Question
How does an engineer determine digit of cells for concrete box girder bridges?

Answer
If depth of a box girder bridge exceeds 1/6 or 1/5 of bridge width, then it is optional to be designed as a single cell box girder bridge. However, if bridge depth is lesser than 1/6 of bridge width, after that a twin-cell or multiple cell is better choice as optional by Jorg Schlaich & Hartmut Scheef in 1982. However, one contain to note that even for wider bridges with small depths, number of cells must be minimized as readily available is not much improvement in transverse load distribution when number of cells of box girder is improved to three or more.