Ambient Vibration Study of Six Similar High-Rise Apartment Buildings

It is of continuing interest to the field of Structural Engineering to examine the nature of structural vibrations. Today; there are numerous needs for such examinations but probably none so important as those associated with the determination of a structure's natural frequencies and associated modal damping values. It is precisely these quantities which are so critical in the prediction of a structure's response due to either strong ground motion or excessive wind loads. It is to this objective, the better understanding of dynamic structural response, that this work is directed.

The material presented herein is concerned with the results of the ambient vibration analyses performed and not with the method of analysis itself. The interested reader will find a detailed description of the method of analysis in the Engineer thesis, "Determination of the Dynamic Characteristics of Full Scale Structures by the Application of Fourier Analysis," (Kircher [1]). The result of an individual on-site analysis is simply the power spectral density function. The power spectral density function (function of frequency) is the magnitude squared of the Fourier transform of the given vibration record (function of time). The vibration records measured on the subject buildings located on Stanford Campus were all produced by means of a small accelerometer. Such acceleration vs time vibration records represent the building's characteristics as excited by random forces and as sensed by the accelerometer's particular position and orientation within the building. The power spectral density function may then be considered a "fingerprint" of any particular building for the given accelerometer orientation.

For those readers interested in a random vibration reference, Crandall and Mark [2] is suggested. Bracewell [3] is an excellent source of information about the Fourier transform and its properties, while Blackman and Tukey [4] probably provide the most sophisticated reference on power spectrum measurement and analys1s. Many other 1nvest1gators have been interested in the application of Fourier analysis for the determination of the dynamic characteristics of buildings. [5,7,8]. All the available references on this topic are not included in the list of references of this report.

The material presented in this report is a result of research, conducted by C. A. Kircher and supervised by H. C. Shah, toward the completion of a doctoral thesis. The essence of this work will provide a chapter of that dissertation to be published later this year. Additional research toward a Ph.D. degree being currently performed by C. A. Kircher and supported by the National Science Foundation Grant GI-34967 and the Department of Water Resources, State of California, Grants DWR B51254 and B51454, includes the following:

(1) The ambient and nonambient analyses of air blast circuit breakers and the accompanying support structures located at the A. D. Edmonston Pumping Plant, California. The purpose of this work is to examine the relationship between the nonlinear dynamic characteristics of the structure and the r.m.s. level of excitation.

(2) The ambient and nonambient analysis of two suspended-floor high-rise buildings located in Mountain View and San Jose, California. The purpose of this work is to compare two identical buildings of unique design and to compare the measured dynamic characteristics with the analytical resUlts calculated by B. Goodno [6] using a combined normal mode and finite element method of analysis.