Consideration of Rotational Mass in the analysis

 Applies To 
 Environment: N/A
 Area: STAAD.Pro Analysis Solution
 Subarea: Seismic Analysis
 Original Author:SANJIB DAS, Global Technical Support Group

By default, STAAD considers translational mass while forming mass matrix. No rotational mass is considered if any moment is applied as member loading in the model. A sample model has been attached (Demo_1.std), since in the model only rotational mass is present as concentrated moments used as member loading the program faces problem with mass matrix formation since all elements are zero. A error will be reported. 

If you want to consider rotational mass, you need to issue SET MASS 1 command which will include rotational mass in mass matrix. You can refer to Demo_2.std. In real life model rotational mass has little effect. It is the translational mass which contributes to the mass participation.