Assessment of AASHTO LRFD Specifications for Hybrid HPS $690\mathrm{W}$ Steel I-Girders

This paper details research conducted to determine the applicability of the 2nd and 3rd editions of the AASHTO LRFD Specifications to hybrid I-girders fabricated from high-performance steel (HPS) $690\mathrm{W}$ $\left(100\mathrm{ksi}\right)$ flanges and HPS $480\mathrm{W}$ $\left(70\mathrm{ksi}\right)$ webs. Specifically, the scope of this paper is to evaluate the applicability of the negative moment capacity prediction equations for noncomposite I-girders subjected to moment gradient. This evaluation is carried out using three-dimensional nonlinear finite-element analysis to determine the ultimate bending capacity of a comprehensive suite of representative hybrid girders. In addition, a design study was conducted to assess the economical feasibility of incorporating HPS $690\mathrm{W}$ $\left(100\mathrm{ksi}\right)$ in traditional bridge applications. This was accomplished by designing a series of I-girders with varying ratios of span length to girder depth ( $L∕D$ ratios) for a representative three-span continuous bridge. Results of this study indicate that both the 2nd and 3rd editions of the specifications may be used to conservatively predict the negative bending capacity of hybrid HPS $690\mathrm{W}$ $\left(100\mathrm{ksi}\right)$ girders, however increased accuracy results from use of the 3rd edition of the AASHTO LRFD Specifications. Thus, it is concluded that the restriction placed on girders fabricated from steel with a nominal yield strength greater than $480\mathrm{MPa}$ $\left(70\mathrm{ksi}\right)$ can be safely removed. Additionally, results of the design study demonstrate that significant weight saving can result from the use of hybrid HPS $100\mathrm{W}$ girders in negative bending regions, and that hybrid HPS $690\mathrm{W}$ /HPS $480\mathrm{W}$ girders may be ideally suited to sites with superstructure depth restrictions.