Parametric analysis and stability study of narrow steel box girders based on Midas FEA

To investigate the key parameters affecting narrow steel box girders, a Midas FEA model of a 40+60+40 m composite girder bridge was established, analyzing the effects of girder height, plate thickness, U-ribs, diaphragms, and stiffeners on deformation, stress, and buckling. Girder height dominates stiffness, reducing vertical displacement by 25.2 % from 1800 to 2600 mm. Diaphragms mainly improve buckling stability (+234.8 %); combining diaphragms, bottom stiffeners, and web stiffeners raises the critical load factor from 1.92 to 30.20. Buckling failure shifts from web to bottom plate to diaphragm with added reinforcements, suggesting a rational reinforcement sequence.