Selecting the Mode Shapes

Go to the Selection of Modes tab next. It controls which modes are taken into account for the spectral analysis.

Modes Shapes for Spectral Analysis

In the table, all 32 modes that were calculated for the modal analysis load case (LC5) are listed (see the Factors for Effective Modal Masses image). They are all activated by default. At the end of the table, you can see the overall sums of the ratios between M_eff,i and ∑ M: 90.9% for f_meX and 93.5% for f_meY.

According to EN 1998-1 [1] 4.3.3.3.1(3), all modes with effective modal masses greater than 5% of the total mass must be taken into account. This implies that masses smaller than 5% may be neglected. Use the button to activate all modes greater than 5% and, thus, deselect minor relevant modes.

Deselecting Modes According to Criterion

As a result, you will notice that the limit of 5% ('0.05' in the box to the right of the button) gives ratios for f_meX and f_meY falling below the 90% rule. The limit must be adjusted: Enter 0.002 to reduce the limit manually.

Adjusting Criterion to Fulfil Requirement

The ratio between M_eff,i and ∑ M now is 0.904 for f_meX and 0.927 for f_meY. 23 modes are sufficient for the analysis instead of 32. Some higher modes can be skipped.