Moment redistribution from interior piers to adjacent spans of straight continuous span bridge girders is an alternate inelastic design procedure, Appendix B6 of the AASHTO LRFD Bridge Design Specifications (AASHTO 2010) that was formerly known as autostress design prior to the 2004 AASHTO Specification (Kayser 2005). This alternative inelastic design procedure recognizes that there is additional capacity beyond the elastic range of the material; when this additional capacity is recognized, localized yielding and inelastic rotation occurs at the section of maximum moment, over an interior pier of a continuous span (Kayser 2005). Moment at this location is automatically relocated or redistributed to the adjacent spans that generally have a lower moment demand (Kayser 2005). Once the moment has been redistributed, a more balanced moment envelope is obtained (Kayser 2005). The reduction in the maximum moment allows the section over the interior pier to be designed for a smaller moment, resulting in a reduced section, fatigue details, material weight, and fabrication costs (Kayser 2005).
This research investigated if Appendix B6 of the AASHTO LRFD Bridge Design Specifications (AASHTO 2010) is an acceptable method for predicting inelastic behavior of a typical single full-length continuous steel hybrid highway bridge I-girder fabricated from HPS-100W flanges and conventional steel webs, with specified minimum yield strength of 100.0 ksi and 50.0 ksi., respectively. Through finite element modeling, it was found that AASHTO (2010) predictions for moment-rotation behaviors are similar for a full-length continuous steel hybrid highway bridge I-girder fabricated from HPS-100W flanges and conventional steel webs, with specified minimum yield strength of 100.0 ksi and 50.0 ksi, respectively. However, further investigation is required to determine if a maximum allowable plastic rotation should be inflicted to prevent the possibility of rupture of the flange.
In addition, this research investigated if varying the grade of steel that the flanges of a hybrid section are fabricated from has an effect on the shear demand on the crossframes in a bridge system. Three different hybrid girder configurations were considered. Each configuration consisted of a top and bottom flange fabricated from one grade of steel and the web fabricated from A572 specified minimum yield stress of 50.0 ksi. The flanges were either material grade HPS-100W with a specified minimum yield stress of 100.0 ksi, HPS-70W with a specified minimum yield stress of 70.0 ksi, or A572-50 with a specified minimum yield stress of 50.0 ksi. Through finite element modeling, it was found that shear demand in the crossframes do not differ by a significant amount due to the change in grade of steel in the flanges of the hybrid section with 50.0 ksi grade steel in the webs. Therefore, the lateral moment redistribution is assumed to be comparable to the characteristics of lateral moment redistribution that is exhibited by lower grade materials.