reinforcing for non-load bearing wall footings

[MikeE55]

A few years ago my engineering firm starting placing longitudinal reinforcing for non-load bearing wall footings in two layers - two bars in the bottom of the footing, and one bar in the top. I am referring to footings that are typically about 24" wide by about 12" thick, and technically are unreinforced concrete with temperature bars only. The purpose of placing the upper bar was to aid in spanning over soft spots in the soil, making sure the footing had at least some reinforcing for stress reversals. I am trying to remember if this is a Code requirement either in the IBC or ACI 318, etc, or if this was a result of attending a seminar where it was suggested. Does anyone else know if this is a Code requirement or just good engineering practice? Thanks.

 

[KootK]

Q1) What's the nature of the wall above? Wood? CFM? CMU? Concrete? I've assumed light frame.

Q2) Is your footing integral with the SOG or separate and beneath it? I've assumed integral.

1) I know of no code requirement for this.

2) In the absence of a stiff wall above the footing, the logical place for the rebar is probably in the top of the footing, where it would prevent visible cracking.

3) Despite #2, in my experience there are usually only bottom bars specified. I assume that's easier to chair or something like that.

 

[phamENG]

No code requirement, and I'm not even sure I'd call it good engineering practice. That single bar isn't going to do a whole lot. If you have legitimate concerns about 'soft spots' then it would be more beneficial to have a geotechnical engineer observe footing excavations and ensure proper removal, backfill, and compaction of those soft spots.

 

[MikeE55]

Kootk, The wall above the footing is typically masonry - usually a brick veneer with a 8" block backup. It is not integral with the slab. In the past I have seen engineers place two bars either 3" clear from the bottom or 2" clear from the top - since it is temperature reinforcing it is okay to do either, but maybe makes more sense in the top. To me, it is a better detail to use a couple of bars in the bottom, where you are accustomed to seeing rebar in a footing, and then to place an additional bar in the top. This additional bar can be used to brace dowels that will extend up into the masonry. I think it is particularly useful for masonry because if you get a crack in the top of the footing it can propagate up into the masonry. Having a bar near the top keeps the crack from opening up very wide, and helps to protect the masonry.

I am trying to remember where I heard this explained - it may have been at a masonry seminar. phamENG, we always have a geotech on these projects, and that certainly helps, but it is no guarantee that you will not encounter a problem.

Sorry, I am not trying to get someone to do research for me - I was just hoping someone would have had the same experience I did and might remember the origin of putting the bars in both top and bottom.

 

[KootK]

Sorry, I am not trying to get someone to do research for me - I was just hoping someone would have had the same experience I did and might remember the origin of putting the bars in both top and bottom.

That's kind of you but quite unnecessary. If everybody felt compelled to research the snot out of everything before posting, there would be little point to this space. Most of my threads are -- at least partially -- about saving myself some research time.

The wall above the footing is typically masonry - usually a brick veneer with a 8" block backup. It is not integral with the slab.

That's pretty much the Cadillac. Truly, in that situation, I think that you'd be just fine with no rebar at all, top or bottom. The wall is the spanning element. I'd still do bottom though.

I think it is particularly useful for masonry because if you get a crack in the top of the footing it can propagate up into the masonry.

I think that's the job of a strategically placed bond beam at the bottom of the wall. 2-#5 etc, just like concrete walls cast against hard concrete.

 

[MikeE55]

Yes, if you have a concrete wall above the footing then there is no reason to use the third bar in the top of the footing. Also true if you have horizontal bars in the masonry, but typically, we use horizontal masonry type reinforcing only. I haven't had any issues from a footing since I started using the third bar - now if I could just remember who gets the credit for that idea.

 

[jerseyshore]

Rarely do I see footings for a non-bearing wall. Usually we specify haunches integral with the slab with a couple of bottom bars.