It seems appropriate to start the presentation of examples of shapes and forms for shell structures with the folded plate because it is the simplest of the shell structures.

The distinguishing feature of the folded plate is the ease in forming plane surfaces. Therefore, they are more adaptable to smaller areas than curved surfaces which require multiple use of forms for maximum economy. A folded plate may be formed for about the same cost as a horizontal slab and has much less steel and concrete for the same spans. Folded plates are not adapted to as wide bay spacings as barrel vaults. For widths of plate over, say, 12 feet, the thickness of the folded plate must be thicker than for a barrel vault. Some advantage may be gained by increasing the thickness of the slab just at the valleys so it will act as a haunched beam and as an I section plate girder.