ECCENTRICITY
Which eccentricity do you mean:
- In-plane eccentricity due to offset of member centreline from bolt group, weld group and gusset plate centrelines
- Out-of-plane eccentricity due to misalignment of cleat and gusset plates
For CHS connections:
- In-plane, the member and cleat centrelines are coincident with bolt group, weld group and gusset plate centrelines, i.e. no in-plane eccentricity
- Out-of-plane, the eccentricity is only due to the cleat and gusset plates being mounted “side-to-side”. Most references agree that this is negligible for static strength and AISC's LRFD Manual of Steel Construction states that this is supported by testwork on similar statically-loaded welded connections. You can avoid eccentric effects by adopting a “tongue-and-cheek plates” connection, which keeps the cleat and gusset plates coincident. In my experience (mining, minerals processing and industrial structures) this is typical for CHS connections anyway.
EFFECTIVE LENGTH FACTOR
The design model, which uses ke = 0.7, is based on AISC’s Engineering for Steel Construction and Thornton’s Bracing Connections for Heavy Construction, which use an even lower effective length factor of 0.65. Your assessment that it might be treated as a fixed-fixed sway member instead, with ke = 1.2, might have some merit. I’d have to read the references before making a comment though.