SF concrete buildings also a quake danger

Tag : Steel Die Sets
San Francisco Mayor Gavin Newsom's recent push for anearthquake-retrofit study of some of the city's shaky wood-framebuildings means a plan for potentially deadly concrete structuresis on hold for now.
Local engineers estimate that San Francisco has several hundredconcrete-frame buildings that are liable to crumble in a bigearthquake centered close to the city. The precarious structurescome in different shapes and sizes - some are high-end condominiumson Nob Hill, and others are former printing houses South of Market.But they were all constructed before building code revisions in themid-1970s that demanded more steel reinforcement in and aroundbeams and columns, and stronger walls.
City officials have decided to speed an analysis of what to doabout so-called soft-story buildings because the wood-framestructures are ubiquitous in San Francisco - possibly numbering inthe tens of thousands - and probably represent a greater risksimply because of their numbers.
Soft-story buildings include the classic San Francisco apartmentbuilding with a store or restaurant on the first floor, as well asthe Sunset District home built over a garage. They have aplate-glass window or garage door on the ground floor instead of asolid wall, making them susceptible to twisting and buckling.
The inflexible, or "nonductile," concrete buildings did notcollapse in the 1989 Loma Prieta earthquake the way that Marinadistrict soft-story apartment buildings did, but experts say theycould pancake in a quake centered near the city, making it unlikelythat people inside would survive.
"If you look at dangerous buildings, these are high on the list,"said David Bonowitz, a structural engineer who sits on the city'sseismic safety advisory committee. "They often are big andmultistory, and when they collapse, people die."
Nonductile concrete buildings tend to collapse in big temblorsbecause they don't have enough steel in their columns and beams toallow them to bend and twist. Instead, their supports crumble andeven explode under too much pressure. Dismal safety record
The buildings have a dismal earthquake safety record, collapsing inmany U.S. quakes, including the 1971 San Fernando and 1994Northridge temblors, and claiming thousands of lives overseas inMexico, Japan and Turkey.
Oakland's former Cypress Structure was a nonductile concreteroadway. The 1 1/2-mile elevated freeway's top deck collapsed inLoma Prieta, killing 42 motorists and injuring hundreds more.Unstable fill soil at the freeway's base made its weak columns evenmore hazardous. The mangled roadway was torn down, and traffic wasrerouted onto new structures.
On July 8, Newsom introduced legislation to address the morewidespread threat posed by the soft-story buildings, which fordecades had largely been ignored by city engineers and leaders.
Newsom's legislation would waive fees and expedite retrofitpermits, and he has instructed the city's Department of BuildingInspection to help create retrofit guidelines for soft-storybuildings. Some Bay Area cities have required property owners toretrofit their soft-story buildings; Newsom's voluntary measure wasseen as more politically agreeable than a mandatory plan. Get in line
The building department has made soft-story buildings a toppriority, directing consultants to analyze the risks and developretrofit plans. But that means other vulnerable structures willhave to wait, according to department Director Isam Hasenin. Theconsultants don't have the staff to simultaneously analyze eachtype of vulnerable building in San Francisco, he said.
The city hopes to make recommendations on how to retrofitsoft-story buildings by the end of January, but devising a plan forother buildings is now more than 18 months away.
While soft-story retrofits are relatively cheap and easy, it isenormously expensive to identify susceptible concrete buildings andshore them up, according to Craig Comartin, director of a group ofstructural engineers called the Concrete Coalition.
The engineers are working to understand the scope of the concretestructures' danger and the best way to ameliorate it.
A retrofit for a nonductile concrete building requires infillingwith new shear walls, "jacketing" or wrapping columns with steelrebar or carbon fiber, or constructing steel braces - or somecombination of all three.
Comartin, who has advised UC Berkeley's retrofit of 12 concretebuildings over the past 10 years, estimated that typicalstrengthening jobs cost $20 to $150 per square foot of floor space.In some instances, repairs can total 60 to 70 percent of the costof replacing the building, he said. 'It's not cheap'
"It's not always clear from the outside that there is glaringdeficiency," Comartin said. "Sometimes it takes an engineer toreally go through and determine whether or not the building isdangerous. It's not cheap."
The high repair cost explains why no big California city hasapproved a law requiring retrofits of nonductile concretebuildings. The Los Angeles City Council tried in 1995, butresistance from building owners killed the plan.
Greig Smith, an aide for a Los Angeles city leader in 1995 and nowa city councilman whose district includes Northridge, is workingwith Comartin to develop an inventory of that city's nonductilebuildings.
Smith said he intends to introduce legislation in the coming monthsthat will set dates by which owners of the dangerous buildings mustretrofit their property. In the meantime, he said, he's working onincentives, such as using public bonds to provide loans to ownerswho retrofit voluntarily.
"This is not easy to do because it's so expensive," Smith said."Last time we talked about this, I got a lot of phone calls frombuilding owners saying, 'Whoa, wait a minute.' But it's somethingthat needs to be done."