Rectangular HSS beam with AXIAL load and slab on top flange -- what is Ly?

[YCLJ]

Hi,

I'm designing a 12-ft long rectangular HSS beam (say, HSS 12x4) with axial load and the beam is also connected to a slab through shear studs on the top flange. So you can see the top flange is well restrained.

For axial load check, do I consider the HSS unbraced in the weak direction? Or should I assume the slab on top flange is fully bracing the HSS in weak direction?

Any reference to AISC literature is welcome.

 

[BAretired]

I would consider it fully braced.

BA

 

[jayrod12]

I would consider it unbraced,

or alternatively, include a torsion calc that checks the HSS for torsion induced by the buckling load being applied at the centroid and the resistance at the edge (in your example it would be an eccentricity of 6").

The easiest way in my eyes would be to calculate the Cr as unbraced. Calculate the Mr as braced. Then use the standard combined loading checks.

 

[YCLJ]

jayrod12, thanks for your reply.

When you said "Cr", did you mean Fcr, or something else?

As for Mr, I believe you mean the required flexural strength per Ch. H of AISC. So that is clear, unless you say otherwise.

 

[KootK]

I'm with BA on this one. You're absolutely braced for weak axis buckling. The only question is whether or not you're braced for torsional buckling. And, given the torsional restraint provided by the floor deck and the fact that HSS sections are great in torsion, I'm willing to be bet that you're braced for torsional buckling too.

The most technically correct way to investigate this would be as constrained axis buckling (pic below). The results of such an investigation are generally pretty favorable with respect to strength. It's a fair bit of work however. In common situations where there are supported beams tying in at regular intervals, designers typically check unbraced weak axis buckling between beams instead of torsional/LTB buckling. This is done under the correct assumption that the procedure is conservative but not excessively so. For a beam that has no supported members tying in at regular intervals, there may be real economy to be gained by evaluating the situation more carefully.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[DaveAtkins]

I would assume it is braced for moment, unbraced for axial load.

DaveAtkins

 

[jayrod12]

When you said "Cr", did you mean Fcr, or something else?

As for Mr, I believe you mean the required flexural strength per Ch. H of AISC. So that is clear, unless you say otherwise.

I'm not familiar with AISC (Canada Eh) but I mean Cr as in column capacity, and Mr I mean moment resistance for an HSS that is considered braced top flange.

 

[BAretired]

Perhaps it depends on the relative magnitude of the moment versus the axial load. The HSS beam is assumed to be a simple span so that the top is in compression and the bottom is in tension from the simple span moment. To some extent, bending compensates for axial load as it reduces compressive stress in the bottom.

Treating the member as braced for moment and unbraced for axial load would certainly be a conservative approach and, unless a more detailed analysis was warranted, would be a sensible assumption.

BA