The thesis utilized physical testing and computer modeling to determine the feasibility of identifying a change to the mass or stiffness of a steel frame. Physical testing was performed using an accelerometer, linear shaker, and arbitrary function generator. Two methods of laboratory testing were developed: ambient vibration testing (AVT) and forced vibration testing (FVT). AVT was able to preliminarily identify the natural frequencies and mode shapes of the frame. FVT was able to precisely identify four distinct natural frequencies, mode shapes, and damping ratios. The baseline frame then underwent two physical changes: the addition of mass to its roof, and the addition of braces along one of its sides. FVT was used again to determine the natural frequencies, mode shapes, and damping ratios of the newly changed structure. An ETABS computer model was developed to represent the frame. This baseline model produced natural frequencies and mode shapes that closely matched the values determined by FVT. The mass and stiffness of this baseline model were then changed multiple times through the addition of mass and braces at various locations on the model. The frequencies and mode shapes were recorded for each change. Two methods were developed to identify the changes to the steel frame. The first method was able to determine which one of the models best represented a single change to the structure (adding mass to its roof). The second method was able to determine the combination of models that best represented the two concurrent changes to the structure (adding mass to its roof and braces to its sides). Both methods utilized the percent differences of each altered computer model relative to the original, and each method satisfactorily identified its respective physical alteration.