Click for next page ( 67

The National Academies of Sciences, Engineering, and Medicine
500 Fifth St. N.W. | Washington, D.C. 20001

Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement

Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.

OCR for page 66
66 CHAPTER 4 CONCLUSIONS AND SUGGESTED RESEARCH 4.1 CONCLUSIONS to Article of the LRFD Specifications and Com- mentary as developed herein. These provisions in LRFD two- This study has resulted in the recommendation that full column format are provided herein on pp. M-7 and M-8 of width may be used for effective width in composite steel Appendix M. bridge members for most situations of practical interest. This recommendation was determined to be suitable for the Ser- vice as well as Strength limit states, for exterior as well as 4.3 SUGGESTED RESEARCH interior girders, and for skewed as well as right alignments. The simplicity of this recommendation results from an 4.3.1 Bridge Types and Geometries extensive set of analyses on various bridge configurations cul- Not Considered Herein minating in the following two consistently observed trends: 1. Tied-arch bridges were not explicitly modeled in the Full width was typically acting at cross sections where FEM studies performed herein. It is not known how it was most needed, i.e., where moments and hence per- the presence of net tension in the floor system of such formance ratios would be highest; in the cases where the bridges will affect the effective width. Various deck- effective width was less than full width at such cross ing options should be considered in this context (e.g., sections, that cross section had considerable excess flex- cast-in-place and precast prestressed longitudinally ural capacity, and post-tensioned). An extensive "impact analysis" based on Process 12- 2. Curved bridges present another situation of interest. 50 principles revealed that more cumbersome curvefit The forthcoming 2005 Interims to the AASHTO LRFD expressions for effective width, although more accurate, Specifications add curved girder analysis provisions to were not significantly so in terms of the governing rat- the curved girder resistance provisions that were in the ing factor (RF) of the bridge investigated. 3rd Edition of the Specifications in 2004. It is generally agreed that a curved girder bridge should be analyzed Based on a limited number of studies of prestressed con- as a system, such that line-girder simplifications (where crete girder configurations producing similar results, the above effective width is used) would not apply. Approximate simple criterion is thought to be reasonable for such config- methods, however, are explicitly permitted in the 2005 urations as well. Interims for use in analyzing curved girder bridges. The very notion of effective width presumes composite One such approximate analysis method is the V-Load behavior. A question addressed in one of the experiments method. The V-Load method idealizes the curved girder performed herein is whether composite behavior can legiti- as a straight girder subjected to vertical (V) loads applied mately be assumed in negative moment regions without con- at diaphragm locations to complement gravity loads. tinuous shear connectors. It would appear that composite Engineers using the V-Load method will want to know behavior can be attained in negative moment regions, even what value of effective width to use for resisting super- without shear studs being distributed throughout the negative imposed dead-load and live-load effects on the com- moment region, as long as the longitudinal reinforcing steel posite section. The research reported herein simply does is properly anchored and developed. This observation, how- not address that question. ever, is based on only a single experimental specimen. 3. A third situation not explicitly investigated herein is where decks are longitudinally prestressed. Such decks would be designed not to crack under service loads. 4.2 IMPLEMENTATION PLAN Whether the effective width of such decks remaining uncracked at the critical cross section in negative It is recommended that the AASHTO Subcommittee on moment regions extends to the full width is an impor- Bridges and Structures (SCOBS) consider the draft revisions tant question. This question was beyond the scope of

OCR for page 66
67 the parametric study conducted herein and should be to live load and because Byers (whose results served as investigated. Although it is worth mentioning that while a basis for comparison with ours) did not use live load investigating the negative moment region prior to crack- either. Thus, it would be of interest to investigate the ing, the observation was that an apparent smaller beff live-load influence on the effective width. than that of a cracked section whereby the rebars are Influence of cable tensioning during construction on fully engaged in taking the tensile stresses. beff: Construction steps were not considered in this work. 4. A fourth situation involves bridge decks that do not Investigating their influence on beff is recommended. have solid thicknesses that meet or exceed the mini- Negative shear lag in cable-stayed bridges: The phe- mum depth of 175 mm (7 in.) specified in Article nomenon has been observed in various types of struc- of the AASHTO LRFD Specifications. The FEM analy- tures, and further research on the subject is recom- ses upon which this report's recommendations are based mended as far as cable-stayed bridges are concerned. all presumed solid deck thicknesses meeting or exceed- Single versus dual effective width: A single value of ing this minimum depth requirement. Situations with effective width was evaluated herein. Attempts to sepa- other types of decks are thus not included within the rate normal stresses into their "axial" and their "flexural" scope of the recommendations and therefore should be components are also recommended for future research investigated independently. although the difficulty of the task is recognized. 5. The research results presented herein focused primarily Impact assessment in terms of rating factor: As was on slab-on-girder systems. Whether the effective width done for the more common slab-on-girder cases, it would provisions for slab-on-girder systems can or should be be of interest to investigate how the proposed values of reconciled with the effective width provisions for seg- effective width in cable-stayed bridges affect analysis mental prestressed concrete box girders is a reasonable results as measured by rating factor. question. Posited another way, for example, why should A wider range of bridge geometries and cable config- a deck in a segmental box girder experience shear uration: For example, in this project the bridges investi- lag differently than a deck in a tub girder? One would gated (other than the Cooper River Bridge) were no more not expect a difference, which means that one would not than 30 m (100 ft) wide. It would be of interest to deter- expect a different criterion for effective width. Yet there mine values of effective width for additional bridges was no attempt in the present study to reconcile its wider than 30 m and for bridges with different cable results with existing provisions for segmental box patterns, cable spacing, floorbeam spacing, slab thick- girders. ness, and so forth. 4.3.2 Bridge Types and Geometries 4.3.3 Recommendations Originating Considered Herein from Experimental Investigations Within the framework of the parametric study and addi- From the experimental studies conducted as part of this tional cases examined herein, further investigation may be research, the following recommendations for further research appropriate beyond the range limits adopted in this study, i.e., arise: Girder spacings farther apart than 4.8 m (16 ft), or in fact Research is recommended for evaluation of instrumenta- greater than 3.6 m (12 ft) for prestressed girders, tion used for measuring strain on rebars that are embed- Span lengths greater than 60 m (200 ft), and ded in concrete. Skew angles greater than 60 deg. More extensive study, including evaluation of rebar strains, of intentionally composite versus noncomposite Although prestressed concrete girder configurations and slab-on-girder specimens would be valuable. Ideally the cable-stayed bridges were considered in this study, only a specimens would be multi-girder systems. Investiga- few such cases were explicitly examined. Expanding the num- tions and comparisons of global and local composite/ ber of analyses on these types of bridges in order to modify noncomposite behaviors would be useful. As mentioned or increase the credibility of the recommendations contained in the literature review, AASHTO is confusing on the herein for these types of bridges may be desirable. point of composite behavior relating to shear stud design. For cable-stayed bridges, the following are suggested as It is recommended that research in this area includes eval- areas for further research regarding effective width: uation of situations and/or loading that allow noncom- posite beams to be evaluated as composite. Surely the Live-load placement influence on effective width: No details of the steel-concrete bond surface would be of live load was considered in the present study because interest since various conditions exist at that location in the dead load is very large in such structures with respect the field. Perhaps a FEM model with interface elements

OCR for page 66
68 only in the shear stud cluster region would be in order Further investigation of crack patterns and how they since there was no contact in certain regions of the non- relate to composite beams versus slip regions of non- composite specimen during later levels of loading in the composite beams with developed rebar may be useful experiment. This report presents only comparison of one in developing and verifying refinements to concrete composite specimen to one noncomposite specimen, so and rebar material modeling assumptions and friction it is unreasonable to assume that the material presented modeling assumptions used in finite element models of here applies to all conditions. slab-on-girder bridges.