Proposed Modifications To AASHTO Cross-Frame Analysis And .
Project No. 12-113Proposed Modifications to AASHTO Cross-FrameAnalysis and DesignAPPENDIX DPHASE I SUMMARY
NCHRP Project 12-113
NCHRP Project 12-113Table of ContentsTable of Contents . iTable of Figures . iiiTable of Tables . ivD1Introduction . 1D2Background . 3D2.1Review of Code Provisions . 4D2.1.1Legacy Code Provisions. 5D2.1.2AASHTO LRFD Bridge Design Specifications (9th Edition) . 6D2.2Fatigue Loading for Cross-Frames . 13D2.2.1Field Load Testing of Cross-Frames . 14D2.2.2Multiple Presence Factors . 15D2.2.3Proposed AASHTO Updates by Modjeski and Masters (2015) . 16D2.3Stability Bracing Strength and Stiffness Requirements . 17D2.3.1Lateral Torsional Buckling of Doubly-Symmetric I-Girders. 17D2.3.2Stability Brace Requirements. 18D2.3.3Fundamentals of Beam Bracing . 18D2.3.4Total System Stiffness of a Discrete Torsional Brace. 19D2.3.5System Buckling of Narrow Girder Systems . 23D2.3.6Stability of Curved Girder Bridges . 25D2.4Influence of Cross-Frame Member End Connection Eccentricity on Cross-Frame Stiffness . 25D2.4.1The Use of Bent Plate Connections in Skewed Bridges . 26D2.4.2The Impact of Single-Angle Members on Cross-Frame Stiffness . 27D3Industry Survey . 30D3.1Development and Distribution . 30D3.2Survey Results . 31D3.2.1Software and Fatigue Checks . 32D3.2.2Load-Induced Fatigue Cracking. 38D3.2.3Typical Cross-Frame Details . 39D4Commercial Software Review . 45D4.1Software A . 45D4.2Software B . 46D4.3Conclusions and Software Recommendation. 46D5Evaluation of Potential Bridges . 47D-i
NCHRP Project 12-113References . 53D6Industry Survey Outline . 56D7Typical Cross-Frame Details . 61D-ii
NCHRP Project 12-113Table of FiguresFigure D2-1: Primary functions of cross-frames during deck construction . 4Figure D2-2: Primary functions of cross-frames in composite bridges in service . 4Figure D2-3: Cross-frame layout as a function of skew angle, θ. 8Figure D2-4: Recommended offset dimension at skewed supports to alleviate cross-frame force effects . 9Figure D2-5: AASHTO fatigue truck (adapted from Article 3.6.1.4.1). 10Figure D2-6: Common cross-frame analytical models . 20Figure D2-7: Compression X-frame and K-frame force distribution diagrams . 20Figure D2-8: System buckling of twin I-girders systems . 24Figure D2-9: Typical bent plate stiffener used for skewed cross-frame layouts and split-pipe detail proposedby Quadrato (2010) . 26Figure D2-10: Bending of single-angle member subjected to tension caused by eccentric connections(McDonald and Frank 2009) . 27Figure D3-1: Survey response distribution . 31Figure D3-2: Distribution of DOT responses across the United States . 32Figure D3-3: Alternate methods of performing fatigue checks if software is not used . 37Figure D3-4: Responses to: "When checking fatigue of cross-frames in steel I-girder bridges - which of thefollowing does your organization consider?" . 38Figure D3-5: Various cross-frame configurations used across the US . 40Figure D3-6: Typical cross-frame connection that includes gusset plates (detail from Colorado DOT). 44Figure D3-7: Typical cross-frame connection that does not include gusset plates (detail from ArkansasDOT) . 44D-iii
NCHRP Project 12-113Table of TablesTable D2-1: Summary of recommended updates to AASHTO LRFD load factors based on Modjeski andMasters (2015) . 16Table D2-2: Summary of recommended updates to AASHTO LRFD fatigue categories based on Modjeskiand Masters (2015) . 17Table D2-3: Variables in the FEA parametric study for X-frames and K-frames . 29Table D3-1: Software packages used for both straight and horizontally curved I-girder bridge design. 33Table D3-2: Software packages used for design of straight I-girder bridges with normal supports . 33Table D3-3: Software packages used for design of straight I-girder bridges with skewed supports . 34Table D3-4: Software packages used for design of horizontally curved I-girder bridges . 34Table D3-5: Responses to: “To your knowledge, can the analysis software your organization has usedperform fatigue design checks of cross-frames in steel I-girder bridges?” . 35Table D3-6: Responses to: “Do you use the software to check fatigue of cross-frames in steel I-girderbridges?” . 35Table D3-7: Responses to “have you had any difficulties or concerns related to cross-frame fatigue checksmade by the software?” . 36Table D3-8: Reported issues or concerns with fatigue checks performed by commercial software . 36Table D3-9: Summary of cross-frame geometry and member sizes by state. 40Table D3-10: Summary of cross-frame sections utilized . 43Table D5-1: Candidate bridges considered for experimental testing . 48Table D5-2: Pros and cons for straight and normal candidate bridges . 50Table D5-3: Pros and cons for straight and skewed candidate bridges . 51Table D5-4: Pros and cons for horizontally curved candidate bridges . 52Table D7-1: List of pertinent cross-frame details used by state DOTs . 62D-iv
NCHRP Project 12-113CHAPTER D1IntroductionD1The final report of NCHRP Project 12-113 succinctly summarizes the key outcomes of the research forpracticing engineers. As such, many of the detailed results were not included in the body of the maindocument for clarity. The intent of this appendix is to provide a more comprehensive overview of the workcompleted in Phase I of the project. Similarly, Appendix E and F expand on Phase II and III of the project,respectively. For reference, Phase I is comprised of the following tasks, which were previously identifiedin main body of the final report as well as the project Request for Proposals (RFP): Task 1: Conduct a literature review of relevant domestic and international research, guidelines, andspecifications including fatigue loading for transverse members. Document current design practicesand how design software incorporates these design practices. Task 2: Synthesize the results of the literature review to identify the knowledge gaps related to theresearch objectives. These gaps should be addressed in the final product or in the recommendedfuture research as budget permits. Task 3: Propose an analytical program to be executed in two parts as follows. Part 1, to be executedin Phase II, includes modeling and validation of three bridges as described in the field experimentin Task 4. Part 2, to be executed in Phase III, conducts comprehensive parametric studies to achievethe research objectives using validated models in Part 1 of the analytical program. At a minimum,the analytical program should consider the following: oAnalytical and loading studies (finite element analysis) to investigate appropriate fatiguestress ranges for evaluation of cross-frames for right, skewed, and curved bridges, andFatigue I and II;oThe influence of girder spacing, cross-frame stiffness and spacing (including staggered),and deck thickness;oDevelopment of stability bracing requirements for steel I-girders during construction andin-service extending available solutions to include bottom flanges in compression in multispan continuous bridges with non-prismatic girders. The analytical studies should includean evaluation of how to combine stability bracing strength requirements with considerationof other loads such as wind, construction, etc.; andoParametric modeling studies to investigate the effective stiffness of cross-frames, includingissues such as the effect of connection details and connection plate stiffness on cross-framemember stiffness reduction.Task 4: Propose a field experiment, to be executed in Phase II, to achieve the project objectives. Ata minimum, the field experiment should consider experimental verification of analytical models byinstrumenting cross-frames including one right bridge, one skewed bridge, and one horizontallycurved bridge to measure cross-frame member fatigue force ranges under controlled application oflive load. The same field experiment shall be repeated for in-service effective and maximum stressranges.D-1
NCHRP Project 12-113 Task 5: Identify existing articles of the American Association of State Highway and TransportationOfficials LRFD Bridge Design Specifications (henceforth referred to as AASHTO LRFD) thatrequire modification. Task 6: Prepare Interim Report No. 1 that documents Tasks 1 through 5 and provides an updatedand refined work plan for the remainder of the research no later than 4 months after contract award.The updated plan must describe the process and rationale for the work proposed for Phases IIthough IV.Tasks 3 and 4 largely served as an opportunity for the research team (RT) to outline its preliminary PhaseII plans to the research panel. Given that Appendix E covers this material more conclusively and in greaterdepth, the major outcomes of Tasks 3 and 4 are not covered here. An exception to this is in Chapter D5, forwhich a detailed overview of the potential bridges to instrument is provided. Ultimately, three bridges wereselected for field monitoring, as is further discussed in Appendix E.Additionally, Tasks 2 and 5 (identifying gaps in knowledge and AASHTO articles to modify) is coveredextensively in the main body of the report, as it directly relates to the motivation and scope of the researchproject. As such, this topic is not covered in this appendix.Thus, this document primarily covers the general background and literature review, an industry survey, anda review of commercial software packages. The appendix is organized in a traditional report format; it isdivided into seven distinct chapters. Following this introductory chapter, Chapter D2 provides an overviewof the current AASHTO LRFD code provisions pertaining to cross-frames, as well as a summary ofpertinent literature. Chapter D3 discusses the development and results of an industry survey distributed toDepartments of Transportation (DOTs) and consulting design firms. An overview of the fatigue designapproach currently utilized by commercial software packages is provided in Chapter D4. Then, a summaryof the bridges considered for instrumentation in Phase II is provided in Chapter D5. Finally, twosupplementary chapters are included at the end to provide the reader with the additional reference material.In Chapter D6, the survey submitted to bridge owners and consultants is provided for reference. In ChapterD7, the typical cross-frame details submitted by various bridge owners are compiled and tabulated.D-2
NCHRP Project 12-113CHAPTER D2D2BackgroundAASHTO LRFD (2017) defines a cross-frame as “ a transverse truss framework connecting adjacentlongitudinal flexural components or inside a tub section or closed box used to transfer and distribute verticaland lateral loads and to provide stability to the compression flanges. Sometimes synonymous with the termdiaphragm.” Cross-frames serve many roles throughout the construction and service life of a steel bridge.They primarily function as stability braces to enhance the lateral-torsional buckling (LTB) resistance of thebridge girders. Provided that they are properly designed and detailed, cross-frames restrain the twist of agirder cross-section at discrete locations along the length; hence, there are aptly referred to as torsionalbraces. From a stability perspective, the critical stage for bracing often occurs during construction of theconcrete bridge deck when girders are noncomposite. In multi-span continuous bridges where the compositedeck provides continuous lateral and torsional restraint to the top flange, cross-frames also provideadditional stability to bottom compression flanges in the negative moment regions, despite girder instabilitygenerally being much less impactful at this stage.Aside from their primary role as stability braces, cross-frames also resist a variety of lateral and gravityloads throughout the life of a bridge and tie together the girders across the width. Cross-frames resist theapplied torque on fascia girders due to typical deck overhang construction and distribute lateral loads acrossthe structure (e.g. wind). They also restrain differential movement in girders (i.e., vertical deflection androtation) caused by dead and construction loads on the noncomposite system (e.g. externally applied loadsand locked-in fit-up forces) and live loads on the composite system. In the completed structure, the passageof heavy truck traffic subjects cross-frames to cyclic loading conditions. Under repeated loading, crossframes can theoretically become susceptible to load-induced fatigue cracking even for stress magnitudeswell below the yield strength of the material.In horizontally curved bridges, cross-frames are considered primary structural elements and engage thegirders across the bridge width to behave as a unified structural system and to resist the torsion developedfrom the curved geometry. The respective roles of a cross-frame system during construction and in serviceare illustrated schematically in Figure D2-1 and Figure D2-2.To expand on these features of a cross-frame system, this chapter presents several sections. Past and presentAASHTO code provisions related to cross-frame analysis and design are first outlined in Section D2.1.Then, Sections D2.2, D2.3, and D2.4 discuss the pertinent research conducted over the last few decadesrelated to fatigue, stability bracing requirements, and cross-frame stiffness, respectively.D-3
NCHRP Project 12-113Wet concrete(i) Brace compression flangeagainst LTB(ii) Distribute lateral loads(e.g. wind)(iii) Distribute constructionand overhang loadsFigure D2-1: Primary functions of cross-frames during deck construction(i) Brace bottom flange innegative moment region(ii) Distribute lateral loads(e.g. wind)(iii) Distribute vertical loads(e.g. live load)(iv) Control lateral flangebendingFigure D2-2: Primary functions of cross-frames in composite bridges in serviceD2.1 Review of Code ProvisionsThe following subsections provide a brief discussion of the legacy code provisions in AASHTO LRFDpertaining to cross-frames in steel I-girder bridge systems. This is followed by an outline of the modernD-4
NCHRP Project 12-113code provisions pertaining to cross-frames in the current 9th Edition of AASHTO LRFD. Note that onlythose provisions that directly relate to the outcomes of this research project are discussed in detail.D2.1.1 Legacy Code ProvisionsIn 1949, AASHTO Specifications introduced a maximum cross-frame spacing limit of 25 feet in steel girderbridges. The 25-foot spaci
Analysis and Design APPENDIX D PHASE I SUMMARY . NCHRP Project 12-113 . NCHRP Project 12-113 D-i Table of Contents . Officials LRFD Bridge Design Specifications (henceforth referred to as AASHTO LRFD) that require modification. Task 6: Prepare Interim Report No. 1 that documents Tasks 1 through 5 and provides an updated .
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