In an effort to reduce traffic and improve quality of life by creating pedestrian-friendly neighborhoods, major West Coast cities are encouraging developers to build higher buildings to avoid pushing the surrounding suburbs further and further outward.
A recent New York Times article, however, questioned the viability of continuing to encourage vertical construction in cities with high vulnerability to earthquakes, particularly San Francisco and Los Angeles. Both of these cities are located in the highest-risk Class 4 Seismic Zones. San Francisco now has 160 buildings taller than 240 feet and a dozen more are in the planning stages or currently under construction, according to the New York Times.
San Francisco has a building height limit of 970 feet in the area where the new 1,070-foot Salesforce Tower is located, according to Tom Hui, director of the San Francisco Department of Building Inspection and a structural and civil engineer. But the currently tallest building in the city, a joint venture of Boston Properties and Hines that was designed by Pelli Clarke Pelli, was approved because its seismic performance goes above and beyond what is required, Hui notes.
Located adjacent to the downtown San Francisco’s new Transit Center, the foundation of the Salesforce Tower goes down 200 feet, “all the way to bedrock,” says Hui, and uses a seismic force-resisting building system composed of special reinforced concrete shear walls, 24 to 48 inches thick at the central elevator and stair core, and a structural, load-resisting steel column and floor framing support system.
The project’s structural engineer, Magnusson Klemencic Associates, led the development and adoption of performance-based seismic design (PBSD), an approach that results in buildings that are safer and preform more predictably during earthquakes than structures that rely on a strictly code-based approach. PBSD uses non-linear computer modeling to meet the intent of the building code and quantifies the building’s performance during multiple seismic scenarios.
Still, the question for real estate investors may be whether there is an increased risk in purchasing skyscrapers in California.
According to Monica Kohler, a Caltech civil engineering research professor based in Pasadena, Calif., the state made seismic retrofitting of hospitals and schools mandatory following the 6.7magnitude Northridge Earthquake in 1994. But newer, more stringent seismic requirements now also apply to all new buildings, she points out.
While the code may be similar for buildings whether they are under or over 75 feet high, William Strawn, manager of legislative and public affairs with the San Francisco Department of Building Inspection, stresses that much more goes into evaluating a building’s performance during an earthquake than building height.
“Bottom line, structural requirements are more strict as buildings get taller.” As a result, he says, “The new skyscrapers are the safest buildings in the city.”
Strawn notes that San Francisco has some of the most stringent seismic requirements in the state and possibly the world. All new projects must meet basic California seismic code and tighter local code amendments, both of which are updated every three years based on what is learned from earthquakes occurring around the world. The latest local amendments are performance-based, required by AB082 and AB083, which call for a review of projects by a panel consisting of structural and geostructural engineers and an academic professor, who use PBSD modeling to predict how the building’s design and materials will perform during an earthquake, says Hui.
Kohler notes that the City of Los Angeles also calls for the use of computer modeling to determine how buildings will perform during an earthquake. “But no matter how expert the computer modeling, all earthquake are different and they continue to surprise us,” she adds.
A way to monitor a building’s actual performance during earthquakes, she continues, would be to install seismic sensors that measure a building’s shaking during a seismic event. “We’re flying blindly without sensors,” Kohler says. “There may be lots of buildings with fractures from prior earthquakes, but because the building’s motion was not recorded, no one knows where to look for them.”
She also notes that modeling doesn’t always take into account how a building’s underground infrastructure—parking garages, utilities, subway stations—might perform during an earthquake, and how each system might impact surrounding structures.
Strawn agrees. “There is no such thing as an earthquake-proof building, because you just don’t know where or how an earthquake will strike.”
Since 2013, San Francisco has required that older wood-framed multifamily buildings with a soft-story condition complete a mandatory seismic retrofit program. Today, nearly 5,000 of such buildings, representing 90 percent of that building stock, have completed or are in the final stages of the retrofits. Another 900 or so buildings face a September 15 deadline to comply with the requirements.
Such adjustments are particularly important, according to Kohler, because California is due for a major earthquake, which happens along the San Andreas fault about every 150 years, according to soil samples examined by researchers. She notes that mandatory retrofitting is the best way to prepare for such an event.
Following the Northridge Earthquake, Los Angeles began identifying at-risk buildings as well and plans to implement a similar retrofit program.
Kohler points out that while this was not a terribly large-magnitude earthquake, a lot of older buildings collapsed because they lacked the strength to withstand the side-to-side motions with the quake. These included:
- Buildings with a soft first story, like an open garage, because the support columns were too slender to bear the building’s weight;
- Unreinforced concrete, brick and concrete-block buildings, which don’t have the support to move sideways; and
- Non-ductile concrete buildings with little or no steel reinforcing, which can lead to cracking during seismic events.