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Risa Interaction Equation H1-1b
2

Risa Interaction Equation H1-1b

Risa Interaction Equation H1-1b

(OP)
Risa accounts for normal stresses in the weak axis due to torsional warping when calculating interaction equation H1-1b by "using AISC design guide 9". Does anyone know what portion of AISC design guide 9 they use if it is a rigorous method, or an approximation such as the bimoment method?

Thanks

RE: Risa Interaction Equation H1-1b

DG 9 is a fairly exact solution.....you'll see a glitch here and there if a boundary condition (or something else) doesn't match their charts completely. But it's a heckuva lot more accurate than any approximate method I've seen.

RE: Risa Interaction Equation H1-1b

(OP)
Also on a side note. Does anyone know of a reference that shows how to use the bi-moment method for a cantilevered beam?

RE: Risa Interaction Equation H1-1b

2
There are a number of caveats to RISA's treatment of torsional warping.
1) RISA only considers warping on members with fixed ends. So, for pinned beams with torsional restraint, the program doesn't consider warping at all. The St Venant's torsional shear is still added into the shear code check though.
2) The program assumes full warping restraint at the fixed end beams. This tends to be pretty conservative for the calculation of warping stresses, but a bit less conservative for deflection.
3) Torsion is only considered for WF beams and Channels.
4) The loading is assumed to be case 2 which is a constant torque. Akin to a point torque being applied at the member ends only. This is the most problematic assumption for most people. For this reason, when I was a tech support engineer for RISA, I always encouraged users to use an "Equivalent Tee" analogy hand calc whenever they were really concerned. Just to get a physical feel for how these warping stresses develop, so that they don't have to trust a "black box" number from the program.
5) The warping stresses in the flanges are then converted into an equivalent weak axis moment that would have produced a similar flange stress. Then finally this equivalent weak axis moment is used in RISA's H1-1b combined stress code check.



RE: Risa Interaction Equation H1-1b

(OP)
JoshPlum great answer you helped me understand better what the program checks. Do you happen to know the process that they use to convert the torsion into a weak axis moment? I tried to use DG9, but it not only does that find stresses instead of moments, but I was finding stresses ~80ksi which I know is wrong, because I was failing on Risa by only 1.2 on unity with large strong axis bending moments combined with the torsional warping and weak axis bending moment.

Thank you

RE: Risa Interaction Equation H1-1b

RISA just uses a moment that would create the same flange tip stress as the warping stress. Sigma = M/S.... Therefore, Mequiv = S*sigma_warping

It's all based on elastic theory though. So, RISA might show a warping stress greater than yield but end up converting it into a moment significantly less than the weak axis Mp for the member.

RE: Risa Interaction Equation H1-1b

Quote:

Also on a side note. Does anyone know of a reference that shows how to use the bi-moment method for a cantilevered beam?

If you refer to the torsion-flexural analogy with the β modifier.....AISC's steel journal has a good article: 'Simplified Design for Torsional Loading of Rolled Steel Members' (by: Lin), 3rd quarter 1977 (p.98-107). They have charts for β that include cantilever beams with a concentrated torque at the free end and another one with a uniform torsion.

Interesting you ask because the first place I looked was my Salmon & Johnson steel text (2nd edition).....and all the β charts were for everything but a cantilever.

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