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The thickness used in design is less than the nominal thickness.
Per the 2000 AISC Specification for the Design of Steel Hollow Structural Sections, AISC requires that the wall thickness used for the purposes of design calculations be taken as 0.93 times the nominal wall thickness, unless the actual wall thickness is known. For example, an HSS10x10x1/2 has a nominal wall thickness of 1/2 in. and a design wall thickness of 0.465 in. The reasoning behind this is explained in the discussion here from the Steel Interchange in the July 1999 issue of AISC's Modern Steel Construction magazine. The Canadian Institute of Steel Construction (CISC) has an excellent explanation of the issue here as well.