Date of Award


Degree Name

MS in Architecture




Cole C. McDaniel


An assessment of the validity of idealizing a concrete diaphragm as rigid was performed through the modal analysis of three existing buildings. Modal analysis was performed both by physical experimentation and computational analysis. Experimental determination of the mode shapes shows that two of the three buildings’ diaphragms exhibited flexible behavior. The experimental results were compared to computational analysis results and were shown to be similar, confirming that that the two building diaphragms are not rigid. As a standard, diaphragms with aspect ratios less than three are permitted to be idealized as rigid per ASCE 7-05. To determine the effect of the rigid diaphragm idealization, the design forces and roof deflections for each building were determined from the computational model through a spectral analysis for both a model with rigid diaphragms and a model with semi-rigid diaphragms. It was found that the design seismic demands for the two buildings with flexible diaphragms were higher when modeled with semi-rigid diaphragms than with rigid diaphragms. The conclusion is made that idealizing a concrete diaphragm as rigid solely based on its aspect ratio may result in an unconservative estimate of the seismic demands on a building.