isolated wall footing vs thickened slab edge

[gte447f]

I have received a request for a change from an isolated wall footing to a thickened slab edge supporting a CMU wall. Both details are attached. The contractor states that they prefer to pour the slab before building the CMU wall because the slab will protect the subgrade from rain and equipment rutting during construction and will lessen the risk of subgrade remediation before pouring the slab. I prefer to isolate the slab from the wall and footing to lessen the risk of cracks in the slab that could result from possible differential settlement between the wall footing and the slab. I would appreciate folks sharing any relevant experiences and opinions.

 

[BUGGAR]

As well as reduce slab shrinkage due to the edge restraint of a monolithic pour. I agree with you.
What has the Contractor proposed to deal with these issues. Does the owner get a credit? Will the Contractor cover your time involved in this?
Does the wall depend on the slab to support it during construction? Did Contractor bid to brace it or did he miss that and is looking to recover.
Tell Contractor he is free to pour a subgrade mudslab or housekeeping slab if he desires tidy.

 

[Hokie93]

Is this a loadbearing CMU wall? If so, what is the magnitude of the load on the footing? Given the footing width (2'-0) in your original section, I am guessing it is either a non-loadbearing CMU wall or a lightly loaded CMU wall.

What bearing width is the contractor proposing for the bottom of the thickened footing? Graphically, it appears to be on the order of 1'-0. I would recommend making the width no less than 1'-6 or whatever width you require; you are in charge of the footing design, not the contractor.

The proposed revision raises the footing bearing elevation by 1'-0. Are you okay with that? Is the geotechnical engineer okay with that? What is the frost depth at the project site?

Assuming the footing is lightly loaded and the proposed change is coordinated with other disciplines and approved by the geotechnical engineer, I would tend to approve it.

 

[BAretired]

I agree that the slab should be isolated from the wall. If a concrete stem wall is used to support the masonry wall, the floor can be placed before building the wall. Exterior grade should be at least six inches below the underside of brick veneer.

BA

 

[gte447f]

Thanks for the input.

Buggar, I agree with you about the additional restraint for slab shrinkage and I like your option of a mudslab/housekeeping slab to keep things tidy. How would this be executed? Is the subgrade prepared and then a mudslab poured, and then later the final floor slab gets poured directly on top of the mudslab? Would this have any ramifications on the slab design, casting, finishing, etc. that I would need to address.

Hokie93, It is lightly loaded wall, less than 2000 plf. The proposed revision does not change the bearing area or depth. They are proposing to turn the slab edge down onto the strip footing shown in the detail. Have you had good luck with monolithic slabs supporting wall loads without any cracking from slab edge settlement?

BAretired, I like your concrete stem wall idea. I think I will suggest that as an acceptable alternative (maybe with Buggar's mudslab at the contractor's option). This is not really related, but I'm curious about your statement about the the brick veneer 6" above grade. What is that about? I get it for wood construction near grade, but for masonry? What would you do for example along a concrete sidewalk abutting a storefront. In my area it would not be normal to see a concrete brick ledge 6" above the sidewalk.

 

[BAretired]

gte447f,

In my area, it is normal to have the brick flashing and weep holes at the bottom of the brick veneer. In order to drain properly, they should be above grade. Usually, the brick flashing is at the bottom of the brick coursing and projects slightly beyond the outside face of stem wall. It is more of an architectural issue than structural.

BA

 

[gte447f]

BA, Is the attached stem wall alternative like what you had in mind to allow the slab to be poured before the masonry wall is built?

Also, I found a tech note from Meridian Brick that says through wall flashing at the bottom of walls should be placed 2" or 3" above finished grade, then goes on to say, in northern climates, the brickwork should be at least 6" above grade. Sounds like maybe a freeze thaw durability issue in northern climates? Makes sense to me. I'm in the south, and it doesn't seem to be on the radar down here.

 

[BAretired]

The stem wall allows the slab to be separated from the wall so that it can move relative to the wall. The slab may be poured at any time. Usually, the slab is poured after the wall is built. That way, the slab is protected from the weather while curing, but the sequence of operations is the contractor's call.

I'm in the north, so the bottom course of brick is typically 6" or 8" above exterior grade. Freeze/thaw is certainly a consideration for the foundation, but I didn't think it affected the elevation of brick coursing. A band of concrete under the brick looks okay to me but to each his own. I would think twice about setting the exterior grade at the same elevation as the floor, particularly if flooding is a probable occurrence.

BA

 

[gte447f]

Thanks for your feedback, BA. I appreciate it.