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Federal Highway Administration > Publications > Public Roads > Vol. 73 · No. 3 > Developing Advanced Methods of Assessing Bridge Performance

Nov/Dec 2009
Vol. 73 · No. 3

Publication Number: FHWA-HRT-10-001

Developing Advanced Methods of Assessing Bridge Performance

by Hamid Ghasemi, John Penrod, and John M. Hooks

FHWA researchers are working to improve safety, reliability, and other metrics for ongoing analyses of the condition of highway structures.

This span in northern Utah is one of the pilot bridges being studied under FHWA's Long-Term Bridge Performance (LTBP) Program, which is developing uniform performance metrics for highway spans.
This span in northern Utah is one of the pilot bridges being studied under FHWA's Long-Term Bridge Performance (LTBP) Program, which is developing uniform performance metrics for highway spans.

The Federal Highway Administration (FHWA) strives to improve the performance of the U.S. highway system, where performance is described at the system level in terms of safety, reliability, effectiveness, and sustainability. The challenge is to understand and define the performance of various components of the system in terms of a common set of objective metrics, so that engineers can measure and improve the performance of each asset and critical component, and ultimately the entire system.

Transportation professionals currently describe the condition and performance of bridges and pavements by different and unrelated measures (for example, condition ratings, sufficiency rating, and health index for bridges; roughness index for pavements; and level of service for traffic).

"There are no uniform performance metrics or performance indicators for bridges in the United States, despite the fact that the current knowledge base of inventory information and condition data on bridges is by far the most extensive in the world," says Tom Everett, principal bridge engineer, FHWA.

Everett adds, "Currently, bridge inspection standards, methods, and the tools used for managing bridge programs are exemplary. But the level of understanding of how bridges perform and how to measure their performance satisfactorily falls well short of the optimum. Because of a general lack of critical data and an inadequate understanding of the multiple cause-and-effect relationships that govern the many aspects of bridge performance, most attempts at assessing that performance rely only on expert opinion, assumptions, and generalizations."

This problem is particularly exemplified when bridge performance is examined at the component level. A prime example is the performance of different kinds of bridge decks under varying types of environmental conditions, traffic loadings, maintenance practices, and protective measures.

To fill this knowledge gap, the Long-Term Bridge Performance (LTBP) Program has identified several high-priority issues where field investigation and analysis of the existing relevant data will provide quantitative measures of bridge performance. Collaboration with a number of State departments of transportation (DOTs) identified these high-priority performance issues.

The Bridge Infrastructure

A brief look at data from the National Bridge Inventory (NBI) provides a revealing picture of this large and vital public asset. The NBI database contains records for 600,000 structures, of which approximately 473,000 are single-span or multi-span bridges that carry roadway traffic over some other roadway or topographical feature, usually a stream or river. These bridges range from the typical overpass structures to magnificent signature bridges such as the Golden Gate Bridge, the Brooklyn Bridge, and the Sunshine Skyway Bridge.

Within this range, the diversity of the bridge infrastructure in terms of age, type, material, width, and length is broad. Several of these factors and others govern the performance of bridges, help explain differences in performance from one bridge to the next, and help predict future performance.

Age. The average age of the 473,000 bridges in the NBI is 40 years old. Age serves as a rough measure of the aggregate service provided by a bridge during its life. Age data also roughly correlate with periodic advancements in bridge materials, design standards, and construction processes. These advancements generally improve current and future performance.

Service conditions. Service conditions might vary dramatically in terms of traffic volumes carried, truck loadings (including permit loads), and level of vulnerability to natural forces such as environmental, climatic, and hydraulic impacts, including ice, debris, wind, and seismic loadings. In many cases, the potential exists for significant damage from vessel or vehicular collision.

Type, frequency, and effectiveness of preservation, maintenance, repair, and rehabilitation. These factors vary significantly based on the level of experience and knowledge of the bridge personnel from the owner/agency, quality of work that is often performed by contractors, funding available for bridge programs, and agency priorities.

The NBI records also describe bridges using many different attributes and parameters, such as kind of material and design load.

Goals of the LTBP Program

Launched in 2008, the FHWA LTBP Program is a 20-year (or longer) major research effort addressing a number of objectives, such as achieving a deeper understanding of bridge performance, developing methods to measure it reliably, and using those measurements to improve the Nation's bridge infrastructure and thus the performance of the transportation system. FHWA will use the knowledge gained from the LTBP Program to solve a variety of problems related to bridge condition assessment and management, to develop new measurement tools, and to advance knowledge of bridge design, construction, inspection, maintenance, and preservation.

Specific goals include determining how and why bridges deteriorate (that is, advances in predictive models); determining the effectiveness of various management practices and maintenance, repair, and rehabilitation strategies; examining the effectiveness of durability strategies for new bridge construction, including selection of materials; and facilitating improvements in management practices through the use of quality data.

LTBP Strategic Plan

The LTBP Program is a large and complex undertaking that requires a carefully thought-out process for its success. A well-defined strategic action plan, with seven key steps, provides the program's overall direction.

This overpass in northern Virginia near the District of Columbia is another one of the bridges selected for the pilot study and is typical of the structures that the LTBP Program will be studying.
This overpass in northern Virginia near the District of Columbia is another one of the bridges selected for the pilot study and is typical of the structures that the LTBP Program will be studying.

This strategic plan is based on a heuristic (experience-based) approach in which the FHWA researchers have to define bridge performance first before initiating the data collection phase. Rather than establishing linear steps, the strategic plan requires ongoing quality control and assurance back to earlier phases as each of the remaining steps is reached. In other words, the process is iterative and will be refined multiple times during the life of the program, yielding new information.

Step 1: Defining Bridge Performance

The transportation user community expects bridges to accomplish their purpose (that is, to perform) in a satisfactory manner as measured against several objectives. Bridges should present a minimal safety hazard to users and minimal obstruction to the free flow of traffic during normal service, produce a minimal negative impact on the local and global environments during construction and subsequent maintenance work, ensure an acceptable level of risk against catastrophic failure, present an aesthetically pleasing appearance, and accomplish all of these objectives with minimal life-cycle costs.

Transportation agencies strive to meet these objectives based on the best available understanding of how bridges perform under various sets of service conditions. The logical starting point for the LTBP Program's path to improved understanding of bridge performance is to break it down into specific issues and evaluate the existing gaps in knowledge that hinder understanding.

In the early development of the LTBP Program, the researchers clearly understood that the definition of bridge performance must be responsive to the needs of the primary stakeholders: the State and local DOTs and private road authorities that own and manage the bulk of the Nation's bridge infrastructure, along with Federal agencies and the bridge engineering community at large. These stakeholders are the ones who will apply the knowledge and lessons learned from the LTBP Program. To best serve these stakeholders, one of the early decisions was to establish an overall definition of bridge performance that addresses four broad categories: structural condition, structural integrity, functionality, and costs.

Diversity of Bridge Characteristics
NBI Item Number of Types
Kind of material, main span, and/or approach span 10
Structure type, main span, and/or approach span 23
Design load 10
Bridge posting 6
Deck structure 9
Wearing surface 9
Membrane 5
Protective system 9

A critical factor in developing a more specific definition of bridge performance was outreach to the States. Members of the LTBP Program held focus group meetings at the offices of 10 State DOTs. The focus groups consisted of DOT experts responsible for the design, construction, inspection, management, and maintenance of bridges. The purpose was to determine the aspects of bridge performance that are considered the highest priorities by these DOTs.

The major findings were remarkably similar from State to State. Regardless of the geographic region, high on the list of priorities were performance issues related to concrete decks, joints for bridge decks, scour at substructures, and deterioration of concrete substructure units. The approaches each DOT takes to these problems differ widely, but there was a consistent desire to understand the performance issues better and to refine or revise the approaches to ensure better performance.

Based on the LTBP Program's internal research and on the input from stakeholders, the researchers identified 20 bridge performance issues and grouped them into different categories. The knowledge gained through the program might increase or decrease the number of issues. Bridge experts from the program grouped the issues into like categories. (See "High-Priority Performance Issues" on page 32.)

Step 2: Identifying Data To Be Collected

The effort to define bridge performance leads somewhat seamlessly into the next three steps, which are in turn interrelated. The process by which the second step is achieved involves addressing each of the high-priority bridge performance issues by identifying the knowledge available to analyze each issue, the critical gaps in current knowledge, the specific parameters that might be useful in characterizing the issue, and the methodology required to obtain high-quality data for each parameter.

Plan superimposed on a U.S. map. The diagram shows eight small icons formed in a W-shaped path represented by a heavy black line between the icons, overlaid on a background map of the United States. The icons start in the western part of the country and flow toward the east in a zigzagged way along the black line, representing the various steps in the LTBP Program's strategic plan: Step 1 (with a notebook and pen icon): Defining Bridge Performance; Step 2 (disc icon): Identifying Data To Be Collected; Step 3 (computer): Developing the Data Management System; Step 4 (protractor icon): Designing the Experimental Program; Step 5 (hardhat): Pilot Study; Step 6 (magnifying glass): Analyzing and Modeling Data; Step 7 (computer): Disseminating Findings; and Program Outcome (the LTBP identifier, indicting that the program has met its objectives).

Also required is deciding among visual inspection, destructive or nondestructive testing, and sensors for long-term monitoring; determining whether practical, effective, and affordable technology is available to gather the necessary data; and establishing whether standard testing methods exist or if custom protocols must be developed. The final stage in step 2 is adopting and developing specific data collection protocols for each of the chosen data collection methodologies.

For almost all bridges, a large amount of data has been generated, collected, and stored because of previous activities related to that bridge. This "legacy data" may include design specifications, as-built plans, inspection reports, maintenance history, traffic data, crash data, weather data, etc. To the extent possible, this data will be extracted from existing files, reports, and electronic databases for use in evaluating various aspects of performance.

In addition to legacy data, the LTBP Program will gather a large volume of quantitative field data gleaned from detailed visual and hands-on inspections, nondestructive testing, embedded sensors for periodic long-term monitoring, and live load tests. Together with additional bridge-related data (for example, geospatial and weather data), this will represent an immense source of information helpful for a better understanding of bridge performance and deterioration.

Flow chart. The flow chart illustrates the main categories of bridge performance issues: Structural Condition, Structural Integrity, Functionality, and Costs. A blue rectangular box labeled 'Bridge Performance' appears above four smaller yellow boxes representing those four categories. Arrows point from the blue box down to each of the four smaller boxes. From left to right, the smaller boxes are labeled: 'Structural Condition: Durability and Stability,' Structural Integrity: Safety and Serviceability,' 'Functionality: User Safety and Service,' and 'Costs: Users and Agency.'

To exploit this information efficiently, however, the bridge engineering community needs a new generation of data management and analysis tools. These new tools will be created as part of the development of the data infrastructure described in step 3.

The process of accomplishing step 2 and the results achieved provide critical input into steps 3 and 4, while feedback from those steps helps refine and improve the conclusions achieved in step 2.

Step 3: Developing the Data Management System

The LTBP Program aims to provide a single source of information for researchers, bridge owners, and other stakeholders seeking detailed information from heterogeneous data sources to develop more accurate bridge performance models. The data and information collected in the LTBP Program will provide a more detailed and timely picture of bridge health, improve knowledge of bridge performance, and ultimately help promote the safety, mobility, longevity, and reliability of the Nation's highway transportation bridges.

To support these goals, the LTBP Program developed an open, scalable, and extensible data management and analysis infrastructure. Openness will be achieved by designing software components with documented interfaces that use service-oriented architecture components for intercomponent communication. Scalability will be achieved by allowing seamless integration of additional hardware to address increasing data volume and usage requirements. Extensibility will be achieved by designing the data infrastructure in a modular and flexible way that enables system administrators to add new data sources and types with characteristics similar to existing data sources.

The researchers will use state-of-the-art data warehousing and mining techniques to facilitate efficient verification and large-scale testing of new research hypotheses. The data integration framework is based on a hybrid model in which the various data sources are linked and brought together in the same data warehouse. A software middle layer, mapped to the actual database, ensures a smooth and transparent integration of various data sources. In addition, the middle layer also can handle federated data supplied by other systems or databases via Web services. The core LTBP data warehouse itself is distributed across several computers in order to ensure a fast querying process.

High-Priority Performance Issues
Category Issue
Decks Performance of untreated concrete bridge decks
Performance of untreated concrete bridge decks
Performance of precast reinforced concrete deck systems
Performance of alternative reinforcing steels
Influence of cracking on the serviceability of high-performance concrete decks
Joints Performance, maintenance, and repair of bridge deck joints
Performance of jointless structures
Steel Bridges Performance of coatings for steel superstructure elements
Performance of weathering steels
Concrete Bridges Performance of bare or coated/sealed concrete superstructures and substructures (splash zone, soils, or exposed to deicer runoff)
Performance of embedded or ducted prestressing wires and posttensioning tendons
Performance of prestressed concrete girders
Bearings Performance of bridge bearings
Foundations and Scour Direct, reliable, timely methods to measure scour
Performance of scour countermeasures
Unknown foundation types
Structure foundation types
New Construction Performance of innovative materials and designs
Risk Risk-based management approach
Functionality Operational performance of functionally obsolete bridges

To support the needs of various user groups, the proposed data infrastructure will employ recent advances in visualization technologies, such as Internet applications that enable users to interact with the data in dynamic, interactive ways from anywhere at any time; an interactive, map-based user interface; and a set of automated interfaces for programmatic access to data. For example, one Web-based application enables users to search for bridge structures based on cross-domain criteria, such as weather statistics, traffic information, and condition ratings from the past 20 years.

Step 4: Designing the Experimental Program

The work included in this step provides the detailed framework for each experimental study that will address one of the high-priority bridge performance issues. The thought process behind each study also provides input into the final stage of step 2 (adopting and developing specific data protocols for each of the chosen data collection methodologies). Once each specific study is designed, the final approach to data collection on the critical parameters can be revised as necessary. The revision might mean eliminating or adding parameters to fine-tune the data collection protocols and/or modify the testing frequencies.

A well-designed experimental program is the key to collecting data that will support a better understanding of bridge performance. The design begins with postulating the critical questions about what governs the performance issue being studied. For example, when considering bridge decks that are untreated (that is, have not been overlaid with a wearing course or treated with a sealing compound), critical questions might include the following:

  1. What is the current condition of the deck concrete?
  2. What is its permeability?
  3. What is the rate of deicer application on the deck?
  4. What is the existing chloride profile in the deck?
  5. What is the rate of corrosion, if any, on the reinforcing steel?
  6. What are the annual climate fluctuations in the vicinity of the bridge?
  7. What is the truck load history that this deck has been exposed to?

In addition to current values, how will the value of these parameters or measurements change over time? A proper experimental study can be developed only after such questions are addressed.

Each experimental study will be developed to assess the relationships of one or more of these critical issues to some aspect of the performance of the deck. For example, studying annual climate fluctuations might elicit data that suggest a significant relationship between freeze-thaw cycles and a measured level of scaling of the surface of a concrete deck. Each study's parameters will include types of testing to be done and the instrumentation and sensors to be deployed. For example, answering the question about the condition of concrete decks might require a field survey of a deck using the conventional chain drag method (chain dragged across deteriorated concrete makes a hollow sound) or mapping cracks visible on the surface or taking advantage of technology such as ground penetrating radar (high-frequency electromagnetic waves detect cracks, voids, and delamination). The LTBP researchers also will develop data collection protocols, such as standard test methods, frequency of testing, and duration of the study. Finally, the researchers will select a representative sample of bridges for the study.

Step 5: Pilot Study

Concurrent with the latter stages of steps 2 through 4 is the beginning of the second phase of the LTBP Program, the pilot study (step 5). The primary objective is to validate the methods and protocols developed for data collection under the first phase of the program. To date, the researchers have selected an extensive array of nondestructive evaluation (NDE) sensors for long-term monitoring. In addition, they developed a protocol for visual and hands-on inspection of the LTBP Program bridges and protocols for each of the testing and monitoring regimens.

Just as bridge selection will play a vital role in the long-term data collection phase of the program, the data collection phase is also crucial in the selection of the bridges for the pilot study. To validate the program protocols in as many environments as possible, the LTBP Program selected seven States that provide a fair representation of the environmental conditions and types of structures throughout the United States. The team kept the sample size to a minimum in order to focus on refining and testing the protocols and guidelines. The selected States are California, Florida, Minnesota, New Jersey, New York, Utah, and Virginia.

These workers are conducting a detailed evaluation of the northern Virginia bridge deck. On the left, a worker is using an impact-echo and complementary ultrasonic testing to detect and characterize delamination. The worker on the right is using ground penetrating radar to assess concrete alterations resulting from deterioration.
These workers are conducting a detailed evaluation of the northern Virginia bridge deck. On the left, a worker is using an impact-echo and complementary ultrasonic testing to detect and characterize delamination. The worker on the right is using ground penetrating radar to assess concrete alterations resulting from deterioration.

The LTBP Program has initiated the instrumentation and monitoring of bridges in Utah and Virginia to test and validate the LTBP Program inspection protocols and guidelines. The Virginia bridge, located on U.S. Route 15 over I 66 at Haymarket, VA, which is a short drive from the District of Columbia, is an overpass typical of the bridges that the LTBP Program will focus on. The bridge is a continuous two-span, built-up steel girder bridge constructed in 1979. The average daily traffic is approximately 16,000 vehicles (as provided by the Virginia DOT), of which an estimated 6 percent is truck traffic. The bridge has a cast-in-place concrete deck with no overlay and no stay-in-place forms that allow visual access to the deck's top and bottom sides. The deck itself is beginning to display early signs of deterioration, which will be key in validating NDE techniques and other instrumentation focused on identifying deterioration in bridge decks.

The second bridge, located in northern Utah near Utah State University, is a single-span, precast concrete girder bridge with integral abutments. This bridge (I- 15 over Cannery Road just west of Perry, UT) has a waterproof membrane with an asphalt overlay, which will enable the program to validate that data collection methods will work with overlays.

As mentioned earlier, many States have indicated that failing joints are a major concern -- one that the focus groups indicated leads to additional performance issues. By selecting the Utah bridge, the LTBP Program will obtain an early look at a jointless structure, which will help researchers identify key performance issues to be investigated on similar spans.

Researchers in this van are conducting a GPR survey on a bridge deck using two perpendicular horn antennas while traveling at speeds up to 30 miles per hour (48 kilometers per hour). The horn antennas enable dual polarization of electromagnetic waves, which provides more accurate reflection from transverse rebars when the longitudinal rebars are the top ones, which is often the case with slab bridges.
Researchers in this van are conducting a GPR survey on a bridge deck using two perpendicular horn antennas while traveling at speeds up to 30 miles per hour (48 kilometers per hour). The horn antennas enable dual polarization of electromagnetic waves, which provides more accurate reflection from transverse rebars when the longitudinal rebars are the top ones, which is often the case with slab bridges.

Although the program has not yet identified the other five specific bridges in the pilot study, it has determined the types of structures that will be studied, including new construction, concrete box beams, prestressed bulb tee, simple span steel girder, and adjacent concrete box beam girder bridges. These structures make up approximately 70 percent of the bridge population in the inventory. In addition, the pilot project will look at bridges with cast-in-place and precast decks, plus those using various forms of overlay and stay-in-place forms.

Step 6: Analyzing and Modeling Data

By studying as much variety as possible through a limited sampling, the LTBP Program will validate the data collection methods, guidelines, and protocols for visual inspection, instrumentation, and NDE developed while addressing the performance issues identified through the focus groups. This effort will provide the opportunity to refine and streamline the data collection process before rolling out to a larger sampling as part of the long-term data collection phase.

The research team does not view validation of the protocols, methods, and guidelines for data collection as independent from the long-term data collection phase. The selection and instrumentation of the pilot bridges and subsequent data collection need to be consistent with the objectives of the long-term data collection phase. The information gathered during the pilot study will feed directly into the long-term phase and provide early answers to questions that can be researched fully in the near future.

Step 7 and Beyond

During the pilot study phase, FHWA will review and refine the test protocols as appropriate. As the pilot study phase proceeds, FHWA will review the refined test protocols, research reports, and findings from the pilot study, and then disseminate them to stakeholders (bridge experts from DOTs, academia, and private industry) for review and evaluation. This process is expected to continue during the pilot study and beyond.

The long-term data collection on a much larger sample of bridges (that is, between 200 and 600 bridges depending on future program funding levels) will commence after the pilot study. The information gathered from such a large sample of bridges over the long term will help FHWA achieve the objectives of the LTBP Program.

Why Improve Performance Measurements At All?

Bridge performance is a concern for virtually everyone: commuters, tourists, deliverers of goods and services, emergency responders, national security officials, and other travelers. It is also of concern to legislators who create transportation programs and provide funds for the design, construction, inspection, maintenance, repair, and replacement of bridges. Stakeholders include administrators who manage bridge programs within transportation agencies, engineers and planners who design and build highways and bridges, and maintenance and management engineers and other personnel who maintain these structures at a satisfactory level of service.

Bridge performance measures can have multiple uses, depending on the perspective and responsibilities of those who are affected. The average traveler looks for reassurance about highway safety, rapid assistance from first responders, and reduced traffic congestion. Legislators might use specific performance measures to assess the ability of public agencies to implement the transportation decisions and programs that they create and fund, plus the overall effectiveness of those programs.

Engineers and planners need to factor performance into bridge planning, design, and construction by applying lessons learned from the performance of previously built structures. Bridge maintenance and management personnel use measures of performance to evaluate policies, practices, techniques, and materials that they employ. News outlets use simple performance measures and statistics to inform the public and key bridge constituencies about critical issues related to the transportation system.

More reliable performance measures will enable bridge owners to evaluate congestion and traffic safety more accurately. Performance measures also help provide an accurate determination of load capacity and any resultant need for load restrictions; identify clear links between a specific action and a change in performance level of some bridge feature; and improve knowledge of how and why bridges deteriorate (that is, lead to advances in predictive models). Better performance measures also can improve understanding of the effectiveness of various maintenance, repair, and rehabilitation strategies, as well as management practices; determine the effectiveness of durability strategies for new bridge construction, including selection of materials; and facilitate improvements in bridge management practices using high-quality, quantitative data.

Finally, improved performance measures will help transportation personnel evaluate bridge serviceability and durability; improve design, construction, and maintenance strategies; establish priorities for resource allocations within the transportation system and within the bridge infrastructure; evaluate organization-wide policies and programs; improve system reliability and accountability; and establish risk-based evaluations of bridges vulnerable to catastrophic failure.

The LTBP Program will continue to reach out to subject matter experts, policymakers, stakeholders, industry, and academia to help refine the program's findings and to collect the necessary data and knowledge to meet the Nation's bridge needs and goals. The pilot program began in September 2009 and will last 2 years. Steps 1 through 6 will be refined and updated as new information is gathered from field investigations and input from stakeholders during the program.

Two plots. These two plots of two different bridge decks show electromagnetic waves traveling through a reinforced concrete deck toward rebars, which are metallic and therefore the strongest reflectors. The waves are attenuated in part due to the geometry of wave propagation, but mostly due to concrete deterioration and the presence of moisture and chlorides. The relative magnitude of the attenuation is a qualitative indicator of the degree of deterioration. The areas plotted in hot colors describe areas of high attenuation and deterioration, while those in cold colors describe the opposite.
These two plots show electromagnetic waves traveling through a reinforced concrete deck toward the rebars, which are metallic and therefore the strongest reflectors. The waves are attenuated in part due to the geometry of wave propagation, but mostly due to concrete deterioration and the presence of moisture and chlorides. The relative magnitude of the attenuation is a qualitative indicator of the degree of deterioration. The areas plotted in hot colors describe areas of high attenuation and deterioration, while those in cold colors describe the opposite.

Dr. Hamid Ghasemi manages the FHWA LTBP Program. He joined the Office of Infrastructure Research and Development (R&D) at FHWA's Turner-Fairbank Highway Research Center in 1994 and was named FHWA's Engineer of the Year in 2001. His work emphasizes seismic-related issues, structural health monitoring, post-hazard evaluation, computer modeling, and structural analysis. He received his doctorate in structural engineering from the University of Kentucky.

John Penrod is currently the FHWA LTBP Program pilot study manager. He earned his B.S. in civil engineering from Georgia Institute of Technology, has 8-plus years of design experience, and is a licensed professional engineer.

John M. Hooks is a consultant at Highway R&D Services. He has more than 40 years of experience in bridge engineering and research, both with FHWA and subsequently as a consultant on several projects.

For more information, visit www.fhwa.dot.gov/research/tfhrc/programs/infrastructure/structures/ltbp/ or contact Hamid Ghasemi at 202 493 3042 or hamid.ghasemi@dot.gov.

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