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Reinforcing Bridges

Since the 1971 Sylmar earthquake ruined the Golden State Freeway overpass at the San Diego Freeway and damaged other roadways, The California Department of Transportation has undertaken a statewide program to retrofit existing bridges. But it was the Loma Prieta quake in 1989 that galvanized the current $2-billion program, making bridge retrofitting California’s highest priority in transportation. With Phase I nearly completed and Phase II well underway, no country has strengthened more bridges than the state of California.

In the San Fernando Valley, work has been completed on 27 of the 38 bridges targeted for reinforcement, and 11 projects are either under construction or in the design phase.

The efforts have paid off. Deputy Secretary for Transportation John Pimental points out that all of the 115 bridges in the earthquake-impacted area that had undergone retrofitting survived the Northridge quake in 1994.

Adding a Hinge

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Hinge seat extensions were developed after the 1989 Loma Prieta quake proved that roadbed joints were vulnerable to separation. Hinges are secured under stress points, attached along one side only to allow joints to expand.

San Fernando Valley Construction

March 1991: 2 bridges completed

Nov. 1991: 1 bridge completed

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Sept. 1993: 9 bridges completed

Aug. 1994: 6 bridges completed

Oct. 1994: 3 bridges completed

Jan. 1995: 5 bridges completed, 5 under construction, 6 in the design phase

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March 1995: 1 bridge completed

Caltrans’ Approach

Caltrans uses computerized models to estimate the amount of force a structure might be subjected to in an earthquake based on soil type, bridge design and proximity to a fault line to determine which retrofitting strategies to use for a particular bridge. These may include:

1. Restraining cables to strap the column to the road bed.

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2. Steel jackets wielded to the existing column, with a layer of grout pumped in to fill the spaces in between.

3. Enlargement of bridge footings to provide additional support. More piles are added, particularly in sandy soils.

Statewide Projects

Caltrans’ seismic retrofit program is being done in two phases:

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Phase I

No. of bridges: 1,039

Cost: $758 million (estimated)

Projected date of completion: End of 1995

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* * * * *

Phase II

No. of bridges: 1,209

Cost: $1.5 billion (estimated)

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Projected date of completion: End of 1997

Prioritizing Retrofit Work

Caltrans’ criteria for deciding which bridge is worked on first:

1. Structures built on soft soils nearest to faults were given first priority.

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2. Structures carrying heavy traffic or those that could collapse onto a crucial traffic artery or railroad, or block access to a hospital or other critical facility.

3. Bridges built during the 1950s and 1960s during the height of freeway construction. Those built after 1971 and those built in the 1940s were judged not to need retrofitting.

Retrofitting in Progress

Under construction:

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Golden State Freeway

* Pacoima Wash

Scheduled completion: late 1995

California 27

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* Topanga Creek bridge

Scheduled completion: mid-1996

Simi Valley Freeway

* Pacoima Wash

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* Laurel Canyon Driver over-crossing

Scheduled completion: late 1995

San Diego Freeway

* Raymer Street bridge

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Scheduled completion: late 1997

In the design phase:

Hollywood Freeway

* Whitsett Avenue over-crossing

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* Whitsett Avenue underpass

Scheduled completion: late 1997

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Source: Caltrans

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Researched by MAGGIE BARNETT / Los Angeles Times


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