Design method for cross-section behaviour of hybrid I-girders under uniform bending

Hybrid steel I-girders, characterised by different strength grades for the flange and web, offer the potential to optimise material utilisation in various loading scenarios. The current American and European standards for steel bridges (AASHTO specification and Eurocode 3) have incorporated specific provisions for the design of hybrid I-girders with the web strength grade lower than the flange. Based on the data obtained from the validated numerical investigation, the limiting slenderness ratio for cross-section classification and applicability of the designated bending moment resistance (flexural strength) in standards are assessed. The assessment results reveal that the compact/Class 1 sections with flange yield strength of 690 N/mm² fail to meet the current codified ductility requirement, and the non-compact flange slenderness limits in AASHTO specification are demonstrated to be excessively lenient. A reduction factor of 1.2 is suggested to narrow down the Class 1 width-to-thickness ratio in Eurocode 3 for I-girders with compression flange steel grade equivalent to S690. Furthermore, the design moment expression of the Direct Strength Method (DSM) is proposed for the cross-section resistance design of hybrid I-girders, and the design approach for the Continuous Strength Method (CSM) is also derived through analytical analysis. Instead of addressing the slenderness of the flange and web individually, the DSM and CSM adopt the concept of overall cross-section slenderness, which allows for a more accurate consideration of the flange-web interaction and offers the potential to streamline the calculation process for cross-section resistance. The statistical and reliability analyses indicate that all the design methods could achieve the reasonable moment resistance predictions for the cross-section resistance of hybrid I-girders.