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For the past 15 years, the Atwood Building in downtown Anchorage has been outfitted with instruments to measure how it performs during big earthquakes.
When the earth shakes, rattles and rolls in south-central Alaska in a big way, like it did on Nov. 30 when a magnitude 7.0 earthquake and numerous aftershocks hit a few miles north Anchorage, seismologists are often interested in one particular building.
The 20-story Robert Atwood Building, built in 1982, is the second tallest building in Alaska and houses numerous state government offices. In 2003, the U.S. Geological Survey outfitted the steel-frame structure with a network of sensors located on 10 levels of the building: the 2nd, 7th, 8th, 13th, 14th and 19th floors, plus the roof, street, parking and electrical/mechanical levels.
According to a USGS factsheet on the Atwood Building:
Specifically, 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.
Because south-central Alaska regularly experiences earthquakes, including some major seismic events like the one on Nov. 30, the Atwood Building is one of the best places in the U.S. to observe how taller steel-frame buildings perform.
In this visualization of the building during the 7.0 quake, the USGS points out that “relative to the height of the building, the motions are magnified by a factor of 100 to show how the building deforms.”
What was it actually like to be inside the Atwood Building when the quake hit? Ask former Gov. Bill Walker, who had only a few days left in his governorship and was in an elevator when the shaking started.
Elevators in the building are notoriously noisy, and at first he thought the quake was cables slapping together in the elevator shaft. He looked at his security detail and rolled his eyes, he said. The rattling continued. Debris fell from the ceiling and a message of some kind flashed on a monitor. He kept expecting the elevator to stop, but it didn’t. He’d lived through the 1964 Good Friday earthquake in Valdez.“This could be the big one,” he thought.When the elevator finally reached the ground floor, the door opened only 8 inches, he said. A security guard had to pry it open.
There were no fatalities or serious injuries from the Nov. 30 quake.
There are numerous structures the USGS has outfitted with seismic performance sensors in the Lower 48 states, including the City-County Building in Salt Lake City; City Hall in Berkeley, California; and the Los Angeles County Department of Public Works building in Alhambra, California. The USGS has also partnered with the Department of Veterans Affairs to monitor the seismic performance of VA hospital buildings in the Lower 48 states and Puerto Rico, including those on the West Coast in Long Beach, Portland, Palo Alto, San Francisco, San Diego and Seattle.
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Michael Grass is Executive Editor of Route Fifty and is based in Seattle.