Validation of modal analysis: frequencies / periods

Dynamic analysis troubleshooting > modal analysis > protocol > frequencies

 

After a modal analysis, check the values of frequencies in the calculation protocol of eigen frequencies.

Use a simplified method to estimate the fundamental frequency (or period) of the structure, in order to perform a plausibility check of the modal analysis results.

In the case of seismic analysis, most design codes provide simplified formulas to estimate the fundamental frequency of a building. It is therefore very easy to check, that the value of the fundamental frequency obtained from the modal analysis is similar to hand calculation.

For instance, a simple approach for estimating the fundamental period is given in EC-EN 1998-1-1 Section 4.3.3.2.2(3) equation (4.9):

where d is the is the lateral elastic displacement at the top of the building, in [m], due to the gravity loads applied in the horizontal direction. The estimated fundamental period T₁ is given in [s].

 

Next step: check relative modal masses