This and the companion paper present an analysis of the amplitude and time-dependent changes of the apparent frequency of a seven-story reinforced-concrete hotel building in Van Nuys, Calif. Data of recorded response to 12 earthquakes are used, representing very small, intermediate, and large excitations (peak ground velocity, vmax = 0.6 2 11, 23, and 57 cm/s, causing no minor and major damage). This paper presents a description of the building structure, foundation, and surrounding soil, the strong motion data used in the analysis, the soil-structure interaction model assumed, and results of Fourier analysis of the recorded response. The results show that the apparent frequency changes form one earthquake to another. The general trend is a reduction with increasing amplitudes of motion. The smallest values (measured during the damaging motions) are 0.4 and 0.5 Hz for the longitudinal and transverse directions. The largest values are 1.1 and 1.4 Hz, respec- tively, determined from response to ambient noise after the damage occurred. This implies 64% reduction of the system frequency, or a factor '3 change, from small to large response amplitudes, and is interpreted to be caused by nonlinearities in the soil.

Apparent Periods of a Building. I: Fourier Analysis

Should average shear-wave velocity in the top 30 m of soil be used to describe seismic amplification?

The aim of this work is to estimate the fundamental translational frequencies and relative damping of a large number of existing buildings, performing ambient vibration measurements. The first part of the work is devoted to the comparison of the results obtained with microtremor measurements with those obtained from earthquake recordings using four different techniques: horizontal-to-vertical spectral ratio, standard spectral ratio, non-parametric damping analysis (NonPaDAn) and half bandwidth method. We recorded local earthquakes on a five floors reinforced concrete building with a pair of accelerometers located on the ground and on top floor, and then collected microtremors at the same location of the accelerometers. The agreement between the results obtained with microtremors and earthquakes has encouraged extending ambient noise measurements to a large number of buildings. We analysed the data with the above-mentioned methods to obtain the two main translational frequencies in orthogonal directions and their relative damping for 80 buildings in the urban areas of Potenza and Senigallia (Italy). The frequencies determined with different techniques are in good agreement. We do not have the same satisfactory results for the estimates of damping: the NonPaDAn provides estimates that are less dispersed and grouped around values that appear to be more realistic. Finally, we have compared the measured frequencies with other experimental results and theoretical models. Our results confirm, as reported by previous authors, that the theoretical period–height relationships overestimate the experimental data. Copyright © 2008 John Wiley & Sons, Ltd.

Empirical estimate of fundamental frequencies and damping for Italian buildings

Wave travel times of vertically propagating waves are measured in the Van Nuys 7-storey hotel, located in the Los Angeles metropolitan area, from impulse response functions computed by deconvolution of the recorded earthquake response. East-West response data from 11 earthquakes over a period of 24 years are analysed. Changes in wave travel times are used to infer about local (between sensors) and global changes of structural stiffness, from one event to another, and with time during the earthquakes that damaged the building (San Fernando and Northridge). Also, wave travel times are used to estimate the fundamental fixed-base frequency of the building, f1, which is compared with independent estimates of the soil–structure system frequency fsys during the same earthquakes and during five ambient vibration tests. The analysis shows that monitoring only the changes of fsys can be misleading for structural health monitoring and can lead to erroneous alarms, while monitoring changes of f1 over suitably chosen time windows (before, during, and after excitation by strong earthquake motions) can be a powerful and robust tool for structural health monitoring. It is concluded that, under favourable conditions, this method can be used as a tool for global and local structural health monitoring. Copyright © 2007 John Wiley & Sons, Ltd.

S.S. Ivanovć

Papers

Experimental evidence for flexibility of a building foundation supported by concrete friction piles