Minimum T&S Reinforcement - ACI 350-01 vs ACI 318

[bootlegend]

I'm noticing the below from ACI350-01 section 7.12.2.1 posted below. (Are excerpts allowed?)

Concrete sections that are at least 24 in. may have the minimum shrinkage and temperature reinforcement based on a 12 in. concrete layer at each face. The reinforcement in the bottom of base slabs supported
on soil may be reduced to 50 percent of that required in Table 7.12.2.1.

Suppose I have a 48" thick mat. Traditional ACI318 T&S reinforcement would be .0018*48"*12" = 1.037in2/ft.

ACI350-01 suggests that I could consider only the top 12" since the section is greater than 24" thick. I would end up with .005*12"*12"=.72in2/ft as a worst case if joints are spaced far apart.

Am I interpreting this correctly? ACI350-01 is intended for environmental engineering concrete structures so I expected more stringent requirements. This is a pretty significant difference for thicker mats.

 

[bones206]

My interpretation is that 0.72 in²/ft reinforcement is required on each face of the 24" thick section.

 

[JedClampett]

We limit our temperature and shrinkage reinforcement to #8's at 12 inches T & B for thick slabs on grade.

 

[bootlegend]

My interpretation is that 0.72 in2/ft reinforcement is required on each face of the 24" thick section.

I agree, but with a 48" thick mat you would need 1.037 in2/ft on each face, assuming both faces see tension. This is more than the 0.72in2/ft required by ACI350-01.

We limit our temperature and shrinkage reinforcement to #8's at 12 inches T & B for thick slabs on grade.

Is their a reference or basis for this?

 

[hokie66]

Your 0.72 is only for the top half.

 

[bones206]

I believe the 1.037 in²/ft would be distributed on each face of the 48" thick section using ACI 318 (0.52 in²/ft on each face).

Using ACI 350, you end up needing an additional 0.20 in²/ft on each face, based on worst case joint spacing.

 

[bootlegend]

Your 0.72 is only for the top half.

The 1.037 in2/ft is for only the top face as well if you stick with the minimums in ACI318-14. 7.6.1.1 calls it minimum flexural reinforcement and notes that T&S steel can be split among both faces but explicitly states that you need .0018 at each face if that face sees tension. So you can still end up with significantly less reinforcement if you use ACI 350 for design.

 

[JedClampett]

ACI 350-89 Section 2.5.

 

[bones206]

Codes manage to be so confusing about the most basic requirements sometimes. So ACI 350 Section 10.5.4 says:

For structural slabs, mats, footings, and walls of uniform thickness, the minimum area of tensile reinforcement in the direction of the span as required by 7.12. Maximum spacing of this reinforcement shall not exceed the lesser of three times the slab thickness, or 12 in.

In a roundabout way, they are inferring that the minimum area of reinforcement on the tension face of a one-way flexural member is equal to the total area of T&S reinforcement required by 7.12. Hopefully this closes the loop for you.