Restraint of compressive reinforcement that is required for stiffness (but not for strength)

[bugbus]

I have a simply-supported composite girder comprising a steel structural section supporting a concrete slab.

In the hogging region (which is the whole span), I am having to rely on the longitudinal reinforcement in the slab to increase the stiffness of the girder. However, the concrete alone is sufficient to satisfy ultimate strength in bending.

Is there any particular requirement to provide restraint to these compression bars when the maximum compressive stress they will be required to sustain is in the order of 200 MPa (~30 ksi)? I should note that the longitudinal bars are the 2nd and 3rd layers in the slab (i.e. they are between the transverse bars, which are layers 1 and 4).

There is no doubt in my mind that if I were to rely on the bars to undergo yielding (or anything close to it), then I would have to provide restraint in the same way as for a column or wall, but I tend to see this as a different problem.

 

[centondollar]

If you have a simply-supported composite girder in which the slab is "on top", steel beam is "below slab" and loads are applied vertically downwards, they you have sagging in the entire span. Furthermore, the stiffness of the girder will not be markedly changed by reinforcement in the concrete slab; the largest (by 2 orders of magnitude, at least) increase in stiffness, provided that you have installed sufficient amounts of shear studs onto the steel beam top flange, will be given by the composite action of the hardened concrete slab. You can check this by calculating rebar contribution to 2nd moment of area and comparing it to the steel beam and concrete slab contribution to 2nd moment of area.

Regarding confining reinforcement (closed links), it is not necessary. The confinement is needed for confining concrete in columns (high risk of buckling of main rebar due to large axial compression), for ductility (energy absorption capacity) of beams/plates/columns (earthquake resistant design), to counteract torsion and shear, and to restrict cracks caused by tension caused by shear force. Your beam is probably not (check this yourself) in torsion, and its shear capacity is handled by the steel beam, and its concrete part is almost completely in compression - I do not see the need for any additional confining rebar.

EDIT: I should add that you will need rebar (longitudinal rebar in the direction transverse to the beam span direction) in the concrete slab to restrict cracking arising from shear stud and concrete interaction. When shear studs takes longitudinal shear in the sagging zone (slab in compression), they distribute this into two diagonal compression struts and a transversal tension force, which causes cracking.