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Focus
| Accelerating Infrastructure Innovations |
Publication Number: FHWA-HRT-11-007
Date: October 2010
Five countries in 17 days. Advancing bridge safety and serviceability took on a global flavor in 2009 with an International Technology Scanning Program tour to Europe designed to bring back best practices and emerging technologies for implementation in the United States. Sponsored by the Federal Highway Administration (FHWA), in cooperation with the American Association of State Highway and Transportation Officials (AASHTO) and the National Cooperative Highway Research Program, the tour featured an 11-member team representing FHWA, State departments of transportation, academia, and structural engineering design consultants.
The team visited Austria, England, Finland, France, and Germany from May 29 to June 14, 2009, to gather information on safety and serviceability practices and technologies related to the design, construction, maintenance, and management of bridges. In the United States, the implementation of the Load and Resistance Factor Design and Construction (LRFD) Specifications for bridges has resulted in greater reliability of bridges, more efficient designs, and a more uniform factor of safety. The LRFD Specifications incorporate analysis, design, and construction methodologies for bridges with load and resistance factors based on the variability of loads and material properties. These load and resistance factors are calibrated using statistical data on loads and materials. The Load and Resistance Factor Rating (LRFR) Specification has built on these advances to improve the safety and reliability of existing bridges through the use of state-of-the-art rating methodology for bridge loads. The reliability-based calibration of the LRFD and LRFR specifications was performed at the strength limit state. The service limit state for the LRFD and LRFR specifications was back-calibrated to past specifications.
"To get the maximum life out of our bridges, we wanted to learn from other countries who are also taking new approaches to implementing and refining new codes." |
"The LRFD and LRFR specifications resulted in a national, positive impact on the way we design and assess bridges in the United States. However, we are far from being done. Our next challenge is to quantify and improve long-term bridge performance to improve not only the safety but also the serviceability of bridges. The latter is important as it improves rideability and durability and prolongs the life of bridges," said scan team cochair Firas I. Sheikh Ibrahim of FHWA.
The scanning team's goal was to identify best practices and quantify the level of safety and performance used in other countries to assure bridge safety and reliability, improve serviceability, and maximize mobility. The scan involved reviewing international design practices, calibration procedures, and load-carrying assessment practices.
"To get the maximum life out of our bridges, we wanted to learn from other countries who are also taking new approaches to implementing and refining new codes," said scan team member Harry Capers of Arora and Associates.
The team also examined how the countries visited perform structural analysis to design and assess bridges. In the United States, a one-dimensional structural analysis is used for typical bridges to determine the effects of vehicular loading on individual girders. This analysis involves the use of empirical live-load distribution factors to convert the three-dimensional bridge into a one-dimensional problem and to approximate the vehicular load each girder carries. Bridges in all of the countries visited, in contrast, are typically analyzed using refined analysis. Refined analysis does not use empirical formulas but rather uses sound structural theories to determine the structural effects of the live loads. "Not only will the use of refined analysis improve the safety and serviceability of bridges, but it will also result in more efficient bridge designs, as well as more educated bridge management decisions," said Ibrahim.
In Austria, for example, load effects on bridges have been determined using the 2-D finite element method for about 40 years. In Finland, meanwhile, it is common to use 2-D and 3-D models in evaluation calculations. For critical bridges on the country's road network, load tests are performed to confirm the actual structural behavior and to verify the models used in calculations.
"We found a higher comfort level in using more sophisticated design tools. Engineers in the countries visited are more accustomed to using refined analysis," said Capers. "These advanced techniques can provide us with a better analysis of how structures are performing and the safety of the structures."
Team members noted that the enhanced accuracy afforded by the use of refined analysis can result in a significant improvement of the rating results of existing bridges, as approximate methods of estimating the effects of force on a bridge are conservative and more uncertain compared to refined analysis. "Refined analysis can give us a better understanding of how a bridge behaves under traffic loads," said scan team member Jugesh Kapur of the Washington State Department of Transportation.
The team also looked at the use of enhanced reliability analysis to assess safety. The countries visited quantify safety in a manner similar to that used in the United States, but it is stated as a probability of failure rather than as a reliability index, return period, or factor of safety.
The Route S33 Bridge over the Danube River under construction in Austria.
Quality control and quality assurance practices were another primary topic of interest. Members of the European Union commonly require independent bridge design checks. This is a standard practice in all of the countries visited, though the degree to which checks are conducted vary. In the United Kingdom, for example, it depends on the complexity of the work and the risk to the owner, while in Germany it is a mandatory requirement contained in the national building code. "Germany's elaborate quality control and quality assurance system certainly stood out to us on the tour," said Kapur.
The Hohenzollernbrücke Bridge in Cologne, Germany.
The agencies visited share many practices and standards that they believe have contributed to the serviceability and durability of their highway bridges. For example, all of the countries visited use a waterproof membrane system on their concrete bridge decks for corrosion protection. Health monitoring of bridges is used, meanwhile, to alert agencies to maintenance needs. Finland, for example, uses instrumentation on special types of bridges, such as long-span bridges, as well as bridges with a history of issues. In France, permanent instrumentation may be installed to survey bridges that have significant deficiencies and reduced load-carrying capabilities.
The scanning tour team has made the following recommendations for consideration by transportation agencies in the United States:
In support of these recommendations, FHWA is currently developing a 1-day training course on the use of refined analysis for bridge analysis and design.
To download a copy of the scanning tour report, Assuring Bridge Safety and Serviceability in Europe (Pub. No. FHWA-PL-10-014), visitwww.international.fhwa.dot.gov/links/pub_details.cfm?id=675. For more information on the scanning tour, contact Firas I. Sheikh Ibrahim at FHWA, 202-493-3053 (email: firas.ibrahim@fhwa.dot.gov).
The scan team visits BASt, Germany's Federal Highway Research Institute.
