Bar Placement In One-Way Slab

[spats]

I have a situation where I have a CIP one-way slab spanning between intergral beams (the beam is the same depth as the slab). The top & bottom reinforcing in the slab is 3/4" clear, which put it to the outside of the longitudinal reinforcing in the perpendicular beam.

What is bothering me is the fact that the bottom reinforcing in the slab is below the bottom reinforcing in the beam... it just doesn't look right, and I can't find any ACI 318 code provisions that address this condition. Of course the vertical load in the slab is transfered to the beam by the shear strength of slab concrete itself, and does not rely on the bottom bars being mechanically supported on top of the beam bars. Shear is less than 1/2 the shearing strength of the slab, so there is no shear reinforcement. The slab is pourly monolithic with the beam, and the slab top steel is mechanically supported on the top steel in the beam.

What am I missing here? It seems like it should be OK, but my eyes tell me different.

 

[hokie66]

Maybe your problem is with your nomenclature. If the slab and beam are the same depth, you don't have a beam at all, just a strip of slab which is more heavily reinforced than the other parts of the slab. Think of it as a flat plate. The "beam" is like a column strip. In flat plates and flat slabs, you always have reinforcing which terminates below other layers.

 

[beton1]

How thick is the slab/beam? Loading, spans etc?
Without any other info, you may want to 'crank' the bars to get the slab reinforcing to sit on top of beam bottom bars.

 

[slickdeals]

Is if the effective depth of reinforcing assumed in the beam design is violated by placing the slab bars below the beam bars.

 

[spats]

The beam and the slab are only 6" thick. The beam is designed for the reduced "d". It only spans about 8' across a corridor.

Thanks for the perspective, hokie66. I like the way you think.

Part of the reason I'm asking this question is the fact that the issue came up during a site visit, and they were verbally instructed to drape the slab bottom bars over the top of the beam bottom bars... unfortunately, several floors were poured without doing that, and I had a concern.

 

[NS4U]

If I understand this correctly, you are saying that you have a one way, and at the supported ends of the slab you are installing reinforcement perpendicular to the slab reinforcement.

You are calling the supported end of the slab a "beam" since the reinforcement at this edge is perpendicular to the reinforcement of the slab.

Couple of thoughts here:

1. Off the bat- at the midspan of the "beam" where your slab reinforcement ties into the "beam" you will have cracking at the bottom of the beam, (and thus cracking where your slab bars ties into its support- "the beam"). this may affect performance for the slab due to inaddequate anchorage/development.

2. You are assuming the load goes from the slab, to the beam, and then from the beam to the support. While this is a valid load path, this probably isn't how the load will want to behave. The situation you are describing here has a beam that is only slightly stiffer than the slab (same thicknesses, but different reinforcement). I suspect that the majoirty of load will first try to go directly to the supports (of the beam) and then only after sufficent cracking happens (the result of improper (for lack of a better term) reinforcement layout) will the load go to the beam then support. Thus poor slab performance.

3. You are designing the slab support a beam. Do you have addequate shear capacity in the beam? You need to meet the Vc/2 requirement for beams, but not slabs. I am surprised you don't need atleast minimum shear reinforcement in your beam.

4. Can you just huanch the beam a bit? Like 1 or 2 inches just to get the beam bars on the outside of the slab bars? I agree that it isn't eccomoical to provide 1 to 2 inches of extra cover in your slab steel just so you can put the beam bars on the outside. Consider reducing the slab thickness (but leaving the beam thickness as is)?

 

[spats]

Thanks NS4U. Regarding your points:

1. I understand what you mean, but in theory the bottom slab bars are not in tension at the beam. They are, however, lapped with a Class B lap splice, so there is continuity provided. I don't feel there is an anchorage/development issue.

2. I still feel like I have a proper load path... not sure what you mean by "poor slab performance".

3. The slab and beam spans are short, so there is not a lot of shear, and Vc/2 is satisfied mostly because of the width of the beam.

4. I can't do this because, as you can see from my last post this is, unforunately a done deal.