Experts Stress Retrofit Need

Fifteen noted California earthquake scientists say in a new report that the most important lesson of December’s San Simeon earthquake is that older buildings should be seismically retrofitted.

In Paso Robles, where the most serious damage occurred in the magnitude 6.5 temblor, none of the unreinforced masonry buildings that had been retrofitted collapsed, the scientists pointed out. A building that had not been retrofitted did collapse, resulting in the quake’s only two fatalities.

The scientists also found that buildings in the middle of blocks seemed to fare better than those at the ends of blocks.

The 28-page report will be published in full in the March/April issue of the journal Seismological Research Letters. The lead writer was Jeanne L. Hardebeck of the U.S. Geological Survey. Rakesh Goel, a civil engineering professor at Cal Poly San Luis Obispo, was the main contributor on the subject of retrofitting.

“The relatively few lives lost in the San Simeon earthquake can be compared to the more than 30,000 casualties just four days later in the magnitude 6.6 Bam earthquake in Iran, mostly due to the collapse of unreinforced masonry buildings,” the scientists said.

“Part of the difference is that the strongest shaking from the San Simeon earthquake probably occurred in the sparsely populated epicentral region [25 miles from Paso Robles], while the fault in the Bam earthquake ran directly through a sizable city,” they acknowledged.

But, they added, “California was also more prepared. Some potentially hazardous buildings in Paso Robles had been retrofitted and performed well in the earthquake.”

“However, there are still many unreinforced masonry buildings in California cities. The tragedies in Paso Robles, and in Bam, emphasize the importance of retrofitting unreinforced masonry buildings before another earthquake strikes.”

Discussing the situation in Paso Robles, the scientists noted that “most buildings in the downtown business district are very old, some built more than a century ago, constructed of unreinforced masonry. This type of construction is known to be especially vulnerable to earthquakes, and it is not surprising that such buildings suffered extensive damage.”

In an interview, Goel said retrofitting need not involve huge cost. He said even the buildings that received only a “basic,” not an elaborate, retrofit, survived the quake well.

In the report, the scientists said, “In general buildings on street corners performed poorly compared to other buildings. Configuration of these corner buildings with windows on the street sides and solid walls without openings on the other two sides created plan asymmetry, i.e., large eccentricity between floor center of mass and center of rigidity.”

The resulting motions during the quake “imposed much larger demands” on walls located on street faces of these buildings, leading to greater damage.

Also, they noted, unreinforced masonry buildings located in the middle of a block, even those not retrofitted, did not suffer catastrophic failure like the Acorn building, a structure that collapsed, killing two people. That building, they observed, was at the end of a block.

“Most buildings in the affected area are constructed without any gap between them,” the scientists said. “It seems that the adjacent buildings provided confinement to each other and prevented collapse, except to those buildings sited on the corners.”

However, they added, unreinforced parapets and facades were damaged because of the ground motion of both mid-block and corner buildings.