Minimum Area of Steel required for Spread Footings

[imsengr]

Hi everyone,

I'm designing isolated spread footings for an industrial plant and my research and discussions with colleagues are giving me differing answers to the minimum amount of steel area required for footing.

Group I says the area of steel must not be less than the temperature or shrinkage steel, i.e. rho, or the reinforcemet ratio, must be greater than 0.0018, and as long as it is that, the steel area is ok. In fact, in the footing examples provided in Notes on ACI 318-89 (Pg. 24-11) and ACI 318-95 (Pg. 371), this appears to the cases.

Group II says that the footing is a flexural member, and thus, must meet the minimum reinforcement for flexural members. The old requirement was 200/fy, but the newer ACI Codes called for not less than:

3*SQRT (f'c)/fy * bw * d, and not less than:

200bwd/fy

Now, which I should I follow? Advice from structural engineers who work a lot with foundations, and anyone else, is most welcome.


Thanks, y'all.

 

[structuralaggie]

It is a flexural member, so it should be desinged as one. The equation for As min is correct (10-3).

However, look down at 10.5.3. If As provided is greater than 1.33 As req., you don't have to use equation 10-3.

Unless you have a big spread footing, the temperature and shinkage will control, since bending stresses will be low.

 

[struktur]

It depends on the thickness and situation. Basically the min reinforcement was determined so that capacity with reinforcement will at least equal with plain concrete.

 

[imsengr]

My spread footings do not need to be really big as they are seated on weathere limestone with a bearing capacity of 100 KSF. I am using 8' square footings. Some of my tension loads are significant - about 400 Kips. As structuralaggie said, my bending stresses are not really large, and temperature and shrinkage steel do control. I was wondering if this is what I should use, or should I use the min. reinforcement for flexural members.

However, as also pointed out, ACI 318-05 provides an "out" in Section 10.5.3 in that if the steel provided is 1/3 larger than that obtained by analysis, then I do not need to satisfy Section 10.5.1 and 10.5.2, which would otherwise more than double the steel area that I would need!

The temperature steel requirement is more than 1/3 the steel required as obtained by analsis so this section applies in my case.

 

[structuralaggie]

You are conforming to the flexural requirments if 10.5.3 is met. This also comes into play when for grade beams. Often they are deeper, like 3', for other reasons than strength. The contractor will be laughing at you if you spec the minimum steel based on 10-3 for something like this.

For spread or strip footings the min. temp. reinf. will control a lot of the time.

 

[UcfSE]

I would use the minimum as specified for footings and slabs, or provide 1/3 more than the calculated requirement.

 

[Taro]

Group I is correct. See ACI 318 section 10.5.4.

 

[DaveAtkins]

And keep in mind that the 0.0018 is not really rho (which is multiplied by b by d). The 0.0018 is multiplied by b by h.

DaveAtkins

 

[imsengr]

Thanks for all the responses, guys. Keep 'em coming.

DaveAtkins, thanks for the note about multiplying tne 0.0018 with h (overall height (thickness) of footing, rather than d (eff. footing depth (to steel layer)). Could you please provide the ACI reference to this?

I noted in Section 7.12.2.1 of the current ACI that "... area of shrinkage and temperature reinforcement shall provide at least the following ratios of reinforcement area to gross concrete area, but not less than 0.0014. (b) slabs where Grade 60 deformed bars or welded wire reinforcement ar usesd = 0.0018 .." Doesn't this mean that we can use the 0.0018 as we would rho? Hope you can assist.

Thanks, y'all.

 

[civilperson]

An alternative to reinforced concrete is Structural Plain Concrete for footings, using tensile strength of concrete as 0.55 x 5 (square root of f'c). No reinforcement is required.

 

[DaveAtkins]

Since ACI 7.12.2.1 states that 0.0018 is to be multiplied by "gross" concrete area, it must be multiplied by b and h, not by b and d.

DaveAtkins