Inner workable flat dimensions of an HSS Rect. 1

[TexasBACON]

Hello all,

First time posting. Forgive me for any inconceivable mistakes.

I am needing to know the inner workable flat dimension length for a HSS 6"x 4"x 3/8". What I am bolting a W6x15 post to a 6" long side of the HSS with two 1"Ø A325 bolts that are spaced 3 1/2" apart, center-to-center. The bolts will use F436 washers that have a 2" O.D. (outer dim.) Just need to know whether clearance will be an issue.

AISC Steel Manual 14th ed. gives only the outer workable flat dimensions for HSS rect and sq. Any tips on where to look/how to consider this will be greatly appreciated.

Thanks!

 

[jayrod12]

Likely. Standard radius at the corners is 2t, therefore you are going to lose 1 1/2" minimum leaving you with 4 1/2" of usable flat surface on the inside. with 3 1/2" bolt spacing and 2" diameter washers, you need at least 5 1/2". You won't even have 5 1/2" clear between inside face of the side walls.

 

[TexasBACON]

Thanks @jayrod12 (not sure how to properly tag you and reply directly to your comment).

It in the Steel Manual, it says the outer corner radii are taken as 1.5tdes. I took this as the worst case and considered the 6" -2*outer corner radii - 2*tdes = 4.275" which still does not give me enough clear spacing.

 

[jayrod12]

I use 2t as a conservative number, it allows for manufacturing tolerances. Either way, you need 5 1/2" and you won't have that clear between the side walls.

 

[dik]

Welcome to the group...

Dik

 

[JLNJ]

Isn't the inner flat essentially the same dimension as the outer flat? I'd use the outer flat as tabulated by AISC.

 

[TehMightyEngineer]

This image here gives you kinda a rough idea of the various interior radii at the corners for various thickness. I also typically assume 2t.

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

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[CANPRO]

There could also be a raised seam on the inside face where the closed tube was welded together. Might not be an issue for you, but if you're wondering about available flat surfaces on the inside, it is something to consider.

 

[Once20036]

I recently attended a webinar from the steel tube institute.
They recommended using 2*tdes for section properties, 1.5tdes for checking b/t and h/t, and 2.25*tnom for the workable flat.
I`d expect the inner flat to match the outer flat.

 

[dhengr]

TexasBACON:
1.) RE: the W6x15: k₁ = .375", so (2)(.375) = .75", plus 2" for the two half washers; means that the min. gage “g” for the beam could be 2.75", not your/the 3.5", the usual gage. So, you could pick up about .75" clr. on the WF bolt centers.
2.) RE: the HSS 6"x4"x3/8": I suspect the outside corner radius is more like 7/16 - 9/16" and the inside corner radius is more like 1/8 -3/16". Thus, you would have about 6", less (3/8 + 3/16)(2), or 6" less (1&1/8") = 4&7/8" on the inside flats. But, as mentioned above, there might be some weld bead/flash in there. (4&7/8") > (2.75" min. bm. gage + 2" for washers) = 4.75". This HSS material is from some pretty old ref. material, and the way standards and manufacturers have changed lately, it is just my best guess. I would see if I couldn’t get some actual measurements on some of the material which was going to be used.
3.) One of the biggest problems might be wrench clearance and swing inside the HSS. So, you better check that too.
4.) Another trick..., you wouldn’t bother many people if you clipped 1/8" off the outer edges (the O.D’s.) of the washers so they cleared the radii a little easier. Then that flat spot runs parallel to the radius toe.