Seismic Retrofit

Background

Caltrans has had a seismic evaluation, design and retrofit program for a number of years (see the Caltrans Seismic page). Prior to the Loma Prieta Earthquake in 1989, Phase I of the Caltrans retrofit program consisted of the hinge and joint restrainer program, which was completed in 1989. Bridges with single column configurations were known to be vulnerable and, in 1987, research was begun on methods for retrofitting columns.

Following Loma Prieta, the retrofit program was expanded considerably. Senate Bill 36 provided $80 million from the 0.25 cent sales tax for the retrofit program ($60 million for State bridges and $20 million for Locals). This legislation also authorized an increase in the level of seismic research ­­ from $0.5 million/yr. to $8 million initially and $5 million/yr. thereafter.

Following the 1994 Northridge earthquake, the U.S. Congress passed legislation allowing the use of Federal Highway Bridge Replacement and Rehabilitation Program (HBRR) funds for the seismic retrofit of non­deficient bridges (as defined by the National Bridge Inspection Standards).

Screening Process

The seismic retrofit program in California has evolved into a highly complex system. The screening program that began in 1990 incorporated a risk algorithm that evaluated the major factors that affect seismic performance, such as structural details, earthquake fault proximity, soil conditions, etc., as well as factors for hazard and risk.

Priority Categories

On January 1, 1991, a report was submitted to the Governor in response to SB 2104. This report included a method for segregating the screened bridges into priority groups as a means of identifying those most critical and in need of immediate repair, versus those which would be repaired as a means of reducing future damage. State owned bridges are grouped into the following categories:

• 1. Category 1 ­ Seismic Safety Category 
  The adopted seismic safety criteria for this category is ANO COLLAPSE; NO DAMAGE; NO SECONDARY DAMAGE OR INJURIES BECAUSE DAMAGED STRUCTURES PRECLUDE MOVEMENT OR ACCESS BY RESCUE AND RECOVERY PERSONNEL.@
• 2. Category 2 ­ Seismic Protection Category
  Bridges in this category are those that will be needed for disaster recovery and therefore meet the seismic safety criteria of ANo collapse.@ These bridges can sustain damage but must be reopened to service within a relatively short period of time, whether through completion of repairs or opened with the aid of temporary shoring.
• 3. Category 3 ­ Seismic Protection Category 
  Bridges in this category are needed to maintain the transportation system in its normal manner but are not needed for disaster recovery. These bridges will be repaired and reopened for service within a reasonable amount of time.

Current Status

State Bridges ­ There are 1,039 bridges in Category 1, the Seismic Safety Category, that consists of 259 single column and 780 multi­column retrofit projects. This group of structures has been designated as the Phase I retrofit program.

The Phase I program was accelerated, with an initial target goal of being under construction by the end of 1994 and completed by the end of 1995. By December 31, 1994, 1,021 bridges were under construction, which amounted to 98.3% of the goal.

As of August, 1997, construction was completed for 1009 Phase I structures, which represents 97%. Current total estimated cost for completion of Phase I is $814 million.

The remaining bridges that have been screened have been designated as the Phase II retrofit program, which has as a target of 94% construction complete by the end of 1997. Currently, there are 1,155 bridges in the Phase II program, with a total estimated cost of approximately $1.05 billion. Most of this money will come from the $2 billion bond which was passed in 1996.

City and County Bridges ­ As of August, 1997 there were 1,114 local bridges statewide that are in the Local Bridge Seismic Retrofit Program. This number is dynamic in nature and continues to change as further screening takes place. Los Angeles County and Santa Clara County have retrofit responsibility for 320 bridges (28%). Caltrans has responsibility for the remaining 794.

Toll Bridges ­ The Caltrans toll retroft program involves the most complex strucutres in the state and, consequently, the most complex seismic retrofit schemes ever undertaken. It has been determined that seven of the State's nine toll bridges will need some form of retrofitting. Six of the toll facilities have 14 construction contracts, in various stages, with an estimated cost of $780 million. The remaining facility, the San Francisco ­ Oakland Bay Bridge, will be partially retrofitted and partially replaced at an estimated cost of $1.5 to $1.7 billion.