Q. What is Multiple-cell box girder?

Multiple-cell box girder: cells connected by top flanges vs. cells connected both by top and bottom flanges

When the depth of a box girder bridge exceeds 1/6 or 1/5 of the bridge width then it's recommended to be designed as a single cell box girder bridge. Though if the bridge depth is smaller than 1/6 of the bridge width then a twin-cell or multiple cell is a better choice. Though even for wider bridges with small depths, the number of cells must be minimized since there isn't much improvement in transverse load distribution when number of cells of box girder is increased to three or more. 

For multiple-cell box girders there are usually two arrangements. First one is that independent cells are connected by their top flanges only while other one is that the cells are connected both at the bottom and top flanges. From structural point of view it's recommended to adopt the second arrangement. For the case of cells connected by top flanges only, their flanges are heavily stressed in transverse direction owing to flexure that can't be effectively distributed across the cross section.

Fig. Box girder with cells connected by top flanges and cells connected both by top and bottom flanges.