Moment Capacity of Composite Floor Girder

[Perception]

Hello,

I have a few questions on computing the moment capacity of a composite steel deck + concrete slab beam system as follows:

1) If a girder only has simply supported beams framing into it on one side, are the beams considered as lateral bracing points assuming web shear plate connections are used? My understanding is that decking will not provide adequate lateral bracing to a girder since the ribs typically run parallel to the top of the girder.

2) How is the moment capacity of a beam/girder at an angle computed? For example in plan view if a beam is slanted 30 degrees to the horizontal, what assumptions do you make for the direction of the deck across the top of the beam and how much space you have available for shear studs? Can decking be installed such that it's slanted 30 degrees to match the beam?

3) How is the moment capacity of a beam//girder computed when the decking changes orientation over the top of the member? Do you assume either a parallel or perpendicular deck condition and ignore the other deck condition on the member?

4) Is composite design typically considered for concrete slab + steel decking systems with joists? If so, how is the tensile capacity of the joist taken into account when computing the moment capacity of the combined section? Intuitively I think you could utilize the tensile capacity of the bottom chord to compute the moment capacity, but I'm not sure.

Thanks

 

[MrHershey]

If you've got headed studs tying your girder and concrete topping together, then I'd argue that your girder is continuously braced regardless of beam spacing or deck orientation. Or your unbraced length is the same as your stud spacing anyways.

Once the concrete hardens, it'll be a lot stiffer than your deck. And in order to actually have any lateral movement of the top flange, you'd have to shear off your shear connectors.

 

[Perception]

Mr. Hershey,

This is true after the concrete has hardened, but before the concrete hardens the girder would have to support the dead load + construction live load on the floor without help from the decking + slab; unless the floor is shored.

 

[SethGuthrie]

Ram Structural System - Ram Steel is used for a lot of composite designs like this. We have an option in the criteria for the deck parallel to brace the girder top flange. By default we only use the perpendicular decks to continuously brace the beam.
I'll attach some excerpts from the Ram Steel Beam manual pdf to elaborate on how we handle the other situations you describe.

 

[Perception]

Thank you Seth. I'm curious what RAM does when the beam is sloped at greater than 10 degrees. I guess you could detail how many ribs would actually fall onto the girder, and thus find the space available for installing shear studs.

Just to clarify if you don't consider a girder to be braced by decking, does a beam framing in on one side provide an adequate LTB brace point? Or, do you have to have beams frame in on both sides for the girder to be considered braced?

 

[SethGuthrie]

If the beam is nearly parallel to the deck, less than 15 degrees, we treat that as parallel. For deck angles 15 to 90 degrees to do the trigonometry. We also offer a tool where the user can specify that the deck is split or deformed in order to have any stud placement.

A beam framing into a girder provides bracing in the program always. Additional brace points can also be user specified. A few more tips here FYI:

https://communities.bentley.com/pro...sis_and_design__wiki/2053/ram-steel-beams-faq

 

[hokie66]

The beams will brace the girder adequately.

I would not go for much composite action in the edge girder. The studs are prone to to rip out.

 

[Perception]

That's interesting hokie66. Is there a AISC provision that addresses this or a study to back this up?

 

[hokie66]

I don't know about AISC, but there is research. I have read it, but since I am not much involved with composite structures, I won't try to do the detective work for you. It depends on whether you are trying for fully composite or partially composite, and involves adding reinforcement transverse to the studs.