Development of LRFD Specifications for Horizontally Curved Steel Girder Bridges (2006)

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AASHTO’s Guide Specifications for Horizontally Curved Highway Bridges (hereafter referred to as the “Guide Specifications”) was first published in 1980. The specifications were based on work conducted in the late 1960s and early 1970s by a group of researchers called the “Con- sortium of University Research Teams” (CURT). The research work resulted in guidance on the analysis of curved bridges and equations for determining the strength and checking the stability of curved girders. An updated version of the Guide Specifications was published in 1993. The 1980 Guide Specifications was written in the allowable stress design (ASD) format. The 1993 Guide Specifications was written in both the ASD and the load factor design (LFD) format. As a result of the work on the National Cooperative Highway Research Program (NCHRP) 12-38 project, the Guide Specifications were updated again and the updated version, written in the LFD format, was published in 2003. In 1999, the NCHRP 12-52 project was initiated to develop design provisions for curved bridges in the AASHTO load and resistance factor design (LRFD) format. These provisions were intended to be incorporated into the specifications to extend the specifications’ cov- erage to curved bridges. Statistically calibrating the curved bridge design provisions was required to ensure smooth merging of these provisions into the then-existing straight girder design provisions. The original organization of the NCHRP 12-52 project called for a two-phase approach. Phase I was intended to produce curved bridge design provisions that were based on the information available at that time. These specifications were intended to be revised in Phase II based on the results of the then-ongoing research on curved bridges. This research was funded by the Federal Highway Administration (FHWA). Several universities collabo- rated with the FHWA in conducting this research. Phase I of the NCHRP 12-52 project produced curved bridge design provisions as planned. It also produced two design examples, one of a box-girder bridge and the other of an I-girder bridge. However, at that time it became clear that the FHWA-sponsored research would produce a new set of design provisions that would be applicable to both straight and curved bridges and that would have some terms of the equations “dropping out” when applied to straight bridges. The new set of provisions was considered to be a significant improvement toward streamlining the design provisions. It was decided not to publish the design specifications developed in Phase I of the project and to develop a new set of specifi- cations and design examples based on the results of the FHWA-sponsored research in Phase II. These provisions were approved by ballot of the AASHTO Highway Subcommittee on Bridges and Structures (HSCOBS) in 2003 and 2004 for straight girders and curved girders, respectively. The straight girder provisions were published in the third edition of AASHTO LRFD specifications in 2004. The curved girder provisions were published in the 2006 interim to the AASHTO LRFD specifications. 1 S U M M A R Y Development of LRFD Specifications for Horizontally Curved Steel Girder Bridges

2In addition to the recommended specifications that were subsequently adopted by AASHTO, the NCHRP 12-52 project resulted in the following: • The statistical calibration of the load and resistance factors for curved bridges. This calibration indicated that the factors developed for straight girders are applicable to curved girders. • The comparison of resistance analysis conducted using the AASHTO Guide Specifica- tions for curved bridges to those conducted using the new LRFD-based design provi- sions. Twenty-one existing bridges provided by several state DOTs and 11 simulated bridges were used in this comparison. The comparison indicated that member propor- tions will not be significantly altered in unanticipated ways and that anticipated changes manifested themselves in the example bridges. • The updating of the I-girder and box-girder bridge design examples. These examples, originally produced in the NCHRP 12-38 project, were updated in Phase I of the project and then updated again in Phase II based on the new design provisions developed in Phase II of the project. The curved bridge design provisions, the statistical calibration work, and the comparison between the existing designs and those conducted using the new provisions are included in this report. The two design examples are available on the AASHTO website at http://www.transportation.org/sites/bridges/docs/Box%20Girder.pdf and http://www. transportation.org/sites/bridges/docs/I-Girder.pdf.