Analyse the bridge considering dead load, live load, safety barrier load, vehicle load etc. Find maximum bending moment and shear forces. Consider some box section depth based on length of girder. Calculate section properties. Check bending & shear strength. Design connections. Also check for fatigue strength.

Broadly these are the steps.

Thank you for your answer. What about the cross section of the box girder bridge? Is there any requirements that i have to follow?

Based on the design code you are following there are lot of guidelines. You can refer to the code. Also for typical general arrangements of a bridge you need to refer some literature/ manuals.

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