Rotation Capacity of I-Shaped Beams under Alternating Axial Forces Based on Buckling-Mode Transitions

During earthquakes, I-shaped beams (I-beams) in a braced structure are subjected to the flexural moment synchronized with reversed axial forces transmitted from buckling restrained braces. A prior study used the modified width-Thickness ratio to evaluate the rotation capacity of I-beams that failed by local buckling under alternating axial force. Generally speaking, I-beams in braced frames are designed with larger sections to resist severe stress conditions. The buckling mode might thereby be switched to web-shear buckling, leading to inaccurate estimation of the rotation capacity when using existing equations. For this study, a classification index of buckling mode is constructed considering axial force and plate interactions. Moreover, a novel index and formulas are proposed for evaluating the rotation capacity of I-beams under alternating axial forces, including buckling-mode transition effects.