Temp and Shrinkage Reinforcing Distribution

[marinaman]

How do you guys distribute temperature and shrinkage reinforcment in thick machine slabs?

For example, I've got a machine slab that is about 60' long, 2' thick, and about 12' wide. I've completed all the analysis for the loads from the machine on to the mat foundation, including the use of soil perameters from the soils report we developed specifically for this machine foundation.

ACI Chapter 7 states that my minimum temp and shrinkage reinforcment should be 0.0018 times the area....or...about (24") x (12") x (0.0018) for about 0.5184 square inches per foot width.....each direction.

Per analysis (not including temp and shrink), I've determined I need about (1) #6 bar at 12" o.c., each direction, each face. That gives me about 0.44 square inches of steel, each direction, each face.....and....about 0.88 square inches per foot width.

My question is.....does ACI chapter 7's minimum reinforcing steel number of about 0.5184 square inches per foot width mean that I need that much per face of slab......or just I need that much steel across the section (in other words, I could've used, for temp and shrinkage, (1) #5 bar in each face and met the temp and shrinkage requirement per (2) x (0.31 square inches) = 0.62 square inches.

Just trying to be sure I don't need 0.5184 square inches per face (top AND bottom) each direction.

 

[ash060]

The temp steel required is a total for footings. So you can put 1/2 each face of what you calculated or 0.2592 in^2/ft/face. Check ACI 15.10.4 for some clarification

 

[dik]

Are there any joints in the slab/footing?

 

[marinaman]

No joints.

This is a structural mat slab to support a large machine.

 

[marinaman]

Has anyone on this board designed a mat machine foundation of this type? I'd like to get a feel for the amount of cracking I can expect in this foundation.

The minimum temp and shrinkage percentage per ACI 7 is 0.18%.

I've provided 0.34%

Well above the minimum......but.....I guess I'm just a worry wart.

 

[JAE]

ash060 - I don't believe that is entirely correct. 1/2 each face is ok except where there is significant flexure in the mat.

Then you are into 10.5.4 or ACI where the chapter 7 minimum (0.0018 x Ag) must be applied wherever there is tension.

See this thread for a long, and intensive, debate on the subject:
thread507-251751

 

[JedClampett]

If you're planning on pouring this without joints, I would up the steel even beyond your design. In ACI 350, where they're very concerned about cracking, for widely spaced movement joints, they use .0052 steel ratio. This tops out at #8's at 12 inches, each face.

 

[dik]

YOu may want to increase the longitudinal reinforcing to 0.5% or 0.6%, then, if no joints.

Dik

 

[marinaman]

After reviewing the comments left here on this board, I have gone back and checked this design. I also believe I need to provide alittle more detail:

I performed the analysis of this mat via RISA Foundation. This mat supports a long printing press. Governing the design of this mat is differential settlement along the length of the pad....not flexure. The press manufacturer has give a very significant and restrictive criteria for differential settlement along the length of the mat. Varying the thickness and width of the mat foundation for this machine, I was able to meet the deflection criteria. The cost of that is that the mat ends up being thick and wide. This is why, thru trial and error with RISA Foundation, and with the subgrade modulus provided by the geotechnical engineer, I finally settled on a 58' long, 12' wide and 2' thick mat foundation.

Reinforcing needed to meet my deflection criteria is essentially the least I can get away with regarding temperature and shrinkage. Even at the low amount of steel I placed in the mat ((1) #6 at 12" o.c. top and bottom each direction) my actual bending capacity is nearly 20 times the factored moment.

Given this data....and the comments listed on this board, I have gone back and reviewed what I know to be provisions of ACI that could effect the minimum steel needed for this foundation....here's what I think I know:

ACI 318 Chapter 7.12.2.1
Minimum steel for temperature and shrinkage is (Gross Area) (0.0018) = 6.22 square inches

I've provided (26) #6 bars for 11.44 square inches.....Easily more than required by 7.12.2.1....but....just to be on the conservative side, I added (1) more bar, top and bottom.....so I now have a total of 12.32 square inches total......6.2 square inches per face....thus meeting 7.12.2.1 even on each face.

ACI 318 Chapter 15.10.4
Says for Nonprestressed mat foundations, I need to meet 7.12.2.1.....as stated above, I meet that.

ACI 318 Chapter 10.5.4
10.5.3 states that 10.5.1 and 10.5.2 do not apply if I have provided greater than 1/3 more reinforcing than required by analysis. I have....thus....10.5.1 and 10.5.2 do not apply.
10.5.4 says I must meet 7.12.2.1....which, as stated above, I do.

ACI 336 Chapter 6.14
No numeric recommendation

ACI 350 Chapter 7.12.2.1
ACI 350 does not apply to this mat. 350 applies to "conveying, storing, or treating liquid...." With structures that are "Subject to uniquely different loadings, more severe conditions, and more restrictive servicability requirements than nonenvironmental structures".

Even though ACI 350 does not apply here....I still looked at it, just to see what it says about holding cracks closed. JedClampets comment, taken from Table 7.12.2.1, with no joints in the mat for 58'....actually falls under the MAXIMUM steel required and not minimum. Being 24" thick, the verbage of this section states that a 50% reduction in table 7.12.2.1 can also be taken for the bottom mat....and....that being 24" thick, you can apply table 7.12.2.1 as 12" thick to each face, top and bottom, at 0.005. The bottom line.....I'd need 8.64 square inches on top (maximum) and 4.32 square inches on the bottom, maximum. The mat is not restrained....other than by friction of the soil below....so I think 0.005 is too much and overly conservative.

ACI 224 Section 3.54
This section contradicts itself....in saying the structure must meet ACI 318 and ACI 350....then turns right around and says you need 0.60%. What the?

Here's what I think.....

I think the bottom, at (1) #6 at 12" o.c. is ok. It meets all provisions of Chapter 7, 10, and 15.....and ACI 350.

The top probably needs alittle more. I'll probably go from having 13 top bars to having 16......getting above the 350 Table 7.12.2.1 requirement for minimum r/f at 40' joint spacing and staying below the 0.005 maximum. I will add this only because this mat is so long. If it were shorter, I would ignore 350 all together and stick with ACI 318 provisions.

Just my 2 cents.

 

[TXStructural]

Temperature and shrinkage is ONLY required perpendicular to computed flexural reinforcement, it is not a substitute for flexural reinforcement, nor in addition to it. Minimum flexural reinforcement (to be considered a flexural member) is found in ACI 318 10.5. A quick computation of the basic limit of 200 bw d / fy (about 1/2 sq inch per sq ft) gives 11.5 sq inches of bar, or #9 @ 12" (or #6 @ 5") in the long direction.

If this is acting as an unreinforced footing, and your intent is only to hold any cracks tight, you will want the reinforcement closely spaced near the upper surface, leaving sufficient cover for the exposure conditions. ACI 350 uses twice the 318 minimum T&S to restrain crack widths more certainly.

A true reinforced mat foundation should comply with ACI 318 chapter 10.

 

[marinaman]

TXStructural - How do you interperete 10.5.3? Does 10.5.3 not apply to this mat?

 

[JAE]

TXStructural - a mat foundation falls under 10.5.4. which uses the T&S reinforcement amounts given in 7.12.
It is not technically temp. and shrinkage reinforcement - just the amount specified.

So for a mat foundation, with flexure in it, the minimum As required would be 0.0018Ag in the tension face...or that required by analysis whichever is larger.

10.5.1 through 10.5.3 would not apply to a mat foundation but could be applied if an engineer felt like it - as long as 10.5.4 was met as well.

 

[ash060]

JAE

I was referring to minimum reinforcing steel only, of course if you are into bending than chapter 10 applies. The note about half on each face comes from the commentary for that section.