Question: A colleague, upon checking calculations for shear in a tube, asked me if I know of a reference for the 0.93 factor he sees being applied. Do you know of this?
Answer: Cold-formed HSS in the US is manufactured to the standard ASTM A500. This production standard has no tolerance on mass/weight and allows a wall thickness under-tolerance of -10%. Manufacturers have thus become accustomed to producing the finished product with a wall thickness close to 0.90 times the nominal thickness (as this saves on the amount of steel used). However, the section properties (e.g., A, I, r, S, Z) used in structural calculations – for safety – need to be determined on the basis of the actual wall thickness, not the nominal. After consideration of all structural reliability issues, the American Institute of Steel Construction (AISC) determined that a wall “design thickness” of 0.93 times the nominal thickness would be appropriate, and would place HSS on a similar structural reliability level as other hot-formed open sections and then enable the use of common resistance/safety factors. Naturally, the design wall thickness of 0.93*tnominal applies to all structural calculations, including those for available shear.