SALT LAKE CITY (AP) Renovations that will let the Utah Capitol ride a major earthquake like a buoy at sea entered the final year Nov. 1 at a critical stage.
Construction crews can only hope an earthquake doesn’t occur before February, when they finish transferring foundation pillars onto a set of shock absorbers.
Geologists said a quake of 7-magnitude or greater is overdue here. And any ground shaking in the meantime could tear apart or bring down the 67,500-ton Capitol.
“It’s a very unusual project,” said Parry Brown, a partner of Salt Lake’s Reaveley Engineers & Associates Inc. “There aren’t many buildings of this type that are historic structures located in high seismic areas.”
The Utah Capitol, a smaller replica of the U.S. Capitol, is a brittle monolith of concrete and marble finished in 1912 with almost no reinforcing steel, forcing engineers to devise a suspended chassis and rigid interior walls that will help hold it up.
Crews are installing 280 shock absorbers — engineers call them base isolators — that will let the building pitch with ground motion and slide laterally up to 2 ft. (.6 m) and rebound, absorbing the jolt of an earthquake.
The steel-and-rubber sandwiches, locked rigidly in place, can be activated only after the final isolator is installed and after the building is allowed a month or two to settle. That will leave the building vulnerable until February, officials said.
Although strong Utah earthquakes are infrequent, separated by hundreds of years, the record shows them to be fairly regular — and violent. In an instant they can raise the Wasatch mountains 10 ft. and drop the valley by 5, ripping open the land like a zipper.
Geologists said the Salt Lake City segment of the Wasatch fault operates more predictably, snapping every 1,270 to 1,442 years.
Careful studies show the Wasatch fault, among dozens of faults in Utah’s heavily populated Wasatch corridor, last slipped 1,286 years ago.
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 when the shaking finally stops.
Crews also are digging a hole for a $15-million underground parking garage. Capitol architect David Hart expects all major construction to be completed a year from now, with finishing touches continuing until the Legislature returns in January 2008 from a pair of marble office wings.
Those buildings built two years ago cost $45 million with a parking garage.
“The schedule looks to be holding very well,” said Hart, who is overseeing every aspect of the $212-million project.
Although engineers spent years analyzing the building’s design and retrofit, excavations still brought some surprises.
One was found under the Capitol’s rotunda, the heaviest part of the building, where 9 million lbs. (4 million kg) of dead weight sit on each of four foundation pillars. One footing was especially shallow, making it more difficult for crews to dig it out safely to install a new pillar, said Dave Marshall, general superintendent at the site.
The rotunda was recently lifted by 1/16th of an in. (.15 cm) for installation of the base isolators, a scary moment that Brown called no small feat. Engineers had calculated the building could be safely hoisted that much, but they couldn’t guarantee it.
The building didn’t crack.
“This is definitely a flagship project for us,” said Douglas Welling, president of Salt Lake City’s Jacobsen Construction Co. Inc., a general contractor working jointly with Arizona-based Hunt Construction Group.
“It’s been a collaborative effort to make this an immensely successful project. We’re on schedule. We’re managing the budget of the owner to guarantee a price, and we’re going to finish it within budget,” he said.