By confining the edges of the corrugated sheet so the sheet can't expand while pounding a 3cm steel pipe across the grain, these sheets stay almost flat instead of becoming cylindrical. Cylindrical elements don't lay parrallel to each other when overlapped. Conical elements do. The cross-ribbing gives the sheet enough rigidity to work with them much more easily. The sheets are very floppy without the added ribs and this makes assembly difficult.