IN THE CANS Key to the steel-plate, shear-wall core are large-diameter pipe columns filled with 10,000-psi concrete. (Photo courtesy of JAJones/Absher)

At the $215-million U.S. Federal Courthouse in quake-prone Seattle, the designers are proving beyond doubt that it's not always a crime to deceive for appearance's sake. Their "secret" weapon in their cabal to create a building that sings out liberty and justice but provides top security and seismic safety is a hybrid shear-wall core. The system, considered a first, combines steel plates, braces and beams into cells "guarded" at the corners by giant steel "cans" filled with concrete. The spine, which has the pluses of a pure concrete core without its penalties of weight, mass and a slow work pace, is hailed as the project's unseen hero.

The architect, calling the 118-meter-tall tower's slender and lightweight SPSW "creative innovation," finds it liberating. Under tremendous budget pressure, "we're struggling every day to achieve absolutely essential security," says Steven McConnell, principal of project architect NBBJ, Seattle. "The level of innovation and the serious money [the core] saved meant more public benefits," he adds.

(Photo courtesy of Michael Dickter/Magnusson Klemencic Associates)

The prime security feature that masquerades as architecture is a main lobby reflecting pool that, with an infrared security screen, invisibly guards against intruders. When penetrated, the system locks down the lobby and alerts security. The prime feature that allowed the architect to fulfill its federal mandate for a sense of openness in the public realm is the absence of a heavy-braced or moment-resisting frame along the "front" half of the building, made possible by tucking the lateral-load-resisting system into the heart of the 23-story tower.

"The core gave the architect flexibility," says Brian Dickson, project manager for the local structural engineer, Skilling Ward Magnusson Barkshire (SWMB). "Once the discipline of the core was set, things became easier," he adds.

Thanks to the SPSW system, there are no perimeter columns cluttering the tower's face. This "transparency," which allows floor-to-ceiling glass, is accomplished through a 7-m cantilever along the front face and a 2-m cantilever around the corners. It allows a clear view into the courtroom lobbies, obstructed only by seven lines of 3.8-centimeter rods. The rods tie the cantilevers together, which minimizes deflection and vibration.

There's more. Because the maximum 3.8-cm-thick steel core doesn't gobble up as much real estate as would an equivalent 0.6-m-thick concrete core, the architect was able to fit the courtrooms into the given floor plate.

The plate core came to the rescue again during design when it became apparent that the courtrooms, which abut the core walls, were shy of floor space. No problem. The structural engineer "shrank" the core footprint to squeeze out that critical space. The supercolumns stayed put, just outside the core's footprint. If moved, they would have invaded the elevator shafts and reduced overturning resistance.

Additionally, the steel system weighs less than would an equivalent concrete core, which minimizes the cost of foundations. Concrete's greater mass means more force would have been exerted on the building in an earthquake. "You have to resist that," says Dickson.

The system's performance in cyclic tests at the University of California, Berkeley, was also astonishing. The inelastic drift was measured at 3.3%. That's "significantly" higher than other comparable systems, says Abolhassan Astaneh-Asl, the Dept. of Civil and Environmental Engineering's principal investigator. Drift value for a moment frame may "get to 3%," he says. Concrete frames don't get beyond 2%.

Astaneh had expected the tests, performed on two- and three-story half-wall specimens, with a supercolumn, to take one day and last through 20 push-pull cycles. But the tests lasted 2.5 days. One specimen went through 80 cycles; the other, 60. "It's unbelievable to structural engineers how much drift these walls could take," says Astaneh.

The test's long duration put a strain on the actuator, he says. It slipped ¹/₈ in. Transducers shut down the system, which was adjusted. There was reportedly no damage. Click here to view renderings.

Another innovation is a system of steel catenary cables embedded in floor slabs. They are designed to help the structure resist progressive collapse if a column is lost. For security reasons, even before the Sept. 11, 2001, terrorist attacks, GSA instructed the team to remain silent about the cable system's details (ENR 6/11/01 p. 7).

But all are free to talk about the core. Bill Bishop, general superintendent for the local joint venture contractor, JAJones/Absher, estimates it was 30% faster to build than an equivalent concrete core.

Steel erector Adam Jones, president of The Erection Co. Inc., Arlington, Wash., calls the system "the thing of the future" for high seismic zones, despite some fit-up difficulties. "Dollarwise, it more than holds its own," he says. A steel-framed building with a concrete core takes about twice as long to build, says Jones. He adds that an all-steel frame avoids the troublesome coordination problems between concrete and steel work.

The owner, the U.S. General Services Administration, is pleased with design and construction. "We've had several projects over budget and lots of claims in the past," says Rick D. Thomas, project manager in the Auburn, Wash., office of GSA's Public Buildings Service. The courthouse, 58% complete, is on time and slightly under budget.

NBBJ claims the 56,222-sq-m courthouse, which includes the 23-story courtroom tower and an independent nine-level office wing, is one of the few large-scale and complex jobs in the country designed using object-based CAD. The architect focused on sustainability. Public spaces are cooled using energy-conserving displacement air ventilation, more common to Europe. Courtrooms, lobbies and offices are designed to maximize daylighting. Waste is minimized during construction. Steel, concrete and insulation have recycled content. And NBBJ got the end-users to agree on a standard size courtroom, which it claims is a first. This allows standardized spans and stacked courtrooms.

Though never before used in tandem, the parts of the core system are not new. The engineer developed the composite supercolumn for Seattle's tallest building, the 76-story Columbia Center (ENR 3/15/84 p. 28). The SPSW system was first intended for a 51-story high-rise in San Francisco but the building never got off the ground.

In the system, supercolumns--large-diameter steel pipes filled with 10,000-psi concrete–take gravity loads and resist overturning forces. Facing SPSWs in the 9.4x 17.4-m core's short direction and facing braced frames with discrete steel plates in the long direction resist seismic loads. The SPSWs include an in-plane backup moment-resisting frame that works in conjunction with a perimeter moment frame on the north half of the facade. The frame is primarily for occupant security as the building line is close to the street.

In plan, the core looks like a rectangle with a circle outside each of the corners. SPSWs are framed by wide-flange columns and beams, with steel plate infill panels. Each wall has an opening midspan for access into the core. Braced frame walls are inset 0.5 m from the inside face of the supercolumns and welded directly to the SPSWs. SPSWs are bolted and welded to the supercolumns. Pipes range in diameter from 1.7 to 1.1 m.

A full braced frame was ruled out because the nearly 7-m floor-to-floor height would have rendered it inefficient. A pure plate system was also ruled out as uneconomical because of large plate sizes.

This SPSW system is not one of the predefined systems in the 1997 Uniform Building Code. It was approved for use under alternate system provisions, says the engineer. The engineer used an R factor of 7.5 to design the system, as indicated for special concentric braced frames.

The plate wall is not simply a shear element. During extreme loading conditions, plates will buckle diagonally and then exhibit diagonal tension-field shear characteristics to resist lateral forces. This increases shear capacity, which is a benefit.

Supercolumns are connected to concrete foundations through reinforcing dowels set into drilled shafts. Core wall columns are connected through embeds in the concrete.

The engineer wanted to shed as much core gravity load as possible to the supercolumns to keep the SPSWs from behaving as columns. The weight would also reduce overturning forces. To accomplish this, the engineer specified that crews erect the core and fill the cans with concrete before welding the SPSWs to the foundation.

Construction, set for completion in March 2004, got off to a rough start in July 2001, mostly due to unanticipated soil contamination. Thanks to selective overtime and a switch to shotcrete foundation walls, the team picked up 42 of the 46 days lost in 46 working days. "We threw a bottoming-out party for the slab-on-grade pour," says JAJones/Absher's Bishop.

Steel erection began six calendar days late, on April 1, 2002. Bishop reports that the core erection was complicated because of tight tolerances. Some bolted connections had to be switched to welds. But the design called for specific welds in some areas and none in others. Therefore, the structural engineer had to be consulted to make changes.

Steel was erected two stories at a time using one tower crane, in a repetitive sequence that began with erection of the 13-m-tall pipes. Next, crews erected each plate wall, typically in a single pick, and bolted it to a supercolumn. Crews then erected the braced-frame walls, followed by the perimeter moment frames, floor beams and metal deck. While the crew moved to the next two floors, concrete crews pumped material into the pipes from the bottom up to avoid interfering with steel work.

To accommodate column shortening, the bottom of the SPSW was not welded to the floor beam until the concrete deck topping was poured eight floors above the plate wall.

The building was erected at a pace of two floors every five working days. The Erection Co. completed the job in six fewer working days than the 91 it had scheduled.

"We think it is an excellent system," says Jones, that could be made even better by modifications to the connections. That would "relieve some of the liability of the fabricator for fit-up," he adds, by relaxing erection tolerances.

"Skilling's all for the improvements," Jones continues, saying that the culture of collaboration and innovation created by John Skilling, who died at age 76 in 1998, still exists. "They're top of the line and always have been," he says. SWMB is often called an architect's engineer. Praise from all corners of the courthouse reinforces that reputation.