Research on horizontally curved steel box girders

Horizontally curved continuous steel girder bridges have become increasingly popular in modern highway systems including urban interchanges due to efficient dissemination of congested traffic, economic considerations, and aesthetic desirability. Composite box girders render an attractive alternative to the open I-shaped girder due to the superb torsional rigidity from a closed section and advantages in maintenance.; AASHTO Guide Specifications for Horizontally Curved Steel Girder Highway Bridges with Design Examples for I-Girder and Box-Girder Bridges (AASHTO 2003) significantly alleviate the difficulties associated with the original form of the AASHTO Guide Specifications (1993). However, the provisions in the current AASHTO Guide Specifications (2003) are primarily based upon research performed as part of the CURT (Consortium of University Research Teams) project in the early 1970's. Currently, the bridge engineering community focuses on a consorted effort of consolidating various forms of bridge design specifications into a single LRFD (Load and Resistance Factor Design) form. AASHTO LRFD Bridge Design Specifications (2004) do not include provisions on horizontally curved steel girders.; Steel box girders are at their critical stage during construction because the noncomposite steel section must support both the fresh concrete and the entire construction loads. A lateral bracing system is usually installed at the top flange level in the open-top box girder to form a quasi-closed box, thereby increasing the torsional stiffness. Single diagonal and crossed diagonal bracing systems are typically considered for lateral bracing systems. Internal transverse bracing (also known as internal cross-frames) are provided in the box and usually spaced every one or two lateral bracing panels to retain the original box cross shape, thereby controlling the magnitude of the distortional warping stress level within a prescribed range. External bracing between the interior and exterior box girders may be necessary in the case of very sharply curved bridges in order to control the deflections and rotations of the girders, thereby facilitating the placement of the concrete roadway deck. In composite girders, according to basic structural mechanics, the stresses (whether they are bending stresses and/or shearing stresses) are additive; however, superposition is not valid for the bending moments and the twisting moments because of different cross-sectional properties at each stage of construction.; The present research addresses many issues about the bracing systems of steel box girders including: (1) lateral bracing systems with different arrangements; (2) internal transverse bracing; (3) coupled behaviors of lateral and internal bracing; (4) intermediate external bracing between box girders. Analytical and numerical investigations have been conducted on a number of hypothetical bridge girders. Findings from these studies have been incorporated into design equations amenable to AASHTO LRFD format. The validity and reliability of these newly developed equations have been verified by three-dimensional finite element analyses.