Peak Floor Acceleration (PFA) is a necessary requirement in Industrial buildings for the design of equipment, piping, cable tray and ventilation duct supports. The seismic analysis of equipments needs the determination of PFA at the points where the equipments are fixed on the supporting structure. The classical spectral methods (SRSS, CQC) do not allow this determination which can be done in using the time integration method. This last approach needs to perform numerous calculations with various realizations of the seismic excitation. This paper aims to present a method allowing calculating the response properties of a linear elastic shear building structure loaded by a transient excitation. This approach is based on an approximation of the structures transfer function which validity is studied through comparisons with numerical (time integration) simulations. A variety of different stiffness distribution patterns is used to assign stiffness of each story level of a structure; and the effects of these stiffness distribution patterns on the properties of transfer functions are discussed. This method is easily extended to the determination of the peak floor acceleration of multi-story structures designed in compliance with the national design codes.