Composite slab on a steel frame 1

[CovertShear]

Hello all.

I am evaluating an existing 3 storey steel frame structure with a composite slab (see attached floor-plan section).

As per usual, structural documentation is not available, and the information I gathered is as follows:

• Steel profiled decking, thickness circa 3mm (1/10 inch).
In situ cast concrete, total slab thickness 75mm (3inch).

• Steel frame is a combination of double welded U-Beams and secondary I-Beams
Nothing is known about shear connectors (studs).
Spans see attached floor-plan.

• Design load (dead + live) = 8.40 kN/m2 (175.43 lb/ft2)

Now I'm not much of a steel guy, but if I assume load width of the secondary beam 2900mm (9.5ft) ,
and calculate it as a simply supported beam => 8.40kN/m2 x 2.9m = 24.36 kN/m (1669.19 lb/ft)

Assuming bending moment as q*L² / 8 , then the capacity of the support I beam is unsatisfactory (150% circa).

Now Im thinking, even though I dont know much about shear connectors (studs), I shouldn't assume the secondary I beam acts "alone", rather it acts as a composite beam with the concrete slab ?

Anyway, thanks for any insights in advance.

 

[Enginerdad]

Now Im thinking, even though I dont know much about shear connectors (studs), I shouldn't assume the secondary I beam acts "alone", rather it acts as a composite beam with the concrete slab ?

That certainly would be helpful, but if you don't know anything about the shear connectors then I'd say it's very hard to justify assuming fully composite action. If there's no other way to make it work without composite action, then I think you need to do some testing to determine the shear connector situation. You could probably get a good idea of it with non-destructive methods like GPR or X-ray imaging. Either way a composite moment frame is an unusual system, and assuming it to be the case is unconservative.

 

[CovertShear]

@Enginerdad

Thanks for your input. I also feel like composite moment frame doesn't make sense.
I will consider X-ray imaging.