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Publication Number: FHWA-HRT-09-012
Date: April 2009
It started with LRFD, but now get ready to meet LRFR. The nationwide implementation of the Load and Resistance Factor Design (LRFD) Specification for bridges has resulted in greater reliability of bridges, more efficient designs, and a more uniform factor of safety. Now the Load and Resistance Factor Rating (LRFR) system is building on these advances to improve the safety of bridges through the use of state-of-the-art rating methodology for bridge loads.
The LRFD Specification incorporates analysis and design methodologies for bridges with load and resistance factors based on the known variability of loads and material properties. The load and resistance factors are calibrated using statistical data on loads and materials. Since October 1, 2007, the Federal Highway Administration (FHWA) has adopted a policy that new bridges be designed using the LRFD Specification. With the widespread use of LRFD, several State transportation departments have started to transition to LRFR for the load rating of new and existing bridges.
FHWA has also adopted a policy that starting October 1, 2010, bridges designed with LRFD be load rated with LRFR. FHWA considers LRFR to be the preferred load rating methodology for existing bridges as well. "LRFR allows you to better assess the safe carrying capacity of bridges, providing a uniform reliability for all bridges. With LRFR, you know what you're getting," says Firas Ibrahim of FHWA's Office of Bridge Technology. "On average, it provides a load rating factor that is 27 percent higher than the older, less reliable rating methodology." States that have used LRFR have realized many advantages, one of which is the system's ability to accommodate State-specific legal and permit loads, as well as local live load statistics.
FHWA has held several seminars on LRFR and has just introduced a comprehensive training course through the National Highway Institute (NHI). The 2-day course, Load and Resistance Factor Rating for Highway Bridges (Course No. FHWA-NHI-130092), provides both novice and experienced bridge engineers with the fundamental knowledge necessary to apply the most recent American Association of State Highway and Transportation Officials' LRFR Specification to bridge load ratings. Upon completion of the course, participants will be able to describe the purpose of load rating, understand the benefits of LRFR, identify legal loads and describe their use in load ratings, and understand the distribution factors used for LRFR ratings. Additional course topics include LRFR limit states, evaluation factors for LRFR ratings, the LRFR process used for load postings, overload permit classifications, and the LRFR evaluation procedure for overload permits. The course also features load rating exercises that allow participants to demonstrate the application of LRFR requirements. The course fee is $600 per participant. The minimum number of participants is 20, with a maximum of 40.
|State transportation departments are starting to use LRFR for the load rating of new and existing bridges.|
FHWA is developing additional materials to expand the LRFR course into a week-long training session. The new materials will cover resistance evaluation and calculations and will include detailed rating examples for steel and concrete girder bridges and slab bridges. Evaluating fatigue resistance and the remaining life of steel bridges will also be covered. "Many States have expressed interest in receiving additional training and technical assistance in using LRFR," says Ibrahim. Resources available from FHWA also include LRFR Implementation Guidelines, as well as onsite technical assistance.
|"LRFR allows you to better assess the safe capacity of bridges, providing a uniform reliability for all bridges. With LRFR, you know what you're getting."|
The Oregon Department of Transportation (ODOT) began using LRFR in 2004. "We developed our own State-specific manual to use in-house and give to our consultants," says Bruce Johnson of ODOT. While ODOT first used LRFR for bridges designed with LRFD, the agency then started developing guidelines for using the method on older bridges. "We had load restricted some older bridges and wanted to find out if the ratings accurately reflected their capacity," says Johnson. After hiring Oregon State University to perform a study, ODOT decided that LRFR was a good predictor of actual capacity for the bridges. While LRFR showed an increase in capacity, the rating factors were generally lower due to the other factors in the load rating equation. This led ODOT to recalibrate the LRFR load factors using weigh-in-motion data collected in Oregon. "We are now fully implemented with LRFR for concrete structures," says Johnson. ODOT has developed guides for using LRFR for girder bridge types, with the final guide on steel girders to be released this year. The ODOT guides are available online at ftp://ftp.odot.state.or.us/Bridge/LoadRating/Tier-2/Manuals/.
"It takes longer to do the rating using LRFR and it does currently cost more, but we believe that we now have a better assessment of the load capacity of Oregon's bridges," notes Johnson.
The Hawaii Department of Transportation (DOT) also recently implemented LRFR. "We have started applying it to our existing bridges," says Paul Santo of Hawaii DOT. "We think it will provide numbers that are more meaningful and will definitely help us with load ratings and permitting issues. FHWA has assisted us in putting together LRFR guidelines for implementation."
The Louisiana Department of Transportation and Development (LADOTD) has just finished developing guidelines for using LRFR in Louisiana. In addition to using LRFR for new bridge ratings, the LADOTD will be re-rating existing bridges as needed. "There is a learning curve but FHWA's guidance has been very helpful in the implementation process," says Dana Feng of LADOTD.
Washington State was one of the first States to fully implement LRFD, with the Washington State Department of Transportation (WSDOT) using the system to design both superstructures and substructures for steel and concrete bridges since 1999. WSDOT developed its own software, QConBridge and PGSuper, to assist in performing LRFD live load analysis and designing precast, prestressed concrete girders. WSDOT is now adapting its LRFD software to incorporate LRFR. "While there are initial costs to modify the software and ramp up to use LRFR, we don't expect that using LRFR will cost more," says Harvey Coffman of WSDOT. The agency is currently looking at how LRFR will affect its load ratings and whether there are some load ratings that will drop.
In New York State, meanwhile, "our goal is to start using LRFR for all bridges," says George Christian of the New York State Department of Transportation. "We are developing an implementation plan and also looking at how we can use LRFR for handling our large volume of overweight permitting requests."
For more information on LRFR or FHWA's LRFR resources, contact Firas Ibrahim at FHWA, 202-366-4598 (email: firstname.lastname@example.org), or Tom Saad at the FHWA Resource Center, 708-283-3521 (email: email@example.com). For more information on the NHI course, Load and Resistance Factor Rating for Highway Bridges, visit www.nhi.fhwa.dot.gov.
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