...a seismic joint. But the museum did not want visible metal floor plates in the contiguous gallery space. Slip connections allow the buildings to work together in the north-south direction and yet move independently in the east-west direction.

Framing in the museum building is customized and irregular to meet the needs of the temporary office space and the museum. There are many beam penetrations to accommodate ductwork in a squat, 13-ft floor-to-floor height.

Construction wasn’t regular, ei-ther. To help tame the two-headed beast, Sellen was hired at the start of schematic design, beginning in October 2002. WaMu Center’s schedule, from demolition to a temporary certificate of occupancy and including the first two floors of tenant work, was 25 months. The move-in started in mid-March, instead of early February. Sellen plays down a six-week schedule stretch, attributing it to added scope and accommodating museum needs. Sellen and Pine Street are mute on specifics about “some delay claims” and Sellen’s guaranteed maximum price.

In a 1.5-year planning phase, Sellen used 3D building information modeling for heavily congested areas. BIM was used to plan demolition of the site’s existing, eight-story building and for the 95-ft-deep excavation, which included 936 tiebacks. In that area, BIM helped expedite permitting review. The model convinced the city to allow installation of tiebacks closer to some nearby utilities than is typical. BIM also was used to plan construction of the BRBs and to coordinate ductwork with steel framing.

To stop the bleeding caused by steel price escalation, Sellen let the steel contract in March instead of May 2004, to place the mill order early.

Typically, steel erection leads the charge. Not at WaMu Center. Above the third floor, the core became the crux of the critical path because core elevators were needed for occupancy of the lower floors in early 2006. Taking the steel off the critical path above the third floor also provided more time for steel detailing, which was dependent on the still-evolving curtain wall details. Steel detailing was done in phases, from the bottom up.

“The biggest challenge was organizing the details as the design was completed, rather than in the usual sequence,” says Jim McLagan, vice president of steel fabricator Canron Western Constructors, Portland, Ore.

Sellen also had to figure out what to do with 300,000 cu yd of dirt from the excavation. The contractor worked with the city to get an early fill permit for a nearby SAM sculpture park that needed fill, allowing it to recycle 120,000 cu yd of dirt instead of hauling it 20 miles. The strategy returned $250,000 to the project.

Sellen had built two other performance cores designed by MKA that served as a learning curve for WaMu Center. But WaMu Center was bigger and the site more congested than those projects. Sellen’s strategy to build, on average, a floor every three days was to divide the core into two and operate two shifts. Construction of each section was staggered to achieve the maximum efficiencies.

There was no lay down space for tying the rebar, so Canron pretied the steel at the yard and shipped it in elements that were then assembled, during the swing shift, on a platform built over the driveway. Workers then lifted the groupings into position, using cranes.

The ductile core was so thick with rebar that Sellen used self-consolidating, 10,000-psi concrete. That also helped in locations with massive embeds in the core wall to anchor the BRBs.

A structural steel braced frame would have been five weeks faster. Even so, Sellen, which did the concrete work itself, preferred a concrete core because it retained control of the schedule and the quality. “The strategy allowed us to start elevators when the core was at the 20th floor,” says Rich Olender, Sellen’s superintendent.

Construction went smoothly. The trickiest part involved connecting the BRBs. It turned out that both the core and the pipe columns could be within construction tolerances, and the BRBs might still not fit without adjustment. “We had designed enough 2D tolerance into the pin connection but it was a 3-D tolerance problem,” says Klemencic.

Moving Parts. Sliding panels offer relief from sun or let light in. (Photos courtesy of Sellen)

The tower was routine compared to the museum building, says Sellen, which was complicated by the leaseback arrangement. The extension’s curtain wall, especially a “bris soleil” system of movable panels, was considered the “hairiest” part because it first had to work for offices, and then art. The curtain wall was designed for both.

There were numerous other accommodations to banking and art, especially for circulation, security and building systems. For example, museum elevators installed above the fourth floor are blocked off until the museum takes over more of its space. 

There was a mandate to keep SAM dry. For example, the museum’s “roof”—the four permanent bank floors, is waterproofed as if it is an exposed roof, especially because the WaMu space contains a kitchen. And WaMu’s 17th-floor rooftop garden had to have a drainage system that would prevent water from infiltrating the museum space.

The mechanical system, especially for the museum, was dependent on the curtain wall and the mechanical systems in the leased space had to be designed for conversion to museum use. Both systems were supplied under design-build contracts. McKinstry, Seattle, supplied the mechanical systems and Benson Industries, Portland, Ore., the curtain wall. The complexities forced McKinstry and Benson to work closely with each other and with Allied Works and NBBJ. Sellen coordinated.

WaMu expects to substantially complete its move-in by the end of the year. Work is on time and on budget for a doubled SAM, closed since January, to reopen in the spring.

“I’m quite thrilled we did all this,” says Griffin. “The way we laid it out in the beginning, all the pieces made sense. People just had to play their parts.”