Seismic upgrading work begins for Washington state capitol dome

Work is under way to repair the dome of the Washington State Legislative Building in Olympia, which was damaged by the 6.8 magnitude Nisqually earthquake in February 2001.

The $10 million dome repair is part of a more than $100 million program, which also includes mechanical/electrical renovation of the building. It is a four-story masonry structure that consists of House and Senate wings connected to a central rotunda rising 220 ft. above the main structure. The dome is comprised of inner and outer masonry structures.

Michael Wright, a principal with Seattle-based Swenson Say Faget, the structural engineer of record for the project, notes that the building withstood earthquakes in 1949 and 1965. Wright attributes its resilience in the 2001 earthquake largely to seismic upgrades made in 1949, 1965, and 1975. He says these upgrades accomplished about 90% of the work necessary to make the dome seismically resistant.

"We're enhancing the detailing of that work," he says. In 1965, for example, a hollow cylinder concrete wall that supports the dome was constructed, but it was not pinned to the structure below it.

Dome rocks, columns walk

Wright theorizes that the 2001 quake caused the dome to "rock" on top of this wall, allowing 12 freestanding columns that partially support the outer dome to "walk." Prior to the construction of this wall, masonry walls supported most of the weight of the dome. Cracking and crushing, rather than rocking, was then a more typical result of earthquakes, he says.

The earthquake caused five large stones that clad the upper rotunda, each weighing 2,000 pounds, to move away from the building. They were being held in place by a 4-in. patch of remaining mortar.

The legislature was in session and school groups were touring the building when the earthquake occurred. "Though frightened by the sounds of more than 173 million pounds of stones, bricks and building finishes moving in response to the earthquake, everyone exited without major injury," Wright says.

Occupants of the building moved out by last summer, and the legislature is scheduled to return to its chambers for the 2005 session.

Additional Building Team members include project manager NBBJ, executive architect Einhorn Yaffee Prescott, and GC/CM Mortenson.