Unified Approach for LRFD Live Load Moments in Bridge Decks

Current AASHTO-LRFD specifications use many disparate design provisions to establish live load demands in bridge decks. As an example, approximately 17% of Chapter 4 addresses the analysis of decks. One of the AASHTO-LRFD analysis methods for decks uses an orthotropic plate model. The present AASHTO-LRFD orthotropic plate model has a single formulation for the plate torsional stiffness, and this is not generally applicable to all deck types. In this paper, new analytical expressions are developed for moment in bridge decks subjected to arbitrary patch loading considering each of the three cases of orthotropy: (1) relatively torsionally stiff, flexurally soft decks; (2) relatively uniformly thick decks (such as a reinforced concrete deck); and (3) relatively torsionally soft, flexurally stiff decks. Using these newly developed expressions, the AASHTO-LRFD notional live load models were combined with impact, multiple presence, and live load factors to determine maximum strong direction live load moments for the Strength I design limit state. Design equations were developed to estimate the maximum strong direction live load moments without performing cumbersome moving load analysis for common deck orientations. Using the proposed formulations, bridge deck strength design demands can now be treated in a unified way across different deck types using only four equations. Application of these methods can significantly reduce and simplify the analysis of decks and allow bridge engineers to make comparisons across different deck design alternatives.