Plastic design of steel bridge girders

Abstract: This thesis assesses the feasibility of a modified plastic design method for continuous steel or composite bridges with slender I-girders. The feasibility evaluation of the design method is closely connected to the moment-rotation relationship of the I-girders at internal supports. Due to local buckling of the slender I-girders, the full plastic moment resistance cannot be sustained as the rotation develops. On the contrary, the girders will reach a moment maximum, which might be less than the full plastic moment, followed by a decline in moment resistance as the rotation increases. Assuming that the moment-rotation relationships are known for all plastic hinges developing in the bridge, a plastic design method can in principle be employed but modified for these relationships. A fundamental condition for a modified plastic design method to be employed is thus that the moment-rotation relationships can be predicted. A simple plate model is developed in this thesis, which predicts the moment-rotation relationship for girders with compact or semi-compact flanges and slender webs. Included in this thesis is also test results in fourteen slender, high- strength steel girders in three point bending tests. A study on the lateral instability of I-girders and also an evaluation of the modified plastic design method with respect to the formal risk of failure, compared to an ordinary elastic design, have also been included.