به حداقل رساندن دوران تیرآهن بیرونی پل های فولادی در طول ساخت عرشه
Abstract
1- Introduction
2- Finite element modeling and artificial data generating
3- Variable selection and parametric study
4- Determine B/D ratio for overhang construction
5- Validation
6- Conclusion
References
Abstract
Wide flange beams are widely used in the United States for bridge design and construction. During the overhang construction of the bridge, torsional loads are often induced due to the fresh concrete load and the use of a deck finishing machine located on the overhang formwork. These torsional moments sometimes cause excessive exterior girder rotation, resulting in many safety and maintenance issues during the construction and service stages. To prevent these issues, most states have specifications for limiting the rotation. Finite element analysis using shell or solid elements is usually recommended for analyzing bridge girders in overhang construction, which can be tedious and difficult in some cases. This study focused on developing a simple method with minimal calculation to evaluate the ratio of unbraced length to girder depth (B/D ratio). The stepwise variable selection method and a regression analysis were conducted to find the relationship between the exterior girder rotation and bridge geometries. A computer program for automatic finite element modeling in SAP2000 was developed using MATLAB, resulting in 4285 finite element models with different bridge geometries being developed to generate artificial data. By conducting a study of variable selection, three parameters were selected based on level of significance. After conducting a regression analysis based on the selected parameters, a method using the normal weight of girder, overhang width, and rotation limit to determine B/D ratio was developed.
Introduction
In the U.S., wide flange beams are commonly used as supports for a bridge deck slab and traffic loads. Typically, these wide flange beams are uniformly spaced transversely, with the bridge deck overhang cantilevering past the exterior girder. The construction of the deck overhang often requires utilizing overhang brackets to support the fresh concrete, deck finishing machine, and other construction loads. As shown in Fig. 1, deck overhang brackets are generally connected to the top flange and react against the bottom of the exterior girders’ web, with spacings between 0.9 m (3ft) to 1.2 m (4ft) along the exterior girders [1,2]. The deck finishing machine, generally located on the edge of the overhang, creates significant loads on the bridge’s exterior girders during deck construction (Fig. 2). One of the major issues in deck construction is unbalanced loads applied along the overhang portion of the deck slab, mostly due to the weight of the deck finishing machine and fresh concrete [3–5]. During deck construction, the bridge shows less transverse stiffness since the concrete deck slab has yet to provide stiffness to the structure. Also, the construction loads are transferred through the overhang bracket formwork system to the exterior girder, causing significant torsional moments on the exterior girder. These torsional moments can lead to excessive transverse rotation of the exterior girder, and instigate many safety and maintenance issues during both the construction and service stage of the bridge, such as changes in deck thickness [3,6], local and global instabilities [7–10], and potential bridge failure [11–13]. Therefore, AASHTO LRFD Bridge Design Specification (2012) [14] requires that the effect of reactions from the overhang brackets be considered during bridge design, and recommends using a three-dimensional (3-D) finite element analysis using shell or solid elements for the torsional analysis in order to recognize the warping effects. The Guidelines for Steel Girder Bridge Analysis (2011) [15] introduces a method using the equivalent torsional constant, which includes the St. Venant torsional stiffness and warping fixity at each end of a given unbraced length, to determine the rotation of the girders.