Bending coefficient (Cb) in a continuous beam.

[WARose]

I have got a continuous beam (about 5 spans) that I am pulling out the stops to try to make work. (It has a uniform distributed load.) One thing I have thought about is using the "bending coefficient" [Cb]. Sounds good for the positive moment (as the top flange is pretty well braced.....but the bottom flange is only retrained at the supports. Am I just stuck with Cb=1.0 ?

Sorry for what seems like such a rudimentary question.....but in a 20+ year career, I'm not sure I've used anything other than Cb=1.0.

Thanks in advance.

 

[Celt83]

See this thread: Link

Check out AISC’s series “Steel Design after College”: Link

 

[StrEng007]

Celt83 beat me to it. I had the same link in mind.

Also, if your top flange is well braced, that is if you don't exceed Lp of the beam, then the limit state of LTB wouldn't apply for moment compressive stress at the top flange (ie, Cb would not be used for positive flexure)

 

[dik]

Can you use plastic design?

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[KootK]

Having explored this issue extensively in the past, I am now of the opinion that LTB is all but impossible for the kind of beam that you've described so long as the adjacent slabs are comparable-ish. I don't even bother to check it any more in my own work.

 

[WARose]

Thanks to everyone for the replies and I will dig into this.

@dik: I can, but I haven't used plastic design a lot either. IIRC, everything had to be braced for that to work....so I am not sure that will fly considering the situation with the bottom flange.

 

[JAE]

Not to make things worse for you but don't forget to alternate any live loads you have on the various spans.
Required patterns would typicall be:

1. LL on odd spans
2. LL on even spans
3. LL on first two adjacent spans, then skip a span, then next adjacent spans, etc.
4. No LL on first span, then LL on next two adjacent spans, then skip a span, etc.
5. LL on first span, skip a span, then LL on next two adjacent spans, Skip, etc.
6. LL on all spans.

 

[WARose]

Good point JAE. Pattern LL isn't something I have forgotten.

 

[dik]

For continuous, regular, stuff, it cannot be beat. I've been using it for over 50 years, for multispan continuous regular spaced stuff. Lighter material (or stronger), fewer pieces to handle (added cost savings). It's a matter of bracing which usually can be done with existing framing or L sections. It may give you the added capacity you require. With plastic design, alternate loading is not an issue and my have increased the loading on the original design. Just some thoughts.

I did a warehouse, over 50 years ago, for an engineer who had lost his license. He was one of my profs at U. I'd done a Gerber solution and he asked why I had done that in lieu of plastic design. I redid the design using plastic design and I was sold on it, and have used it since. That was my only Gerber solution. When I first started I was told by other engineers that it was more expensive... same with HSS sections. This is not the case.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[dik]

Not an issue with plastic design... deflections are approx 1/3 that of simple span and are usually manageable. It only affects the moments at the splice location and generally 1/4 the Mr (code requirement) governs. The only minor downside is that the section has to be Class 1 which may require a slightly heavier section.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik