Multiplate is made in sheets that are bolted together. This is frequently referred to as standard corrugated metal. This value may be satisfactory for many computations of discharge. However, more precise values are given in the accompanying table, which shows values derived from tables and graphs published by the Federal Highway Administration for culvert design and that apply to both annular and spiral corrugations, as noted in the table. Values from this table should be used by U.S. Geological Survey offices in computation of discharge through culverts.
Experimental data show a slight lowering of the n value as pitch increases.
The n values for these three types of corrugation are also given in the table. Standard corrugated helical pipe is formed from a 24-inch wide strip of steel. Tests on helically corrugated pipe indicate a lower coefficient of roughness than for annualarly corrugated steel pipe. Roughness values for helical corrugated pipe are listed in the accompanying table and are based on American Iron and Steel Institute recommendations. Multiplate construction is used with both steel and aluminum.