Effective Slab Width for Composite Steel Bridge Members (2005) / Chapter Skim
Currently Skimming:
Pages 59-65
The Chapter Skim interface presents what we've algorithmically identified as the most significant single chunk of text within every page in the chapter.
Select key terms on the right to highlight them within pages of the chapter.
From page 59... ...
Based on the assessment, draft criteria are recommended and illustrated in the context of positive and negative moment region worked examples. 3.2 ASSUMPTIONS AND IMPLICATIONS Some key, almost paradoxical, assumptions underlie the use of the notion of effective slab width, e.g., 1.
|
From page 60... ...
3.3 DESIGN CRITERIA DEVELOPMENT Candidate design criteria were derived by performing regression analyses based on the beff /b values extracted from the finite element parametric study in the vicinity of the maximum positive and negative moment sections. Candidate effective slab width criteria for positive moment sections were derived initially from the simple-span cases, while the candidate effective slab width criteria for negative moment sections were derived from the multiple-span continuous cases.
|
From page 61... ...
3.4.2 Rating Factor in LRFR The general expression for rating factor in LRFR is as follows: where C capacity (C = φCφsφ × R: Strength limit state, C = fR: Service limit state) fR allowable stress specified in the LRFD code R nominal member resistance DC dead-load effect due to structural components and attachments DW dead-load effect due to wearing surface and utilities P permanent load other than dead loads LL live-load effect IM dynamic load allowance γDC LRFD load factor for structural components and attachments γDW LRFD load factor for wearing surfaces and utilities γP LRFD load factor for permanent loads other than dead loads (1.0)
|
From page 62... ...
Thus, even to consider the notion of effective slab width in negative moment regions without shear studs makes no sense. Complicating this issue is the ambiguity of the current AASHTO specifications on whether negative moment regions without continuous shear connectors can be considered to be composite when longitudinal deck reinforcing steel is developed and anchored to clusters of shear connectors in moment inflection regions.
|
From page 63... ...
Of course, if the ongoing NCHRP Project 12-62 develops more significant skew correction factors for the AASHTO LRFD transverse live-load distribution factors, then there may not be such offsetting effects. 3.5.2 Cable-Stayed Bridges In light of the results tallied in Table 11 for the first four analyzed cable-stayed bridges and the longitudinal variation of effective slab width seen in Figures 62 through 72, a reasonable and conservative lower bound set of effective width values for cable-stayed bridges may be summarized as follows:
|
From page 64... ...
is in fact the most liberal of all effective width provisions in all known international codes. This proposal is based on an extensive and systematic investigation of bridge finite element models that are more sophisticated than the models upon which other codes are based, that are corroborated by experimental results both by others and by the authors, and that explicitly investigate the negative moment region much more extensively than previous researchers have done.
|
From page 65... ...
In summary, the process that has been followed in arriving at the proposed full width criteria has involved each of the following: • Formulating a new definition of effective width which for the first time accounts for the variation of stresses through the deck thickness as well as both moment and force equivalence between the finite element model and the line-girder idealization wherein beff is used; • Performing judicious finite element modeling and analysis, using appropriate levels of detail (e.g., approximating "smeared" rather than discrete deck rebar and cracking, yet explicitly representing deck thickness using four brick elements through the thickness) ; • Corroborating that finite element modeling approach with experimental data produced by others as well as by the authors; 65 • Designing a suite of bridges according to industry guidelines to support the parametric study; • Performing a systematic parametric study of finite element models of these bridges that produced results from which effective widths according to the new definition could be methodically extracted; • Formulating various candidate criteria for effective width, based on regression analysis, that intentionally span the gamut between simplicity and accuracy; • Applying Process 12-50 in a systematic assessment of impact of those various candidate criteria in order to recommend which criteria were most appropriate; • Proposing specific draft code and commentary language for implementing those criteria in AASHTO LRFD Article 4.6.2.6.1, for consideration by the AASHTO Subcommittee on Bridges and Structures; and • Illustrating the use of the recommended criteria in the form of comprehensive worked design calculation examples.
|
Key Terms
This material may be derived from roughly machine-read images, and so is provided only to facilitate research.
More
information on Chapter Skim is available.