STAAD PRO UNBRACED LENGTH
STAAD PRO UNBRACED LENGTH
(OP)
I am using staad pro to evaluate a moment frame system. I am using the steel code check and I am getting an overstressed ratio on my frame beams that I know is not correct. Since the beam is see a negative bending moment it is taking the beams capacity as fully unbraced.
Is there a way in staad to have the program consider the bending inflection point to be braced? Or should I just look at the bending diagram and manually but in the UNB value.
Thanks in advance for any help.
Is there a way in staad to have the program consider the bending inflection point to be braced? Or should I just look at the bending diagram and manually but in the UNB value.
Thanks in advance for any help.






RE: STAAD PRO UNBRACED LENGTH
Professional and Structural Engineer (ME, NH)
American Concrete Industries
www.americanconcrete.com
RE: STAAD PRO UNBRACED LENGTH
RE: STAAD PRO UNBRACED LENGTH
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: STAAD PRO UNBRACED LENGTH
I'm not arguing, more for curiousity. But my bet is no, and that is most likely due to not seeing the design loading.
RE: STAAD PRO UNBRACED LENGTH
1) Back in the day when inflection point may have been considered a point of bracing, the Cb value was almost always taken as 1.0. So, there was some conservatism there. If you calculate a larger value of Cb you might still get the same beam to work with a longer unbraced length.
2) It may or may not be reasonable to consider the unbraced length to be the full length of the member. Maybe you've got a full depth shear tab or something else that will adequately restrain that beam for torsion or that bottom flange for lateral translation. That may be used to justify a lower unbraced length.
RE: STAAD PRO UNBRACED LENGTH
I believe Josh has it correct, while the inflection points may have been taken as braced points the Cb factor was likely not considered and thus it likely washed out to approximately equivalent. I also suspect that the typical continuous or moment frame beams have sufficient unintended bracing from beams or decks attached to the member to add some lateral restraint such that if they are under-designed it's not a huge cause for concern in historic structures.
Regarding a re-design or update of an older structure, I fully agree with KootK's assessment.
Professional and Structural Engineer (ME, NH)
American Concrete Industries
www.americanconcrete.com
RE: STAAD PRO UNBRACED LENGTH
a) less load than expected and;
b) greater incidental, torsional restraint than expected (JP's point).
There is also something more demonstable however. That's the difference between regular, code LTB and constrained axis LTB. Constrained axis LTB numbers can be significantly better. If you have rotational bracing at the columns and any splices, and you run a constrained axis LTB check, it should be pretty rare that your capacity would come out less than the values calculated assuming IP as LTB braces.
The main problem with the constrained axis check is that it's not usually built into software and hand calcing it is a fee killer. Josh should really take care of that for us.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: STAAD PRO UNBRACED LENGTH
Hah!
Back to the OP's question, UNB and UNT are your friend in staad. Never leave home without them.
Professional and Structural Engineer (ME, NH)
American Concrete Industries
www.americanconcrete.com
RE: STAAD PRO UNBRACED LENGTH
Thanks
RE: STAAD PRO UNBRACED LENGTH
Not sure what you mean about "until I see no negative bending moment in the beam". How is your bracing influencing your moment diagram?
Professional and Structural Engineer (ME, NH)
American Concrete Industries
www.americanconcrete.com