Novel elevator design gives King Street Center a lift

By LAURA N. SMITH
KPFF Consulting Engineers

For developers, contractors and engineers, there's nothing like an aggressive construction schedule to create challenges and opportunities for innovation.

That has been the case with King Street Center, a prominent new entry in Pioneer Square's continued renaissance. The eight-story, 450,000 square-foot building has been on a fast-track design-to-occupancy schedule that began in late 1997. The developer, Wright Runstad, expects to have the building which includes underground parking ready for occupancy by office and retail tenants by mid-July, 1999.

Members of the project team, including Cindy Edens project manager for Wright Runstad, Mark Garland, senior project manager for Lease Crutcher Lewis (LCL), Richard Lundstrom, project manager for the architects NBBJ and Greg Varney structural engineer for KPFF Consulting Engineers, determined that one way to meet this accelerated construction schedule was to jump-start the installation of elevators.

On this project, as on most others, the installation of the elevators was on the critical path. This is because the installation of the elevators is time consuming and normally can't begin until relatively late in a project when the elevator support structure at the roof is complete. In addition, elevators can be used during the construction phase to speed up construction.

Building a stand-alone elevator tower and core allowed installation to begin early, maximized its use during construction and prompted a lot of questions from passersby who asked: Where's the rest of the building?

On most sites, workers use ladders or an exterior hoist to move up and down a building in the early stages of construction. The use of an exterior man-and-materials hoist, which is attached to the outside of the building, inhibits the completion of the exterior cladding and the building enclosure.

Early installation of the interior building elevators would provide a more efficient movement of workers and materials and eliminate the need for an exterior hoist. The interior elevators could also be used to move lightweight construction and finish materials fixtures and tools, reducing the time the tower crane is devoted to these tasks.

The project team made the decision to build the elevator support structure at the roof and install the elevators as quickly as possible.

This meant the elevator and stair core of the building became a structure within a structure initially rising up ahead of the rest of the building as a self-supporting tower.

This presented KPFF, the structural engineers, and the entire project team, with several unique considerations and challenges, which required a very creative and interactive design approach.

The team proceeded with a plan to build the tower core using post-tensioned, reinforced concrete. The tower consists of moment frames framed by the four columns supporting concrete beams and floors at each level. These floors and beams support the elevator lobby floor and the core's stairs while forming the openings of the elevator shafts.

NBBJ and KPFF revised the floor plan to allow for the repositioning of the four tower columns in the building lobby to accommodate the floor-to-floor height restrictions imposed by the Pioneer Square historic district. In the event of an earthquake during construction, the tower was designed to be capable of withstanding full seismic loads as prescribed by the uniform building code.

To provide a four-month head start for installation and elevator use, the tower would have to be constructed on a fast-track with eight days of construction scheduled for each floor of the elevator core (as opposed to three weeks per floor for the rest of the structure). This was accomplished by keeping the detailing and configuration of the core the same from floor to floor. This repetitive construction pattern allowed workers from K&L Rebar to quickly position the rebar and post-tensioning cables.

In addition, LCL developed an innovative floor-by-floor construction system. A table top was mounted on trusses, which were then positioned on rollers. Each floor slab was poured on top of the table, remaining there until the concrete reached sufficient strength. Then the table was lowered and rolled out from under the completed level. The tower crane then lifted the assembled framework up to the next floor. This saved valuable time for formwork installation.

One additional detail was added to this scheme. During construction the core was isolated from the floors by a closure strip or gap in the concrete floor surrounding the tower frame. This allows the concrete floors to cure and shrink without damaging the completed core. Once the floors have cured, ranging from 14 to 28 days, the closure strip was filled in with concrete. A side benefit of this closure strip was that it provided a convenient location to post tension the floor slabs.

As construction continues, the core will become an integral part of the building framing, acting as one in resisting lateral forces such as wind as well as seismic loads.

In the mean time, the sight of the core tower rising above the rest of the building draws curious stares and comments from the general public and construction workers at nearby projects.

According to Jerry Lorrigan, job superintendent for LCL, People walking past would look at the building rendering and then look up at the core tower and ask: "Where's the rest of the building?"

"It's an unusual construction process, but it has really worked at this site. Using this tower concept has allowed us to stay on top of an aggressive schedule, in spite of the soft sub-grade and the construction debris from Old Seattle," Lorrigan said.

According to Cindy Edens, the major benefit to the project was enabling the project team to meet the tight schedule requirements.

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