One year ago last November, sitting at a desk in his first-floor office in the 20-story Robert B. Atwood Building in downtown Anchorage, building manager Dan Reynolds felt the ground move. It began with a jolt in his spine -- then spread throughout the building. "It was a pretty good quake," Reynolds recalled this week. "The blinds started moving, swaying back and forth."
No one inside the Atwood tower could have known but they'd just begun to ride the 2002 Denali fault earthquake, which measured 7.9 on the moment magnitude scale and shook for about 100 seconds.
At its epicenter on the south side of the Alaska Range far to the north, the ground was sliding apart sideways along a 200-mile-long crack that partially bisected the state from west to east. Roads and airstrips buckled, cabins and stores rattled apart. People ran for cover. When it was done shaking, Denali would rank as the largest earthquake in the world that year, as well as the largest "inland quake" to strike North America in nearly a century and a half.
A couple hundred miles to the south, however, the quake's effect on Anchorage seemed less profound. Most of the people in town experienced only moderate shaking, depending on where they stood. Standing in the Atwood Building -- with more than 200 vertical feet of steel and glass directly overhead -- Reynolds had reason for concern. He wasn't new to earthquakes.
Forty years ago this week, as a teenager in his parents' home in northeast Anchorage, Reynolds had weathered the Good Friday earthquake -- at 9.2 on the moment magnitude scale, the second-largest quake ever recorded. More than a hundred people lost their lives as the quake displaced an area about a third the size of California. It damaged thousands of buildings.
In Anchorage, homes perched on the Turnagain bluff west of Downtown tumbled toward the sea. A woman in one of the homes died when she fell into a crevice and ground closed above her. Another woman died when the J.C. Penney department store collapsed. A man died in an airport control tower. All of the tallest buildings in town suffered structural failure.
But Anchorage grew back.
As it did, soil engineers began to pay greater attention to the potential for earthquake-induced landslides and ground liquefaction, advising against certain building sites. And structural engineers developed earthquake-resistant building techniques, recommending stronger materials and designs that give tall office buildings a better chance of withstanding seismic trauma. The Atwood Building is a case in point.
Built in 1982 during the heyday of Alaska's oil boom by Texas billionaire Nelson Bunker Hunt, the Atwood Building -- originally called the Hunt Building, then the Enserch Building, then the Bank of America Building -- seemed a vast improvement on past mistakes.
Its framework of structural steel enclosed in a "sheer-wall" of steel panels is at once stronger and more flexible than Anchorage's more brittle, weaker-linked, concrete high-rises of old, of which the quake-damaged and now derelict MacKay Building on East Fourth Avenue is probably the most glaring example.
According to Dave Cole, a soils specialist and project manager for Dowl Engineers of Anchorage, the Atwood Building is designed to flex horizontally -- about one inch per story -- without suffering damage. The higher the story, the more the building bends. On the 10th floor it can rock 10 inches back and forth under severe strain from an earthquake. On the 20th floor it can rock 20 inches.
On the day of the Denali quake, Reynolds could be grateful for two things in his role as the Atwood building manager.
One, that it was a Sunday afternoon, and instead of having the 1,000-plus people who work there on weekdays -- including, sometimes, the governor (on the 17th floor) and several state department heads -- all experiencing various degrees of concern about riding out an earthquake in a highly flexible skyscraper, there were probably less than 20 people in the building.
Two, that his own office was on the ground floor.
Though that had its disadvantages too.
"Obviously you've got all that building above you," Reynolds said.
And just then, the Atwood Building was continuing to shake -- stronger and longer than it ever had before in his three years as building manager. Reynolds began to worry about conditions upstairs.
"People up on 20, they're going to feel it more," he said. "Knowing how bad it was shaking on the first floor, I could imagine what it was shaking like on the 20th floor."
So once the movement subsided, Reynolds decided to check on it firsthand. Riding the elevator to the 20th floor, he found the building's mechanical plant and maintenance center empty. He looked for leaks in the boiler system or possible electric failures. Finding none, he dropped down to the professional offices on the 19th floor.
He didn't see anyone on that level either.
But inside the vacant Wachovia Securities suite on the west side of the building, the telltale signs of a strong quake were fairly apparent. Drawers and filing cabinets had rolled open throughout the office. One of the cabinets, with all of its drawers open at the same time, had toppled over, dumping what appeared to be a dry fish tank to the floor. A lamp was tipped over too.
Reynolds could only wonder what it was like a few minutes earlier on that floor.
Rachel Batres could have told him.
As the continuing-education coordinator for the Alaska Bar Association, which has offices on the 19th floor, she was trying to catch up on some work that Sunday afternoon. But she didn't get very far in her office before the quake struck.
"So it started and you have the usual thought that, 'Oh, this feels like an earthquake,' " Batres recalled. "But it kept on going. And I really felt the building swaying, kind of like a boat, rocking back and forth."
The vertical blinds that shade the windows were banging about five feet from side to side. Drawers were opening and closing. Hanging plants were swaying back and forth. The big plate windows creaked ominously.
"What scared me the most was the sound," Batres said. "The elevator car was swinging back and forth. ... You could hear the 'Boom! Boom!' of the car hitting the side of the shaft."
After the initial jolt of the earthquake had subsided, the upper floors of the building kept swinging back and forth, almost like an inverted pendulum, Batres said. She began to feel a little seasick and crouched on the floor to recover.
SEISMIC ARRAY
Designing a big, tall office building for earthquake country requires both give and take, according to John Aho, a structural engineer and principal project manager for CH2M Hill in Anchorage.
A modern high-rise needs to be stiff enough to resist minor earthquake forces but not so stiff that it breaks. It needs to bend when it really has to -- as perhaps it should under the stress of a Denali-grade earthquake -- but not bend at times when it doesn't.
"That's really important," Aho said last week. "You don't want too much interstory drift" -- the tendency of successive stories to move horizontally under pressure -- "because it'll make people get seasick. And you don't want too much drift after a seismic event, because you get substantial, nonstructural-type damage with interior walls and stuff being thrown around.
"It's a real balancing act."
And if it sounds complicated, it is. Buildings don't just bend on a simple east-west or north-south axis, said Cole, the Dowl engineer. Sometimes they wave like spaghetti, or twist like a corkscrew or rock like a chair.
"They do all sorts of things," Cole said. "They have torsional responses, so they twist. They also have what we call rocking responses, where the foundation can actually rock."
For its own part, the Atwood Building at the corner of Seventh and F streets is now something of a nationwide study in earthquake motion. Since December, it's been wired with seismic monitors in 35 different locations from its basement to its roof. The monitors will be able to detect its slightest deflection at several levels and directions -- producing a three-dimensional picture -- and transmit the data to scientists in their labs.
Aho, who serves as chair of Anchorage's Geotechnical Advisory Commission, calls it the most modern seismic array on any high-rise building in the nation.
Nearby the Atwood Building, deep below the surface of the Delaney Park Strip, other seismic monitors are now recording the character of the adjacent ground motion. The two projects go hand in hand, Cole said.
When an earthquake strikes, different areas of Anchorage project different frequencies of shock waves, depending on the depth of the soil from surface to bedrock. Scientists and engineers are just beginning to learn more about how much that difference really matters in Alaska.
For example, shallow soils, like those near the Hillside, produce short seismic waves that strike with a quick jolt. Deeper soils, like those near the Inlet, produce longer waves that roll upward with a rhythmic beat that might repeat once every one or two seconds.
What engineers generally want to avoid, Cole says, is the mistake of placing a tall building with a long structural frequency (due to the building's height) on deep soils with a long frequency. If ground and building harmonize too closely, a high-rise can lose itself with runaway resonance during a major earthquake.
(A vivid example of the phenomena was the harmonic oscillation that destroyed the Tacoma-Narrows Bridge -- nicknamed "Galloping Gertie" -- when high winds in 1940 began shaking the span at the same frequency as the structural frequency of the bridge.)
Conversely, relatively short buildings are more susceptible to earthquake damage through resonance in shallow soils.
Admittedly, Aho and Cole both say, this means that Anchorage's high-rise buildings Downtown are poorly sited on deep soils near the coast and would be better positioned near the mountains. And major low-rise buildings like the campuses of the University of Alaska Anchorage and Alaska Pacific University are poorly sited on shallow soils near the Hillside and would fare much better during a powerful earthquake in deeper soils by the sea.
If resonance was all that matters.
SWAYING
Back on the 19th floor of the Atwood Building -- still in the throes of the Denali earthquake of Nov. 3, 2002 -- resonance was really beginning to matter to Batres.
It was like she was locked in a metronome.
"It doesn't sound like it went on so long for the real world," she said, "but I think that the whole oscillation effect -- (the motion) went on longer due to the swaying."
Finally, as the quake began to subside, Batres pulled out her cell phone and called her brother, a professor of geophysics at a college in New Mexico to ask his advice.
"Because he specializes in plate tectonics, I thought, well, he'll be reassuring," she says.
He wasn't.
"I said, 'I'm calling from the second-tallest building in the state. I'm on the top floor and I'm freaking out.' ... My brother thought it was hilarious."
But he also advised her not to try to use the elevator that was bouncing back and forth in its shaft. Once the shaking stopped, he said, she should exit by the stairs. That's what she did, Batres said.
And she didn't return to the building that Sunday.
"No, I left for the day," Batres said. "I just thought, 'You know, whatever I'm working on -- it can wait.' "
Daily News reporter George Bryson can be reached at gbryson@adn.com.
