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Levels of Lateral Flange Bending in Straight, Skewed and Curved Steel I-girder Bridges during Deck Placement by Nohemy Y. Galindez Exterior steel I-girders are required to withstand deck overhang loads during construction. This is partially accomplished by checking the flexural limit states for constructibility given by AASHTO. These limit states ensure that the maximum flange bending stresses produced during construction do not exceed the section flexural capacity of the girder. For constructibility design, both the bending stresses and the flexural capacity of the flanges are affected by the loads corresponding to the deck placement sequence. Therefore, stiffness changes need to be considered during the various casting stages to compute the corresponding flange bending stresses and capacities. The specifications take into account this effect by defining separate limit states for discretely and continuously braced flanges. The limit states for discretely braced flanges involve not only the major-axis bending stresses produced by vertical loads but also the lateral flange bending (LFB) due to torsional responses or direct horizontal forces such as those produced by wind. During construction, torsional effects are principally generated on exterior girders by deck overhang loads. In curved girders, it is also required to consider the significant torsional stresses introduced by the curvature, where the loads are eccentric with respect to the supports. Additionally, direct LFB may be induced in skewed bridges at crossframe locations caused by differential displacements or out-of-plane rotations. Some simplified models have been proposed to estimate the LFB in exterior girders during deck placement conditions in straight bridges. However, the use of comprehensive models decreases the uncertainty in the lateral stiffness offered by structural elements such as the cross frames, the interior girders and the deck forms. In addition, the curvature and the skew angle effects have not been directly addressed in these simplified works. AASHTO Specifications recommend approximate equations to estimate the torsional effects due to both deck overhang loads and curvature. For skewed bridges, the provisions recommend using 10Ksi as a conservative estimation of the unfactored LFB in bridges with discontinuous cross-frame lines and skew angles exceeding 20°. However, more precise approximations may be defined for each source of LFB if effects such as the continuity over the intermediate supports and the deck casting sequence are considered. In this work, a comprehensive suite of finite element analyses is conducted on hypothetical three-span straight, skewed and curved bridges to assess the levels of flange bending during deck placement. The parameters varied include the span lengths, the cross-frame spacing, the skew angle and the radius of curvature. In addition, concentrated and distributed loading cases are considered to approximate the torsional effects due to eccentric overhang loading. A comprehensive formulation of the LFB effects due to curvature is also included for both loading cases. Numerical results were compared to current AASHTO Specifications and new approximations were proposed for predicting the LFB stresses. The flexural limit states for constructibility were also evaluated using the numerical stresses. It was concluded that the curvature is the variable that most affects the limit states. Conversely, for the parameters exercised in this study, no significant effects were observed by varying the skew angle. The governing limit state of the casting sequence considered in this study corresponds to the ultimate strength for discretely braced flanges in compression. The yielding limit state controls in short span lengths while the web bend-buckling limit state becomes significant in the pier regions for long span lengths. AASHTO does not include a specific recommendation for the spacing of cross frames in steel bridges. Therefore, the designer needs to either evaluate different configurations to select the most optimum spacing, or follow traditional practice that assures safe results. For that reason, a reliability analysis was proposed in this work to develop a practical method to select the cross-frame spacing for deck placement conditions considering the flexural limit states for constructibility that are affected by the cross-frame spacing. A Monte Carlo Simulation is performed for straight, skewed and curved steel I-girder bridges generating some fragility curves that allow identifying the maximum cross-frame spacing for deck-placement conditions according to the maximum tolerated level of risk.

Levels of lateral flange bending in straight, skewed and curved steel I -girder bridges during deck placement