Bearing wall and column footing - thickened slab or strip/spread fooing 3

[xyyy]

I am designing a light framed two story office building. The walls are light gauged steel stud wall with sheathing. The floor is TJI joists and wood beams. The roof is wood truss with sheathing. The area at each level is about 50'x80'.

There are bearing walls at both exterior and interior. The first level is slab on grade. My question is about the wall and column footing. Should the thickened slab be used as bearing wall and column footing? Or should strip footing and spread footing be used? What I mean of strip footing and spread footing are that the stem wall and pedestal of strip and spread footing are separated from slab on grade by pre-molded joint filler. The allowable soil pressure is 1500 psf. The maximum load on interior bearing wall is 3.8 klf and the load on exterior bearing wall is 2.8 klf. The columns are 6x6. The loads on columns are 18 kips.

This building is located in tropical area. So ground freeze is not a consideration.

Your help is appreciated.

 

[mijowe]

Occasionally I will get a geotechnical report will specifically indicates that the slab on grade should be separate from the foundations. So in that condition I always do. The reason for this is settlement and possible cracking of the slab on grade as a result. if you isolate the two, whether the geotech requires it or not, you can reduce that risk.

 

[a2mfk]

I do a lot of forensic investigations so I see a lot of problems with buildings due to slab and foundation settlement, and slab shrinkage. Though what you describe seems like pretty light wall loads, the two biggest advantages of separating your SOG from your foundation is it allows the slab to shrink away from the foundation with less chance of cracking, and it allows your foundation to move without affecting the slab.

But here in Florida, it is very common to do monolithic foundations on single story or lightly loaded two story structures, especially with foundations.

If you choose monolithic slab-foundations, then I would isolate the columns on pad foundations. But if you do a strip foundation, then I would combine them.

 

[xyyy]

mijowe and a2mfk, thank you for your quick responses. Just get an email from geotechnical engineer saying that "Some settlements can be expected for the footings. Perhaps, as a precautionary measure, footings for bearing walls and columns should not be poured monolithically with slabs on grade for this project.".

I get question about the detail. Attached is details I drew for wall and column footings. I feel there is construction problem because I remember some body said that contractor can not make the stem wall and pedestal and top of slab on grade flush. Any suggestion on that? How should I fix these details? The detached footing details I can find are all for concrete or masonry walls. But the situation I mention will not happened to concrete or masonry wall or column footings. Thanks alot.

 

[AELLC]

See attached -

 

[xyyy]

AELLC, Thank you

 

[AELLC]

Your are welcome -

BTW as an afterthought I am sure the builder would much prefer a post-installed column base such as Simpson ABA66Z

 

[a2mfk]

That is the CYA note that every geotechnical engineer puts in their reports. Of course all soil other than bedrock will compress under loading and thus settlement is going to occur.

Details- I prefer the column/post detail of XXYY, very standard and the way I have always done steel columns is similar. You want to isolate the column and foundation from the slab for settlement reasons, and for slab shrinkage reasons. If you can make your pedestal small enough, the architect can also incorporate a decorative column cover that will hide your joints in the slab. This will prevent future flooring problems with movement at the isolation joint.

If you have high wind uplift, I would prefer a base post connection with an all-thread rod that will extend all the way into your footing. This eliminates the load transfer into the pedestal and then into the foundation. Now assuming you embed the anchor rod 9" into a 12" foundation, you have much higher shear cone values.

 

[xyyy]

The formal geotechnical report came. It is only state that "based on column load of 21.1 k and wall load of 3.16 klf, settlements of on the order of 3/8 inch were computed. Differential settlement is not expected to exceed 1/4 inch". There is NOTHING saying that the slab on grade should be separate from the foundations.

Most of the columns are actually at the end and also part of the walls. There are only two columns are individual columns. The standard column footing detail can be used to separate column footing from SOG. But for columns with walls together it seems quite complicated. I have two wall thickness, 6" and 3 1/2". Also the loads on the 3 1/2" thick wall is no more than 1680 plf which is quite small.

I also get question regarding the difference between thickened slab for wall and column footing and having strip footing and spread footing 1' under top of SOG. I saw the thickened slab footing are commonly used for this type of light frame construction.

I also saw two types of details for connection between SOG and CMU or concrete walls. One type is the is joint filler to separate walls from SOG. And the other type is the is steel rebar dowel connecting walls and SOG. When to use which one?

Thanks alot for all your responses.

 

[a2mfk]

The geotechnical engineer is not going to tell you how to design your slab and foundations.

There are dozens of ways to detail every aspect of a building depending on the material, loading, architectural constraints, etc. You are asking good questions here, but this is where the guidance of an experienced mentor who is familiar with your local practices can help you.