Missing mass: Please refer to the following wiki for missing mass correction.
Scale Factor: The following wiki explains the Scale factor in Response Spectrum load case.
communities.bentley.com/.../28216.what-is-the-scale-factor-f4-that-needs-to-be-provided-when-specifying-the-response-spectra
ZPA: Say, you have applied the following Response Spectrum command.
SPECTRUM CQC 1893 X 0.375 Z 0.375 ACC SCALE 9.81 DAMP 0.05 MIS 0 ZPA 23 SAVE
0 0.12; 0.1 0.198; 0.2 0.2; 0.3 0.2; 0.4 0.186; 0.5 0.165; 0.6 0.15;
0.7 0.138; 0.8 0.126; 0.9 0.118; 1 0.108; 1.1 0.1; 1.2 0.096; 1.3 0.087;
1.4 0.08; 1.5 0.077; 1.6 0.068; 1.7 0.063; 1.8 0.06; 1.9 0.059; 2 0.058;
2.1 0.056; 2.2 0.052; 2.3 0.05; 2.4 0.048; 2.5 0.043; 2.6 0.042; 2.7 0.04;
2.8 0.04; 2.9 0.04; 3 0.039;
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MIS ==> This command requests that the missing mass mode be calculated for this loading direction.
blank or 0 ==> Use default of spectral acceleration at 33 Hz or if a ZPA frequency value is entered, then use the spectral acceleration at that frequency.
ZPA 23 ==> Use the spectral acceleration at 23 Hz.
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There are few things that are to be kept in mind while performing a Response Spectrum analysis. It is to be understood that one cannot actually model all the lateral load resisting elements in the model as the process of calculation of their stiffness is unknown. for an example, you have a reinforced brick wall in your model. You cannot model it in your structure. The frequencies/time period calculated for different fundamental mode shape would be approximate one. One would compute higher time period for the fundamental modes and for them the spectral acceleration data computed will be lesser than the actual on using the Elastic Response Spectrum provided by IS:1893-2002. Stiffer the structure, lesser is the time periods for fundamental modes and higher would be the spectral acceleration data- which would provide higher seismic forces on the model on using the Response Spectrum which has PGA vale of 1.0g . The code has introduced clause 7.8.2 which stipulates that base shear computed from the static method has to be compared with that of Response Spectrum. Code is not allowing to lower the DBE (design Basic Earthquake) below a particular value which is lower bound value and is determined by the static method. The base shear computed from the two process has to be compared. As per clause-7.8.2 of IS:1893(part-I)-2002, the program compares the base shear of Response Spectrum analysis(VB) with that of Static method (Vb). If the base shear of Static method is higher than that of Response Spectrum method, the program calculates the ratio of them and that ratio is reported as “MULTIPLYING FACTOR (Vb/VB)”. The program multiplies all the response quantities like- member forces, displacement etc. with this factor. You need to get this checked in your model. You simply need to add IS:1893-2002 seismic definition added in your model. The rest of the things will be taken care of by the program.
In your model, you have used IS:1893-1984 code in the seismic definition. Thus, the requirement of clause -7.8.2 of IS:1893 Part-I-2002 doesn’t hold good and the program doesn’t compare the base shear of Static method and Response Spectrum method