static load analysis using the multi-modal response spectrum analysis

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