GPS Monitoring of Structures: Recent Advances

Abstract

Relative displacements used to assess stress and drift conditions of structures are difficult to measure directly. Accelerometers are the most common instruments used to monitor structural systems during earthquakes or strong winds, but to arrive at displacements requires processing including double integration. In most cases, such processing does not take place in real-time or near real-time because of requisite steps including choice of filters for signal processing. These choices could introduce errors at periods relevant to structural response. However, recent advances in Global Positioning System (GPS) technology recording at 10 samples per second (sps) allows reliable monitoring of long-period structures such as suspension or cable-stayed bridges and tall buildings. GPS units with a capability of resolving motion at the centimeter level (+ or -1 cm for horizontal, + or -2 cm for vertical accuracy) with sampling rates of 10 sps are now available from several manufacturers. The majority of tall buildings (20-40 stories or more) are flexible steel framed structures whose fundamental period (T, in seconds) can be estimated with the empirical formula T = 0.1 N, where N is the number of stories of a building. Thus, the frequencies corresponding to the fundamental periods of most buildings over 20 stories are 10-20 times the Nyquist frequency of the sampling, which is sufficient to assess accurately its average drift ratio. With this method, displacement is measured directly in real-time. In this paper, we discuss the scientific justification and feasibility of using GPS technology to monitor long-period structures. We describe an experiment simulating the response of a tall building. The results indicate that sampling at 10 sps provides a clear and accurate response history from which drift ratios and dynamic characteristics of the specimen can be derived. The paper discusses in detail how data recorded from tall buildings monitored with GPS units can be used by engineers and building owners or managers to assess the building performance during extreme motions caused by earthquakes and strong winds, by establishing threshold displacements or drift ratios and identifying changing dynamic characteristics. Such information can then be used to secure public safety and/or take steps to improve the performance of the building.