Minimum tie bar spacing in trench footing

[MTLoeb]

I wonder if anyone has a clear reference for the tie bar requirements in a trench footing. For this example lets say we have a 2'-0" deep footing thats 1'-8" wide. Cast against dirt so 3" cover, and (3) #4 bars top and bottom with ties at 16" o.c....Oh No! the clear spacing of our longitudenal bars is 6.3 inches. Thats greater than the 6" allowed per ACI 318-14 chapter 25. Now I lean a direction wich im trying not to put out here, but the tie bar requirements need additional support past 6" clear cover.... but we are only providing this much rebar because of minimums required by the area of concrete. It's not like we need this trench footing to be this wide the whole depth, this is just commom construction practice in some area. you could say, why reinforce concrete that is not structurally needed, or maybe that chapter 25 references a research paper about columns for that section, so it doesnt apply to us....well, what do you do typically if you have this situation, is another vertical #3 tie needed?
Thanks

 

[hokie66]

I think what you have is a typical strip footing, not a beam. If that is the case, the ties serve as support bars, not as stirrups. So I see no technical reason for closer spacing, but then sometimes codes are not logical.

 

[MTLoeb]

Great, Thanks Hokie. techical reasons aside, would you put one in for support? Part of this is certainly constructability, and I've done some framing but I cant say I've poured a footing before, so i really dont know if the #4 bar up there spanning ~12" is a little to wobbly for concrete pouring.

 

[hokie66]

No, that would be fine.

 

[Tomfh]

12” is nothing. If it’s only for support then you can go 3-4 times that.

 

[MTLoeb]

is there any code reference I can point to? the one in chapter 25 loosely points to needing it, although it doesnt seem neccesary. (25 implies columns, but not very well)

 

[StrEng007]

While I agree with most of what you are saying, regarding the ties for support only... it would all go back to the purpose or function of the foundation.

From my experience, a trench footing that has somewhat uniform loading from a wall could be designed conceptually as a continuous wall footing (i.e., transverse bottom bars carry the primary flexure and shear, while longitudinal bars are there for safe practice of any settlement or transition of loading at door openings).

The type of trench footing that I can imagine you're referring to would look like the haunch style footings under a PEMB. For those footings, they often experience heavily concentrated loads for gravity and/or uplift. If that's the case, flexure and shear occur along the length of the footing and the ties (stirrups) would indeed play a critical role in the strength of the foundation. This would be subjected to the design concept behind a beam on elastic foundation.

 

[Smoulder]

(20" - 6" cover - 1" perimeter ties - 1.5" longitudinal bars) / 2 gaps < 6" clear.

If I did the maths wrong, 3.5" cover?