Composite slab within beam depth diaphragm

[bricksandmortar]

I am using a composite slab within the depth of a steel beam supported on angles but I am concerned if this will be sufficient to provide good diaphragm action. If the slab is fully within the depth of the beam would providing either shear studs or rebar welded to the webs be sufficient to provide the diaprhagm action? For the internal beams I am proposing to have rebar through the webs to connect the slabs on each side. I could extend the slab above the top flange of the beam and have the rebar continuous over but it would be preferable to keep it within the depth of the beam for the Architectural requirements. I have sketched some details of the options below:

 

[kostast88]

If you want composite beams, raise the slab and pass the reinforcement above the steels & provide studs. Sketch #2.

The first sketch with welded reinforcement + threaded bars is not clear to me.

In the UK I've come across this quite a lot, especially with concrete on metal deck. I never treated these beams as composite. We could switch to hollowcore precast slab + topping at any time to control costs. Plus, they don't weld the studs on site. Instead they just shot-fire them which always made me nervous regarding composite action.

As far as diaphragm is concerned (assuming concrete on metal deck) the 150mm concrete on both details would be just fine. I assumme this is a low-rise without earthquake considerations and/or significant lateral loads. Do check the buckling of steels thoroughly(right point of application etc).

 

[bricksandmortar]

@kostast88

Thanks for the reply. I am not designing the beams as composite, I was specifically asking about the diaphragm action of a slab within the depth of the beam. In my case I am actually using the slab to prop basement retaining walls but am also interested in the detailing for an edge beam / metal deck slab. The first sketch showing reinforcement through the beam web or shear studs shot fired to the angle are not intended to generate composite action but to ensure a good diaphragm connection. Are you saying that these shear studs / reinforcement are not required for the diaphragm action?

 

[MotorCity]

I would attach studs to the beam. If you try to run rebar through the beams, the rodbusters will hate you.

If you insist on running bars through the beams, the maximum bar length would be the beam spacing so the bars would not be continuous. As for composite action, I agree the slab and beams don't need to act composite in the vertical direction, but they do need to act "composite" in the horizontal direction (in plane) to maintain diaphragm continuity.

I vote studs.

 

[kostast88]

So now you have a significant lateral load on your floor. I'd be interested to see a sketch of your problem. I now understand why you want to provide a good connection.

You need a RIGID diaphragm. This means:
1. If the solid part is more than 70mm, and has minimum rebar in both directions, then it is (EN 1998-1 5.10 (1))
2. “The diaphragm is taken as being rigid, if, when it is modelled with its
actual in-plane flexibility, its horizontal displacements nowhere exceed those resulting from the rigid
diaphragm assumption by more than 10% of the corresponding absolute horizontal displacements in
the seismic design situation.” 4.3.1 (4)

This comes from the seismic code, dealing earthquakes which is a lateral force. And you have a lateral force.

It is hard to say using my imagination but: the axial force in the floor ( say 1/2 the retaining force /m) * floor span between beams , should be transfered as longitudinal shear along the steel beams. If you have the transfer length required (which you might) you don't need the studs. But if you don't, then you'd like to add them - I'd put them preferably on the web of the steel beam, if you have the tolerance.

 

[bricksandmortar]

I am just trying to ensure the force is transferred across the ground floor slab to the opposite retaining wall so that it is in equilibrium. I don't think I need to have shear studs on the web for this but am more concerned with transferring the compression force through the slab to beam junctions especially if I have the deck parallel to the beam. In this case, the concrete could leak at the junction as the gap may not be sealed completely and therefore the concrete may not be properly compacted.

 

[MotorCity]

I did not realize you had a retaining wall at each end of the diaphragm. Now I question the need or function of the diaphragm. What sort of lateral force resisting system are you assuming that you have? Retaining walls are sort of like a cantilevered system where all of the lateral force is resisted by cantilever action.

 

[bricksandmortar]

@MotorCity

I am designing the retaining walls as being propped by the ground floor slab.

 

[BridgeSmith]

Is the length of the retaining walls really that short that it's advantageous to run a beam longitudinally between them, rather than running beams across from one retaining wall to the other?

Even if that is the case, your diagram shows the steel decking/form pans oriented in the wrong direction to span from the retaining walls to the steel beam.

Anyway, with the way you have the slab, beam and retaining walls shown, you don't need to connect the slabs through or with the beam. The slabs on each side are pushing against each other with the steel beam in between. The only loads would be horizontal compression and the vertical shear due to the weight of the slabs. If the bottom of the slab isn't supported directly by the bottom flange of the beam, then all you need is a spacer to take up the difference. I'm not seeing the necessity for reinforcement through the beam, shear studs to connect the beam to the slabs, or any other attachment that ties the left and right slabs together.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[bricksandmortar]

I am not concerned with the span of the decking, only the ability to transfer the compression force through the beam. The slab is forming the ground floor over the basement and will need to transfer the force both parallel and perpendicular to the span of the decking. My main concern is if the concrete at the junction leaks due the decking not being tight to the beam and therefore has voids / weakness around the edges.

 

[BridgeSmith]

The slab is forming the ground floor over the basement and will need to transfer the force both parallel and perpendicular to the span of the decking.

If you say so. However, be aware the steel form pans only span one direction supporting the load of the wet concrete. We use them all the time on bridge superstructures, and they don't seem to have any problems keeping the concrete in at the end of bridge.

I guess I'm just struggling to understand why, if the span is short enough in the direction of the beam span for the steel deck form pans to span that direction, as you've shown, why you need the beam. If the span is longer than what the forms can span, then they'll need to be oriented the other direction and span from the beam to the retaining walls.

Btw, if you have rigid support on 3 sides of the slab, and a steel beam spanning on the 4th side, I don't think you can realistically expect it to behave as a two-way slab.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[kostast88]

BridgeSmith is right, you want the decking the other way. But then again I guess you have 4 sides.

Compression will be transferred through the conrete either way.

Will there be leakage? I doubt it, builders provide end caps at the edge gaps. Looks like that: Link

The only thing that could bother me is having earth pressures at rest on one side (K0) and active earth pressures on the opposite side (Ka). Then you would have the difference in force taken by the rigid beam as bending around the weak axis. Combined with the bending around the major axis. Could concrete separate from the beam in this case? The answer is no because the slab is always being compressed from everywhere. I don't believe it would deflect that much.

Which is why a couple of studs on the web of the beam would take the headache away. Alternatively raise the floor (sketch #1)

PS - I believe your beams are small and no significant thermal effects come into play.
PS2 - YOur starter bars are in the way of installing the floor. You may want to rethink this edge connection.