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One of the main difference between  steel I girder bridges and box girder bridges is the existence of a stiffened plate at bottom flange

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In the continuous support zone  of a bridge, bottom flange of steel box girder is subjected to axial compressive load and design of stiffened bottom flange.

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In compression flanges, according AASHTO STD if b/t ratio exceed 45, longitudinal stiffeners should be considered.

According AASHTO specification, If the longitudinal stiffeners are placed at their maximum w/t ratio to be designed for the basic allowable design stresses of 0.55Fy and the number of longitudinal stiffeners exceeds 2 , then transverse stiffeners should be considered. Transvers stiffener give sufficient support to longitudinal stiffeners.

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