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.
Tests show somewhat higher n values for this metal and type of construction than for riveted construction.