Utah Capitol Getting Shock Absorbers

Utah Capitol Getting Shock Absorbers

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SALT LAKE CITY (AP) -- The Utah Capitol is being raised -- 1/32nd of an inch -- as engineers start the critical phase of a retrofit to cushion the 67,500-ton building against an earthquake.

Geologists say the Salt Lake Valley is due for a magnitude 7 earthquake, and workers are preparing to install the first of 280 rubber-and-steel mounts under the Capitol's footings. The "base isolators" will allow the Capitol to sway and list with a quake, like a buoy at sea.

"It is without a doubt the most complex seismic retrofit done in Utah," said Parry Brown, a partner for Salt Lake's Reaveley Engineers & Associates. "The most complicated part of the project was coming up with a load-transfer system -- lifting the building so we can cut off existing columns and remove foundations, which will be replaced with a new foundation and the seismic base isolators."

The hoisting will take another 20 months, "like watching concrete dry," said architect David Hart, executive director for the Capitol Preservation Board, who is overseeing every aspect of the $212 million project.

The emptied Capitol is expected to be ready for occupancy again by January 2008. Gov. Jon Huntsman, his executive staffs and the part-time Legislature will move back in from a pair of Capitol wings constructed as part of a campus-wide renovation plan.

The contractor working at the Capitol, Jacobsen Construction Co., has started another seismic upgrade for the oval-shaped Mormon Tabernacle concert hall on Temple Square in downtown Salt Lake City. That building was assembled 137 years ago using wooden beams tensioned by leather straps.

Hart said the Capitol overhaul is in many ways more complicated than any seismic retrofit, including work done on San Francisco's City Hall and Asian Arts Museum.

The Utah Capitol is a brittle monolith of concrete and marble finished in 1912 with almost no reinforcing steel, forcing engineers to devise a new, rigid framework and interior walls that will hold it up. San Francisco's City Hall had enough steel to make structural changes easier to fashion, he said.

Engineers planned several precautions as workers excavate the Capitol's foundation and cut into its concrete footings. First, workers had to inject grout at high pressure into the rocky, granular soil under the footings. They are laboriously cutting into each footing in a 14-step, staggered process to ensure support remains for the building, a smaller version of the U.S. Capitol.

"To our knowledge this hasn't been done before," Hart said.

Workers are jacking up parts of the building to slide the rubbery mounts under new footings, starting about a month from now. Reaveley said they can lift sections of the building as much as 1/16th of an inch before risking facade cracks, but Hart said he's trying to keep the lifting to 1/32nd of an inch.

Engineers and architects can only hope an earthquake doesn't snap as they transfer loads from old to new footings and a new gridwork of foundation beams.

"Earthquakes occur randomly in general, but the Salt Lake City segment of the Wasatch fault historically has seen large earthquakes every 1,250 to 1,300 years," Brown said.

"The last one recorded geologically with dating techniques was 1,250 to 1,300 years ago," he said.

Geologists can't say with certainty when the next quake will strike, but they say the threat is real and constant and that a large quake could occur a century from now -- or tomorrow. And that threat comes from only one nearby fault line. The clock is different for each of dozens of active bedrock faults ensnaring Utah's heavily populated 120-mile Wasatch corridor that runs north and south of Salt Lake City.

Engineers say the Utah Capitol is doomed without a suspension system. The building is structurally sound -- but only if it doesn't move.

To assess the danger of a quake, the state hired a consultant to drill wells that went 350 feet deep under Capitol Hill -- about 35 feet lower than nearby Great Salt Lake. The first surprise was the drilling never reached bedrock.

"It's all sand and silt. That tells you a lot," Hart said. "It's a big sandbar, is what it is, from Lake Bonneville." The prehistoric lake covered much of Utah after the last ice age; the Great Salt Lake is a tiny remnant.

The second surprise came when a computer analyzed sounding devices threaded into the wells to replicate ground movement from an earthquake.

Geologists have long suspected a large earthquake in the fractured Salt Lake basin could collapse the Capitol, snapping marble columns and unhinging the Capitol dome. But worse, the computer results indicated a distant quake, if large enough, could set off a domino effect of quakes up and down the Wasatch Range, affecting a corridor that is home to nearly 2 million people.

The worst-case scenario means a powerful jolt as distant as Nephi, Utah, about 70 miles south of Salt Lake City, could progressively set off faults northward to Salt Lake and beyond to the Idaho border.

With that in mind, engineers are designing the Capitol to stand up to a 7.3-magnitude quake -- although the building would be unfit for occupancy afterward.

"This building will rock and roll a little bit. There may be a couple things that fall -- the plaster might get cracked, glass might break -- but everybody will get out of it, and then we can come back in and take whatever time we need to rebuild it," Hart said.

(Copyright 2005 by The Associated Press. All Rights Reserved.)

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