Corner reinforcement in slabs

[MonsieurR]

Hi all,

I'm currently reading ACI 314-11 (GUIDE TO SIMPLIFIED DESIGN FOR REINFORCED CONCRETE BUILDINGS) and it appears that point 7.3.8 recommends the use of corner reinforcement at the exterior suported slab corners for a distance equal to one-fifth of the longer clear span of the slab. The area of the reinforcement should be sufficient to resist a moment equal to the required positive flexural strength, per unit of width, in the slab panel, in accordance with 7.3.8.1 and 7.3.8.2.

Does anyone know the reason of this?
Why do we need it in first place?
Why does the distance need to be 1/5 of the span?
Why does the area of reinforcement need to be equal to the positive flexural strength?

Best regards.

 

[JAE]

Look at the ACI 318-11, section 13.3.6. The commentary there states:
"Unrestrained corners of two-way slabs tend to lift when loaded. If this lifting tendency is restrained by edge walls or beams, bending moments result in the slab. This section provides steel to resist these moments and control cracking...."

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[KootK]

Does anyone know the reason of this?
Why do we need it in first place?

Like JAE mentioned, tying the corner down results in some oddball bending moments. See below for crack patterns etc.

Why does the distance need to be 1/5 of the span?
Why does the area of reinforcement need to be equal to the positive flexural strength?

Based on elastic structural analyses of slab systems, that's what was determined to be approximately required for adequate performance. It compares reasonably to rebar quantities and bar extensions that you might get from coefficient slab design methods and the like.

I see this detailing in the typical details for slab and beam systems but, to be frank, I almost never see the recommendations followed to the letter for two way flat slab and flat plate designs. Usually, designers seem to run nominal top bars (#5 x4' @12" for example) around the edges of wall supported slabs and, where those bars turn corners, you get a two way mat that looks a bit like the code recommendation. Usually that mat is short and insufficiently dense however. I've not seen any detrimental consequences of this practice to date.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[dik]

If the slab flexes in two directions, there is a tendency for the corner to 'lift'; this is called a 'corner lever' condition. The added reinforcing addresses this tendency to uplift.

Dik

 

[KootK]

Another thing to keep in mind is that the whole setup is only as good as your slab is tied down. Lots of designers seem to be migrating to details that utilize top bars with just standard hooks rather than true bent corner bars down into the supporting walls.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[dik]

KootK... I try to keep the same reinforcing in both directions in the corners... with an 'isotropically' reinforced slab, the bending moment is the same regardless of the angle of the crack.

Dik

 

[KootK]

I do the same. In fact, I've never actually seen the 45 degree bars out in the wild. Retaining contractor respect pretty much makes it a no brainier I think.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[dik]

I have the 45deg bars for my standard details for stiffened slab edge foundation. Only place I've ever used them.

Dik