Abstract: The similarity and difference of the stiffness equations for buckling and period eigenvalue analysis of structural systems are presented. The buckling factor is related to the period of the for different structural systems. Because there are a lower limit required by codes for the buckling factor of a real structure, the upper limit of the period is thus determined through the relation of period and buckling factor. The period is related to the square root of the total height multiplied by a factor, this equation changes slightly for different structural systems.