reidential basement floor 4

[rittz]

My client who is also the owner-contractor has requested that no reinforcing be specified in the 4” sab in a residential 4-plex we are designing. (a first for me). Any comments about that?

 

[BUGGAR]

Without getting into all the technicalities, you could just have him write that into your engineering services agreement, and hopefully that will shield you if there are problems. But some lawyers will still nail you because ACI mentions non-reinforced concrete which they interpret as being engineered work just because it's in the code and you are responsible regardless.

We don't design to engineering principles anymore; we design to legal principles.

 

[jayrod12]

Here north of the 49 this is very common.

Besides the unsightly cracking and zero resistance to poor base prep there should be no reason it can't work

 

[SlideRuleEra]

If moisture or high water table is a concern NOT having steel reinforcement can be an advantage. By definition, a four inch slab cannot have adequate reinforcing steel cover - the slab is too thin. Steel corrosion can quickly become an issue.

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[steellion]

We have this come up from time to time. The best thing you can do is create a paper trail. Have written direction from the owner to use an unreinforced slab. Follow up with a memo that states that your recommendation is to use reinforcement to help control cracking, but you will design the slab as unreinforced at the owner's request as it is a serviceability issue, not a life safety issue. Then you're covered when they come back a few years from now complaining that their slab has cracks everywhere.

 

[KootK]

See below for ACI's recommendations. Slabs on grade are tricky because, while you can say what may be better or worse, it's almost impossible to guarantee a specific level of performance without being prohibitively conservative.

Like others have suggested, I think that the key is managing client expectations. I have this conversation with my clients early. Four times out of five they go cheap, and do not call me about subsequent cracks. The fifth time, they go expensive, and nobody harasses me about being a wasteful ninny.

Maybe you should blow your owner-contractor's mind and suggest one of these 3.5" slabs.

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.

 

[Lion06]

Or show the slab as reinforced with typical WWR and let him chose to omit it. Then it's clearly at his discretion and you've done your due diligence in showing the right thing in the drawings.

 

[XR250]

In these cases, I state, " 4" slab per code"

 

[ajk1]

Since you cannot achieve the code required 75 mm of cover to the rebar for concrete poured against soil, you cannot put rebar in a 4" thick slab and be in compliance with the intent of the Code unless you use stainless steel bars, which seems going rather overboard for a house.

A 4" unreinforced slab on grade that is not acting to support structural members above, should work fine if there is a proper compacted granular base, a competent native undisturbed soil with no soft areas, that has been inspected and approved by a geotechnical engineer, no water problem or uplift, not some very unusually heavy concentrated load, and the concrete mix is properly proportioned without too much water.

If the forgoing conditions are not met, then you are kidding yourself if you think rebar will make it ok. Keep in mind that all concrete cracks due to shrinkage, irrespective of whether it is reinforced or not.

A disadvantage of the rebar is that if you ever have to break thru the slab to get at leaking underground drain piping, as I have has to do, it will be more costly to do so because they will have to cut the rebar. I would not put rebar in, not because of the cost, but because it will be of only marginal benefit and will not meet Code for required cover, if the slab is 4".

 

[XR250]

@ajk1;

The slab is likely being placed on a vapor barrier on washed stone. As such, it is likely not considered cast against soil

 

[Signious]

@XR250

In my (limited) experience, the provisions for slabs against soil vs form are due to the roughness of the material. Placing bar 3" away from soil runs under the assumption that the mean distance between the soil and the bar will be 3", and the minimum 1.5". With this in mind unless the washed stone bed is very smooth, I would count it as against soil.

 

[jayrod12]

I've always had the same understanding as XR250. Otherwise you could barely get a 5" slab on grade to work for cover and bars in both directions.

 

[Splitrings]

I can see no reason to reinforce a 4" slab in a residential application. Reinforcing in this application will only keep cracks held tightly together. It will not prevent cracking. How wide can the cracks become? The slab is constrained on all sides. Simply cut control joints to control where the cracks form. If you are adding reinforcing to increase strength, it is much more economically to increase the slab thickness.

As ajk1 stated, provide a uniform subgrade and place concrete as dry as practical. The modulus of rupture is about 12-15% of the compressive strength of concrete. The wetter the concrete is placed the weaker the slab will be in flexure. Plus all the other disadvantages to wet concrete, like increased permeability.

 

[KootK]

I interpret the sacrificial inch thing the same as Signious does: as a means of accounting for non-level substrate. If anything, I would say that the vapor barrier helps by possibly allowing one to assume that the slab on grade is an interior element on the underside.

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.

 

[XR250]

The 3" separation from the soil is to prevent corrosion of the rebar. If you have a vapor barrier, that is not as much of a concern.

 

[KootK]

2" separation is intended to prevent corrosion of the rebar, just like in cases where there is a dirt/concrete interface but the concrete is not cast against the soil. I believe that the extra 1" is there to account for variations in the elevation of the substrate when concrete is cast against the soil. That's why, when we design a serious slab on grade with negative moments, we assume that bottom inch does not contribute to the concrete compression block.



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.

 

[boo1]

Kootk residential slabs in FL are 3-1/2"

 

[rittz]

Thanks to all for taking the time to put your thoughts to paper. It gives me some ammunition to form my rebuttals. Much appreciated.

Rittz