ABSTRACT
California has high incidences of damaging earthquakes. Eighty percent of the state’s population lives in the seismic zone with the greatest probabilities of strong ground motion. Most earthquake-related death and property loss result from damage to structures. Many old buildings remain from the early years before the building codes had significant provisions for seismic resistance. In particular, many unreinforced masonry buildings pose real hazards to human life. Seismic retrofit greatly reduces the life risk at a fraction of the building’s replacement cost. Risk analysis provides a basis for deciding if retrofit makes sense as a risk-reduction strategy.
The risk analysis provides estimates of the cost of preventing a quake-related death. Estimates for the typical cost of preventing a death are as follows: for unreinforced masonry bearing wall buildings, $0.6 million; for unreinforced masonry infill wall buildings, $3.7 million; and for nonductile concrete frame buildings, $9.6 million. The uncertainty in these results is about a factor of 10. The building-to-building variability introduces another factor of 50 to the distributions. Surveys of Americans indicate that they value incremental risk reduction at $3 million to $7 million per life saved. On this basis, retrofit of unreinforced masonry bearing wall buildings is a good way to save lives. Retrofit of unreinforced masonry infill wall buildings makes sense where local conditions indicate a high hazard. In light of the uncertainty, requiring the retrofit of all nonductile concrete frame buildings is not a good way to save lives.
Some local governments in California have taken action against the dangers of unreinforced masonry buildings. Long Beach is a pioneer, passing an ordinance in 3421971 that required retrofit or demolition of buildings. The city of Los Angeles passed a mandatory retrofit ordinance for its 8000 bearing wall buildings in 1981. The city is now gearing up to take on infill wall buildings. The state passed the Unreinforced Masonry Building Law in 1986, which required cities and counties to establish mitigation programs by 1990. Many of these programs require retrofit, but other programs are ineffective.
