In contrast to folded plates where the thickness is based on the design of a slab element, the thickness of the barrel shell is usually based on the minimum thickness required for covering the steel for fireproofing, plus the space required for three layers of bars, plus some space for tolerance. If these bars are all half inch rounds, a practical minimum would be 3 1/4 inches. Near the supports the thickness may be greater for containing the larger longitudinal bars.
If more than one barrel is placed side by side, the structure is a multiple barrel structure and if more than one span, it is called a multiple span structure. Any number of continuous barrels or continuous spans are possible except that eventually provision should be made for expansion joints in a large structure.