U.S. Department of Transportation
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Federal Highway Administration Research and Technology
Coordinating, Developing, and Delivering Highway Transportation Innovations
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|Publication Number: FHWA-HRT-07-004 Date: May/Jun 2007|
Publication Number: FHWA-HRT-07-004
Issue No: Vol. 70 No. 6
Date: May/Jun 2007
FHWA is embarking on a 20-year effort to collect more and better bridge data to improve management now and far into the future.
|(Above) This truck is exiting a freeway ramp typical of those to be monitored.|
According to the National Bridge Inventory (NBI) database, nearly 600,000 highway bridges stitch together the Nation's highway network, making the system dependent on their integrity if it is to continue to provide seamless transportation to the traveling public. Skillfully managing these various structures is vital to the movement of goods and people around the United States.
Given the wide range of bridge structure types, material characteristics, age, daily traffic, and climatic conditions, management requires knowledge in many areas, including performance characteristics, deterioration models, and quantitative data. "The magnitude of capital investment in bridges and their impact on safety, mobility, and national security clearly require a proactive approach in management of the U.S. bridge inventory," says Tom Everett, principal bridge engineer for the Federal Highway Administration (FHWA).
Many initiatives for improving bridge management have been undertaken, especially since the infamous collapse of the Silver Bridge at Point Pleasant, WV, in 1967. The latest initiative is FHWA's Long-Term Bridge Performance (LTBP) program, which was authorized in 2005 under the Safe, Accountable, Flexible, Efficient Transportation Equity Act: A Legacy for Users (SAFETEA-LU). The program is strategic and ambitious, a 20-year research effort that has both long-range and short-term goals. It will be similar to the Long-Term Pavement Performance (LTPP) program that began in 1987 through the Strategic Highway Research Program. The framework of LTPP offers components that could prove useful in the new LTBP program.
To initiate the bridge program, FHWA is developing specifics regarding program goals and criteria, and is publicizing them throughout the United States. FHWA drafted a framework with help from the Center for Innovative Bridge Engineering at the University of Delaware. Stakeholders in the public, private, and academic segments of the highway bridge community, including international interests, are discussing the framework before final approval. Through a series of workshops, FHWA will seek views on all aspects of bridge performance data collection and analysis, long-term monitoring, bridge selection criteria, sensing technologies, nondestructive evaluation (NDE) tools, and expected outcomes and products.
When interstate construction began in the late 1950s, the bridge inventory in the United States already was large, and the bridges were aging. Little consideration was given to assessing bridge conditions at the national level, and there was no uniformity to evaluating performance. This situation continued until 1967 and the failure of the Silver Bridge. The collapse, which spilled rush-hour vehicles into the Ohio River and killed 46, resulted in FHWA's establishment of the National Bridge Inspection Standards (NBIS) and National Bridge Inspection Program (NBIP) in 1970.
NBIS set minimum bridge inspection criteria and prescribed the methods, frequency, and qualifications for performing inspections. The standards required that qualified professionals inspect highway bridges at least once every 2 years and that results be reported annually to FHWA. The agency has maintained this information in its NBI database since the early 1970s. The database currently contains records on some 472,633 bridges and 124,846 culverts with a span greater than 6 meters (20 feet). The mean age of all bridges in NBI is 40 years.
The scope of NBIP reporting has increased over the years, and the database is now a comprehensive source of bridge inventory and conditions. NBIP is adequate for managing a national program aimed at eliminating bridge deficiencies but not for detailed bridge performance measurement or optimizing maintenance programs. This is mostly due to the subjective nature of the data contained in the NBI database and the lack of detailed, element-level (rather than system-based) bridge information.
The NBI database does not include conditions of specific elements such as protective systems and deck joints. Nor does it provide information on local damage or deterioration. Also, the condition and appraisal ratings are qualitative and too general for developing plans and estimates for repairs or rehabilitation work.
Many States have responded to these limitations by collecting element-level bridge data. Although element-level inspections provide more detailed and useful information for bridge management, especially at the State and local levels, the data collected are still limited in several respects. Most significantly, the data are based primarily on visual inspection, augmented only with limited mechanical methods such as hammer sounding or prying.
Many types of damage and deterioration must be identified and measured to determine whether a bridge is safe. Many of these factors are visually detectable when they are at an advanced stage of deterioration. Simply looking at a bridge probably will not determine, for example, whether it has been overloaded. Frozen bearings, corrosion, and fatigue damage can exist without visible indications. Voids in prestressing ducts also can remain undetected. Also, inspectors do not typically collect quantitative information on the operational aspects of bridges, such as congestion, crash history, or use by heavily loaded trucks.
|This closeup photo shows the spalled concrete and rusted rebar on the pier cap of a stringer bridge.|
"The LTBP program will begin to fill a few of the information gaps that exist in our current program," says FHWA's Everett. "Specifically, the LTBP program will provide quantitative bridge performance data that will improve designs and enhance models utilized by bridge management systems. Effective bridge management tools and efficient designs are essential to maximizing the benefits of our investments."
If the Nation's bridge network is to meet increasing traffic and freight demands, future bridge management systems will require improved life-cycle cost and performance models, better understanding of deterioration, and confirmation of the effectiveness of maintenance and repair strategies. These advances will require higher quality quantitative performance data for the development of new models and decisionmaking algorithms. This is where the LTBP program comes in.
"We believe that this is important research for a number of reasons," says Marc Ansley, chief engineer for the Structures Research Center at the Florida Department of Transportation. "Current knowledge of how different characteristics affect the durability of bridges is very limited. Also, there are limited data on the long-term behavior of post-tensioned bridges considering time effects — creep and shrinkage — in combination with thermal variations." Finally, Ansley says, "The effectiveness of short-term durability testing to adequately describe the actual performance of a bridge component over a projected 75-year or longer life is unknown."
The LTBP program's overall objective is to collect, document, and maintain high-quality quantitative performance data over an extended period of time from a representative sample of bridges nationwide. This performance data will enable bridge owners to address a variety of condition management problems, including assessments of how and why bridges deteriorate; the effectiveness of various maintenance, repair, and rehabilitation strategies, and management practices; and the effectiveness of durability strategies for new bridges, including materials selection. Quantitative data also can be used to improve deterioration models and enhance life-cycle cost analysis; support optimal allocation of resources (in conjunction with decisionmaking tools and algorithms); support performance measures for serviceability and structural safety; assess the operational performance of bridges (focusing on congestion, delay, and crashes); and facilitate the validation and improvement of design provisions.
FHWA's vision for the LTBP program is threefold, with collection of data being the common thread. First, a number of bridges that represent the majority of structure types in the NBI database will be subjected to a long-term (at least 20 years and preferably longer) program of detailed inspection and evaluation. The resulting database will support improved designs, predictive models, and bridge management systems.
Second, a subset of the above bridges will be outfitted with instruments to permit continuous monitoring of operational performance under all conditions.
Third, decommissioned bridges will undergo "autopsies" to help improve the knowledge base and the ability to determine the capacity, reliability, and failure modes of bridges in a variety of conditions. These conditions could include damage due to corrosion, overloads, alkali-silicate reaction, fatigue, and fracture. In all three approaches, the LTBP program will take advantage of state-of-the-art sensing and NDE tools.
The LTBP program's framework has two components: program management and technical execution. Management and administration, which will be modeled somewhat after the LTPP program, will involve a prime contractor overseeing the program's day-to-day operations. The contractor will have responsibility for all subcontractors who conduct detailed inspections; annual meetings with stakeholders to review and assess program activities; training for data collection by subcontractors; and outreach and collaborative opportunities to mine data and develop new models, tools, and algorithms. FHWA will form an advisory committee to help guide the program. State responsibilities will include providing access to bridges and bridge files, and supporting safety and traffic control measures. The prime contractor or subcontractors will interact directly with bridge owners and coordinate and conduct bridge inspections.
The technical component of the program concerns the specific data to be collected, bridge sampling, performance measures, technology to support data collection, data quality and collection strategies, and data mining and analysis. Specific data will include damage and deterioration due to overloading, scour, corrosion, deflection, congestion, and myriad other factors. The program also will seek reliable quantitative data on maintenance and rehabilitation activities and life-cycle costs. This data should reflect the type, cost, timing, and effectiveness of upkeep. Cost data should include both direct and indirect user costs.
The LTBP program was created as a 20-year research effort, with funding authorized for fiscal year (FY) 2006 through FY 2009. Under current funding, FHWA envisions that most of the LTBP funds will be devoted to detailed periodic inspection, evaluation, and monitoring of bridges. Continuous monitoring and autopsies of decommissioned bridges will be put on hold pending more funding. This will limit information collection to critical data, with clear knowledge of how this data will be used in the future. Data selection should be based on the relationship between bridge performance and deterioration, and on the most important factors that limit bridge performance over the long term. To identify true causal relationships, FHWA will need to include some nonbridge factors, such as load history (for example, truck weights), maintenance activities, geometrics, and so forth. All data should provide a fundamental understanding of bridge behavior, capacity, failure modes, and reasons for performance deficiency.
On data quality and selection strategies, sampling must be used because monitoring and collecting detailed data on the half million bridges in the NBI simply is not feasible. To gain a representative sampling of the Nation's bridges, a study is needed to help guide selection from the NBI database and others such as the Freight Analysis Framework database, which contains quantitative information on average daily traffic and annual truck traffic, tonnage, and traffic volume. The U.S. Department of Transportation created the Freight Analysis Framework as a comprehensive database and policy analysis tool to examine geographic relationships between infrastructure capacity and freight movement for the truck, rail, water, and air modes, and various combmodities. Other potentially useful databases include the Highway Performance Monitoring System, Highway Safety Information System, and LTPP system.
Current classifications of bridges are based on the structure types and materials. For the LTBP program, consideration will be given to selecting bridges for study by groups based on performance characteristics that may include physical conditions and traffic and structural capacities. For example, although continuous stringer bridges with integral abutment and simple-span stringer bridges are both slab-on-beam bridges, the former will perform better in terms of durability, reliability, and reserve structural strength.
After classifying bridges and tapping all relevant databases, the final representative sample may be characterized by age distribution, material types (steel, concrete, prestressed concrete, high-performance materials), foundation types, location in different climatic and environmental zones, exposure to different hazards (such as floods, earthquakes, and hurricanes), different annual truck traffic and weights, network of bridges, and different maintenance strategies. Again, the total number of bridges to be inspected and monitored will depend greatly on funding levels, an unknown at this time.
In terms of data collection, NDE and structural health monitoring tools and techniques for testing, monitoring, and evaluating bridges will be crucial to augment visual inspections. FHWA envisions that the LTBP program will help foster technology development and integration. Selected technologies should enable data to be recorded in a format useful to State departments of transportation when they are evaluating maintenance and repair needs.
Much of the technology for monitoring bridges is already available as a result of research and development by FHWA and other agencies. In addition, FHWA will develop protocols for data collection, storage, documentation, archiving, access, and dissemination. FHWA will develop and document the protocols to ensure uniformity within the LTBP program and with the hope that they become standards for all future inspection and evaluation programs for infrastructure performance.
|This laser system in the foreground is measuring the deflection of beams during a load test.|
The immediate short-term needs of the program include the formation of an advisory committee, workshops to seek feedback from the bridge community for formulating future directions and activities of the LTBP program, selection of a lead-support technical contractor to help FHWA in day-to-day operation of the program, and preparation for testing and evaluation of the first set of pilot bridges. This program will generate an extensive and quality bridge database that will help improve the mobility, safety, and reliability of the Nation's bridges.
"FHWA is launching the LTBP program at a critical time," says Bojidar Yanev, executive director for bridge inspection and bridge management at the New York City Department of Transportation. "As a result of the steady improvement in NBI over the last quarter of the 20th century, a second generation of bridge management has become inevitable. Just as computerized data management made NBI possible, the rapidly developing structural monitoring techniques and the growing varieties of construction and maintenance options are making possible the integration of network and project considerations into a new type of adaptable and efficient bridge management system. And just as NBI could only have been established on the Federal level, so is the LTBP program uniquely suited for meeting the new and advanced infrastructure needs."
Hamid Ghasemi manages the LTBP program for FHWA. He joined FHWA's Turner-Fairbank Highway Research Center in 1992 and has been involved in numerous research studies and projects addressing the needs of the bridge community, with emphasis on structural health monitoring, posthazard evaluation, seismic issues, and structural analysis. He was named FHWA's Engineer of the Year for 2000. He received his doctorate in structural engineering from the University of Kentucky.
For updated information on the status of the program, see the LTBP program Web site at www.tfhrc.gov/structur/ltbp.htm. For more information, contact Hamid Ghasemi at 202-493-3042 or firstname.lastname@example.org.