Moment-Rotation Characteristics of Laterally Restrained Hybrid Steel I-Girders under Moment Gradient

Hybrid steel I-sections, featuring a lower-strength web compared with their flanges, present an economically efficient design for I-girders that optimizes material use. This study investigates the impact of low-strength grade webs in hybrid I-girders through a combined experimental and numerical analysis of the moment-rotation characteristics of laterally restrained I-girders subjected to a moment gradient. Testing was conducted on nine compact I-girders with Q690 flange plates, incorporating varying web strength grades of Q690, Q460, and Q355. Additionally, validated numerical modeling was employed to explore the influence of web strength grade on I-girders with compact flanges but noncompact and slender webs. Results from the experimental and numerical analyses demonstrate that compact web I-girders with a hybrid design exhibit superior rotation capacity compared with homogeneous high-strength counterparts, whereas hybrid I-girders display increased vulnerability to flange vertical buckling, characterized by vertical deformation of a compressed flange plate into the web plate. This susceptibility arises from the early onset of plasticity and stiffness reduction in the low-strength web of hybrid I-girders. Furthermore, the study assesses the flange vertical buckling models stipulated in Eurocode 3 and proposes potential modifications based on numerical analysis results. The detailed examination of the role of web strength grade and the fundamental investigation into flange vertical buckling offer promising avenues for advancing structural design practices and enhancing the overall comprehension of structural behavior in I-girders.