Modal identification of concrete shear wall buildings

In structural design, assumptions regarding the dynamic properties of a building are often required to simulate its response to dynamic loads. However, the assumed properties may be significantly different from those of the actual building due to differences between the idealized structure and in situ conditions. Building codes provide simple equations to estimate a building's fundamental period; but these equations do not capture the complex distribution of mass and stiffness within a building. Further, dynamic response is often simulated using truncated modal superposition by assuming constant viscous damping in all low-frequency modes. Clearly, reliable data on the dynamic properties of actual buildings would help engineers select realistic values of these properties in structural analysis. In this study, the in situ dynamic properties of 27 reinforced concrete shear wall (RCSW) buildings in Montreal were examined. For each building, the natural periods, mode shapes, and modal damping ratios of up to six vibration modes were identified from ambient vibration records. The results show that the equation included in several building codes to estimate the fundamental sway period of RCSW buildings could be improved. Further, simple models to predict natural periods, which were calibrated to the Montreal building data, are presented. Finally, measured damping values are shown to be scattered between one and five percent of critical viscous damping for these buildings and this result is used to suggest damping ratios that can be safely assumed in structural analysis and design of RCSW buildings.