Staad.Pro - Seismic loads by response spectra

You need to create mass modelling over here. You need to defined the loading for dead load of the model along with required % of LL in X, Y and Z direction in the RS load case. There has been another option of defining this loads in mass reference load case. If you have a mass reference load case, the program will from mass matrix on the basis of that loading that will be used in RS loads.

Thank you for replying. What I understand: In the 12 Load case ( RS) I have to add dead loads and other necessary loads in 3 directions by the required %. And repeat loading method is not correct.

Yes, you are correct.

Excuse me...

1. I have done seismic loading with the method you mentioned. I have 1 note from the output, it says: ***NOTE: MASSES DEFINED UNDER LOAD#      10 WILL FORM

            THE FINAL MASS MATRIX FOR DYNAMIC ANALYSIS.

 EIGEN METHOD   : SUBSPACE

 -------------------------

 NUMBER OF MODES REQUESTED              =   300

 NUMBER OF EXISTING MASSES IN THE MODEL = 38229

 NUMBER OF MODES THAT WILL BE USED      =   300

 Does it mean the analysis is not correct?

2. And one more question. Some permanent loads are applied to plates in their local axis. Then I want to apply these loads in X/Y/Z. In input, loads have been indicated such as PR -50 kN. In the seismic load case, I write these loads as PR GX 50 kN, PR GY 50 kN, and PR GZ 50 kN. Is that correct?

Excuse me...

1. I have done seismic loading with the method you mentioned. I have 1 note from the output, it says: ***NOTE: MASSES DEFINED UNDER LOAD#      10 WILL FORM

            THE FINAL MASS MATRIX FOR DYNAMIC ANALYSIS.

 EIGEN METHOD   : SUBSPACE

 -------------------------

 NUMBER OF MODES REQUESTED              =   300

 NUMBER OF EXISTING MASSES IN THE MODEL = 38229

 NUMBER OF MODES THAT WILL BE USED      =   300

 Does it mean the analysis is not correct?

2. And one more question. Some permanent loads are applied to plates in their local axis. Then I want to apply these loads in X/Y/Z. In input, loads have been indicated such as PR -50 kN. In the seismic load case, I write these loads as PR GX 50 kN, PR GY 50 kN, and PR GZ 50 kN. Is that correct?

Response to Q-1. Please check if % of the sum of mass participation in the direction of application of RS loads is at least 90%. if not, increase the number of codes to be considered in the analysis. It is evident that program is not facing any problem in modal analysis.

Response to Q-2. It is suggested that you apply permanent plate loading in Global direction and then use them in mass modelling in three orthogonal directions.

Thank you! but this structure is a storage tank (circular) and that means Hydrostatic pressure acting on local axis in the tank wall (vertical plates). My only choice is to apply loads on local axis