Schematic of Atwood array and boreholes

The Advanced National Seismic System (ANSS) structural monitoring system in the 20- story Atwood Building is one of the most sophisticated systems in the country. Sensors are deployed on the parking and street levels, the electrical and mechanical levels, the roof level, and the 2nd, 7th, 8th, 13th, 14th, and 19th floors. Six instrumented boreholes and a seventh surface sensor are located a city block away. The data from these instruments provide the input signal of the seismic waves that shake the building.

The instrumentation within the building is designed to record (1) lateral swaying, (2) twisting, (3) rocking, and (4) drift (displacement) between selected pairs of adjacent floors and average drift between any number of floors. The swaying, twisting, and drift are related directly to the shaking of the building from seismic waves, and the rocking is related to interaction of the building with the underlying soil. Measuring the rocking is particularly important for incorporating the effect of soil-structure interaction in the design of structures.