Q: What is Scale Factor given in Response Spectrum Cases definition dialog box for IS:1893 code?
A: It should be noted that the spectrum ordinates defined using in-built IS:1893 code option as well as other code options are normalised values. It means that these are without any units. These are the values of spectral accelerations divided by acceleration due to gravity. So the spectrum must be converted to a specific set of units which are used in the model. This is done by using a scale factor with a value of acceleration due to gravity in the current units of your model. At the same time it must be noted that the design spectrum of IS:1893 is actually MCE level spectrum and it must be reduced by dividing it by a factor of 2R where R is response reduction factor. So in the first run the value of scale factor should be SF=I*g/(2R) where I is importance factor. After first run, check the base shear developing in the model and if the computed value of base shear is less than code specified minimum base shear than enhance the value of scale factor used in the first run so that resulting base shear would match with the code specified minimum value.