HIGHWAY BRIDGES

Abridged from Earthquake Spectra,
Vol 11, Supplement C, Chapter 6,
April, 1995
Used by permission of EERI

Introduction

Most of the metropolitan Los Angeles transportation system survived the Northridge earthquake with minimal or easily repairable damage. However, extensive damage to or collapse of several highway bridge structures caused widespread disruption after the earthquake. At least one death and several injuries were direct consequences of the failures. The relatively low death and injury rates are attributed to the early local time (4:31 A.M.) of the earthquake.

This text summarizes observations on performance of the highway transportation system during the Northridge earthquake. The emphasis is on freeway bridges. This discussion includes: a review of freeway bridge construction, design specifications, and the retrofitting program; an overview of the performance of bridges; detailed descriptions of several bridges that sustained notable damage or collapse; an overview of widespread damage to bridge abutments; and freeway bridge reconstruction. A concluding section summarizes the main observations and their implications.

History of Freeway Bridge Design, Construction, and Retrofit

The metropolitan Los Angeles freeway system comprises 528 miles of freeway containing 2,523 freeway bridges. Besides the freeway system, there are approximately 1,500 street bridges under county jurisdiction and another 800 under city jurisdiction. The earliest of the freeways date from 1940; freeway construction has continued up through the time of the earthquake. Table 1 indicates chronologies for freeways in the epicentral region (Brodsly 1981). The design, construction, and retrofitting practices have varied widely over this period.

Table 1

Seismic Design Codes

Caltrans (California Department of Transportation) has always compiled its own seismic design criteria. It formulated the first code requirements in the United States for seismic design of bridges in 1940. The Caltrans criteria from 1940 to 1965 are quoted below (Housner 1990).

1940 "Provision shall be made for seismic stresses resulting from earthquake. The seismic force shall be considered as an assumed horizontal force applied at the center of mass in any direction that will produce a maximum stress in the member considered. The assumed horizontal force shall be a percentage of the dead load and will be determined by the Designing Engineer."

1943 ". . . structures ... shall be designed to resist a seismic force (F) in accordance with the following formula: F = CW, where F is the seismic force to be applied horizontally in any direction at the center of gravity of the weight of the structure, W = dead load of the structure, and C is: 0.02 for structures founded on spread footings with a bearing capacity exceeding 4 tons/ft. squared or better, 0.04 for structures founded on spread footings with a bearing capacity less than 4 tons/ft squared, and 0.06 for structures founded on pile foundations."

1965 ". . . structures ... shall be designed to resist earthquake forces (EQ) in accordance with the following equations:

EQ = KCD

where:

EQ = force applied horizontally at the center of gravity of the structure. This force shall be distributed to supports according to their relative stiffness.

K = Numerical coefficient representing energy absorption of the structure:

• =1.33 for bridges where a wall with a height to length ratio of 2.5 or less resists horizontal forces applied along the wall.
• = 1.00 for bridges where single columns or piers with a height to length ratio greater than 2.5 resists the horizontal forces.
• = 0.67 for bridges where continuous frames resist horizontal forces applied along the frame.

C = 0.05(T)-1/3 (maximum value of C = 0.10).

• Numerical coefficient representing structure stiffness.

T = 0.32(D/P)l /2 for single-story structures only.

• Period of vibration of structure.

D = Dead load reaction of structure.

P = Force required for one-inch horizontal deflection of structure.

The EQ forces calculated above shall never be less than 0.02D. Special consideration shall be given to structures founded in soft materials capable of large earthquake movements, and to large structures having massive piers."

Following the 1971 San Fernando earthquake, Caltrans initiated revisions in its seismic design criteria. At that time, approximately 352 bridges in the state inventory were in various stages of construction or had completed plans so that changes could be made only by negotiating with the contractors (California Department of Transportation 1976). Caltrans wrote change orders to add hinge restrainers, increase the amount of steel spirals and ties in reinforced concrete columns, and add other strengthening measures. For bridges that were in design stages then, the earthquake design forces were increased by a factor between 2.0 and 2.5 times the previously used forces.

At the same time, Caltrans commenced development of new criteria for future bridge designs. The new criteria introduced effects of fault proximity, site conditions, dynamic structural response, and ductile details for reinforced concrete construction. The criteria applied to design of all new bridges beginning in February 1974 (California Department of Transportation 1976). Although the criteria have evolved since 1974, the basic design forces and detailing procedures in the 1974 provisions are very similar to those in use by Caltrans at the time of the Northridge earthquake.

According to the Caltrans criteria, design response quantities (displacements and forces) are obtained from design response spectra referred to as ARS spectra, where A relates to expected bedrock acceleration, R relates to normalized rock response, and S relates to the soil amplification spectral ratio. The ARS spectra represent 5% damped linear-elastic design response spectra for maximum credible events. The design forces are reduced by an adjustment factor Z based on ductility and risk assessment. Figure 1 compares force design spectra for various Caltrans codes for peak rock acceleration of 0.6g.

Another important aspect of post-1971 Caltrans codes is incorporation of ductile design for reinforced concrete. Shortly following the 1971 San Fernando earthquake, Caltrans developed improved reinforcement details, including closer spacing and improved detailing of column transverse reinforcement, requirements for top reinforcement in footings and pile caps, and controls on locations of column longitudinal reinforcement splice locations. Since around 1980, standard practice is to design the column to have a shear strength exceeding the shear required to develop the plastic moment capacity in the columns. The procedures have continued to evolve into the 1990s. The current design criteria specify details and proportions that are intended to lead to ductile flexural response of columns, and elastic response in all other elements, in the design earthquake.

In 1991, Caltrans engaged the Applied Technology Council to review and revise standards, performance criteria, specifications, and practices for new bridges. The ATC-32 Project (Improved Sesimic Design Criteria for California Bridges) was only partially completed at the time of the Northridge earthquake, so results of that project had not entered current practice. Some of the design procedures described in preliminary drafts of the ATC-32 report have been used in the reconstruction of bridges damaged by the Northridge earthquake.

Bridge Seismic Retrofit Programs

Following the 1971 San Fernando earthquake, Caltrans initiated a three-phase Bridge Seismic Retrofit Program. The first phase of the program involved installation of hinge and joint restrainers intended to prevent deck joints from separating (Roberts 1992). Joint separation and subsequent unseating were judged by Caltrans engineers to be the highest risk. This phase included installation of devices to fasten the superstructure elements to the substructure, as well as to interconnect adjacent superstructure segments across in-span hinges. This phase was essentially completed in 1989 after approximately 1,260 bridges on the State Highway System had been retrofitted.

Shear failures of columns in the I-605/I-5 separation bridge in the Whittier Narrows earthquake in 1987 reemphasized the inadequacies of pre-1971 column designs, and led to basic research programs on column retrofitting in 1987. Phase 2 of the Bridge Seismic Retrofit Program also began then. Its focus was on retrofitting columns in single-column bridges because of the perception that they were more vulnerable than multicolumn bridges.

The 1989 Loma Prieta earthquake and resulting bridge damage led to acceleration of the Bridge Seismic Retrofit Program, and initiation of Phase 3, on multicolumn bridges. In July 1990, the Caltrans Division of Structures completed an initial vulnerability analysis of the 24,000 state, county, and city bridges and produced a prioritized list for retrofitting. The list contained nearly 7,000 state and 4,500 city and county bridges that required further analysis and evaluation (Roberts 1992). This list has been reduced based on subsequent analyses.

The status of bridge seismic retrofitting at the time of the Northridge earthquake was reported by the Los Angeles Times (1994) as follows. Out of 1,313 state bridges identified for retrofit, 251 retrofits had been completed at the time of the Northridge earthquake. The County of Los Angeles had identified 216 bridges that could require retrofitting, of which only six retrofits were complete at the time of the earthquake. The City of Los Angeles has a $97 million program to retrofit 187 bridges, of which only $12.9 million had been spent as of 17 January 1994.

Freeway Bridges Performance: General Observations

The Office of Structure Maintenance and Investigations of Caltrans inspected 850 bridges and reported damage to 212 structures in the Los Angeles area (California Department of Transportation 1994). Reported damage includes hinge and hinge restrainer damage, abutment damage, shear key and bearing damage, deck spalling, column and girder damage, approach slab damage, partial collapse, pier wall damage, pile cap damage, and pile damage. Of the damaged bridges, Caltrans estimated repair or reconstruction costs of over $100,000 for each of 55 bridges, and $3,000 to $100,000 for each of 121 bridges.

There was partial or complete collapse on five bridges:

• Route 14/5 Separation & Overhead (Bridge #53-1960F) and North Connector Overcrossing (Bridge #53-1964F), both at the Interstate 5/State Road 14 Interchange (Golden State/Antelope Valley Interchange)
• Gavin Canyon Undercrossing (Bridge #53-1797R/L) on Interstate 5
• Mission-Gothic Undercrossing (Bridge #53-2205) on State Road 118 (Simi Valley Freeway)
• La Cienega-Venice Undercrossing (Bridge #53-1609) on Interstate 10 (Santa Monica Freeway)

All of these structures were of reinforced and/or prestressed concrete, with construction completion dates ranging from 1964 through 1976. Several had been retrofitted with hinge restrainers following the 1971 San Fernando earthquake. Collapse scenarios in all cases appear to involve column flexural/shear failures or unseating at in-span or abutment hinges.

Three bridges sustained major to moderate damage to reinforced concrete bridge columns but did not collapse:

• Fairfax-Washington Undercrossing (Bridge #53-1580) on Interstate 10
• Bull-Creek Canyon Channel Bridge (Bridge #53-2206) on State Road 118
• Ruffner Avenue Overcrossing (Bridge #53-2396) on State Road 118

These structures were of reinforced and/or prestressed concrete.

There was damage to bridge abutments throughout the epicentral region. The observed damage included shear key cracking and failure, spalling or failure of abutment backwalls and wingwalls, approach settlement, and approach slab buckling.

Damage in steel bridges comprised pounding damage between adjacent elements, buckling of cross bracing, bending of cross-brace gussets, bearing damage including anchor bolt and restrainer fractures, and damage to supporting abutments and pier walls. More information on damage to steel bridges is contained in Astaneh et al. (1994).

Performance of Retrofitted Bridges

The epicentral region contained several bridges that had been retrofitted with hinge restrainers and/or column jackets. Most of the retrofitted bridges performed adequately although, in some cases, hinge restrainers did not perform properly. Column jackets, whether made of steel or fiberglass, performed well in all cases. Examples of performance observations are given below.

Gavin Canyon UC (Bridge #53-1797L/R). This concrete box-girder bridge was on I-5 approximately 14 km from the epicenter. The bridge had been retrofitted in 1974 with restrainer cables and diaphragm bolsters across the skewed in-span hinges. The bridge collapsed, apparently from unseating and restrainer failure.

Interstate 5/State Road 14 Interchange-Separation and Overhead (Bridge #53-1960G) and South Connector Overcrossing (Bridge #53-1963-F). These curved concrete box-girder bridges were about 12 km from the epicenter.They had been retrofitted with restrainer cables as part of the damage restoration contracts following the 1971 San Fernando earthquake. The restrainers had been designed with cables grouted in pipes, leaving little room for cable elongation. Both bridges had major hinge damage; some spans were supported tenuously on their 14-inch hinge seats. Nuts were missing from some of the restrainers. Hinge diaphragms (which had not been retrofitted with bolsters) were heavily damaged at some locations. (California Department of Transportation 1994).

Interstate 10/Interstate 405 Interchange-Southwest Connector OC (Bridge #53-1630G). This connector (and others at this interchange approximately 20 km from the epicenter) had been retrofitted with steel column-jackets. The bridge sustained rocker damage at abutuments, with some cracking and spalling of the return wall. No damage to the jacketed columns was apparent. The bridge had been instrumented for strong-motion recording.

Cadillac Street Off-Ramp. This off-ramp structure is approximately 300 feet from the Fairfax-Washington collapse on I-10. The columns had been retrofitted with steel jackets. Concrete in the gap between the jacket and the soffit showed signs of spalling, indicating some ductility demand. Otherwise, the bridge appeared undamaged.

Other examples of bridges with steel or fiberglass/epoxy jacketed columns include the I-10 at the Santa Fe Exit, 1-5 at the Griffith Park Ramp, and the I-405/SR134 Ramp. The jacketed columns performed well in all cases.

Table 2 Summary of bridges with moderate or major damage

Observations Related to Construction Era

Caltrans design criteria and details have varied considerably over the years, as described previously. Trends in the relation between damage and construction era may provide insight into construction eras where potential vulnerabilities are most likely and where future evaluation and retrofitting efforts should focus. Table 2 lists bridges with damage classified as collapse, major damage (without collapse), and moderate damage. The terms "major damage" and "moderate damage" are subjective. Major damage generally refers to cases where column spalling and rebar buckling extended over a length of one column diameter or more, or to cases where severe hinge damage and near unseating occurred. Moderate damage refers to cases where column spalling or shear cracking occurred without buckling, or where abutment/pier damage was substantial.

The data in Table 2 show collapse and major damage in structures constructed as late as 1976. In many cases, the early hinge retrofits performed well, but in others, collapse (Gavin Canyon UC) or severe damage (Bridges #53-196OG and #53-1963F on I-5/SR14) was associated with inadequate performance of these earlier hinge retrofits. Column jackets performed well in all cases.

Roadway Bridges Under County and City Jurisdiction

The Los Angeles County Department of Public Works reported serious damage in only four out of 1,500 bridges on the county system.

Column and abutment damage on the Sierra Highway bridge over the railroad tracks
• Damage to abutments on the McBean Parkway over the Santa Clara River
• Damage to approaches of the Old Road bridge above the Southern Pacific railroad
• Buckled girders and other damage along the Old Road bridge over the Santa Clara River

The City of Los Angeles reported that 62 out of a total of 800 spans were damaged, and two required closure. Damage to the two closed bridges included:

• Column damage to a 70-foot pedestrian bridge over Wilbur Avenue at Collins Street in Tarzana requiring street closure
• Approximately 2 feet of settlement of high approach fills near Nordoff Street over the Southern Pacific rail line in Northridge.

Other damage to city bridges consisted of approach-fill settlement, pavement cracking, shear key damage, superstructure rotation, rocker bearing damage, and architectural damage.

Updated December 17, 1997.
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