Composite steel beam?

[Milanaz92]

Hi guys, I’m new here. I’m designing continuous W-beams that are 25’-0” long. 21’-0” of the the beams are composite with stud and deck on top. Deck stops at 21’. The remaining 4’-0” of beams are enclosed with reinforced concrete (old school) and concrete slab on top. I wanna know if I can treat this beam as fully composite? Also what do you guys think will happen at 21’?

 

[bob33]

Continue the studs to the ends of the beams and make the concrete continuous. Switch from deck as a form to formwork at 21'.

 

[bob33]

Regarding what will happen at 21', there may be a tendency for shrinkage and differential deflection to cause cracking between the two types of slab. Make sure the longitudinal reinforcing is continuous and add top rebar between formed slab and deck slab for any potential localized negative moments.

 

[JoshPlumSE]

Hmmmm, what do you mean by continuous? Moment connections at each end? Or, a cantilever on one side.

If it's a 25' long simply supported composite beam, where 4 ft of the beam is non-composite that is the simpler concept. If it were 25 ft long composite beam with fully composite action then you will have a certain moment of inertia. This moment of inertial is dependent on the ability of the shear studs to transfer the shear force between slab and beam. So, if the end of the beam has no shear stud transfer (where shear force is highest) then this is problematic and I tend to think of it as a non composite beam.

If the beam has a slab on one side and not the other, then this is okay, it just reduces the moment of inertia of the beam and hence reduces the stiffness of the beam. But, it can still be composite. It might take some engineering judgment, however, to determine what the effective width to use for the overall beam.