The parameters used to generate IBC (International Building Code) compliant design response spectra are specified only approximately for a given site in most cases and values may even be erroneous. So, the question arises about the effect of approximation or error in the values of these parameters on the predicted internal forces in the structural elements of a building calculated on the basis of the provisions of IBC. This paper explores this issue and presents the sensitivities of design response spectra and maximum moments induced in the structure to these IBC parameters. Within the critical range of short time periods, the variation in peak acceleration of design response spectra for various site classes is found non-uniform. Only peak acceleration is affected by varying SS. Thus, for buildings with the fundamental time period of around 0.6 sec and greater, the parameter SS of IBC is immaterial. Also, for a given site class, the design response spectra are almost identical in the peak acceleration region for all values of S1. The maximum moment is insensitive to SS except for the site class E. This implies that any error in the evaluation of this parameter will not affect the design of the structure.