GPS monitoring of structures in real-time: Recent advances

Mehmet Çelebi¹, Will Prescott¹, Ross Stein¹, Ken Hudnut¹, Steve Wilson^2
1U.S. Geological Survey, Menlo Park, CA 94025, USA.
Tel. : +1-415-329-5623,
Fax: +1-415-329-5163,
E-mail: celebi@usgs.gov
²Leica GPS, 23868 Hawthorne Blvd., Torrence, CA 90505, USA

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 Hz 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 (± 1 cm for horizontal, ± 2 cm for vertical accuracy) with 10 Hz sampling rates are now available off-the-shelf 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 accurately assess its average drift ratio.