RC Slab

[breaking_point]

When designing a RC slab, it is typical to spec the same reinforcement top and bottom. When analyzing this section, do you assume the concrete contributes nothing (i.e. a neutral axis of 0) when analyzing the stress blocks as the compression steel would balance the tension steel? As such, the moment capacity would be the force in the steel multiplied by the level arm between the T and C steel?

 

[dik]

Not normally... depends on how much slab you have... if lots then you can optomise the reinforcing a bit... and, stagger bars (possibly over 2 spans). What type of slab are you looking at? one way over multiple supports?

Dik

 

[JAE]

When designing a RC slab, it is typical to spec the same reinforcement top and bottom.

No - not really - hardly ever.

The bottom steel is designed for the required positive moment and the top steel for the required negative moment at each end.

Concrete design in the US, as defined by ACI utilizes the reinforcement (tension resistance) and the concrete compression side (compression resistance) in tandem to resist the applied moment.

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[Mjkkb2]

Not here in the US where we use ACI design provisions. Typicality the concrete compression block with steel reinforcement tensile force is assumed to create the force couple. So the compression side doesn't require reinforcement. There are provisions for compression reinforcement but I have never done a slab where I needed it. For beams or other frexuraly stressed members with high bending moments it could be done but usually concrete is enough to carry the compression.

 

[rapt]

I might spec equal top and bottom steel with a small slab with lots of re-entrant corners etc where it is not worth trying to define a whole lot of different bar types and specific design and support strips. I did this on my own house slab a few years ago.

But you cannot make the design assumption you are talking about as the compression face steel will probably not be at yield or anywhere near it. In fact it might be in tension in a lightly reinforced slab.

 

[dik]

I might spec equal top and bottom steel with a small slab with lots of re-entrant corners

With any corners... due to flexure you can get 'corner levers' where the slab corners want to 'lift up'.

Dik

 

[breaking_point]

A couple of times on my internship I've had to design a simply supported slab over one span which I conservatively treated as one-way spanning. I calculated the amount of steel required in tension, and the senior engineer told me to specify the same steel top and bottom. When I thought about this and my internal moment resistance from my plane analysis, the compression steel would equal the tension steel and not require any concrete for horizontal equilibrium. Can anyone shed light on this?

 

[dik]

you maybe need something to keep the bars apart <G>. If simple one way slab with only supports at the end, other than for deflection, the top layer of reinforcing is not required.

Dik

 

[KootK]

When I thought about this and my internal moment resistance from my plane analysis, the compression steel would equal the tension steel and not require any concrete for horizontal equilibrium. Can anyone shed light on this?

While not an efficient way to detail a slab in many cases, I think that this logic is correct from the perspective of flexural strength. Utilizing the steel only should yield a lower bound moment capacity. One needs to ensure that the rebar detailing supports this approach of course. Any bar that is being utilized in compression needs to:

1) be anchored appropriately.

2) be spliced appropriately.

3) be prevented from buckling.

The third point is one that I've always wondered about. Compressed rebar in a column requires ties in order to prevent it from buckling. Obviously, your slab is unlikely to have such ties. Of course, anecdotally, slab reinforcement winds up inadvertently in compression all the time and I've never heard of it buckling and popping out of the slab. My guess is that slab compression rebar doesn't see the same kind of strain levels that column rebar does.




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.

 

[rapt]

Kootk,

You final sentence is answered by my last sentence. The compression strain in compression face reinforcement in a normal slab is very low. It would normally be near the neutral axis.

 

[Ingenuity]

I did this on my own house slab a few years ago.

Whaaaaat! You did not post-tensioning it?

 

[dik]

Whaaaaat! You did not post-tensioning it?

Ever try to tension a 6' strand? Span is so short, with anchorage losses...you almost have no stressing at all

Dik

 

[hokie66]

dik,
That was a bit of Aussie humour on Ingenuity's part. Those two know a bit about PT.

 

[dik]

Yup... knew that... Canadian humour thrown in...

Dik

 

[rapt]

It is a bit bigger than 6' dik! I need to be able to lay straight in bed.

Actually, it was initially designed as PT, but a combination of house builders not being used to it and perimeter reinforced core filled block walls and lots of sloping edges and being too far from a major city to make PT economical forced a change to RC. So the Concrete industry got to provide extra concrete instead. Neighbours think it is the local cyclone shelter.

PS humour not allowed on this site thank you. Please desist or I will have to red flag you, and myself.

 

[dik]

Rapt... I feel chastened... no confession this week...

Dik

 

[KootK]

You final sentence is answered by my last sentence.

I see it. Somehow, I just never connected those dots before. Thanks for that. In cases where yield strains would be more likely to materialize in flexural compression steel, are you then of the opinion that it needs to be restrained laterally like column bars?

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.

 

[rapt]

The strains in the compression steel would need to be pretty high to require lateral restraint.

The column rules assume the steel is at yield in compression, so the axial force in the bars is the full yield.

The Australian code does not require lateral restraint to be considered if the axial stress is below half of the yield stress.

I would assume in most normal slabs this would be the case. A very deep heavily reinforced slab might be different, not not your normal every day office/retail/parking/residential slab.

 

[dik]

The Australian code does not require lateral restraint to be considered if the axial stress is below half of the yield stress.

Makes a lot of sense... as long as you have more than 1/2" concrete cover, the rebar is restrained.

Dik