Slab on Grade - 0.5% steel positioning 2

[cec17]

I am working on a jointless mat foundation with an exposed top face, and trying to figure out steel placement.

ACI 360-10 only makes reference to 0.5% steel "by cross-sectional area" once in a jointless slab, as far as I can tell. They don't seem to give guidance if this steel is supposed to be placed at the top only, or if it can be distributed between both faces.

ACI 224 goes on to recommend 0.6% and also doesn't seem to give guidance on placement.

This Concrete Construction mag article (Link) says to use 0.5% to 0.6% "near the top of the slab". It makes sense that it would be required at the top, where cracks propagate. My concern becomes when slabs are a bit more thick (24") for anchor rod embedment. We cant feasibly provide 0.5% in the top mat for a 24" thick slab (#7 @ 5" ??). I am debating providing 0.5% by cross sectional area, but using something like #5 @ 12" for the bottom mat, and making up the difference with bars spaced 8 - 10" OC at the top side.

Is there any published ACI language, not an article, that says where the 0.5% steel goes?

 

[OldDawgNewTricks]

Is it a nonstructural slab-on-ground as implied by the post title or is it a structural mat foundation as stated in the first sentence of the post?

If it is a mat foundation, then ACI 318 13.3.4.4 would require reinforcement located close to each tension face (presumably both top and bottom faces will experience tension). The amount of reinforcement at each face should be the greater of the required structural reinforcement or the temperature/shrinkage reinforcement. If T&S controls, it is common practice to either provide equal amounts at both faces or 2/3 of the reinforcement at the top and 1/3 at the bottom.

 

[hokie66]

If it is a mat foundation, I would use half top, half bottom.

But for a general slab on ground, the only force which causes shrinkage cracking is friction with the subgrade. So cracks most likely propagate from the bottom.

 

[human909]

There are three big unanswered questions here:
-Why is your slab/matt so thick?
-Is crack control beyond T&S minimums requirements required? Would some surface cracking be a big client concern?
-Are you required to meet minimum tension reinforcement requirements. (if so this will likely dominate as you'll have 2 faces and 2 directions of tension)


I often find myself stuck in situations like this as I commonly design matt foundations for light, slender structures with tightly spaced columns. Wind over turning dominates and 500mm-800mm thick foundations are often required when 250mm might do for strength requirements. I've seen codes interpreted in different ways on whether the minimum tension reinforcement requirements need to be met in such circumstances. While in the past I've met minimum tension reinforcement, partly because of guidance from more experienced engineers. However I'm looking for future design to take advantage of clauses that allow to no meet minimum tension requirements. If the reasons for such a thick footing is simply overturning then brittle failure of the sort minimum tension steel aims to avoid is not of concern.

I haven't referenced any codes as I don't deal with ACI. But generally AS isn't grossly different from ACI as far as I'm aware.

 

[Smoulder]

@cec17 ACI 350 gives 0.375% with 2/3 in top and 1/3 at bottom. This comes from 0.5% base requirement but half of the bottom discounted for slab on ground.

@human909 You can go below minimum for footings if brittle failure doesn't cause collapse. You'll have to judge if your case falls under that relaxation.

 

[EngDM]

My concern becomes when slabs are a bit more thick (24") for anchor rod embedment. We cant feasibly provide 0.5% in the top mat for a 24" thick slab (#7 @ 5" ??)

I'd be treating this area more like a top mat for a drop panel, whereby you increase the reinforcing steel in the thick area only, in addition to your slab mat. You'll may need some flexural reinforcement anyways to engage enough of the concrete slab depending on your anchor rod uplift.

 

[canwesteng]

ACI 350 also lets you reinforce just the outer 6" as well for big blocks of concrete. There is also nothing stopping you from putting #7@5", it's more than feasible. They are mats with bigger bars at 5",6", and those have bars at the bottom too.