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6 CHAPTER 2 FINDINGS This chapter is organized as follows. The survey sent to The literature review confirmed the need to examine neg- gather information from various bridge-owning jurisdictions ative moment regions specifically, given the little previous is described. Insights from the literature review that are par- research on the subject. Criteria originally developed for the ticularly relevant to study of effective width are summarized, positive moment region were co-opted for use in the negative including comparison of several codes in use on the interna- moment region without explicit study of the unique aspects tional scene. New definitions of effective width developed of negative moment region behavior as regards effective in this research are summarized, as are verifications of the width. This is a key reason for the negative moment region finite element modeling approach employed. The Finite Ele- subassemblage experiments described later in this chapter ment Method (FEM) based parametric study is at the heart and in Appendix F. of the research conducted herein, and its principal features and results are summarized along with those of "special case" bridges, which are beyond the limits of the parametric study. 2.1.2 Comparisons of International Code Key experimental results and their role in corroborating the Provisions for Effective Width FEM-based parametric study are provided as well. Other findings of interest from the literature review con- cerned the various codes and specifications for effective width in use internationally (Ahn et al., 2004). Figure 3 shows a 2.1 SURVEY AND LITERATURE REVIEW comparison of various international codes in graph form, FINDINGS while Table 1 provides a complementary view of the simi- larities and differences of these codes. Most have a limitation 2.1.1 Survey Results on effective width based on span length. Given that a span length parameter is present, the notion of "effective span A survey was distributed in the summer of 2001 to the state length" was invented to enable such criteria originally devel- bridge engineers and TRB representatives in all 50 states. oped for positive moment regions to be co-opted for appli- Replies were received from approximately 40 of these states. cation in negative moment regions. This redefinition of span The replies are tallied in Appendix A along with a copy of length is one example of issues that have arisen in applying the survey form itself. Replies indicate no leads regarding positive moment criteria to a region where those criteria were other studies investigating effective slab width. A few replies not originally even intended to apply. Figure 4 shows how indicate a few recently constructed bridges with large girder several international codes define the notion of "effective spacings. Where maximum girder spacing policies are explic- span length." itly stated, they are generally conservative, with a 3 to 3.6 m The historical review presented in Appendix B and Appen- (10 to 12 ft) limit being common. In some cases, the stated dix C indicates that the 12t limitation in the AASHTO effec- reason for this limit is to facilitate eventual deck replacement. tive width formulation (AASHTO LRFD S188.8.131.52, AASHTO Where more liberal limits are stated, those limits are based on, Standard Specs 10.38.3.1) has been in AASHTO (then for example, maximum spans of stay-in-place forms (approx- AASHO) since the 1940s, the early days of composite beams. imately 4.6 m = 15 ft) or maximum girder spacings allowed Even that formulation is based on empirical research pub- for the use of empirical deck design (4.1 m = 13.5 ft). lished in the World War I era--long predating composite Personal contacts in Europe (Switzerland, France, and the beams. That research was for reinforced concrete T-beams, U.K.) and Japan were consulted. European and Japanese lim- not steel beams with composite concrete decks. Also in those its on girder spacing are more liberal than those in the United days, highway vehicle loads were small, bridge decks were States, as described later in this chapter. In Japan, for exam- not mandated to be a minimum of 175 mm (7 in.) thick, and ple, girder spacings are permitted up to 6 m when supporting bridge floor systems had closely spaced longitudinal stringers. prestressed decks (although no known field test results on Design and construction practices obviously have changed bridges with girder spacings larger than 3 m are available). significantly since then. For example, almost all international
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7 ion experimental work is cited by these codes to accompany the analytical background. The limited test data points to the desirability of experimental verification of analytical results. Table 1 highlights the various parameters that have some influence on effective width. In general, there is a tradeoff between accuracy and simplicity. For example, the value of effective width depends on whether the applied loading is dis- tributed or concentrated--but only the British and Japanese codes recognize this distinction. Another example is whether distinct values of effective width are to be used depending on whether service or ultimate loading is applied--here only the British code and Eurocode recognize the distinction. The review of design criteria presented above brought to light several distinct philosophies underlying the various effective width code formulations being used internationally, ranging from simple (e.g., Canada, which presumes line-girder analysis) to relatively complex (e.g., the British BS 5400, Figure 3. Effective flange width of simply-supported which does not presume line-girder analysis and which also span. distinguishes between point loads and distributed loads). As one might expect, there is a tradeoff between simplicity and accuracy--especially when the full spectrum of possibilities building and bridge codes for steel-concrete composite beam must be accommodated even within the context of line-girder members in the last few decades have departed from any kind analysis (e.g., interior and exterior girders, positive and nega- of thickness limitation in their effective slab width formula- tive moment regions, linear and nonlinear realms of behavior, tions. These various considerations taken together suggest box and I-girders, and absence or presence of axial load, the latter being the case for cable-stayed and tied-arch structures). that the 12t limitation in the current AASHTO provisions for effective width can be liberalized. The research team is not advocating a blind copycat 2.1.3 Other Aspects of the Literature Review approach. The background for the various international code provisions typically consists primarily of parametric analytical Detailed description of the literature review is provided work that most recently is finite element based. Little compan- in Appendix B. Much of the classical literature in this area TABLE 1 Comparison of provisions be Provisions AASHTO BS 5400 Canadian Japanese Eurocode 4 Distinguish UDL vs. Point Load N Ya N Yb N Distinguish Exterior vs. Interior Girder Y Y N N N Expressed as One-Sided N Y Yc Y Yc Distinguish M(+) region from M(-) region N Y N Y N Distinguish I Girder from Box Girder Yd N N NA NA Distinguish Strength (Ultimate) vs. Service N Ye N N Yf Value Modified at Supports N Y N N Y Value Modified for Concrete Cracking N Y N N Y Y : considered N : not considered NA : not applicable a use UDL (Uniformly Distributed Load) case for highway bridges b use PL case especially for internal supports of continuous girders c effective flange width is divided into central part and side parts d use different provisions for concrete segmental box girder bridges e use effective flange width for service limit state, use full width for ultimate limit state f use effective flange width for service limit state