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 I I 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,f(1), which is compared with independent estimates of the soil-structure system frequency f(sys) during the same earthquakes and during five ambient vibration tests. The analysis shows that monitoring only the changes of f(sys), can be misleading for structural health monitoring and can lead to erroneous alarms, while monitoring changes of f(1), 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 (c) 2007 John Wiley & Sons, Ltd.
