Historic Theater's Retrofit a Tough Act to Stage
The project was being bid out in 2012 while the local construction market was heating up and price estimates were going up quickly. Because of the GC-CM arrangement, Pankow alerted the city that subs would be bidding higher than what they were estimating, says Lee Jones, Pankow project manager. The contractor and the city collocated offices in the second floor of the building. This enabled the team to make decisions quickly as surprises popped up.
Ronald Hamburger, senior principal and head of structural engineering for the Western region of Simpson Gumpertz & Heger, took a performance approach to the building's seismic retrofit rather than following the prescriptive requirements of the code. The engineer used nonlinear dynamic analysis to model expected cracking patterns at various degrees of structural deformation during shaking of the lightly reinforced concrete perimeter walls, clad with terra cotta.
The main element of the seismic retrofit system consists of reinforced concrete shear walls between and tied into existing steel columns and beams that surround the theater's auditorium. Hamburger specified 4-in.-dia shear lug pipes, inserted vertically into the shear walls, to attach the walls to the mat foundation.
The shear lugs are set into greased sockets in the 4-ft-thick ballast slab at 2-ft intervals. The connection allows the shear walls to rock at their bases, thus limiting the forces transmitted to the entire building during a quake. "The system will perform as well or better than anchoring," says Hamburger.
In places, clay-tile partitions surround existing columns. Crews filled the cavities between the tile walls and the columns with lightweight, expansive foam. This provides stability for the walls, considered highly vulnerable without the addition of steel bracing.
Crews embedded as many as six layers of high-tensile-strength fiber-reinforced-polymer-fabric resin strips in the roof and floors for tensile reinforcement. This was an improvement over the steel plates used for the same purpose on the opera house, says Lamont.
FRP has the same function as steel plates, but it is feather light, has virtually no thickness and is much easier to install, she says. The FRP also avoided uneven floor surfaces and kept existing floor elevations.
Another element of the seismic upgrade was horizontal steel bracing in the attic. Crews had to maneuver 30-ft-long braces, picked by crane, through openings in the skylight, avoiding catwalks and plaster ceiling supports. Chain falls, rollers and overhead trolley systems helped move each brace into place after it was unhooked from the crane.
Steel columns that extended from the stage to the roof were demolished to widen the performance area by 12 ft on each side. Crews then installed a new truss that runs across the widened stage, bearing on new supports.
Surveyors laser-scanned the attic and boiler room to provide a more accurate building information model of those spaces for field coordination. In the crowded basement, instead of relying on the drawings from the 1900s, design consultants created 3D digital models of the existing building.
"Some thought this was too much effort, but it really helped," Jones says.
Protecting four murals painted in 1915 was the scariest challenge, says Lamont. The pieces are estimated to be worth $8 million.