Practical Lessons from the Loma Prieta Earthquake (1994) / Chapter Skim
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6. Highway Bridges
6. Highway Bridges
Pages 165-218
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From page 165... ...
Immediately after the February 9, 1971, San Fernando earthquake, CALTRANS began a comprehensive upgrading of their Bridge Seismic Design Specifications and Construction Details. CALTRANS's bridge design specifications 165
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From page 166... ...
The initial phase of the CALTRANS Bridge Seismic Retrofit Program involved installation of hinge and joint restrainers to prevent deck joints from separating. Separation was the major cause of bridge collapse during the San Fernando earthquake and was judged by CALTRANS engineers and other investigators to be the highest risk to the traveling public.
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From page 167... ...
Hinge-Joint Restraining Devices As previously stated, the initial phase of CALTRANS' Bridge Seismic Retrofit Program involved installation of hinge and joint restrainers to prevent deck joints from separating. This was identified as the major cause of bridge collapse during the San Fernando earthquake (LeBeau et al., 1971)
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From page 168... ...
The use of grade 60, A 706 reinforcing steel in bridges has recently been specified on a few projects on a trial basis. In the eight counties declared disaster areas after the Loma Prieta earthquake, there are approximately 800 bridges designed after 1972 using the newly revised seismic-design criteria and confinement details.
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From page 169... ...
Older Bridges Designed for Pre-1972 Seismic Forces and Design Criteria The major causes of bridge damage in the Loma Prieta earthquake were the criteria and details for which they were originally designed. There were over 4,000 bridges on the combined state, county, and city systems in the eight counties that were declared disaster areas after the earthquake.
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From page 170... ...
Dynamic Response of Deep, Soft Foundations The effect of the dynamic response of deep, soft soils in the structure foundations also proved to be a contributing factor to the collapse of the Cypress Viaduct and must be analyzed and included in future design procedures, especially for long, tall structures with relatively high periods of vibration. The effect of incoherence in the foundation response is also an important factor in the design of very long structures such as the San Francisco-Oakland Bay Bridge and the mile-long freeway viaducts.
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From page 171... ...
Inadequate Beam-Column,Ioint Reinforcement Research and analysis conducted subsequent to the Loma Prieta earthquake have shown conclusively that the lightly reinforced column-pedestal detail unique to the Cypress structure was the main cause of the total collapse. Immediately following the earthquake, the Structural Engineering Department at UC Berkeley requested that CALTRANS's Division of Structures salvage a section of the damaged Cypress Street Viaduct that had not collapsed for the purpose of conducting a series of seismic performance experiments.
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From page 172... ...
Pedestrian-Bridge Performance During the vulnerability screening and seismic analysis of bridges subsequent to the Loma Prieta event, the CALTRANS Office of Earthquake Engineering staff has noted that the class of single-column-supported pedestrian bridges have universally required seismic-retrofit strengthening. They are generally lightweight and very narrow, offering little lateral stiffness, thus rendering them especially vulnerable to seismically induced lateral forces.
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From page 173... ...
Seismic-retrofit details to strengthen existing bridges were developed and proven with this research program and their good performance in three recent earthquakes in California in April and June 1992, prove the validity of the ductile design and retrofit approach adopted by CALTRANS. After the Loma Prieta earthquake caused 44 fatalities on the state highway system, capital funding for seismic retrofitting was increased from $4 million to $300 million per year.
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From page 174... ...
Development of Vulnerability-Analysis Algorithm In order to set priorities for more than 24,000 bridges in the state for order of seismic-retrofit upgrading, CALTRANS engineering staff developed a risk-analysis procedure and adjusted it over the next three years as more information became available. Identification of bridges likely to sustain damage during an earthquake was an essential first step in the Single-Column phase of the Bridge Seismic Retrofit Program, which had begun just prior to the Loma Prieta earthquake.
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From page 175... ...
3. Define the minimum ground acceleration capable of causing severe damage to bridge structures.
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From page 176... ...
Important features of this first step are the ease and cost with which it could be carried out and the data base that could be developed, highlighting bridge characteristics that are associated with structures in need of retrofit. This database will be utilized to confirm the assumptions made in the Single Column phase of the retrofit program.
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From page 177... ...
During 1992, advances were made in the procedures employed by CALTRANS to prioritize bridges for seismic retrofit, and a new, more accurate algorithm was developed. The most significant improvement to the prioritization procedure is the employment of the multiattribute decision theory.
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From page 178... ...
Table 6-2 contains the newly adopted seismic performance criteria for the California State Highway System. Development of Acceleration Response Spectra for Deep Soft Bay Muds The damage patterns experienced during the Loma Prieta earthquake of October 17, 1989, reemphasized the importance of the influence of soil and founda
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From page 179... ...
HIGHWA Y BRIDGES First Level Criteria MULTI-ATTRIBUTE DECISION PROCEDURE | PRIORITIZATION | RATING ~ 1 ACTIVITY HAZARD 60%1 140% Second r IMPA Criteria 179 Prioritization ~ (Activity) (Hazard)
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From page 180... ...
The Loma Prieta event has been called the geotechnical engineer's earthquake for that reason. The damage to state highway bridges at Watsonville and along both the east and west shorelines of the San Francisco Bay followed that pattern as shown in the following figures.
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From page 181... ...
In the future, the role of the two methods for other bridges shall be reviewed by a CALTRANS approved consensus group. Functional Evaluation Ground Motion: Probabilistically assessed ground motions that have a 40% probability of occurring during the useful life of the bridge.
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From page 182... ...
Figures 6-5 through 6-8 show the results of soft-bay-mud response on the Struve Slough Bridge near Watsonville, where the ground movement of the soft muds caused the piling to shear off below the bent caps. Figure 6-9 shows the 100-year-old shoreline on the west bay side of San Francisco Bay; most bridge damage occured to those structures that were built along the bay shore over the deep, soft muds.
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From page 183... ...
~~- ~ BR~ :: I:: 7~:= e ~~ FIGURE 6-3 Noah section of Cypress Street ViaducL >A ss@e~ . FIGURE 6-4 Present and 1880 sho[eUne near Cypress Sl~c[ure.
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From page 184... ...
7~ FILMY 6~5 Collapsed Stave Sloupb Bridge. FIGURE 6-6 Evidence of large ground movement
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From page 185... ...
HIGlIWA Y BRIDGES 185 FIGURE 6-7 Piling sheared at girder soffit.
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From page 186... ...
Seismic ground motions have been predicted in the deep, soft soils with an analytical procedure, and the predictions have been confirmed with the actual recorded motions at four sites around the San Francisco Bay. In addition to developing the new set of design response spectra for deep, soft soils, CALTRANS has also initiated a program to identify and map the soft soil sites in the state.
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From page 187... ...
CALTRANS Research Project R-7 was initiated to address this area in depth. For the major bridges crossing the bays from San Diego in the south to Antioch at the extreme northeast end of the San Francisco Bay and estuary, CALTRANS has engaged consultants to conduct site-specific complete hazard analyses using a probabilistic approach to define several levels of design earthquakes for bridge seismic-design purposes.
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From page 188... ...
and Eidinger and Abrahamson (19911. Foundation Design for Liquifiable Soil Sites While soil liquefaction was not a contributor to bridge damage, it was apparent near several major structures in the east bay and must be considered and dealt with in future seismic design for bridges.
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From page 189... ...
Ductile Column Design Bridge columns designed prior to the 1971 San Fernando earthquake typically contain very little transverse reinforcement. A common detail for both circular and rectangular columns consisted of #4 (.5-inch diameter)
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From page 190... ...
The use of grade 60, A 706 reinforcing steel in bridges has recently been specified on a few projects on a trial basis and will probably become common practice by the end of 1993. Research has been conducted to confirm the design criteria that were adopted after the 1971 San Fernando earthquake.
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From page 191... ...
Those first projects were advertised within two months of the Loma Prieta event, and work was started in early 1990 and completed by the end of that year. Figure 6-1 1 is a completed columnjacket-retrofit detail, the end result of this research.
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From page 192... ...
The most elaborate and expensive tests conducted to date have been the half-scale and third-scale models of the proposed retrofit details for the doubledeck viaduct structures in San Francisco. Models, using two different seismic retrofit techniques, were tested at both UC San Diego (half scale)
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From page 193... ...
They tested a model of the outrigger joint of the I-980 structure that was damaged in the Loma Prieta earthquake by using the identical prototype details. They replicated very closely the actual field damage, then tested the retrofitted joint as CALTRANS had redesigned it and confirmed the validity of the redesigned joint details.
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From page 194... ...
l red ~ :: ~ F ~~ ~ ~ 1 - \~: ~ L~:~ :~ :s :: ~ ~~ ~— : ~~:~::~: ~ :n __1 - . :: ~ ~ 194 PRACTICAL LESSONS FROM THE LOMA PRIETA EARTHQUAKE 1 1 FIGURE 6-12 Half-scale model of edge beam retrofit in test stand.
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From page 195... ...
Bond Development Length of Large-Diameter Reinforcing Steel Bars Under Dynamic Loading Peer Review Panel concerns about the adequacy of the American Association of State Highway and Transportation Officials' (AASHTO) design code provisions for bond development length for number 14 and number 18 reinforcing steel bars, which are typically used in large bridge columns in California, prompted additional research testing.
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From page 196... ...
Review and Revise Existing Seismic Design Specifications and Details One of the research projects that was funded is the Applied Technology Council ATC-32 Project (CALTRANS Research Project R-11) to review and revise the entire CALTRANS Bridge Seismic Design Specifications.
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From page 197... ...
Both these structures were designed many years prior to the development of the modern seismic design specifications for bridges. Both were designed for lateral-force requirements of 0.06 g to 0.10 g and could not be expected to withstand the seismic forces that even a moderate earthquake such as Loma Prieta produced.
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From page 198... ...
First, CALTRANS prioritized the bridges using the vulnerability prioritization procedure that is discussed above. This prioritization procedure ensures that the most vulnerable bridges are scheduled for seismic retrofitting first.
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From page 199... ...
Response of Deep, Soft Soils One of the most important lessons learned from the Loma Prieta earthquake and the research that has followed is the importance of calculating the seismic response of soft mud foundation sites for use in design of a bridge. Geotechnical engineers and geologists have been sounding the warnings for some time, especially after the 1985 Mexico City earthquake.
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From page 200... ...
The continuation of research in this area will be aimed at reducing the conservatism that permeates ductile column and joint design in California today. Prior to the Loma Prieta earthquake, there had been little or no research into the performance of very large column-cap joints and confinement details for large joints.
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From page 201... ...
Seismic Design and Retrofit of Bridges, Seminar Proceedings, Earthquake Engineering Research Center, University of California at Berkeley and California Department of Transportation, Sacramento, California, 1992.
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From page 202... ...
Seismic Design and Retrofit of Bridges, Seminar Proceedings, Earthquake Engineering Research Center, University of California at Berkeley and California Department of Transportation, Division of Structures, Sacramento, California. Der Kiureghian, A
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From page 203... ...
1992. CALTRANS Procedures for Development of Site-Specific Acceleration Response Spectra", Seismic Design and Retrofit of Bridges, Seminar Proceedings, Earthquake Engineering Research Center, University of California at Berkeley and California Department of Transportation, Division of Structures, Sacramento, California.
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From page 204... ...
In Seminar Proceedings, Seismic Design and Retrofit of Bridges, Earthquake Engineering Research Center, University of California at Berkeley and California Department of Transportation, Division of Structures, Sacramento, California. Mellon, S., et al.
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From page 205... ...
In Seminar Proceedings, Seismic Design and Retrofit of Bridges, Earthquake Engineering Research Center, University of California at Berkeley and California Department of Transportation, Division of Structures, Sacramento, California. Sweet, J
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From page 206... ...
Seismic Design and Retrofit of Bridges, Seminar Proceedings, Earthquake Engineering Research Center, University of California at Berkeley and California Department of Transportation, Division of Structures, Sacramento, California. Fenves, G.L., F.C.
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From page 207... ...
1990. Post Loma Prieta Earthquake Initiative, Seismic Analysis of an Elevated Portion of the Bay Bridge Distribution System Structure.
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From page 208... ...
1992. Research Based Bridge Seismic Design and Retrofit Program, Criteria, Standards and Status.
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From page 209... ...
Recent Lifeline Seismic Risk Studies, Report: Technical Council on Lifeline Earthquake Engineering, Monograph 1, American Society of Civil Engineers, New York, November. Werner, S.D., S.A.
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From page 210... ...
As a major part of the comprehensive earthquake vulnerability evaluations of important transportation structures we have a need to first determine the foundation response. This research is a direct result of problems with structures constructed on deep, soft soils during the Loma Prieta earthquake.
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From page 211... ...
R-19. Development and Implementation of Improved Seismic Design and Retrofit Procedures for Bridge Abutments.
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From page 212... ...
Contract Amount $125,000. Contract Completion Date October, 1993.
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From page 213... ...
This is the chief reason we learn from earthquakes. My interest stems from what the lessons from the Loma Prieta earthquake mean nationally.
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From page 214... ...
The I-280 Southern Freeway viaduct in San Francisco is a project I am intimately familiar with, having been working on it since just days after the Loma Prieta earthquake. This is a structure on a very high acceleration site and the poorest soils in the Bay Area.
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From page 215... ...
Use of Simplified Nonlinear Analysis Methods for Seismic Analysis There are many options for nonlinear analysis available. I would like to encourage the use of a more simplified type of analysis, as it has a lot of benefits and is a good tool to use.
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From page 216... ...
In this case, the panel reviewed the seismic design criteria and the geologic-hazard report, the design and performance criteria, applicable and available research results, analysis and modeling assumptions, design details, construction documents, and constructability. The panel did not perform a check on the drawings' calculations; therefore peer review cannot be considered a substitute for independent plan checks.
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From page 217... ...
There was not anything in the bridge field during the Loma Prieta earthquake that gave us a good test, but it is a good principle. There are things we need to learn about it yet particularly the increased vulnerability at the joints and the means to address that.
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