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 2
2 CHAPTER 1 INTRODUCTION AND RESEARCH APPROACH 1.1 PROBLEM STATEMENT AND PROJECT Conventional multi-girder system OBJECTIVES · Deck System: Conventional Cast-in-Place The phenomenon of shear lag is shown in Figure 1a. Shear · Conventional concrete (e.g., f c = 21 MPa or 28 MPa) lag can result in underestimating the deflections and stresses · High-Performance Concrete (HPC) at the web-flange intersections of a girder in calculations Prestressed, either constant depth or variable depth, based on line-girder analysis and the elementary theory of and often prestressed longitudinally as well as trans- bending, which assumes that plane cross sections remain versely, on potentially very wide girder spacings plane. It is traditional to obtain correct values of maximum · Alignment: deflection or stress from the elementary theory by using an Right effective slab width concept in which the actual width of each Skew flange is replaced by an appropriate reduced ("effective") · Span Location: width (Moffatt and Dowling, 1978; ASCE, 1979; Garcia and Positive Moment Region Daniels, 1971), labeled beff in Figure 1b. The determination Negative Moment Regions considered composite of effective slab width directly affects the computed moments, where sufficient shear studs and longitudinal rein- shears, torques, and deflections for the composite section and also affects the proportions of the steel section and the num- forcing steel are supplied · Applicable Limit State, e.g., ber of shear connectors required. The effective slab width is thought to be particularly important for serviceability checks Service II and Fatigue (elastic), and (e.g., fatigue, overload, and deflection), which can often gov- Strength I and perhaps Strength II (possibly inelastic). ern the design. Figure 2 summarizes the various influences that the effec- For girder spacings 2.4 m (8 ft) or less, the effective width tive width beff has in the design and rating of a composite computed according to the current AASHTO provisions gen- beam. Both sides of the basic LRFD methodology, iiQi erally includes all of the deck. With the increasing use of Rn, are influenced by the effective width for all limit states wider girder spacing, however, the contribution of the addi- involving flexure of a slab-on-girder composite beam. Thus tional width of deck is not fully recognized by the current cri- it is not possible by inspection to determine the net effect of teria. The AASHTO Guide Specifications for Segmental Con- any proposed change to effective width for a given bridge, let crete Bridges recognize the entire deck width to be effective, alone for a suite of bridges. A systematic parametric study is unless shear lag adjustments become necessary. Field mea- necessary. Just such a study is at the heart of the research surements of modern composite steel bridges indicate that results presented herein. recognition of more of the concrete deck is often necessary In AASHTO bridge design specifications (AASHTO, to better correlate actual with calculated deflections. 2004), the effective slab width for interior girders of all types The above criteria apply to all types of composite interior of composite bridge superstructures, except for orthotropic and exterior steel bridge members. In addition to their com- deck and segmental concrete structures, is specified as the mon use on multi-girder bridges, composite deck systems least of the following: (1) one-quarter of the effective span can participate structurally with tied arches or cable-stayed length, (2) 12.0 times the average depth of the slab plus the bridges. Thus, the effective width of the slab may well differ greater of web thickness or one-half the top flange width, and among some of these cases from more conventional multi- (3) the average spacing of adjacent beams. These criteria cur- stringer I-girder bridges. The effective width of decks using rently apply to all types of composite interior and exterior high-strength concrete may also be affected by the larger steel bridge members with any combination of the following: elastic and shear moduli of the concrete. Distinctions for the effective width of slab to be used may be needed · Conventional or High-Performance Steel Girder System: Tub-girder · In positive and negative bending, Two-girder system · At the AASHTO serviceability and strength limit states,