Three Buildings, Three Different Approaches
It is not surprising that architects in the earthquake-prone San Francisco Bay Area incorporate robust and sophisticated seismic technologies into their buildings. Some of the earthquake-resisting strategies in the region’s high-profile new-construction projects, such as the coupled shear-wall system inside the twisting and turning tower at the de Young Museum or the base isolators below the just-completed Oakland Cathedral, naturally attract attention because of their unusual design. But older, more conventional structures are continuously being subjected to seismic retrofit and they require at least just as much engineering finesse.
One of the area’s innovative seismic retrofit projects is part of developer TMG Partners’ plans to transform a vacant and fortresslike tower at 680 Folsom, in San Francisco’s South of Market neighborhood, into desirable office space. The overhaul, designed by Skidmore, Owings & Merrill (SOM), involves installation of completely new building systems and replacement of the seemingly impenetrable precast-concrete cladding with a high-performance glass curtain wall.
Even though the steel moment frame is too flexible for a vertical addition under current code requirements, Berkeley-based structural engineer Tipping Mar found a way to add two floors to the 12-story building and increase the size of each floor plate with four horizontal extensions. Plans call for enlarging the 1960s-era tower by about 30%, adding 125,000 sq ft to the current 400,000 sq ft. Even so, the renovated 680 Folsom will not appear bigger, say its designers.
The articulated floor plate will allow “the building to be read as three separate volumes instead of one big hulking mass,” says Leo Chow, associate director of design in SOM’s San Francisco office.
The new configuration also should provide TMG with an extra edge as it tries to lease the tower before moving forward with construction. The horizontal additions should make the building more attractive to corporate tenants since the protrusions provide the possibility of additional corner offices on each level, explains Chow.
Early in the design process, the team considered a retrofit scheme that included a perimeter bracing system, but they worried that such a strategy would add to the building’s visual bulk. Instead, they opted for constructing I-shaped, post-tensioned concrete spines at each end of the tower’s core. The addition of these spines, or “flexural walls,” will provide strength and stiffness, and displacement will be reduced, explains David Mar, principal of Tipping Mar.
The spines will be “self-healing,” according to Mar. The post-tensioning tendons inside the concrete will allow them to flex at the base and realign without sustaining damage. Mar and his team have set the strength and stiffness of these spines to protect nonstructural components, such as the glass facade. “We worked with the manufacturer to make sure maximum drifts would not damage the skin,” he says.
This strategy will help the renovated building exceed the requirements of the seismic code, which emphasizes preventing loss of life rather than minimizing property damage. Even after a powerful earthquake, 680 Folsom should be both operational and safe to occupy.
This resiliency, along with features such as a green roof, a high-performance skin and energy-conserving mechanical systems, is part of the project’s bid for LEED Gold certification. Even though the rating system has no credits pertaining to seismic design, the team has applied for innovation points based on the renovated building’s durability. “We are getting 680 Folsom ready for the next 100 years,” says Mar.
Less Invasive
In China Basin, along San Francisco’s southern waterfront, structural engineers from Simpson Gumpertz & Heger (SGH) and architects from HOK faced a design problem similar to that posed by 680 Folsom: The owners of a three-story, 300,000-sq-ft building wanted to expand but were limited by the seismic capacity of the existing concrete structure. However, the client, real estate investment firm McCarthy Cook, had one additional, and significant, requirement: Construction could not disrupt the operations of the bioscience laboratories that already occupied the building.
The restriction ruled out construction of shear walls, a solution similar to that proposed for the vacant 680 Folsom, since the strategy could not be deployed without displacing the tenants from the mid-1980s structure. To avoid invasive interior construction, the firms devised an alternative approach that included two new steel-framed stories over the roof of the existing building, on top of a system of seismic isolation bearings. At 150,000 sq ft, the addition would be three times the size of the one the building had been originally designed to accept.
Bearings like those at China Basin typically are used under structures, for both retrofit applications and new buildings. SGH engineers say the project, completed last spring, is the first in the U.S. to incorporate seismic isolators in a location other than the base of a building and in conjunction with a vertical addition.
Better Performance
Implementation of the strategy required only minimal construction on the lower floors to strengthen columns so that they could accept the increased gravity loads. But even without major interior retrofit work, the addition “made the existing building better from a seismic standpoint,” says John Sumnicht, a senior principal in SGH’s local office. The new floors act like a mass damper. “During strong earthquake shaking, the new stories will tend to counterbalance the movement of the lower floors and actually reduce the amount of seismic forces and displacement demand on the existing structure,” he says. The isolation system is designed to allow the addition to move as much as 45 in. in either direction relative to the existing structure below—1.5 times the displacement required by code.
The 87 isolators are positioned on top of the existing building’s columns and sandwiched between two grids of steel beams. The system is composed of 33 lead-rubber bearings, and 54 elastomeric slider bearings. The two types are combined in order to tune the structure’s period—the time it takes the addition to move from center, to the extreme right, to the extreme left, and back to center again, explains Sumnicht. Lead-rubber bearings alone would make the period too short; replacing some of these with sliders lengthens the period, he says. “This is one of the ways seismic isolation works. When the period is lengthened, the building is not shaken so violently.”
Because the existing building is a lab, it has a high concentration of roof-mounted mechanical equipment and...
...utilities. Designers needed to closely examine clearances around the isolators so they would not collide with, and damage, utilities during a quake. Contractors also had to take special care not to disrupt building services during construction. “Coordination at the roof level was a nightmare,” says Sumnicht.
For the architects, the biggest challenge was design of vertical circulation—the locations where core elements such as stairs and elevators extend through the new construction. “In order to accommodate movement, we had to consider what would connect to the existing construction and what would connect to the new construction,” says Mark Borchardt, senior associate in HOK’s local office. In the end, the architects decided to extend the cores from the existing building and surround them by a 36-inch moat to prevent core elements from crashing into surrounding construction during a temblor. A tunnel-like device that allows movement in any horizontal direction bridges the moat and connects the cores to the new floor plates.
Sliding Slowly
Across the bay at the University of California, Berkeley, engineers are planning a very different kind of retrofit for the school’s 85-year-old Memorial Stadium. The engineering problem presented by the stadium is an unusual one, in part because of the building’s heritage. It was designed by Beaux-Arts architect John Galen Howard and was placed on the National Register of Historic Places in 2006. But the characteristic that most clearly distinguishes the building from most other structures slated for seismic retrofit is its location straddling the Hayward Fault. The approximately 40-mile-long fault runs along the western edge of the East Bay hills, separating the North American Plate from the Pacific Plate. It bisects the 72,000-seat stadium, tracing a roughly north–south diagonal from one end zone to the other.
Hayward is the type of fault known as “strike-slip.” According to geologists, during a quake on this kind of fault, the earth on each side of a ground rupture moves mostly in a horizontal direction, with one plate sliding past the other. Other types of faults, such as “normal” and “reverse” faults, involve primarily vertical movement, while “thrust” faults involve a combination of angled and vertical movement.
A major quake has not occurred along the Hayward fault in 140 years, yet the earth around it is moving very slowly but steadily at a rate of about 3⁄16 in. per year. Evidence of this “fault creep” can be seen in the stadium’s nonductal reinforced-concrete structure, especially under the seating bowl at expansion joints, where adjacent columns and beams no longer align.
“Incrementally, the eastern half of the building is moving south, and the western half is moving north,” explains David Friedman, senior principal with Forell/Elsesser, the stadium project’s structural engineer. The San Francisco-based firm has devised a retrofit plan to accommodate expected horizontal surface displacements of as much as 6 ft that could occur during a powerful temblor.
The seismic retrofit is part of a multiphase development plan for the stadium designed by the Los Angeles office of HNTB Architecture. It includes a new plaza and the partially below-grade Student-Athlete High Performance Center (SHPC), to be built along the western edge of the stadium.
The training and sports-medicine facility had been stalled by several lawsuits and protests by local residents opposed to the removal of a grove of trees on the site. But construction of SHPC is now cleared to move ahead in early 2009. The seismic retrofit project, currently in schematic design, will follow as a second phase.
The goal of Forell/Elsesser’s retrofit scheme is to allow the portions of the stadium directly over the fault to move independently from the rest of the building. It includes replacing two wedge-shaped pieces of seating bowl with bunkerlike “rupture blocks” built on top of plastic sheeting and separated from adjacent parts of the structure by a 5-ft gap. During a strong Hayward quake, the blocks may twist or tilt, but occupants will be protected, explains Friedman.
Protective Steps
Construction of the blocks will involve several steps. After first reinforcing the stadium’s historic perimeter wall, contractors will increase the density of the soil with “rock columns.” They will then install the plastic sheet and a 30-in. mat slab before installing steel framing and concrete shear walls to support the wedgelike pieces of the seating bowl above.
As the final step in the retrofit, workers will reconstruct the upper portion of the western half of the seating bowl. The eastern half, which is built into the Berkeley hillside, along with the part of the seating bowl closest to the field, are slab-on-grade construction. As such, they are not subject to collapse during a temblor, and therefore are not slated for replacement as part of the stadium retrofit project.
Underpinning the scheme is extensive research and collaboration with the university’s own experts in fault-rupture mechanics, says Friedman. But the solution they developed does not rely on high-tech devices such as base isolation or high-damping bearings, he points out. For all its sophistication, “the retrofit involves a fairly conventional use of materials and seismic systems.”
