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

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31.1 Introduction The history of specification development related to hori- zontally curved girder bridges spans more than 30 years. Dur- ing the late 1960s and early 1970s, a group of researchers called the “Consortium of University Research Teams”(CURT) dev- eloped guidance on the analysis of curved girder bridges and characterizations of the strength and stability of curved girders. This work lead to software products and the design provisions that became codified in the 1980 AASHTO Guide Specifications for Horizontally Curved Highway Bridges (hereafter referred to as the “Guide Specifications”) (1). These specifications were initially produced in the allowable stress design (ASD) format. In 1993, an updated version of the 1980 Guide Specifications was released (2). The 1993 Guide Specifications was written in both allowable stress design (ASD) and load factor design (LFD) format. Recognizing the need to update the technology in these earlier design specifications, a major research effort was initi- ated by the FHWA that involved both experimental and ana- lytic investigations of curved steel bridges. The experimental work undertaken in the FHWA’s Turner-Fairbank Highway Research Laboratory involved a three-girder, single-span struc- ture. The experimental program has been widely reported and has been ongoing for over a decade (3, 4, 5). The FHWA’s experimental program was augmented by tests of single girders at several universities and by large-scale finite element analysis with nonlinear materials, plate out-of-flatness, and nonlinear geometries (6). Comparisons were made between the results of tests on the curved girder test frame and analytic results. One of the pivotal features of the finite element analysis was its ability to accurately represent the capacities due to local flange buckling and lateral torsional buckling, both in terms of the resistance of the cross-section and in terms of the deflected shape. The agreement between the analytical and experimental results was excellent and made it possible to augment a limited number of experimental tests with hun- dreds of analytic investigations. An update to the 1993 Guide Specifications was prepared under NCHRP Project 12-38 (7, 8) and was published in 2003 (9). The state of the art of curved girder specifications and a review of the intervening 10 years’ advances in under- standing the resistance of curved sections resulted in a more modern specification, which was written in the LFD format. The provisions for I-girders retained some of the features introduced in the 1980 Guide Specifications, but made sig- nificant advances in the recognition of the need to directly interrelate the lateral flange bending stress, or “warping” stress, with the vertical bending stress. This interrelation was done by subtracting part or all of the lateral flange bending stress from the resistance of the cross-section. The need for additional stud connectors in composite sections due to the radial component of shear between the deck and girders was also recognized. Recognizing the need to include curved girder bridges in the AASHTO LRFD Bridge Design Specifications (10),AASHTO asked the NCHRP to initiate Project 12-52 to develop rec- ommended state-of-the-art design specifications for hor- izontally curved steel bridges. The recommended design specifications were required to be statistically calibrated and to be written in the load and resistance factor design (LRFD) format. The recommended specifications were also required to incorporate research results accumulated over the years, including the results of the FHWA research. To ensure a smooth transition to the new provisions, the design example for an I-girder bridge and the design example for a box-girder bridge (which were both developed under the NCHRP 12-38 project) would have to be updated to reflect the application of the new provisions. In addition, to fur- ther investigate the effect of the new design provisions on the required girder sections, design comparisons were conducted for a large number of existing and simulated bridges. C H A P T E R 1 Introduction and Research Approach

1.2 Research Objective The request for proposals (RFP) for the NCHRP 12-52 project stated the research objective as follows: The objective of this research is to prepare specifications for the design and construction of horizontally curved steel girder bridges (for both I- and box-girders) in a calibrated load and resistance factor design (LRFD) format that can be recommended to AASHTO for adoption. The specifications shall be based on the Recommended Specifications for Horizontally Curved Steel Highway Bridges developed under NCHRP Project 12-38,which are in a load factor design format, supplemented by the results of the FHWA large-scale curved I-girder tests as they become available. 1.3 Scope of the Study The scope of the study was generally determined by the tasks identified in the RFP as the tasks anticipated to be encom- passed by the research. The task description, copied from the RFP, is as follows. PHASE I Task 1. Review and evaluate pertinent domestic and inter- national research, on the basis of applicability, conclusiveness of findings, and usefulness for the development of LRFD Specifica- tions for horizontally curved steel girder bridges. The focus of this Task shall be primarily on the review and evaluation of all pertinent material from NCHRP Project 12-38 and the FHWA steel curved girder bridge project. Task 2. Review the background of the calibration procedures used in NCHRP Project 12-33,“Development of a Comprehensive Bridge Specification and Commentary.” Using these procedures, the predictor equations from the NCHRP Project 12-38 specifi- cations, any available test data, and the target reliability index and load factors from the AASHTO LRFD Bridge Design Specifications compute resistance factors. Task 3. Apply the available knowledge from Task 1, the results of Task 2, and the design and construction specifications for hor- izontally curved steel girder highway bridges developed in NCHRP Project 12-38 to develop a set of draft changes to the AASHTO LRFD Bridge Design Specifications and the AASHTO LRFD Bridge Construction Specifications. Fold these curved girder provisions into the existing provisions for tangent girder bridges as efficiently as possible. Identify significant differences between the provisions for curved and tangent girders and any gaps in available knowledge for consideration in Phase II. Identify and incorporate provisions in the NCHRP Project 12-38 specification, not specific to curved girders, that should be in the LRFD Specifications and identify provisions (whether or not specific to curved girders) to be deleted. Task 4. Use the draft specifications developed in Task 3 to evalu- ate critical sections of at least 10 existing representative structures that capture variations in major geometric parameters (e.g., span, curvature, skew, beam type, location and type of transverse com- ponents, and connection details). These same structures should be evaluated with the draft specifications from NCHRP Project 12-38 for comparison. Task 5. Provide an Interim Report documenting the results of Tasks 1 through 4. A meeting with the Project Panel will be held approximately one month after delivery of this report, which must be submitted before July 1, 2000. Task 6 and Task 7. Under Task 6, based on the comparisons in Task 4 and on review comments from the Project Panel, revise the draft LRFD design and construction specifications developed in Task 3. Using the revised draft LRFD Specifications, rework the two design examples produced under the NCHRP 12-38 project. Provide a summary of the differences between the existing design examples and those prepared using the proposed LRFD Specifi- cations. Under Task 7, submit the material developed in Task 6 for Panel review recognizing that the recommended specification must be submitted before December 1, 2000. Task 8. Prepare a final report that documents the entire Phase I research effort and includes the recommended specifications, revised based on the Task 7 reviews. The design examples shall also be included in an appendix. PHASE II Task 9 and Task 10. Under Task 9, develop a work plan to resolve differences and to fill in the gaps in knowledge identified in Task 3 of Phase I and submit for Panel review. Items to be considered include, but are not limited to, the effects of bearings and staged construction, and additional design conditions (sublimit states) and load combinations needed for curved girders. The latest infor- mation from the FHWA curved girder project should be evaluated in preparing this work plan. This work plan should prioritize activ- ities needed and should include cost and time estimates for com- pletion of each activity. The contractor should anticipate meeting with the Project Panel to discuss the proposed work plan. It is antic- ipated that not all the activities will be funded. The Project Panel will identify the activities to be funded.Under Task 10,complete the activities approved in Phase II, Task 9, for implementation. Task 11. Provide further proposed revisions to the draft spec- ification based on the results of Phase II, Task 10. Expand the Phase I calibration to include a greater range of design parameters and to include results from the FHWA project, as they become available. Evaluate critical sections of the 10 structures used in Phase I, Task 4, and at least 30 additional existing representative structures.Use the load factors of the AASHTO LRFD Bridge Design Specifications to compute the resistance factors corresponding to target reliabilities from 2.0 to 4.0 by intervals of 0.25. Task 12. Develop commentary on the selection of the level of analysis necessary for specific design situations and evaluate the need for adjustment factors to be applied when less sophisticated analysis procedures are used. This material should be based on the findings from modeling at least three curved bridges (two I-girder and one box-girder). Analyses should use at least three levels of sophistication: V-load or M/R, grid analysis, and finite element three-dimensional analysis. Task 13. Submit the recommended specifications and the two design examples modified as necessary. Note: This material must be submitted before December 1, 2001. Task 14. Submit a final report that describes the entire research effort in Phases I and II. 1.4 Research Approach Phase I was intended to incorporate into the Guide Speci- fications the basic provisions developed in the NCHRP 12-38 project, which were published as the 2003 Guide Specifica- 4

tions (9), with only minor updating. It was anticipated that the FHWA-funded research would result in a new set of resist- ance equations, and Phase II was intended to incorporate these equations into the specifications. In Phase I, the following work was completed: • The literature search conducted under NCHRP Project 12-38 (11) was updated. The updated literature search (12) is included as Appendix A. • Initial calibration studies were conducted that determined that the loads and load factors currently in the LRFD specifications could be retained and used with curved sys- tems (13). This work is included in Appendix C (which is available online at http://trb.org/news/blurb_detail. asp?id=5965). • Specification provisions based on the recommendations of the NCHRP 12-38 project were revised to fit within the then- existing edition of the AASHTO LRFD specifications (14). • The two design examples originally prepared under the NCHRP 12-38 project were updated (15, 16), one for a curved I-girder bridge and one for a curved box-beam bridge. The original scope for Project 12-52 envisioned publishing the recommended specifications developed under Phase I of the project for use until such time as the FHWA curved girder project produced sufficient results to write an updated spec- ification under Phase II of the project. In 2001, the NCHRP panel directing Project 12-52 became satisfied that the FHWA project was moving quickly enough that within about 2 years it would be possible to develop provisions based on that work. This work was anticipated to result in updated resistance equa- tions.Rather than publishing design provisions and then super- seding them in approximately 2 years, the panel decided not to publish the Phase I provisions and instead work toward a schedule that envisioned the adoption of Phase II provisions in 2004, so that it would be available in ample time for the 2007 FHWA deadline for use of AASHTO LRFD specifica- tions on all federally funded projects. The decision not to publish Phase I design provisions deemed the work conducted under Phase I, with the exception of the initial calibration work,obsolete.Therefore, the literature search (Appendix A) and the initial calibration work (Appendix C) are the only work from Phase I covered in this report. The remaining work was part of Phase II. 5