RISA 3D 7.0 Code checking discprepancy
RISA 3D 7.0 Code checking discprepancy
(OP)
So we just received the latest version of RISA 3D, v7.0 which now includes code checking from the AISC 13th. I thought I would do some code check comparing and noticed a pretty substantial difference between the LRFD 3rd edition and AISC 13th (LRFD) code check when looking at WT sections in flexure. For example, a simply supported WT8x20 unbraced, with a 10K load at mid-span yields a unity check of around 1.43 based on LRFD 3rd ed. code in RISA. However, by just changing the code to AISC 13th (LRFD), the code check drops by nearly half to 0.7. The thing is, both code checks reference the exact same equation in their respective references (H1-1b, and hand calcs tend to agree more with the AISC 13th (LRFD) code check, but I would think RISA would show exactly the same code check because the equations are the same for singly symmetric members in both books. Seeing as how hand calcs tend to agree more with the AISC 13th ed. code check, it looks like the LRFD 3rd code checking for WT sections was perhaps not done correctly in the previous versions of RISA as well. Has anyone else had a chance to look at RISA 7.0, or can somebody confirm? Was thinking maybe I should send an email to RISA to let them know of the situation.






RE: RISA 3D 7.0 Code checking discprepancy
RE: RISA 3D 7.0 Code checking discprepancy
Great job verifying with a hand calc! It's scary how many people don't do that. If the 3rd Ed. implementation was wrong, that's REALLY scary considering how long ago RISA added that.
I know of one multi-zillion dollar fix that was required because an engineer did not check what another program was doing with WTs. The program's WT implementation was flat wrong and the trusses started to collapse when they started sitting the precast on it. Problem was that there were dozens of these trusses already up.
RE: RISA 3D 7.0 Code checking discprepancy
I really don't mean to hijack this thread, but 271828, do you have a reference, case study, or any further information about the failure you mentioned? I only asked because our office is currently designing a staggered truss building with WT sections as the diaphragm chords. We're detailing the precast plank to brace the trusses during construction, but I would be interested in reading more about that issue.
I wish they had a personal message feature on these forums, but I guess that would attract recruiters. Ah well...
RE: RISA 3D 7.0 Code checking discprepancy
This was about 6 years ago, so that software isn't being used nowadays. It wasn't a Spec. problem. The software was just plain wrong, spitting out phiMn that weren't even close. I'm sure it's been fixed by now.
It comes down to the age old issue: The design engineer was in a big hurry like we all are and didn't verify the results. I've done that as I'm sure most here have, but he was unlucky.
I also wish we had a PM feature.
RE: RISA 3D 7.0 Code checking discprepancy
"The difference is likely related to the different ways that the two codes handle slender elements. It looks to me that the 13th edition doesn’t care about web slenderness for bending. In the 3rd edition, Chapter F specifically does not apply to member with slender elements. Therefore, the Qs value is taken into account.
I have little doubt that the 13th edition code check is more on par with what the code writers’ intentions were. However, baring a specific section of the code that deals with Slender sections, we cannot use those same equations for LRFD 3rd."
Haven't had time to research this yet, but I suppose it does sound reasonable - what do you guys think? I did notice that in the RISA output for both code checks, a Qs value is calculated but perhaps not taken into account with the 13th ed. code check.
RE: RISA 3D 7.0 Code checking discprepancy
The Commentary to F9 talks about this stuff.
I don't have my 3rd Ed. here, so I can't look at the difference between the two formulations.
RE: RISA 3D 7.0 Code checking discprepancy
RE: RISA 3D 7.0 Code checking discprepancy
RE: RISA 3D 7.0 Code checking discprepancy
RE: RISA 3D 7.0 Code checking discprepancy
RE: RISA 3D 7.0 Code checking discprepancy
RE: RISA 3D 7.0 Code checking discprepancy
It appears that RISA is using the Qs factor since part of the stem (nearest the flange) is in compression but it still uses the 1.5My instead of 1.0My for the limit. This seems inconsistent to me at this point. But I bet what they are doing is still overly conservative since you say the AISC 13th gives a ratio of 0.7.
2) Next, I flipped the tee to where the tip of the stem is in compression. I calculated 2.149, and STRUDL gets 2.148. In this case, I used the Qs reduction with the 1.0My limit as it of course should be.
3) Next, I tried RAM Advanse and I got a stress ratio of 2.0 and it didn't matter whether the stem was oriented up or down.
RE: RISA 3D 7.0 Code checking discprepancy
RE: RISA 3D 7.0 Code checking discprepancy
RE: RISA 3D 7.0 Code checking discprepancy
RE: RISA 3D 7.0 Code checking discprepancy
RE: RISA 3D 7.0 Code checking discprepancy
RE: RISA 3D 7.0 Code checking discprepancy
"No limiting stem width-thickness ratio, ?r, is provided in this section to account for the local buckling of the stem when it is in compression. The reason for this omission is that the lateral-torsional buckling equations (Equations F9-4 and F9-5) also give the local buckling strength as Lb approaches zero"
What I am not convinced of is should the limit be 1.0My or 1.6My according to the 13th even if only a small part of the stem is in compression adjacent to the flange? It would seem to me as it would be 1.6My. Penalizing the entire section because only a small portion of the stem above the N.A. is in compression seems very conservative and would exclude most all tee sections from using the 1.6My limit for bending.
RE: RISA 3D 7.0 Code checking discprepancy
RE: RISA 3D 7.0 Code checking discprepancy
RE: RISA 3D 7.0 Code checking discprepancy
Haynewp - my hand calcs yielded the same, and now I can see how RISA obtained the value that it did.
Thanks for all the replies, its interesting to get comparisons from other programs as well and see how they interpret the code differently for things such as this.