Mat Foundation Reinforcement Spacing

[BadgerPE]

Hey all,

Before I get started I am just going to say that this post is not an attempt to open the 0.0018bh vs 4/3As reqd vs 200/fy debates that have been posted before.

By analysis, I need 0.0018(12in)(18in)=0.39in^2/ft for reinforcing both faces of an 18" mat foundation. That works out to be #7 bars at 18" o.c. which meets the requirements of ACI 318-08. My question is, would it be better practice to use the #7 bars at 18" o.c., or use #6 bars at 12" o.c. which would increase the qty of steel reinforcing by about 10%?

Just curious what others feel is a better method to use. I have several of these footings and a 10% savings in steel might be worth it, but not at the expense of structural integrity.

 

[Teguci]

I would even go as far as increasing the amount of steel by 10% in order to get the 18" spacing purely from a construcatbility/cost point of view. As for structural integrity, you are talking about a mat foundation and not a 4" thick slab on grade. Although the commentary doesn't delve into the reasoning, I would guess that the 18" spacing is both a bookend requirement (limits testing results to 18" rebar spacing) and a servicability issue for exposed elements (small poorly distributed shrinkage cracking between rebars spaced more than 18" apart). Since a 3.6" thick slab can be designed with rebars at 18" on center and shrinkage cracking in a buried mat slab is a non-issue (add exceptions here) I don't think an 18" spacing in an 18" slab is anything to get concerned about (unless you're designing a bank vault or a hardened military asset).

 

[JAE]

The only other aspect is the #7 bar requires longer development lengths/splices so the edges of the mat might have to extend past piers/piles to get developed - but I'll leave that to you.

Also...glad you use the correct amount in the mat (i.e. 0.0018 on each face) assuming you have moment in both + and - directions on the mat.

 

[DST148]

In general I agree with others.
We typically restrict spacing to 12" or less, especially for top reinforcing in mat foundations. In most cases, the mat (soil supported or pile supported) also served as the top of the cellar / basement and was not buried in the ground.Therefore, we use smaller bar sizes and restrict the spacing of the top reinforcing from crack point of view. Also with a rebar spacing at 12" or less, it makes walking over the rebar-mat a bit easier during inspections and concrete placement.

 

[dik]

For an 18" thick slab, I'd go with the #7's... fewer bars to place and less, but adequate, reinforcing steel area...

Dik

 

[dik]

Should have added that there is more 'foot' space for stepping through the bars in placing...

Dik

 

[hokie66]

A vote for the 12" spacing, strictly for constructability. The concreters (and hopefully the inspector) have to walk on the top mat, and 12" spacing is more easily negotiated without falling through the holes.

 

[FixedEarth]

I have only done few mat foundations. However, I remember first checking the needed Area of steel for maximum positive and negative moments. Then comparing to see if I met the minimum A(s)requirement. Did the design procedures change? Thanks.

 

[slickdeals]

Regarding walking on a mat, I have read/seen WWR mesh placed over the top mat to make walking easier. If using bars at 12" o.c. adds more cost, I think it might be easier to use larger bars at 18" o.c. and providing a WWR mesh on top. It will also help with shrinkage cracks on the top surface.

More good information can be found in the Concrete International article

http://www.concrete.org/PUBS/JOURNALS/OLJDetails.asp?ID=51683594

It’s no trick to get the answers when you have all the data. The trick is to get the answers when you only have half the data and half that is wrong and you don’t know which half - LORD KELVIN

 

[hokie66]

I think adding mesh on top of the bars is an unnecessary and possibly detrimental step. If the mesh is light, maybe the kind that comes in rolls, it is almost impossible to keep it flat, so it may compromise cover. I don't see how it would help with shrinkage cracks.

 

[DST148]

For a 48" thick mat, we did provide 6 x 6 WWF over top reinforcing. The clear cover to top reinforcing was kept at 2". The mesh was not counted upon for any structural purposes. It was project manager who insisted on using WWF for 48" thick mat.

 

[slickdeals]

This is from the CRSI article:

"It’s common practice to place sheets of welded wire reinforcement (WWR) between the two layers of reinforcing steel within the top mat. The WWR will allow laborers to walk on the mat before and during concrete placement (when the top bars will be buried in the concrete), preventing them from falling through the mat. The WWR is sacrificial and is not usually considered in the structural design computations."

It’s no trick to get the answers when you have all the data. The trick is to get the answers when you only have half the data and half that is wrong and you don’t know which half - LORD KELVIN

 

[Teguci]

So if you have WWR, why not use it structurally? And, if you have WWR, use #8 @ 2'-0" oc or #9 at 2'-6"oc and use the WWR for distribution of shrinkage cracking for exposed tops of foundations. The stiffer bars will reduce the number of required chairs for the top layer of rebar.

 

[thoughtofthis]

I can see it both ways, but it just means additional material costs and more bars to tie. Is 12" that much easier to walk on? What is the bottom bar spacing? The article emphasizes aligning top and bottom bars as much as possible.

Also, the WWR placed in the article refers to a dual layer of top reinforcing, which could hold the WRR in place. With only a single layer, the WWR will be wavy and could effect cover.

 

[hokie66]

As stated in the CRSI article quoted by slickdeals, the mesh is between the orthogonal layers. There are not two layers in each direction. This would cancel my question about keeping the mesh in place during construction, but I still think it unnecessary. Having walked on thousands of reinforcement mats, I think 12" is reasonable.

 

[faisal45]

I think 12" is reasonable. development length of #6 is lesser than
#7. and ofcourse it will be easier to walk.

Thank you.

 

[asixth]

I prefer to space reinforcement no greater than 12" for crack control. More reo tonnage equals more cost (both material and labour) but that is just my preference and is what is typically done in my local industry. Not saying that it is the best way of doing things.