Plain concrete footing design 3

[Anaximandar]

Does anyone have an example of plain concrete wall footing design? I was going through ACI 318-05 and it doesn't seem that min. reinf. provision applies to footings. If my footing is 12" thick and 24" wide with 12" pier on top - shouldn't arch effect create compression and nearly eliminate tension at bottom allowing design for bending without reinforcement? I know I still have to put longitudinal reinf. bar at least at top of pier and bottom of footing. What about temperature and shrinkage reinf?
Thanks

 

[WillisV]

Chapter 22 of ACI-05 covers structural plain concrete.

 

[CTW]

I don't have ACI 318-05 near me, but if my memory serves me correctly, the minimum steel in footings is permitted to be taken as that required for temp/shrinkage.

 

[miecz]

I don't believe that ACI requires temperature and shrinkage reinforcing in a plain concrete footing.

 

[swivel63]

how much more expensive is it to just throw some minimum steel top and bottom in it?

 

[mitchelon]

If it is plain concrete, it does not require reinforcement and it is designed as such. It is cover in ACI Chp22. Your plain concrete footing is basically going to be thicker. To give you an example taken from a PCA handbook, same column, same loads, same soil, same concrete – reinforced footing 9’-10” square and 2’-3” thick; plain (unreinforced) footing 9’-10” square and 3’-9” thick.

 

[Anaximandar]

Thank you for your replies, it makes a nice discussion. I finished the design using ch22, ch15 and ch14 of ACI, which, by the way doesn't specifically address unreinforced footings. Temp. and shrinkage reinf. is also not covered for footings and foundation walls. It is interesting how we are conditioned (through education, mostly) to avoid plain concrete. FYI, according to ch22 of ACI 12” thick footing can take 4.1 kip-ft moment without any bars (Mn=5*(f’c)^0.5*(Section modulus)). Again thank you for your comments.

 

[miecz]

A little history might help. As I remember it, ACI318 used to be titled "Building Code Requirements for Reinforced Concrete." Plain concrete was covered by a separate publication. I believe it was with the 1999 revision that plain concrete was incorporated into ACI 318 and the title was changed. Chapter 14, a carryover from the earlier code, is meant to apply to reinforced concrete. Think of Chaper 22 as a stand alone entity and the rest of ACI 318 as a reinforced concrete code.

 

[WillisV]

Also note that some local building codes have overriding provisions requiring say #4@48 carry bars and minimum 0.0018 type longitudinal steel.

 

[Lion06]

Anaximandar,

I assume you mean 4.1 k-ft per ft of width, with 3000 psi concrete? Also, I know you don't mention it, but your calc bears out that you are ignoring the bottom (2) " of concrete per 22.4.8.
This seems like such a small moment. I know I learned this stuff in school, but has anyone ever really designed (and used) a plain concrete footing?

 

[Anaximandar]

Thank you, jmiec, I did not know that, as I started practice with 99 code.
WillisV, I do know some local codes have certain overrides (such as Chicago Building Code limits lintel deflections to L/600 or 0.3in, whichever is greater), but I couldn't find any in IBC 2003 (adopted code by the municipality with jurisdiction) regarding reinforcement in footings.
MYerges; yes, it is per foot of width and I reduced the section per 22.4.8. Moment of 4.1 kip-ft is the ultimate moment using 0.9 reduction factor for 4000 psi concrete, since tension in concrete governs. However, since I had room to play - I used 0.65 factor to obtain 3.4 kip-ft, because it could be argued that without reinforcement a brittle failure occurs, therefore requiring more reduction.
I think I will get an answer to this question, because I issued the drawings and calcs for city review. Final design was; plain concrete in footing (12" thick), but temperature and shrinkage reinf in foundation wall (13" thick). foundation bears @ 42" depth, so foundation wall is only 2.5 feet high. Thank you all for discussion, if i get comments from the city reviewer soon - I'll post them.

 

[Anaximandar]

Sorry, that should've been "whichever is smaller" in lintel deflection limit.

 

[WillisV]

Anaximandar - agreed nothing in IBC - I was speaking more of county level building codes which sometime adopt the IBC with a few minor "revisions". One of which I have seen is being a minimum number of bars in footings. I typically don't rely on plain concrete as it is fairly easy to just hook the dowels for the wall or pier reinforcing out into the foundation anyway. But hey if it works it works - thats why I mentioned Ch 22 in the first place.

 

[dik]

The ACI design handbook for the 1971 code has a formula:
kd = 2.53*SQRT(qs/SQRT(f'c))

and the thickness of the footing:
t=kd*overhang

The design manual also has a table of kd coefficients. The reference may predate 1971...

Dik

 

[ash060]

For plain concrete footings go to the PCA Notes and there is a chart in the plain concrete section for footings. All you need is the factored bearing pressure and you are in business. Also for example a 2' wide footing with a 8" bearing wall on it only has 5" to develop a bar which is very hard to do, so you will be using plain concrete anyway. (24"/2 - 3" Cover - 4" (half wall width) = 5") This number will be a little different for masonry, but you get the idea. Plain concrete is good.

 

[LowLax]

The office I work at consistently designs wall footings without any rebar. We put 2 bars at the top of the wall and 2 at the bottom, creating a deep beam. We assume the load is going to transfer at a 45 degree angle through the footing and into the soil.

We've gotten a few calls from inspectors that are concerned we made a mistake. The design makes logical sense to me. I only check it if I have a project with bad soil and my wall footings need to be wide. We always reinforce spread footing however.

 

[COEngineeer]

Same here, we do what LowLax does. Especially on footings that is burried only to get frost depth (3-4 ft), there is no reason to put steel in the footing. The only steel we put is the bent dowell @ 48 into the wall.

Sometimes for taller wall (for basement) I still use no reinforcement in the footing because I assume the slab in the basement is going to restrick the bottom of the wall so that it will act like it was pinned at the bottom. So I design the basement wall like if it were pinned at the bottom (instead of fixed in most cases).

 

[miecz]

Same here, 2 bars, top and bottom of the the wall, plain concrete footing. Interesting, I continually have to explain the theory to other PEs.

If enough engineers throw reinforcing in the footings because it's cheap, then building inspectors and other engineers start believing it's a requirement.

 

[Taro]

The "arch effect" does not eliminate tension in the bottom of the footing. If you draw a strut and tie model of the load path, you will see that the inclined compression struts at each side of the footing create a net tension across the bottom of the footing. For equilibrium, this force must be resisted by the concrete in tension, passive resistance/friction from the soil, or rebar.

 

[Anaximandar]

Taro, you are right (off course). When I said "nearly eliminates" (should've said "significantly reduces"), I was referring to tension that needs to be taken by reinf. bars. Remaining tension is small and can be taken by the concrete, which was the point of the entire discussion here. We seem to assume that concrete can take NO tension, which is not true. ACI allows design of plain concrete structural elements.
Strut and tie model is a very rough approximation and is ONLY valid after the concrete cracks and stresses are redistributed. At low stresses (i.e. less than modulus of rupture) strain within the D-region (D-region is within the distance equal to height of member) such as brackets or footings where cantilevered part is equal or less to the footing thickness, St. Venant's principle is NOT valid, plains DO NOT remain plane and strain DOES NOT vary linearly with depth. Therefore, Final Element Analysis is required to find actual stresses. Assumption that strain function is linear is conservative, and, if stresses obtained by this linear analysis are less than allowable tensile stress in concrete - we do not need reinforcement.
Sorry for long reply. This is starting to turn into discussion on how much of theory, computational accuracy and computer aid we should use in everyday practice. We should start a new thread.