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Steel Meets Concrete in U.S. Courthouse

An unusual structural system combines steel plate and concrete-filled pipe to produce a cost-effective seismic-resistant frame for a federal courthouse.

June 01, 2006 |

When officials at the U.S. General Services Administration first presented the program requirements for the new federal courthouse in Seattle, the building's structural engineer knew exactly what to pull out of its bag of tricks.

GSA wanted a building that balanced safety and security features with open, welcoming public space and expansive, unobstructed views from interior spaces. The agency also wanted a 70% efficiency ratio and a 100-year durability standard in an earthquake-prone region, all the while adhering to a construction budget that was in line with GSA's standards.

Seattle-based Magnusson Klemencic Associates answered with a first-of-a-kind composite structural system that combines steel plate and concrete-filled pipe to create a compact, super-strong core that consolidates the building's gravity-, wind- and earthquake-resisting systems. This eliminated the need for a full perimeter backup moment frame, resulting in open and flexible floor plates. The solution also resulted in an optimized lateral system that integrates key architectural and mechanical amenities, keeping the surrounding area free for precious courtroom space.

The steel plate/composite shear wall system combines steel plate wall panels, steel wide-flange beams and columns, and concrete-filled steel pipe columns. One-inch-thick steel plates are welded between the five-foot-diameter pipe columns on the north and south faces; conventional steel framing fills the east and west sides. This core runs nearly the full height of the building. (A conventional moment frame supports levels 22 and 23.)

During an earthquake, the steel plates are free locally to buckle or bend, preventing damage to the surrounding framework that holds up the building. With every cycle of plate buckling and bending in an earthquake, the seismic forces are further resisted, according to Brian Dickson, PE, senior associate and MKA's project manager for the 23-story, $175 million courthouse, which opened in late 2004. Dickson says this new approach takes advantage of the compressive strength of concrete and the seismic energy-absorbing properties of structural steel.

A composite structural system serves as the core of the 23-story, $175 million federal courthouse in Seattle. The unusual design combines steel plate wall panels, steel wide-flange beams and columns, and concrete-filled steel pipe columns. One-inch-thick steel plates are welded between the five-foot-diameter pipe columns on the north and south faces; conventional steel framing fills the east and west sides.
Photo: Michael Dickter, Magnusson Klemencic Associates; Diagram: MKA

The unusual structural design was honored last month with a Grand Conceptor Award at the 2006 Engineering Excellence Awards, sponsored by the American Council of Engineering Companies.

Dickson says the GSA conducted extensive testing of the system before approving its implementation. Eight months prior to construction, a half-scale replica of the system was tested at the University of California, Berkeley, where jacks pushed and pulled with a force of 1.2 million pounds on the model in a manner consistent with tests performed by the Federal Emergency Management Agency after the 1994 Northridge earthquake.

"It performed better than what we thought," says Dickson. "It has exceptional seismic force-resisting characteristics. It's a very ductile system."

MKA specified the system twice previously, but neither project (a residential tower in San Francisco and an office building in Bellevue, Wash.) was ever built. The third time was a charm for MKA's design, as GSA officials liked the compactness and seismic performance of the system. They also liked the cost savings over more traditional structural approaches.

Because the system is mostly steel, it is lighter than all-concrete structures, or even many traditional steel-braced frames. "Because there's less building mass, there's less seismic demand," says Dickson. "Those demands need to be resisted somewhere, and that happens in the foundation in most cases." He estimates that the system resulted in a cost savings of 10% in foundation costs.

Furthermore, at the time of construction (in early 2001), the cost of steel plate was substantially lower. "It used to be that plate was relatively inexpensive by comparison, so we were utilizing a low-cost material that was very simple to fabricate," he says. "In today's market, this may not be the case, because the cost for steel plate is considerably higher."

Concrete-filled pipe, on the other hand, is still "hard to beat" from a cost perspective, especially in high-rise construction, according to Dickson. "One of the great benefits of concrete-filled pipe is that you're able to rely on the pipe as a form for the concrete, as your confining steel—which becomes more important in areas of high seismicity—and also as your vertical reinforcing," says Dickson.

"If you were trying to find an equivalent all-steel solution, it could be two to three times as costly, if not more than that," says Dickson. "The concrete-filled pipes that we used are about five feet in diameter, so you would have to start bundling or building up structural box columns, which tend to be very costly by comparison."

Apparently, the GSA is happy with the results in Seattle. The agency is planning to implement the system in the new federal courthouse in Los Angeles. For Dickson, imitation is the ultimate compliment. "The GSA is really smart about these things," he says.

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