longitudinal stiffener at bottom flange
![](https://20civil.ir/wp-content/uploads/2023/08/Longitudinal-stiffeners-in-bottom-flange-steel-box-girders-20civil.ir_-1.jpg)
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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
![](https://20civil.ir/wp-content/uploads/2023/08/Steel-Superstructures-bridges-I-plate-girder-steel-box-girder-bridges-20civil.ir_.jpg)
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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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![](https://20civil.ir/wp-content/uploads/2023/08/continuous-span-bridge-steel-box-superstructures20civil.ir_.jpg)
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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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![](https://20civil.ir/wp-content/uploads/2023/08/Longitudinal-and-transverse-stiffeners-in-steel-box-superstructures-20civil.ir_.jpg)