RISA torsion Analysis.

[Margana]

Hello,

I'm new to the forums, but have had the chance to use RISA a bit. I have a question concerning Torsional Analysis in RISA. I've tried to find this somewhere in the help files but couldn't so here it goes.

I'd like to know what equations are used to analyze a torsional effect acting on steel members. I assume that torsion is taken into account but cannot figure out what analysis is used, and whether or not this factors into the unity or shear check for the design results.

And along those same lines is there anywhere where RISA mentions what equations are being used for different scenarios. I know under the details section it mentions H1-1a but according to the steel manual (13th ed) this equation doesn't account for torsion. Although the torsion in my model is negligible so it most likely will not be the governing case I just want to make sure that it is accounted for before going back to the bldg. department.

Thanks for any help.

 

[Stazz]

Torsion is covered in the help. It states that it uses the AISC Torsion Design Guide: Case 2 (Concentrated torques at ends of member with fixed ends). But I'm not sure on the specific equasions or interaction.

Here is what's in the help:

A twisting of the member induces torsional forces and stresses. The primary reference used in the development of RISA-3D's torsional calculations was Torsional Analysis of Steel Members, available from the AISC. The equations used for torsional stresses won't all be repeated here, but they can be found in the reference. RISA-3D models warping members using CASE 2, as shown in the Torsion reference.

 

[Margana]

Perfect. That set me on the right track...

And luckily being an AISC member I was able to DL the torsional design manual for free to see what eqn's they were using.

Thanks

 

[JoshPlumSE]

Margana -

While RISA does an extremely good job with Case 2, this can be overly conservative (for stress) in most cases sometimes. It may even be unconservative in some cases.

Therefore, whenever a non-case 2 torsion occurs, I usually suggest following up the RISA analysis with a quick hand calculation. I usually use the Equivalent Tee analogy specified in the AISC design guide (Section 4.1.4 and figure 4.4).

One of the nice things about the equivalent Tee analogy is that it allows you to account for boudary conditions that are in between the assumed perfect restraint for warping and the assumed perfectly un-restraned warping restraint.... Though I have to admit that I rarely do so.

Josh