Strip footing / thickened edge slab 1

[mjl23]

Any suggestions on where to look for design examples/references to design a footing for residential construction? Standard construction around here is to use a thickened slab edge or strip footing. The typical strip footing you see in books has a symmetrically placed wall above the footing.

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Standard construction detail here is to have the outside face of the masonry wall line up with the outside face of the footing.

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To increase the bearing capacity the footing gets widened towards the interior side of the footing. I presume some some eccentricity gets applied to the footing in this situation. I have some ideas on how to do it, I just thought there might be some published examples to confirm my assumptions.

Thanks.

 

[csd72]

This depends 100% on local building practices. Where is the building?

 

[mjl23]

Florida. But, I wouldn't think the geography would matter for the analysis.

 

[DaveAtkins]

Here is how I solve this problem:
1. Find the linear wall load (klf or plf).
2. Assuming the soil under the slab edge is at its limit (e.g, 2000 psf), calculate the width (perpendicular to the slab edge) of slab edge that is required to balance the linear wall load.
3. Find the centroid of the soil pressure calculated in number 2 above. Calculate the eccentricity between this centroid and the linear wall load.
4. The moment that must be resisted by the rest of the slab-on-grade (inside of the thickened slab edge) is equal to the linear wall load times the eccentricity calculated in number 3 above. Remember to factor loads for concrete design.

DaveAtkins

 

[csd72]

mjl23,

This is an international forum.

Geograpy does matter as different soils are predominant in different areas and this effects the type of footings that are used.

I agree with daves post, though the slab may not need to go into bending if the footing is deep enough for a push/pull effect.

Another option is to step the footing on the inside face of the wall and continue it out past the outside edge of the wall- but this is a more expensive option.

csd

 

[UcfSE]

csd, the location does not matter. The OP gave you the type of footing. He or she did not ask if it is common or popular.

mj, look for a text on foundation engineering. Das and Coduto are two good examples that come to mind. In those, look for design of footings with eccentric loads. In your case, you'll need to find the load on the footing and its eccentricity. From there the text should give you enough information to put 2 and 2 together.

 

[mjl23]

Thanks. The stated methods was about what I was thinking. I was a bit surprised I couldn't find any examples specific to this. From some searching around I see where the detailing for this varies by building department (location of longitudinal bars in the thickened edge, presence of additional transverse bars in the perimeter).

Typically there's a dowel at the construction joint for transfer. Also typical seems to be to specify wire mesh in the slab and extend it towards the outside edge. Not so typical is the additional transverse reinforcing previously mentioned and the placement of the longitudinal bars varies. By 'typical' I'm referring to the various prescriptive building codes or literature.

While I was able to find several example detail drawings, I was unable to find any that had example calculations to back up these drawings.

Thanks again.

 

[UcfSE]

In FL, it is not typical to have transverse bars in a residential thickened slab, nor do you find a construction joint typically either.

 

[csd72]

UcfSE,

Have you ever heard of a raft slab? It has an edge detail that looks exactly the same as the section shown but works completely different. There are also variants where the lower section is poured first and then the upper section is poured later with a couple of dowel bars between.

US practices are often completely different to other parts of the world (and note the OP never mentioned the country).

csd

 

[mjl23]

By "construction joint" I was referring to the joint between the wall and the footing. I think it is quite common for there to be a dowel there.

I also realize that "In FL, it is not typical to have transverse bars in a residential thickened slab,", that's why I said "Not so typical is the additional transverse reinforcing". I guess that's why I was seeking out something analytically to show me why the code was requiring it since it seems more typical not to have this bar.

 

[UcfSE]

mjl, sometimes the building department requires things that have no real use or helpful function. That happens a lot with footing reinforcement from my experience.

Is this a cip wall you are talking about?

csd, have you used many raft slabs in residential construction? The OP told us what standard construction is in his area, so I don't see why the "where matters". We were given what we needed.

 

[csd72]

UcfSE,

No need to be difficult, if it was that clear then I would not have asked the question.

Raft slabs are the most common home foundation in many parts of Australia, so until I knew that it was in Florida I had a deeep suspicion that this was where it was.

mjl23,

I did not know if this was your first job or your 10000th job. Until I know otherwise, I assume that it is your first job, so apologies if I sounded condescending.

csd

 

[mjl23]

I appreciate all insight.

This particular project is in Florida and is a masonry wall (grout filled cells with rebar).

Thanks again.