Question regarding Composite Beams 1

[Alves21]

I am starting to study composite beams and I have some doubts on this subject.
First, from my understandment, as long as I am only working with positive moments I can consider both primary and secondary beam as composite beam. If I am working with moment frame beams I should ignore composite action and design as simple steel beam. Also, all the beams should have stud bults, including moment ones. Are those assumptions correct?
And what about axial load verification, should I take this in concideration? I have lateral wind loads acting on columns. If so, what about the unbraced length of the beam since the span is almost 9m, should I take this in consideration or the slab has some influence on this?
From my studies I've heard of diaphragm effect of the slab that I think has some relation to this.

Thanks in Advance!
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[Agent666]

You can get some composite action in the negative moment regions, but generally I'd say its not worth it. Even for a continuous beam the positive moment regions can still be considered as being composite.

My local standard recommends for moment frames leaving out the studs at beam ends within yielding regions. This might be a length of 1.5 times the beam depth. One thing that shouldn't be ignored is the stiffening effect of having composite beams as part of moment frames in determining building periods and hence seismic loads. It can result in a 20% increase in the stiffness for beams for example.

Slab provides some restraint to beams under axial loads. However, generally I'd say most engineers transfer loads into the diaphragm slabs via studs and deal with the transfer of loads within the slab itself. But if you have significant axial loads then you might need additional restraint to the bottom flange.

 

[JoshPlumSE]

"First, from my understandment, as long as I am only working with positive moments I can consider both primary and secondary beam as composite beam."
Yes, primary and secondary beams can both be composite.

"If I am working with moment frame beams I should ignore composite action and design as simple steel beam."
I always ignore composite behavior for beams that are are part of moment frames. The backspan of a cantilever beam is a more difficult question. Even so, I usually ignore composite behavior there too.

"Also, all the beams should have studs, including moment ones."
Usually, yes. The purpose is to connect the deck / diaphragm to the beams. Though the spacing of the studs could vary significantly between the composite and non-composite ones.

"And what about axial load verification, should I take this in consideration? I have lateral wind loads acting on columns. If so, what about the unbraced length of the beam since the span is almost 9m, should I take this in consideration or the slab has some influence on this?"
The deck / diaphragm will brace the beam's top flange against Lateral Torsional Buckling (i.e. beam buckling due to moment), but it won't brace the bottom flange at all for beams with negative moment. If you need bottom flange bracing you need to come up with a way to do that.

As far as column type buckling due to axial load in a beam, the deck / diaphragm doesn't do this.... at least not alone. But, where you have other beams connected, it is easier to consider those locations a brace point when they've all got studs connecting them to the deck.

 

[r13]

Anybody knows any good composite design textbook, or technical publication, which will suit the OP's need (study and learn).

 

[Guest090822]

"Steel Structures - Design and Behavior" by Salmon & Johnson has a chapter on composite beam design.
Any Mechanics of Materials college textbook should be able to explain the basic theory too.