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Seismic Vulnerability of the National Highway System

Seismic Vulnerability Study of Highway Systems:
Tea-21 Research Project

Presented at the 15th U.S.-Japan Bridge Engineering Workshop, November 1999

Gokhan Pekcan, Ph.D.
Acting Senior Program Officer, Multidisciplinary Center for Earthquake Engineering Research, State University of New York at Buffalo, 102 Red Jacket Quadrangle, Buffalo, New York 14261-0025

W. Phillip Yen, Ph.D.
Research Structural Engineer, Federal Highway Administration, HNR-10. Turner-Fairbank Highway Research Center, 6300 Georgetown Pike, VA 22101

Ian M. Friedland, P.E.
Associate Director for Development, Applied Technology Council, 555 Twin Dolphin Drive, Suite 550, Redwood City, California 94065 (formerly Assistant Director for Transportation Research, Multidisciplinary Center for Earthquake Engineering Research)

 

Abstract

In the fall of 1998, the Multidisciplinary Center for Earthquake Engineering Research was awarded a 6-year, $10.8 million contract by the Federal Highway Administration of the U.S. Department of Transportation to conduct a series of studies related to the seismic performance of the U.S. national highway system. The research conducted under this contract is intended to provide improved tools for evaluating and assessing the social costs and impacts of earthquakes on highway systems and bridges, and to reduce the amount of damage that may occur to existing and future highway structures from a moderate-to-significant seismic event. This paper summarizes the motivation behind and objectives of the main group of tasks undertaken within the scope of this program: development of loss estimation methods for highway systems; preparation of a manual for the seismic design and retrofitting of long span bridges; development of earthquake protective systems and a systems design manual for bridges; and specialized ground motion, foundation, and geotechnical studies.

Introduction

A highway system is typically comprised of a network of urban expressways, surface streets, intersections, computerized signals, drainage systems, utility right-of-ways, and excavations and embankments, all of which can be impacted by earthquakes. Transportation networks must continue to function during and after the occurrence of a natural disaster such as an earthquake, so that lifelines can continue to provide emergency services and to minimize loss of life and economic distress. As evidenced in past earthquakes, highway transportation is often disrupted as a direct result of damage or collapse of highway system components. Of all the components of highway systems, bridges are usually considered to be the most vulnerable to earthquake induced damage and collapse. A survey of available statistics reveals that more than one-third of U.S. highway bridges may be vulnerable to damage and/or failure due to earthquakes. In fact, more than 70 percent of the nation’s 480,000 highway bridges were built prior to 1970, and can be regarded as vulnerable to moderate-to-major earthquake ground motions. Recent earthquakes, particularly the 1989 Loma Prieta, 1994 Northridge and 1995 Great Hanshin (Kobe) earthquake in Japan, have demonstrated the need, i) to find new and improved ways to design and construct bridges and highways which can withstand the force and displacement demands placed on them, ii) to cost-effectively retrofit existing seismically vulnerable structures, and iii) to develop tools and technologies which can adequately predict the economic and social impacts from damaging earthquakes, to assist in effective decision-making regarding operation, retrofit, and design strategies.

Recognizing the shortcomings evident in both existing highway bridges and current bridge design specifications, the Federal Highway Administration (FHWA) initiated a comprehensive seismic research program in cooperation with the Multidisciplinary Center for Earthquake Engineering Research (MCEER, formerly the National Center for Earthquake Engineering Research – NCEER), in 1992. That program included two contracts, one focused on existing highway construction and seismic vulnerability and the other on the development of improved methods and details for future bridge construction. Among the products of these studies are a set of manuals for seismic evaluation and retrofitting of conventional highway bridges and systems (MCEER, 1999a, 1999b, 1999c), and a series of recommendations for improving future seismic bridge design specifications (Rojahn et al., 1999).

Despite these gains in knowledge, there is still some distance to go before seismically-resistant highway systems are a reality. Some classes of bridges have yet to be studied in a systematic way; such as long span bridges, and structures on or crossing difficult sites continue to be problem. Some of these structures are very old and typically possess non-ductile structural details. Many are located on either unknown foundation types or incorporate lightly reinforced concrete substructures. In many cases, poor soil conditions exist, yet soil-foundation-structure interaction during an earthquake is a highly complex phenomenon and is still poorly understood. Hence, given the current state of knowledge, it is difficult, if not impossible, to quantify the seismic capacity and a highway bridge with much certainty. Therefore, unrestricted access to these "lifelines" cannot be assured at this time, which could have a major impact on emergency services following a damaging earthquake, and the resultant impediments to post-earthquake recovery.

Moreover, the high cost of retrofitting the Nation's bridges is a major disincentive to owner agencies who are faced with extensive upgrade programs. A more intelligent way of prioritizing a bridge inventory, and selecting bridges to be retrofitted, is required to ensure that scarce resources are utilized in the most cost-effective manner. For this purpose, a seismic risk assessment procedure has been developed by MCEER under one of the earlier FHWA contracts. To reach its full potential, however, the methodology must be extended to assess both direct and indirect costs of earthquake damage to highway systems and components such as tunnels, slopes, pavements, and retaining structures, and it must be validated and calibrated against actual earthquake experience-data. Concurrently, improved retrofit technologies are needed which are more efficient and reduce the overall cost to acceptable levels compared to standard strength-based retrofitting techniques.

To address these and other related needs, MCEER was awarded a 6-year, $10.8 million contract by the FHWA in the fall of 1998. This contract, which was authorized under the 1998 Transportation Equity Act for the 21st Century (TEA-21), will address research on the seismic vulnerability of the U.S. national highway system. This new contract will draw on and extend the work that has been conducted under the two prior FHWA research projects being conducted by MCEER, and on work done by the California Department of Transportation (Caltrans) and others.

Seismic Vulnerability Of Highway Systems

The earlier FHWA contracts were primarily focused on the seismic design and retrofitting of typical highway bridges found throughout the United States. The TEA-21 contract focuses on several special issues considered as critical to the future of the Nation's highway transportation infrastructure. TEA-21 research addresses:

The project will also address a series of special studies, including: i) development of post- earthquake, non-destructive assessment technologies for retrofitted bridge components, ii) technical assistance and support for NCHRP Project 12-49, which is developing a new seismic design specification for highway bridges, and iii) designing and implementing a seismic instrumentation network for a major cable-stayed bridge currently under construction across the Mississippi River between Cape Girardeau, Missouri and the State of Illinois.

The objective of this six-year research program is to study the seismic vulnerability of highway systems and components, and to develop cost-effective methods of retrofitting system components and facilitate the implementation of these methods in the field. The program has a national focus and addresses structure types and issues not currently being addressed in other research programs. Various experimental and analytical studies are expected to result in a number of manuals, guides, and design and retrofitting recommendations for evaluating and improving the seismic vulnerability of highway systems and components. Some of the new and improved knowledge and technologies may have applicability to other modes of transportation and opportunities for the transfer of this information to these other modes will be pursued.

The program is comprised of eight primary tasks as follows:

Task A - Project Administration
Task B - Loss Estimation Methods for Highway Systems
Task C - Seismic Design and Retrofit Manual for Long Span Bridges
Task D - Earthquake Protective Systems
Task E - Foundation and geotechnical Studies
Task F - Special Studies
Task G - Technology Exchange and Transfer
Task H - Project Reporting

The objective and focus of the research being conducted under Tasks B through G are summarized in what follows.

Loss Estimation Methods For Highway Systems

Loss estimation methods have moved to the forefront as earthquake hazard reduction tools. As a result of technological advances in data collection and management, relational data analysis, and software development, lost estimation methodologies can be utilized very efficiently. Much of this advancement in the highway field has been as a result of the seismic risk assessment (SRA) methodology developments under the FHWA-sponsored MCEER Highway Project. Acknowledging that the assessment of socioeconomic impacts caused by the failure and disruption of transportation systems was essentially absent until very recently, the current state of knowledge requires a continuing rapid advancement.

The role of loss estimation methods is three-fold. First, loss estimation methods can be used to estimate the seismic vulnerability of existing transportation systems, in particular highway networks. Second, based on this analysis, loss estimation methods can be used to project future benefits, in terms of reduced losses, from the execution of retrofit programs. Finally, loss estimation can be effectively applied in a post-earthquake environment, i.e., they can be used to help assess optimal recovery strategies allowing a comparison of relative benefits associated with each strategy or action.

The research being conducted by MCEER is focussed on the development and application of loss estimation methods for this broad range of purposes. The approach for integrating loss estimation methods into the current overall program is to build on what has been accomplished previously under the MCEER Highway Project, by enhancing that seismic system risk assessment models and expanding the types of losses that are considered (including indirect losses), incorporating new developments and data where appropriate, and ensuring the usability of the methodology so that it is presented as a flexible and user-friendly software tool with ample documentation. The importance of expanding the methodology to address other highway system components and transportation systems is also recognized. Consequently, the research plan for this task is envisioned to comprise three major subcomponents.

Subtask B1 - Loss Estimation Methodology Development, Additional Refinement of the on-going SRA Project; System Calibration and Validation.
Subtask B2 – Extend to Include Other Highway Structure Components
Subtask B3 – Extend to Include Other Types of Transportation Systems

Subtask B1 is comprised of five parts: i) validation, calibration, and refinement of the current SRA methodology; ii) development of direct and indirect loss modules; iii) calibration and validation of the complete system; iv) completion of the system software and documentation; and v) development of a demonstration and training workshop.

Under the earlier FHWA project, a major focus has been on bridge damage state modeling and traffic state analysis, with limited additional work in direct loss modeling and other highway elements. As part of Subtask B1-1, refinements in the current methodology will be made to address several limitations such as the applicability of models outside of the current demonstration city (Memphis, Tennessee), enhancements in network analysis uncertainties, and more rigorous treatment of site-specific assessments of certain seismic hazards. In order to extend the current SRA methodology to incorporate accurate assessment of direct and indirect economic losses, further model development will also take place in Subtask B1-2. To ensure the credibility of these new models, independent calibration and validation studies of the complete-integrated system, including the incorporation of other highway system components and transportation modes (under Subtasks B2 and B3), will be performed in Subtask B1-3 using data from a series of actual earthquakes.

To expand the current SRA methodology to other highway components, damage models will be developed for retaining structures, slopes, tunnels, culverts, approach fills, and pavements, under Subtask B2. Specific model development will include damage state fragility modeling, including uncertainty analysis, repair and restoration modeling, and traffic flow impact modeling based on different levels and types of damage. The goal of this task is to improve the overall modeling of damage and impacts to highway networks by including all key components.

The objective of Subtask B3 is to integrate freight and Urban Mass Transit (UMT) rail transportation systems models with the highway system models. Past experience in the U.S. and Japan has shown that these transportation alternatives can greatly facilitate emergency response and recovery operations after an earthquake and, hence, reduce overall economic losses due to earthquake damage. The end result of this integration will be a multi-modal land-transportation SRA methodology.

Finally, there is a tremendous opportunity to also facilitate cross-disciplinary research to improve particular loss estimation models. For example, the loss estimation methodology can be used to identify the relative importance of long span bridges with regard to overall system performance, as the necessary information (from Task C) will feed into the fragility and damage/loss modeling in this part of the program. In addition to the long span bridge task, the advanced bridge earthquake protective system task may provide important insights and information in the application of the methodology.

Seismic Design And Retrofit Manual For Long Span Bridges

In 1983, the FHWA published the "Seismic Retrofitting Guidelines for Highway Bridges," the first attempt at providing nationally-applicable guidance on screening, evaluating, and retrofitting seismically-vulnerable highway bridges. That report and its update (FHWA, 1995) were intended to address the vast majority of typical short-to-medium span structures found on the U.S. highway system. However, very limited information is available on a national basis concerning evaluating the seismic vulnerability or retrofitting of long span bridges. As previously mentioned, because these structures are usually critically important to the region they serve, society expects higher levels of performance from them than for the more conventional highway structures. This part of the TEA-21 project will address this problem in a consistent manner, drawing on experience from recent earthquakes and the expertise of a team of skilled practitioners and researchers. Hence, the research for this task is comprised of three major components:

Subtask C1 - Design and Retrofit Manual Development

Subtask C2 – Supporting Ground Motion, Geotechnical and Foundation Studies

Subtask C3 – Supporting Structural Studies

For the purpose of this project, long span bridges have been defined as those structures where the main span exceeds 120 m and are classified into the following superstructure types:

In addition, most substructures for long span bridges can be classified into following types: spread footings, timber piles, concrete piles, steel piles, or caissons.

The "Seismic Design and Retrofitting Manual for Long Span Bridges" will be developed under Subtask C1. This manual will be complementary to but distinct from the 1995 Seismic Retrofitting Manual and its current revisions. Some of the issues to be covered in the proposed manual and to be provided by the supporting Subtasks C2 and C3 are discussed in the following.

Structural Issues

Long span bridges generally cross waterways at locations where there is little, if any, redundancy in the highway network. Therefore, operational demands on such bridges are considerably greater. This dictates that long span bridges should behave in a "mostly elastic" fashion and, thus, remain open following a major earthquake. The most prolific of all long span bridges in the U.S. is the truss-type of structure. Therefore, it is planned to primarily, but not exclusively, focus on the truss type of long span bridges in this research.

Establishing retrofitting measures for long span bridges is expected to be challenging. It is necessary to conduct experimental studies on critical subassemblages that are representative of typical long span bridge structures in order to gain an understanding of their expected performance and behavior under dynamic loading. Of particular importance are braced and unbraced sway frames, bearing-pedestal seating systems, and the strength and ductility of bridge piers and foundations.

Subtask C3-1 is addressing issues related to braced piers in steel truss bridges and their substructure connection behavior. A series of analytical and experimental studies are being conducted to ascertain the most critical elements along with the need for retrofitting.

Ground Motion, Geotechnical, And Foundation Issues

There are a number of important considerations for long span bridges that must be addressed. These include the need for studies on improved evaluation of long period ground motion characteristics, developing methods for estimating liquefaction induced lateral spreading loads on caissons and pile supported piers, consideration of soil-foundation-structure interaction from large pile groups and caissons, and characterization of the site response at soft soil sites.

A comprehensive review and study of the factors that influence long period ground motions and their spatial variability that is important for long span bridges will be provided in Subtask C2-1. This task will result in guidelines for incorporating these factors explicitly in the estimation of seismic design ground motions, and to complement the deficient database of ground motion recordings close to large earthquakes with synthetic time histories.

Extremely large pile groups (involving hundreds of piles) are very common for long span bridges and group effects for such systems are believed to be extremely important. Limited pile group test data is available for 3x3 and 4x4 pile groups, which showed that elasticity pile-group theory over-predicts the group effects. Completely ignoring group effects can lead to very serious errors for very large pile groups, while relying on the elasticity approach has been found to be deficient even for small pile groups. In order to address this issue, Subtask C2-2 will establish practical approaches to determine the very large pile group effects by incorporating advances in constitutive modeling and 3-D finite element analysis.

Earthquake Protective Systems

Earthquake protective systems (EPS) provide promise for reducing earthquake losses in highway systems. This family of technologies is continuing to grow at an increasing rate with potential for even greater improvement. Recognizing this fact, Task D is comprised of the following:

Subtask D1 - Current Practice and Performance Review

Subtask D2 - Intelligent Passive Systems (Passive and Adaptive Systems)

Subtask D3 - Manual for the Design and Retrofit of Bridges with Earthquake Protective Systems

The current state-of-practice will be reviewed in light of recent field experience and new developments in the modeling of highly-damped systems under Subtask D1. Subtask D2-1 will provide concurrently detailed reviews of state-of-the-art advanced technologies that may be applicable to the development of intelligent bridge bearings for highway bridge applications. These reviews are expected to lead to the development of a new generation of protective systems. The intent is to apply smart materials to the construction of intelligent passive systems (isolators and dissipaters) which will combine the reliability of passive devices with the real-time responsiveness of active control. Other approaches for introducing "intelligence" into bridges will also be explored, such as the use of smart restrainers and adaptive structural forms.

Since, high damping is generally associated with EPS, the dynamic behavior of bridges with high damping will be investigated on the basis of 3-D nonlinear time history analyses, validated by large-scale model tests, under Subtask D2-2.

A major product from this task will be a manual for the design and retrofit of bridges with earthquake protective systems. The overall task will be coordinated with other tasks such as those concerned with ground motion and geotechnical hazards. In particular, there will be contributions to Tasks C and B (on long span bridges and loss estimation studies, respectively) where fragility functions are required for EPS-bridges.

Foundation And Geotechnical Studies

The research program under Task E includes studies on foundation retrofitting (Subtask E1) and soil retrofitting (Subtask E2). These studies are intended to augment and support research conducted on foundations and soils for long span bridges under Task C.

An earlier contract between MCEER and the FHWA addressed issues such as the improved assessment of seismic load demands on foundation elements, moment capacity of pile footing foundations, and pile-to-pile cap connection details and their movement and lateral capacity. In many cases, however, design approaches are still based on foundation displacement demands arising from bridge response. There is a lack of design guidance on the displacement capacity of various pile types and connection details. This issue is addressed in Subtask E1-1, which will result in seismic evaluation guidelines for assessing the rotational/ductility capacity of plastic hinges for piles within soil and for assessing the strength and inelastic rotational capacity of existing pile-to-cap connections. Recommendations for improving the ductility capability of new pile designs will be provided.

The vulnerability of bridges to earthquake induced liquefaction, lateral ground deformation (or spread) has also been clearly demonstrated in past earthquakes. Several liquefaction mitigation approaches using soil remediation have been studied in the past. However, further research is needed on the mitigation option related to foundation design or retrofit as current methods of soil improvement are very costly and time consuming to implement. The research to be undertaken in Subtask E1-2 will develop several models to simulate the interaction between soil in a lateral spread zone and pile or pile foundation elements and pile caps. General design guidelines on conditions suitable for structural retrofit, or a structural mitigation versus ground remediation option will be established under this task.

Most of the soft soil retrofit problems are generally associated with soil liquefaction, which clearly has been responsible for numerous bridge failures in past earthquakes. Recent research has identified a number of outstanding issues related to the use of ground densification approaches for soil remediation purposes. Accordingly, Subtask E2-1 will address part of this issue by developing an improved remediation technique and design method to mitigate liquefaction hazards in silty soils using stone columns.

Finally, the applicability of new alternative ground remediation techniques will be investigated under Task E. For example, deep soil mixing using cement or cement/lime mixtures as a liquefaction mitigation method is receiving increased attention in the U.S. and Japan. This method has definite applications to bridge sites; therefore, research is coordinated to tailor the technology to bridge applications and to develop design guidelines for its use.

Special Studies

Task F of the project will address a series of special studies including:

Subtask F1 - Post-earthquake Nondestructive Assessment of Retrofitted Bridges

Subtask F2 - Cape Girardeau Cable-stayed Bridge Instrumentation

Subtask F3 - NCHRP Project 12-49 Supporting Studies

Subtask F4 - Earthquake Reconnaissance

Post-Earthquake Assessment Of Retrofitted Bridges

Following a strong earthquake, perhaps the most important question facing bridge engineers is whether or not a particular bridge has been damaged and to what extent. If there are visible signs of damage, is the bridge safe enough to allow continued traffic on it?

Damage is generally self-evident in the form of cracking and crushing of concrete columns, unseating of girders at the bearings and seats, and damage to foundations and abutment back-walls that is evident from pavement distress and bulging soil. However, if a bridge has been retrofitted with either cable-restrainers, column jackets, foundation improvements, or a combination thereof, the damage may not be as visible. In such cases, the engineer may need to resort to nondestructive evaluation (NDE) techniques as a means of damage detection and to assess the severity of damage.

This task will provide a comprehensive review and assessment of such techniques and will develop a manual for conducting post-earthquake evaluations using appropriate NDE methods and technologies.

Cape Girardeau Cable-stayed Bridge Instrumentation

The acquisition of structural response data during earthquakes is essential to confirm and develop methodologies for analysis, design, repair and retrofitting of bridges and other structures. To understand structural response to earthquake-induced ground shaking, it is necessary to obtain free-field ground motion data in the vicinity of a structure, acceleration and displacement data in and around the foundations to identify the soil-foundation interaction effects, and on the structure itself.

The new cable-stayed bridge currently under construction near Cape Girardeau, Missouri, provides an excellent opportunity to install a number of sensors in the free-field, within the foundation, and on various sub- and superstructure elements and subsystems within this long span Mississippi River crossing. Subtask F2 will provide a detailed seismic instrumentation design for the bridge, and will acquire all required equipment. The research team will then provide continued technical support to the bridge designer, construction contractor, and owners during installation of the system.

NCHRP Project 12-49 Technical Support

The AASHTO-sponsored National Cooperative Highway Research Program (NCHRP) initiated NCHRP Project 12-49, "Comprehensive Specification for the Seismic Design of Bridges" in mid-1998. The project is intended to develop and incorporate new probability-based specifications into the AASHTO LRFD Bridge Design Specifications. The project is addressing improvements to the overall seismic design philosophy and performance criteria, seismic loads and site effects, analysis and modeling procedures, and design and detailing requirements. A joint venture of the Applied Technology Council (ATC) and MCEER is the primary contractor for the project.

Work under NCHRP Project 12-49 is intended to incorporate currently available research results and the state-of-practice, and does not include provisions for special analytical studies should they be needed to address important issues or gaps in knowledge. Therefore, under Subtask F3, support will is being provided to the research team conducting NCHRP Project 12-49. This support includes conducting a limited number of special studies identified as critical to the completion of the specifications by the ATC/MCEER Joint Venture. One such study was initiated under Subtask F3-1(a) to conduct a series of parameter studies necessary to address the development of design and detailing provisions for various bridge components, including columns, foundations and abutments. The parameter studies will also evaluate the cost impact of different design decisions in a variety of seismic zones of the U.S. A second subtask (F3-1(b)) is providing support for the design and conduct of a workshop to address various issues related to the development of performance criteria for geotechnical engineering and bridge foundation design.

Earthquake Reconnaissance

When damaging earthquakes occur, it is important to assemble and have on-site an earthquake reconnaissance team as soon after the earthquake as possible. There are many lessons that can be learned and technical information obtained from such an exercise by gathering and documenting data on the behavior and performance of soils, foundations, and structures. The MCEER Highway Project previously supported reconnaissance teams to document bridge and highway system performance and damage, often resulting in the initiation of special studies in order to develop an understanding of this performance and to improve analytical models and retrofit designs.

Consequently, Subtask F4 will provide for support of continued earthquake reconnaissance efforts, and the rapid compilation of reports documenting damaging events and dissemination of this information to the profession. In fact, in late summer of 1999, the project supported reconnaissance efforts in Turkey (***Gokhan – fill in official earthquake names***August 17, 1999 Izmit (Kocaeli) Earthquake) and Taiwan.

Technology Exchange And Transfer

An important element of this project is the transfer of newly developed and improved technologies to the transportation engineering profession both in the U.S. and internationally. As a result, Task G will provide support for the development of workshops on various aspects of the program, and the planning and conduct of national and international conferences. The task will also provide support for outreach and coordination with AASHTO, and State and Federal transportation agencies.

Among the specific items under this task, a series of training courses will be developed to assist in the implementation and use of the "Seismic Retrofitting Manuals for Highway Systems" which are nearing completion under the prior FHWA contract with MCEER. This task will also provide for the planning and conduct of the 3rd National Seismic Conference on Bridges and Highways on behalf of the FHWA. Support will also be provided for the conduct of the 17th, 18th, and 19th UJNR U.S.-Japan Bridge Engineering Workshop.

It is anticipated that a number of demonstration projects will also be organized and conducted during latter years of the research program. These demonstration projects will assist in the implementation and transfer of new and improved technologies developed under this research program.

Conclusions

The FHWA-sponsored research program recently initiated by MCEER will address a number of important aspects related to the seismic vulnerability of highway systems. The overall research program includes six major task areas, which have been summarized in this paper. Further developments and achievements as a result of this project will be discussed in subsequent papers and presentations.

The results of this work are expected to have significant short- and long-term impacts on the performance and economics associated with the earthquake performance of highway systems, and it is expected that some of the advances made in this program will have direct application to other surface transportation modes. As the research is still in its early stages, initial results and products are not yet available.

References

FHWA ( 1995), Seismic Retrofitting Manual for Highway Bridges, FHWA Publication No. FHWA-RD-94-052, May.

MCEER, (1999a), "Seismic Retrofitting Manuals for Highway Systems - Volume I: Seismic Risk Analysis of Highway Systems." (in progress)

MCEER, (1999b), "Seismic Retrofitting Manuals for Highway Systems - Volume II: Seismic Evaluation and Retrofitting Manual for Highway Bridges." (in progress)

MCEER, (1999c), "Seismic Retrofitting Manuals for Highway Systems - Volume III: Screening, Evaluation, and Retrofitting of Retaining Structures, Slopes, Tunnels, Culverts, and Pavements." (in progress)

Rojahn, C., Mayes, R., Anderson, D.G., Clark, J.H., D'Appolonia Engineering, Gloyd, S., and Nutt, R.V., (1999), "Impact Assessment of Selected MCEER Highway Project Research on the Seismic Design of Highway Structures," Technical Report MCEER-99-0009, Multidisciplinary Center for Earthquake Engineering Research, State University of New York at Buffalo.

FHWA ( 1995), Seismic Retrofitting Manual for Highway Bridges, FHWA Publication No. FHWA-RD-94-052, May.

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