HSS design

[ag89]

I have a few questions for the design of HSS.

1. Why are the property tables different for HSS in the LRFD design specification from the properties given in the AISC ASD 9th ed for TS sections?

2. In design, to allow for the torsional effects of eccentric loading, the LRFD spec combines the loads for flexural + axial + the square of the sum of shear and torsion (Eq 7.2-1). Can I use the same interaction for ASD, using stresses instead of factored loads? I am designing the structure using ASD.

3. The allowable stress in LRFD for torsion (this specific case) is 0.6FY. Can I use this allowable stress for torsion design?

4. Basically, from what I can tell, the equation for stress due to torsional loading is: f=T/C.

Any help would be appreciated. Thanks in advance.

 

[Taro]

The HSS properties are different because the specification that governs the fabrication has rather loose tolerances and the manufacturers can consistently roll their plate stock at the low end of the tolerances. Therefore, section 1.2.2 of the HSS spec requires using a design thickness of 0.93 times the nominal thickness unless you verify the actual thickness.

Torsion is well covered in the LRFD HSS spec, AISC's torsion design guide, and many steel texts. Also you can call or email AISC's Steel Solutions Center for more information.

 

[ishvaaag]

Structural codes are tantamount to selfcontained proposals to warrant some level of safety. Then we need only to add to their contents, never modify them if something stays undefined.

Respect Torsion, section K3 of ASD specifies to add (service level, non factored) torsion forces to those coming from all other loads, the resultants

"not exceeding the allowable values".

Then the answer to your question 3) will be true only for the cases in which the ASD defined allowable stress stays at 0.6 Fy, as in some cases happen.

Respect question 2) to proceed as you indicate differs from the directly sum ordered (be it axial or shear stresses the case), yet it might be more rational for the cases in which shear with normal stresses interaction becomes significant.

Respect question 4) I have at least 2 books nearly completely dedicated to torsion in steel structures, so to reduce all to a quotient is simply misrepresentation. Occasionally structural designers meet torsion and need the full array of tools, not just a closed form formula.

 

[Imagineer]

Ag89,

Q1) Taro is correct.

Q2) If you have the HSS Connections manual, why not simply design it as LRFD?

Q3) No, because that is not the "allowable stress". You have to remember that LRFD has factors on both the load and resistance. If you use factor for only one side then you eliminated the safety check on the other side.

Q4) No. IIRC, the torsion equation uses Q. Unfortunately, don't have a reference right with me but it is something like T=PQ/Ic????? I'm sure that is not correct put the correct equation looks like that.
Imagineer

 

[JAE]

I believe that the torsion stress equation is:

t = Tc/J

where:
(English units....sorry for those of you on the SI)
t = shear stress (psi)
c = distance from center of section to outer fiber (in.)
T = torque (in-lbs)
J = polar moment of inertia of the section (in4)

 

[ag89]

From the responses, I gather there is no HSS specification implementing ASD. Like imagineer said, I guess I will just design it using LRFD. I was hoping to keep everything in ASD, since the whole structure is designed that way. I guess the only part of the puzzle I am missing is the allowable stress for torsion based on ASD.

Thanks for the input

 

[ishvaaag]

ag89, you have all what ASD has to say about torsion forces and allowable stress in section K3. The allowable stress is the same that applies without torsion, only that torsion -longitudinal and transversal- stresses are to be added (summed) in every of the pertinent checks it happens to concurr; the allowable stress stays then in ASD that specified so to be for the case.