Propping Up Local Bridges : Transportation: Caltrans is banking on $950 million in Prop. 1A bonds to retrofit highway structures here and throughout the state vulnerable to quakes.

Dozens of public bridges in Orange County are being retrofitted and strengthened to keep them from collapsing during an earthquake, and Caltrans officials hope voters will approve a $950-million bond issue next week to allow the strengthening of several hundred additional highway structures here and throughout the state.

Jim Drago, a spokesman for the California Department of Transportation in Sacramento, said 53 bridges, many of them freeway overpasses, either have been or are in the process of being retrofitted in Orange County.

The local bridges are among the 1,039 structures in California that Caltrans plans to retrofit by 1995 at a cost of $750 million, Drago said. The construction is financed through the state’s gasoline tax.

Caltrans officials said it costs an average of $500,000 to retrofit each bridge, and requires about a year of work.

Typically, the process involves jacketing the existing columns in steel casing or steel-reinforced concrete to prevent them from cracking or disintegrating during earthquakes. Most bridge failures occur when short columns break or crack.

Experts say that during a quake, the tall columns under a bridge bend a bit, causing most of the force to be transferred to the shorter columns, which are not designed to absorb the added stress.

Encasing the shorter columns in steel or steel-reinforced concrete makes them more flexible, allowing them to bend as much as five inches, compared to one inch for columns not retrofitted.

“The goal is to make these bridges earthquake-resistant and keep them intact,” said Drago. “Nothing is earthquake proof. We accept the fact that there will be damage in any earthquake, but if we keep them intact they won’t collapse.”

Retrofitting began after the 1971 Sylmar quake. Since then, about 122 Southern California bridges have been fixed. All of the strengthened bridges in Los Angeles County survived the magnitude 6.6 Northridge quake in January.

Drago said that Caltrans hopes voters on Tuesday approve Proposition 1A, which authorizes the state to sell $950 million in bonds to pay for retrofitting of an additional 1,600 highway bridges and overpasses, including seven toll bridges, that are being studied statewide as candidates for retrofitting.

“Some of the bridges . . . are in Orange County, but we don’t know how many will actually need retrofitting. We will produce a list of the new bridges that need to be strengthened by the end of the month,” Drago said.

In Orange County, highway overpasses have already been fixed at the interchange of the San Diego and San Gabriel River freeways. The 7th Street overpass at the Garden Grove Freeway in Seal Beach has also been retrofitted. Caltrans officials said the 7th Street overpass is less than three miles from the Newport-Inglewood Fault, which is considered more dangerous to urban areas than the San Andreas Fault.

In all, there are 46 structures at eight other locations throughout Orange County that still need to be strengthened. One of the biggest retrofit projects in the county is at the San Diego and Costa Mesa freeways interchange in Costa Mesa.