Question: We are looking at using an HSS material with a 70 ksi minimum yield strength that is not currently and accepted material in AISC 360. What do we need to be aware of pertaining to the design and are there other codes that are applicable to this material?
Answer: Some European manufacturers currently are able to provide square/rectangular HSS of very high yield strength. In laboratory tests that have been performed, to date, on high-strength HSS there have been no adverse properties detected, except you need to take account of the high yield-to-ultimate strength ratio (Fy /Fu) in connection design (because reasonable ductility levels are presumed, and the elongation of this product is lower than for ASTM A500 product) and the higher degree of connection deformations.
The design rules in Chapter K of AISC 360 are based on the 2nd. Edition, 1989, IIW recommendations [1], which were valid for HSS up to a nominal yield strength of 360 MPa [52 ksi]. The more recent, 3rd. edition, 2012, IIW recommendations [2] – which are very similar to the 2nd. Edition design rules – have extended the range of application to HSS with a nominal yield strength up to 460 MPa [67 ksi]. However, whenever the HSS nominal yield strength exceeds 355 MPa [51.5 ksi], as in your case, connection design must be performed as follows [2]:
(i) The design yield stress should be taken as the lower of the nominal Fy value and 0.8 of the nominal Fu value.
(ii) All connection resistances, determined using the published formulas, need to be reduced by 10%.
Thus, this philosophy could be applied to design of your high-strength HSS connections, with a yield strength cap of 67 ksi. So, although the AISC 360 Specification does not specifically reference these high-strength HSS this material should be acceptable, with the provisions noted above.
The minimum guaranteed yield strength (Fy ) can be fully utilized in the design of compression members.
References
[1] IIW, 1989. “Design Recommendations for Hollow Section Joints – Predominantly Statically Loaded”, 2nd. Edition, IIW Doc. XV-701-89, International Institute of Welding, Paris, France.
[2] IIW, 2012. “Static Design Procedure for Welded Hollow Section Joints – Recommendations”, 3rd. edition, IIW Doc. XV-1402-12, International Institute of Welding, Paris, France.