Response of an Integrated Building and Free - Field Surface and Downhole Network

An integrated seismic monitoring system with a total of 55 channels of accelerometers is now operating at and in the nearby free-field site of the 20-story steel-framed Atwood Building, in highly seismic Anchorage, Alaska. The building has a single-story basement and a reinforced-concrete foundation without piles. The monitoring system comprises a 32-channel structural and a 21channel site array. Accelerometers are deployed on 10 levels of the building to assess translational, torsional, and rocking and interstory drift (displacement) between selected pairs of adjacent and average drift between floors. The site array, located approximately a city block from the building, and comprising seven tri-axial accelerometers, one at the surface and six in boreholes ranging in depths from 15 to 200 feet (5-60 meters). The arrays have already recorded responses of the building and the site caused by numerous earthquakes both at distances ranging from tens to a couple of hundred kilometers. Analyses of the responses clearly indicate propagation of seismic waves from the lowest borehole all the way to the roof of the building. Although the response data collected to date are of low-amplitude, they reveal the complete seismic behavior and dynamic characteristics of the building and the site. Data from an earthquake that occurred 186 km away are used to identify the fundamental structural frequency as 0.58 HZ (NS) and 0.47 Hz (EW) and the fundamental site frequency as 1.5 Hz. During the low-amplitude responses recorded from this event, waves propagate in approximately 0.5 seconds from the boreholes to the roof. Mode-coupling and beating effects are identified. Soil-structure interaction effects are not observed at the low levels of recorded motion. The integrated array serves as an example for future instrumentation projects to assess their behavior and performances.