ASCE 7-16_Extracting Enough Modes for Response Spectrum

[m_ridzon]

I'm reviewing ASCE 7-16 §12.9.1.1 about extracting enough modes for modal response spectrum method. I'm using the ANSYS software which reports mass participation for the three translational directions (UX, UY, UZ) as well as the three rotational directions (ROTX, ROTY, ROTZ). Does §12.9.1.1 include the latter? It doesn't seem to specify. On some nonbuilding components I have analyzed, I have found that the rotational mass participation may be very low until I extract a gazillion modes, which becomes unreasonable computational expense.

 

[HTURKAK]

- The following para. from NEHRP Recommended DESIGN EXAMPLES FEMA_P-1051;

12.9.1.1 Number of Modes
An analysis shall be conducted to determine the modes of vibration for the structure. The analysis
shall include a sufficient modes to capture participation of 100% of the structure’s mass. For this
purpose, it shall be permitted to represent all modes with periods less than 0.05 seconds in a single
rigid body mode having a period of 0.05 seconds.
EXCEPTION: Alternatively, the analysis shall be permitted to include a sufficient number of
modes to obtain a combined modal mass participating of at least 90 percent of the actual mass in
each orthogonal horizontal direction of response considered in the model.

- For the 3D analysis of building type structures you should consider ;Translational X , Translational Y , and Rotational Z modes assuming X and Y are horizontal and Z vertical coordinates .