Unbraced lengths based on inflection points.

[JRR66]

I'm doing an analysis on an existing building that uses cantilever construction. The girders run over the columns and cantilever. Between the two cantilevers there is a simply supported beam. The roof joist run over the top of the girder cantilever and then there is a simply supported beam that frames into the cantilever. Since there is compression in the bottom flange is it necessary to look at the entire span of the member as unbraced or can you look at the member as being unbraced for the negative bending up until the point of inflection. The joists are continuously braced on the top flange and girders top flange is braced by the joist at 7'-9"O.C.. To look at this as fully unbraced seems unnecessary since the members typically see compression in the top flange for a larger portion of their span.

Please see the link for a sketch:

http://files.engineering.com/getfil...9-3309-4e00-8663-bdb408ea73f0&file=img002.jpg

Any information and references on proper way to address this situation would be appreciated.
Thanks,
John

 

[KootK]

I'd recommend posting a sketch. You'll get more and better recommendations that way. Anyhow:

1) Sounds like classic Gerber system to me. I'll assume that in what follows:
2) Inflection point pseudo bracing is no longer recognized as real bracing as the theory of it is, and always was, bunk.
3) One way or another, you probably need to -- and likely can -- call the columns points of rotational restraint for the girder.
4) Based on what you've written, I assume that you have a wide flange beam framing into the girder perpendicularity at the ends of the cantilevers. If this beam is deep enough (~60% girder depth) and connected suitably, it can be considered rotational restraint for the girder.
5) Add up 1-4 and you've got:

a) simple span beam between cantilevers to design.
b) a cantilever to design that is rotationally braced at both ends at minimum.
c) a continuous-ish beam between cantilevers that is, at minimum, rotationally braced at each end.

Link

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.

 

[JRR66]

KootK

Thank you for the response and information you have provided, along with the link to the article. I have updated my post to include a sketch thanks for that suggestion.

 

[KootK]

Thanks for the sketch JRR. I'll stand by my original comments with one exception: baring some discrete fly bracing at the double angle shear splices, I'm not convinced that you can call the ends of the cantilevers rotationally braced. That will impact your cantilever designs substantially. Check this out: Link

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.

 

[JRR66]

KootK

Thank you for the link to the design example. I started reading through it this morning and will be adding it to my library a great reference document. This existing building I'm working on will be getting a lot of bracing added to it that is for sure.

Thanks again for the help!!