AISC HSS Yieldline Global Force Strength Reduction Factor, Qf 1

[onlym112]

Hello everyone,

AISC 9-16 of the 16th edition manual lays out the requirements for calculating the strength of HSS chords subjected to out-of-plane transverse loads, as well as in-plane and out-of-plane moments. When calculating the strength of an HSS chord member, a strength reduction factor, Qf, is applied to account for large global forces under compression. The formulas given for Qf in AISC Specification K1.3 are only applicable when the wall slenderness ratio (b/t) is less than or equal to 30.

I would like to know how other engineers deal with the strength reduction factor Qf when the b/t ratio is not met. Could you share your thoughts, design processes, and literature related to the matter?

Thank you

 

[DrZoidberWoop]

My response is a cop-out. This is one of those situations that rarely comes up in practice. If the HSS wall is so thin that the b/t or h/t ratio > 30, it's probably not a good idea to run the connection load path through the HSS face.

Options to handle the out-of-plane loads would be
1) Upsize HSS wall thickness from available inventory, at no cost to the customer.
2) Modify the connection so the loads are directed to the perpendicular HSS walls. This could be a collar plate or something similar.
3) Interrupt the HSS for whatever connection is required. Flange plates that go through the HSS come to mind.

 

[SteelTubes]

Upsizing the wall thickness is a good option. Steel Tube Institute has a tool called HSS Connex for determining minimum wall thickness required to meet connection requirements.

https://steeltubeinstitute.org/hollow-structural-sections/hss-connex-tool/

 

[human909]

Fully agree with the above comments. AISC doesn't apply to me but I come to the same result as the two previous commenters regardless, and largely due to connections.

REASON
Until recently the last couple of years I've mostly used HSS as pure compression/tension members and generally selected larger thin walled sections unless geometric constraints restricted things. Often this meant b/t>25 and occasionally >30.

As soon as you delve chords and frames made from HSS you can quickly realise that connections will dictate wall thickness. Those who are designing thin walled chords are probably not accounting fully for the connection challenges an will find that if they ever approach ULS that it will likely fail before ULS.

Lastly. Unless labour is extremely cheap (developing countries) in your locality then upsizing the wall thickness will only add a minor amount to fabrication and erection costs. So don't hesitate to upsize the wall thickness.

 

[onlym112]

Thank you everyone for your insight.