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59 CHAPTER 3 INTERPRETATION, APPRAISAL, AND APPLICATIONS 3.1 INTRODUCTION Given that a simplistic line-girder analysis is insufficient for such situations, the use of idealizations that inherently The objectives of this study were to propose criteria for assume line-girder analysis (such as effective slab width) may effective width provide recommended specifications and be considered questionable at best in these kinds of situations. commentary and provide worked examples illustrating the use of those proposed new criteria. Draft criteria were devel- oped based on applying regression approaches and account- 3.2.2 Some Implications of Wide ing for the different subsets of the parameters varied in the Girder Spacing parametric study described in Chapter 2. Effects of those criteria were assessed, using the Rating Factor (RF) as the The 12t limitation clearly can be removed. Given that the measure of effect. Based on the assessment, draft criteria are challenge to the 12t limitation arises, for typical deck thick- recommended and illustrated in the context of positive and nesses, in the context of girder spacings wider than 3 m (10 ft) negative moment region worked examples. or so, additional implications of wide girder spacings are also of interest. Some of these implications are as follows: 3.2 ASSUMPTIONS AND IMPLICATIONS · The empirical method of deck design is prohibited by Some key, almost paradoxical, assumptions underlie the AASHTO for use beyond a girder spacing of 4.1 m use of the notion of effective slab width, e.g., (13.5 ft.) [AASHTO LRFD S188.8.131.52]. Thus, methods of traditional design, prestressed design, and system analy- 1. Even with the sophisticated computer-aided analysis sis of decks that go beyond line-girder analysis (e.g., available for bridge design in the 21st century, tradi- grillage and finite strip) must be used to design the actual tional line-girder analysis provides an ongoing useful decks. Given that these methods typically go beyond context for analyzing steel girders acting compositely line-girder analysis, some may ask why it should still be with concrete decks. permissible to use line-girder analysis for the in-plane 2. Given that the context of analyses based on effective analysis of the composite girders supporting the result- width is single member line-girder analysis, analyses ing decks. based on effective width are not appropriate for use in · The use of the line-girder-oriented distribution factor for- situations where a line-girder analysis is insufficient mulas is prohibited by AASHTO for use beyond a girder and a system analysis is thus considered necessary. spacing of 4.9 m (16.0 ft) [AASHTO LRFD S184.108.40.206]. · The possible interaction of plate bending with in-plane "effective width" behavior increases. This interaction is 3.2.1 Some Implications of Line-Girder a possible explanation of the "negative shear lag" evi- Analysis Limitations dent in some of the cable-stayed bridge analysis results presented in Chapter 2. Beyond line-girder analysis, system analysis is generally · Longitudinal shear forces that get "funneled" into the considered necessary in the following kinds of situations: shear connectors increase. Whether the current AASHTO shear stud design criteria still apply, in HPS and HPC · Highly skewed and curved girder bridge analysis and composite combinations, may need to be revisited. design, · After-fracture redundancy and load redistribution analy- sis where required (e.g., Daniels et al., 1989), and Some of these implications of the use of wider girder spac- · Detailed design stages for systems where second-order ings in conjunction with high-performance steels and con- effects can be important, e.g., cable-stayed bridges. cretes point to the need to re-examine existing AASHTO