Seismically serene

August 11, 2010

Protecting buildings against earthquakes is not a new concept. Techniques such as the use of shear walls and braced frames have been applied for more than half a century. More recently, methods such as the use of base isolators to separate buildings from the ground have gained popularity. In the wake of Seattle's recent earthquake, however, the subject has gained renewed interest among building owners, code-making bodies and insurers.

Applying these time-tested earthquake-proofing techniques to existing historic buildings, however, poses additional challenges for building teams-whether they involve preserving the building's historic fabric, working in confined spaces or implementing construction while the building is occupied.

What follows is a look at several seismic-retrofit projects, each with its own techniques-and challenges-for resisting seismic loads and mitigating damage from earthquakes.

Isolating L.A. City Hall

Perhaps even more than the "HOLLYWOOD" sign, Los Angeles City Hall is recognized by moviegoers around the globe. The 74-year-old structure that Superman helped bring to prominence has undergone a six-year, $273 million seismic retrofit that will allow the building to thrive well into the 21st century.

A relatively new retrofit approach was applied to the building. Inserted into the building's aging skeletal structure were 414 high-density laminated-rubber base isolators, 107 sliders and 50 viscous dampers placed under the existing columns and walls. The assemblies allow ground movement independent of the isolated structure of up to 2 feet in any direction; at 29 floors and 910,800 square feet, the Byzantine-style building is the tallest in the world to undergo this type of seismic retrofit.

The building's original architect, Los Angeles-based architect/engineer AC Martin Partners, was commissioned to lead the seismic retrofit. The firm has a 95-year history of working on landmark buildings in Los Angeles.

The focus of the work was on isolating from the ground the original columns at the building's base, as well as creating a "moat" around the building. First, the basement was extended down by 8 feet after the existing slab was removed. Workers then excavated down to the existing pedestals and column base plates, where reinforcement was placed around all of the pedestals so that each of the footings would be expanded. This created a much stronger base below the plane of isolation. A new rigid and strong concrete floor diaphragm was also created above the plane of isolation.

Other seismic retrofit measures applied to the building during the project-such as the addition of concrete shear walls to strengthen the structure-are not new but are unique in scale.

"The building's historic fabric had to be respected," says Onik Tahtakran, director of structural engineering for AC Martin. "If you introduce a brand new, stiff bracing system inside the building, the effects of an earthquake could be even worse because of the much larger forces that would be generated during a strong ground motion and the altered path they would follow. If the path of these increased forces is not substantiated or reinforced, you could have local damage in concentrated locations."

Several test isolators will be placed under equivalent loading in test frames at the site. They will be monitored and periodically tested for indications of isolator property changes.

The use of base isolators has recently spread from New Zealand to Japan to the United States. It's a success story in the structural engineering field, Tahtakran adds, and L.A. City Hall has written the most recent chapter.

Oregon State Library gets pinned

Several hundred miles north in Oregon's capital, Salem, another historic structure received a seismic retrofit. The 74,000-sq.-ft., three-story Oregon State Library, circa 1939, sits on the northwest flank of the state's Capitol Building.

Because of extremely tight working conditions-as little as 12 inches of working space-the use of traditional concrete shear walls was ruled out by architect Fletcher Farr Ayotte and structural engineer KPFF Consulting Engineers, both of Portland. Instead, the 18-month, $5.47 million project employed a combination of piles, steel plates and tube-steel columns to shore up the building.

Twelve pin piles, each 40 feet in length, anchor the building. Tube-steel columns were erected upon each pile group for the full three-story height of the landmark building. Steel plates, in 4-by-8-ft. sections, were then installed between the columns to form the shear wall. All the work was done with the building occupied and with historic materials never farther than 4 feet away from the delicate construction.

"Things are never in exactly their most ideal locations, so you need to do special detailing to fit your system into the existing elements," says Keith Robinson, project manager with KPFF. "In this case, it involved an existing concrete frame, fitting the new steel-plate shear walls, and getting them to stack vertically for the various floors. That was complex.

"The pin piles are a fairly new technology," he adds. "We installed them at the end of the shear walls, so they will resist the tendency to tip over during an earthquake."

Much of the critical work was done in extremely tight confines with only 8- or 9-ft. clearances, adds James McKune, vice president of Portland-based Emerick Construction. It required the use of forklift-sized excavators that had to be literally disassembled and taken down an elevator piece by piece to be reassembled in the basement work areas.

Seismic work in the library's ornate two-story reference room, which features four 50-ft. unsupported columns with splices in their midsections, also posed a challenge. Original plans-laced with the structural engineer's traditional "brute-force" approach, as Robinson calls it-envisioned steel angles and plates attached directly to the columns, which would have required removal of historic oak paneling on the interior of the room or 8-in.-thick marble cladding from the exterior. Instead, the design team came up with a novel solution involving core-drilling the columns that left the interior and exterior coverings undisturbed.

From the roof, 3-in.-diameter holes were drilled 32 feet down the column centers. Pretensioned steel reinforcing rods were inserted and grouted in place, providing constant compression in the columns.

The technique proved to be faster and cheaper-approximately $40,000 less-than typical alternative approaches.

Anchoring Amazon.com

Another example of a conventional seismic-retrofit technique applied on a very unconventional scale is found in Seattle, where a public hospital was converted into headquarters for Amazon.com. The 69-year-old building needed reinforcing to protect against future seismic events. The solution was an adjacent 63,000-sq.-ft. addition that met stringent preservationist concerns as well as reinforced the structure. Together with a second addition currently under construction, Amazon.com's headquarters space will double.

Sitting atop Beacon Hill, the original structure is one of the Pacific Northwest's best examples of art deco design. Structural analysis in the early 1990s verified that the building needed seismic reinforcement. Zimmer Gunsul Frasca (ZGF), a Portland-based architectural firm, was brought in to develop options that eventually included constructing the supporting addition.

A C-shaped, 16-in.-thick concrete shear wall was constructed eight stories high along the north side of the original building. On each floor, steel braces connect the shear wall to the adjacent floor of the old structure.

"The beams act like fingers running back into the original building and are strapped onto the floors, tying them to the new, very stiff shear wall," says Dale Alberta, a designer with ZGF. "So in a quake, they would buttress the structure.

"The exterior concrete shear wall was clad with brick and terra-cotta, and punched windows sized to the original windows were used," Alberta adds.

Seattle developer Wright Runstad & Co., which holds a 99-year lease on the property, steered the project through the city's landmarks preservation board and formed the project team, which also included two other Seattle-based firms: general contractor Sellen Construction Co. and structural engineer Andersen Bjornstad Kane Jacobs Inc.

         
 

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