Scale factor is a factor by which the input spectrum data is multiplied when carrying out a response spectrum analysis. Typically when the acceleration spectrum is in terms of “g” ( normalized values), you would use the value of “g” as the scale factor.
Missing mass : As you know in a response spectrum analysis, the final response is obtained using a modal combination of the responses obtained from various modes. These modes attempt to capture the effect of vibration of the masses in different directions. However you may not get 100% of the mass to participate through the considered modes. The missing mass mode is a pseudo mode to include the effect due to these masses.
ZPA : When using the missing mass mode the software needs to know what acceleration to consider for that mode. You can either directly input that acceleration or you can specify a value of frequency representing the ZPA ( zero period acceleration ) so that the software can use the corresponding acceleration for the missing mass mode. If this frequency is not entered, the software defaults to a value of 33 Hz and uses the corresponding acceleration for the missing mass mode.
These terms are explained in more detail in the section 5.32.10.1.2of the Technical Reference Manual
Regarding the warning you mentioned, the clause 7.8.2 of the IS 1893 requires that the base shear obtained from a dynamic analysis like response spectrum be compared against the base shear calculated based on a fundamental period. For the software to be able to do this, you need to define certain additional parameters related to the IS 1893 code. These are explained in detail under the node a. of the section 5.32.10.1.2 mentioned above.
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RE: Problem in Response Spectrum Analysis by using IS 1893:1984
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