RISA Model - Beam not design for lateral-torsional buckling

[patelam]

Hi,

I have designed a simple structure in RISA 3D and have used the software to design column (vertical) elements. On checking the detailed design report for columns in RISA, the flexural design in major axis does not accounts for lateral torsional buckling. It says "Limit state of lateral - torsional buckling does not apply".

Can someone explain if there is the code regulation that restricts the software to design for lateral torsional buckling or I am missing to input the parameter in the model?

 

[DaveAtkins]

I assume that means lateral-torsional buckling does not apply to that particular member size.

I recommend you check the design by hand to confirm this is the case.

 

[JoshPlumSE]

"Limit state of lateral - torsional buckling does not apply".

It's been quite awhile since I used RISA-3D. But, I looked at your model quickly. Some quick thoughts:
a) You're looking at the detail report for that member, right? And, the expanded information for the Flexural Analysis (Strong Axis). It took me awhile to figure out where in the program this information is given.... This new interface is a challenge for me. LOL.

b) Flexural Torsional Buckling (as I understand it) is a COMPRESSION failure that usually only applies to singly symmetric members like WT's or Double Angles. For these members, the weak axis is so weak (in weak axis bending and torsion) that when the flange goes into compression buckling the lowest energy buckling mode will NOT be a pure strong axis buckling shape. Rather it will include a twist of the member as well.... That's while it's a "flexural-torsional" buckling mode. So, my first thought is just that FTB doesn't apply for this member which is a wide flange beam.

c) My next thought is that this is an axial / compression failure mode, not a bending type of failure. So, why in the world would RISA want to include a note about this in the flexural capacity check. I suppose it's possible that this axial failure can affect the flexural strength in some way. If so, that might justify the message.

d) To be clear, the main form of stability (i.e. buckling) failure for beam is usually lateral torsional buckling (LTB). This term is so similar to flexural torsional buckling (FTB) that it's probably common for people to confuse these terms when they are less familiar with the AISC code.

 

[driftLimiter]

check the member unbraced length parameters, and also check if LTB really does apply for that cross-section. You shouldn't trust design check outputs without some form of verification. This member is a good one to verify using another method and compare what RISA is doing.

 

[JoshPlumSE]

Sorry, it appears that I'm the one who was confused. I don't know how I mis-read that you were saying there was a message about "flexural torsional buckling". In fact, I'm looking at your model again and realizing that there was no message about Flexural Torsional Buckling. My brain just read what it thought was going to be there.

Ooops!

Anyways, the unbraced length is listed as 7 ft for the compression flange. Looking at the detail report. When I changed the unbraced length to 10 feet (just to check), the detail report told me that the Lp (i.e. length at which LTB controls) for this beam is something like 8.5 feet.

 

[patelam]

Thanks all for your response. @JoshPlumSE , yes thats true. I calculated LP and it shows 8.5ft, which is larger than Lb hence not check for LTB. But I wish if RISA could give a reason why LTB is not applicable for better clarity to the user.

@DaveAtkins @driftLimiter - Thanks for your response. It helped me resolve the issue.

 

[JoshPlumSE]

Thanks all for your response. @JoshPlumSE , yes thats true. I calculated LP and it shows 8.5ft, which is larger than Lb hence not check for LTB. But I wish if RISA could give a reason why LTB is not applicable for better clarity to the user.

For the life of me, I can't understand why they don't at least report Lp when LTB doesn't control. Just so we can understand why it doesn't control.