Note that this specification has an upper limit on F[sub]y[/sub] of 70 ksi. (this affects seismic designs, mostly, older HSS had no upper limit so the R[sub]y[/sub] on these sections could be less than convenient).
Note also that the reduced design thickness t[sub]des[/sub] for HSS does not apply to A1085 designations.
Olson said:
Traditionally, ASTM A500 allowed for a wall thickness tolerance of -10% of the value specified. Hence, manufacturers have produced tubes with a design thickness of up to 10% less than the nominal thickness required by the standard. This reduction in material led to recommendations made jointly between AISC and the Steel Tube Institute (STI), leading to provisions (AISC 2010 Specification for Structural Steel Buildings ANSI/AISC 360-10, Section B4.2) requiring a reduction in the nominal thickness of all HSS members by 7% for all HSS section calculations. In comparison, A1085 tightens the wall thickness tolerance to -5% and adds a new mass tolerance of -3.5%. These tighter restrictions better align HSS tolerances with other structural members and eliminate the need for the 0.93 factor in calculations. Obviously, these improvements result in more efficient designs when utilizing HSS.
A common application of HSS members is in a braced frame to resist seismic load. HSS sections are often utilized as the bracing element due to their efficiency in carrying both tension and compression loads. This efficiency has come at a price when designing a building with a resistance factor (R) of greater than 3. The seismic provisions of AISC 360 require an engineer to focus on the actual capacity of a member in order to control the failure mechanism of the lateral force resisting system. To realize the actual capacity of a steel brace, a designer must multiply the specified yield strength by an overstrength factor (Ry) to account for inherent overstrength in steel members. R[sub]y[/sub] for A500 is 1.4, while R[sub]y[/sub] for A992 is 1.1. Clearly, the larger R[sub]y[/sub] results in a nearly 30% increase in force the designer must account for. The high R[sub]y[/sub] for A500 is due to the high variability in acceptable yield strength of tubes. A1085 specifies an upper bound on the yield strength of 70 ksi. In time, this upper bound limit will logically lead to better predictability of the material strength, a lower R[sub]y[/sub] factor, and more economical seismic designs utilizing HSS members.
