Ambient and Forced Vibration Analysis of Full Scale Structures

Vibration analysis techniques are used to examine the dynamic characteristics of full scale structures. Specifically, the results of a study of six similar high-rise apartment buildings and a study of two 230 KV air blast circuit breakers are presented. A portable mini-computer system has been used to perform on-site acceleration measurements and corresponding analyses (e.g., calculation of power spectra). This mini-computer system is programmed for data acquisition and discrete Fourier analysis. Forced vibration excitation of the breaker structures is accomplished by a portable electro-magnetic shaker unit. The forced vibration input to the structures include white noise as well as sinusoidal excitations.

The study of six similar high-rise apartment buildings is performed by the examination of power spectra obtained from ambient vibration measurements. Two types of comparisons are made: comparisons between power spectra obtained from different measurements in the same buildings and comparisons between power spectra obtained from different measurements of similar or "identical" design.

Two 230 KV air blast circuit breakers are studied by examining the power spectra obtained from ambient and forced vibration measurements. Sinusoidal forced vibrations are used to identify mode shapes. Comparisons are made between the dynamic characteristics of an unmodified breaker structure and a breaker structure modified by the addition of mechanical dampers to the structural frame. For both modified and unmodified breaker structures, damping is calculated at different levels of excitation of the structures. Comparisons are made of the change in damping with the change in level of excitation.