There are more than 12,000 bridges in the California State Highway system, plus an additional 11,500 city and county bridges. Each bridge is inspected at least every two years by Caltrans' Division of Structures. Some bridges are inspected more frequently.

Since the 1971 Sylmar earthquake struck the Los Angeles area Caltrans has been engaged in an ongoing bridge retrofit program. Using research developed following the 1971 earthquake, Caltrans implemented new bridge design criteria. From 1986 to 1989, a retrofit program developed by Caltrans identified single-column bridges as being potentially the most vulnerable to earthquake damage. Research sponsored by Caltrans at the University of California, San Diego, led to a retrofit procedure that uses steel jackets to increase the strength of columns. Following the 1989 Loma Prieta earthquake in the San Francisco Bay area, Caltrans sponsored accelerated retrofit research primarily conducted at the University of California at Berkeley and the University of California at San Diego. More than $40 million has been spent on seismic research since the Loma Prieta quake. Ongoing research is continuing.

In addition, Caltrans appointed a Seismic Advisory Board of External engineering and scientific experts to advise the department on seismic safety policies, standards and technical practices. Peer review panels of independent seismic and structural experts also are utilized to review earthquake strengthening strategies on major, complex retrofit projects.

Initially, Caltrans' Seismic Retrofit program consisted of restraining sections of 1,262 bridges with steel cables. The work cost over $54 million and was completed in 1989. The Seismic Retrofit program now involves strengthening the columns of existing bridges by encircling certain columns with a steel casing or, in a few instances, an advanced woven fiber casing. In addition to the column casing, some of the bridge footings are made bigger and given more support by placing additional pilings in the ground or by using steel tie-down rods to better anchor the footings to the ground. In a few projects bridge abutments are made larger and the existing restrainer units are made stronger because encasing the columns make them stiffer and can change the way forces are transmitted within the bridge. Many Seismic Retrofits involve "hinge seat extensions" which enlarge the size of the hinges that connect sections of bridge decks and helps prevent them from separating during severe ground movement.

The design of each bridge to be retrofitted is "site specific" or based on the maximum credible earth movement expected at that location. The calculation depends on many factors, including the nearest active earthquake fault, type of geology beneath the bridge and the original bridge design.

The first column Seismic Retrofit project was the $724,000 Orange (57) and Pomona (60) freeway connector project in Los Angeles County, which began in April of 1990 and was completed in February of 1991. By the January 17, 1994, Northridge earthquake, 122 bridges in District 7 (Los Angeles and Ventura counties) had been retrofitted, and all performed as expected during the magnitude 6.8 temblor.

The Seismic Retrofit program is split into Phase 1 and Phase 2. Phase 1 includes 1,039 bridges identified for strengthening after the Loma Prieta quake at a cost of $812 million in construction. By December of 1997, 1,036 (99 percent) of those structures had either been completed or were under construction.

Phase 2 identified an additional 1,364 bridges for strengthening following the January 1994 Northridge earthquake. As detailed engineering of those structures is completed, the number of bridges that will actually require retrofitting will change. Currently, the number stands at 1,155. As of Dec. 1, 1997, a total of 1,112 bridges have been completed or are under construction. The remainder are in various stages of engineering design, review and analysis. Retrofitting of the Phase 2 bridges is estimated to cost $1.05 billion in construction.

A total of 1,114 city and county-owned bridges have been identified as candidates for seismic strengthening. Caltrans is the lead agency for 794 bridges. Los Angeles County is the lead agency on 287 bridges. Santa Clara County is the lead agency for 33 bridges. In all, the state's bridge earthquake strengthening program will involve approximately 2,200 structures, including the state's toll bridges, and will cost more than $4 billion in construction. The breakdown of the cost is as follows:

The toll-bridge retrofit program will cost $2.5 billion.

Phase 1 of the retrofit program will cost $812 million.

Phase 2 of the retrofit program will cost $1.05 billion.

Funding for the bridge retrofit program comes from transportation money generated by the tax on motor vehicle fuel. Under the funding priorities approved by the California Transportation Commission, funding for seismic retrofitting of bridges in Phase 1 and Phase 2 and other safety-related projects have the first call on available transportation resources.

Proposition 192, which passed March 26, 1996, with 59.9 percent of the vote, provides for $2 billion in bonds, including $650 million for retrofitting seven of the state's toll bridges and more than 1,000 other structures identified for strengthening after the 1994 Northridge earthquake.