For reliable and practical application of structural health monitoring approaches in conjunction with dense sensor arrays deployed on 'smart' systems, there is a need to develop and evaluate alternate strategies for efficient problem decomposition to rapidly and accurately determine the occurrence, location and level of small changes in the underlying structural characteristics of a monitored system based on its vibrational signature. Furthermore, there is also a need to quantify the level of uncertainties in the identified system characteristics so as to have a measurable level of confidence in the parameters to be relied on for the detection of genuine changes (damage) in the monitored system. This study presents the results of two time-domain identification techniques applied to a full-scale 17-story building, based on ambient vibration measurements. The Factor building is a steel frame structure located on the UCLA campus. This building was instrumented permanently with a dense array of 72-channel accelerometers, and the acceleration data are being continuously recorded. The first identification method used in this study is the NExT/ERA, which is regarded as a global (or centralized) approach, since it deals with the global dynamic properties of the structure. The second method is a time-domain identification technique for chain-like MDOF systems. Since in this method the identification of each link of the chain is performed independently, it is regarded as a local (or decentralized) identification methodology. For the same reason, this method can be easily adopted for large-scale sensor network architectures in which the centralized approaches are not feasible due to massive storage, power, bandwidth and computational requirements. To have a statistically meaningful results, 50 days of recorded data are considered in this study. The modal parameter and chain identification procedures are performed over time windows of 2 h each and with 50% overlap. Using the NExT/ERA method, 12 dominant modes of the building were identified. It was observed that variations in the frequency estimation are relatively small; the coefficient of variation is about 1-2% for most of the estimated modal frequencies. Chain system identification was successfully implemented using the output-only data acquired from the Factor building. Probability distributions of the estimated coefficients of displacement and velocity terms in the interstory restoring functions (which are the mass-normalized local stiffness and damping values) that were found based on the chain system identification are presented. The variability of the estimated parameters due to temperature fluctuations is investigated. It is shown that there is a strong correlation between the modal frequency variations and the temperature variations in a 24 h period.

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There are a small number of commercially-available sensors to measure rotational velocities in the frequency and amplitude ranges appropriate for earthquake strong motions on the ground and in structures. However, the behavior of these sensors has not been thoroughly tested and characterized for earthquake monitoring purposes. To address this need, the authors, with assistance from colleagues in the U.S. and Taiwan, have developed performance test methodologies and performed initial testing of two such rotational velocity sensors: the Eentec™ model R-1™ and the PMD™ model RSB-20™. Both are magnetohydrodynamic rotational-velocity sensors, the latter with force feedback. Two examples of each sensor and two 19-bit data acquisition units (Kinemetrics six-channel K2) were obtained courtesy of the Central Weather Bureau of Taiwan and the Institute of Earth Sciences, Academia Sinica for testing in late 2006. Both sensor models have three orthogonal sensors with sensitivities of 50 V/rad/s. The data acquisition units also have internal force-balance linear accelerometers.Additional samples of the R-1™ were obtained for further testing in 2007. Performance testing of these sensors consists of: 1) noise floor measurements at both an urban site and a seismically-quiet site; 2) cross-axis measurements on a linear shake table, at strongmotion levels to 1 g; 3) field measurements of known rotations using the NEES SFSI Test Structure at Garner Valley; and 4) field measurements of microtremor and earthquake ground motions at existing seismic array site GVDA (Garner Valley Downhole Array). Results of these tests are analyzed not only to characterize the performance of these specific sensors but also to define performance-envelope needs for rotational sensor deployments.

This paper presents a classification and prioritization of nonstructural systems, including medical equipment, in hospitals based upon the results of extensive surveys of effects of major seismic events. Surveys included damage surveys, interviews of medical and administrative personnel, and solicitation of expert opinion. As part of a larger study on nonstructural mitigation in hospitals, this effort sought to identify the importance and interdependence of various nonstructural systems subjected to earthquakes and other extreme events. Focused information was obtained for the 1994 Northridge earthquake. Additional information was obtained from experiences in the 1995 Kobe earthquake, the 1999 Chi-Chi earthquake, and the 1999 Kocaeli earthquake. Survey results led to a prioritized list of hospital nonstructural systems that can aid mitigation efforts in maximizing the continued functionality of essential medical facilities when exposed to extreme events.

Foundation impedance ordinates are identified from forced vibration tests conducted on a large-scale model test structure in Garner Valley, California. The structure is a steel moment frame with removable cross-bracing, a reinforced concrete roof, and a nonembedded square slab resting on Holocene silty sands. Low-amplitude vibration is applied across the frequency range of 5-15 Hz with a uniaxial shaker mounted on the roof slab. We describe procedures for calculating frequency-dependent foundation stiffness and damping for horizontal translational and rotational vibration modes. We apply the procedures to test data obtained with the structure in its braced and unbraced configurations. Experimental stiffness ordinates exhibit negligible frequency dependence in translation but significant reductions with frequency in rotation. Damping increases strongly with frequency, is stronger in translation than in rocking, and demonstrates contributions from both radiation and hysteretic sources. The impedance ordinates are generally consistent with numerical models for a surface foundation on a half-space, providing that soil moduli are modestly increased from free-field values to account for structural weight, and hysteretic soil damping is considered.