C-F8F8 Round HSS

Round HSS are not subject to lateral-torsional buckling. The failure modes and postbuckling behavior of round HSS can be grouped into three categories (Sherman, 1992; Ziemian, 2010):

• (a) For low values of $D / t$, a long plastic plateau occurs in the moment-rotation curve. The cross section gradually ovalizes, local wave buckles eventually form, and the moment resistance subsequently decays slowly. Flexural strength may exceed the theoretical plastic moment due to strain hardening.
• (b) For intermediate values of $D / t$, the plastic moment is nearly achieved but a single local buckle develops and the flexural strength decays slowly with little or no plastic plateau region.
• (c) For high values of $D / t$, multiple buckles form suddenly with very little ovalization and the flexural strength drops quickly.

The flexural strength provisions for round HSS reflect these three regions of behavior and are based upon five experimental programs involving hot-formed seamless pipe, electric-resistance-welded pipe, and fabricated tubing (Ziemian, 2010).