Composite Floor w/ HSS 1

[JLNJ]

What are the pitfalls (if any) with design of a composite floor system using HSS shapes? Is there any shear "lag" of sorts to consider given that the studs will be welded in the middle of the relatively flexible top horizontal face? Are there any specific/special Code provisions which apply?

Intuitively the design seems straightforward but not having seen it done before I am concerned that there are issues I may not be aware of. I know that HSS have lower moments of inertia compared to a WF with the same weight per foot so I will need to pay attention to deflections during concrete placement.

 

[msquared48]

It's going to be more expensive for the client as more steel will need to be used and the shape is more expensive, but if there is torsion in the beam, the shape will be better.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[paddingtongreen]

It is a good bet that the shear connectors will have to be devalued because of the thin horizontal wall of the tube.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[JLNJ]

I have been thinking about the shear connectors (some of the code prescribed values are quite generous). It does make sense that the shears might be de-valued, but you can put shear connectors on a relatively flimsy beam flange, a pair of them even, out away from the web, and the values don't get knocked down.

 

[JLNJ]

Answer from AISC solution center follows. I actually received two prompt and helpful answers, one from Kurt Gustafson and one from Brad Davis. Posted is Brad's response.

"While there are no specific recommendations, an HSS can be used as a composite beam using the AISC Spec. Chapter I with minor modifications. Your concern about the thickness is discussed below.

The first question is whether or not the composite member can be designed using a plastic stress distribution. The webs must be compact to use such a distribution. (Otherwise, use an elastic distribution per Section I3.2a(b).) For a w-shape, the limit is given at the bottom of Page 16.1-83 in Section I3.2a(a). This limit matches (lambda)p for Table B4.1 Case 9. For the case of an HSS, use (lambda)p from Case 13 instead. The dimension “h” is the depth minus 3 times the design thickness, per Section B4.2(d). The compression flange must also be compact also. Use (lambda)p from Case 12 for that. The second question is whether the wall is thick enough for stud welding. Page 16.1-88, Section I3.2d(6) second paragraph, second sentence gives a limit for that. Finally, this is not in the Spec., but it seems like a good idea to me to put the studs toward the edges of the HSS top wall rather than putting them down the middle of the section. I think I’d put them 1.5”-2” from the edges of the section and stagger them."

 

[JoshPlumSE]

I know of at least one other project that used HSS composite beams. I think it was for a clean room for chip manufacturing or something.

They always like HSS because there are less surfaces for dust build-up and such.