Minimum reinforcement of flexural members for Mat Foundations 3

[oengineer]

10.5 of ACI 318-11 shows the folloing (see image below):

ACI 318-11 in section 10.5.4 states "For structural slabs and footings of uniform thickness, As,min in the direction of the span shall be the same as that required by 7.12.2.1. Maximum spacing of this reinforcement shall not exceed three times the thickness, nor 18 in."


R10.5.4 in the commentary mentions the following: The minimum reinforcement required for slabs should be equal to the same amount as that required by 7.12.2.1 for shrinkage and temperature reinforcement. Slabs-on-ground are not considered to be structural slabs in the context of this section, unless they transmit vertical loads or lateral forces from other parts of the structure to the soil. Reinforcement, if any, in slabs-on-ground should be proportioned with due consideration of all design forces. Mat foundations and other slabs that help support the structure vertically should meet the requirements of this section.

Would this mean that a minimum flexural reinforcement, as stated in 10.5.4 of ACI 318-11, needs to be satisfied along with the temp & shrinkage reinforcement for mat foundations? Or is this just referring to flexural members, like beams?

I ask because that seems to contradict what is stated in Section 15.10.4 of ACI 318-11. Section 15.10.4 states "Minimum reinforcing steel in nonprestressed mat foundations shall meet the requirements of 7.12.2 in each principal direction. Maximum spacing shall not exceed 18 in."

Also, PCA states the following (see image below):

Could some please clarify this?

Comments/suggestions are appericated.

 

[JoshPlumSE]

This is my least favorite portion of ACI-318:

My original interpretation (from earlier versions of the code):
You determine for the flexural mins of 10.5.1. If this is a foundation slab, then you may use 4/3*Asrequired (10.5.3, I believe, from the earlier versions of the code) if it is less than the flexural minimums. Provided that your total reinforcement ratio is at least the 0.0018 required for temp / shrinkage.



New code language interpretation:
Section 10.5.1 doesn't really apply. The flexural min for slabs is 0.0018.

A brief rant about this:
The minimum flexural reinforcement is there to prevent brittle failure of the beam. By making sure the reinforced strength of the beam is greater than the moment that causes cracking. The 4/3*As_req liberalization (which is still allowed for beams) is there to allow beams with less ductile behavior, provided the amount of steel is 4/3 times that actually required for the design forces. Somehow, structural slab foundation are now deemed to be so "brittle" that this 4/3 liberalization isn't allowed. Personally, I understand that for ELEVATED slabs. But, for structural slabs on grade, that doesn't make any sense to me.

If you require reinforcement in both the top and bottom of the slab, then total reinforcement will be something along the lines of 0.0036. Possible double what we would have gotten with older codes. Assuming that Temp / Shrinkage controlled the reinforcement design of the slab foundation.

 

[oengineer]

This is my least favorite portion of ACI-318:

My original interpretation (from earlier versions of the code):
You determine for the flexural mins of 10.5.1. If this is a foundation slab, then you may use 4/3*Asrequired (10.5.3, I believe, from the earlier versions of the code) if it is less than the flexural minimums. Provided that your total reinforcement ratio is at least the 0.0018 required for temp / shrinkage.



New code language interpretation:
Section 10.5.1 doesn't really apply. The flexural min for slabs is 0.0018.

A brief rant about this:
The minimum flexural reinforcement is there to prevent brittle failure of the beam. By making sure the reinforced strength of the beam is greater than the moment that causes cracking. The 4/3*As_req liberalization (which is still allowed for beams) is there to allow beams with less ductile behavior, provided the amount of steel is 4/3 times that actually required for the design forces. Somehow, structural slab foundation are now deemed to be so "brittle" that this 4/3 liberalization isn't allowed. Personally, I understand that for ELEVATED slabs. But, for structural slabs on grade, that doesn't make any sense to me.

If you require reinforcement in both the top and bottom of the slab, then total reinforcement will be something along the lines of 0.0036. Possible double what we would have gotten with older codes. Assuming that Temp / Shrinkage controlled the reinforcement design of the slab foundation.

Thanks for the response.

 

[r13]

When dealing with mat foundation, I tended to think it is a flexural member capable of develop beam action, especially between rigid supports, so the minimum reinforcement for beam applies. For SOG, except rare events, beam action is not likely to occur, thus, reinforced with T&S steel is usually more than adequate. Often times, we shouldn't read the code word by word, when there is conflict, or confusion, engineering judgement prevails.

 

[hokie66]

For your reading enjoyment, or maybe just a bit of mischief on my part.

thread507-251751

 

[Ron]

@hokie66....quite a bit of both

That is a good example of a good discussion thread...something we are seeing less of in here these days

 

[JAE]

Yes with all this Covid time on our hands, by all means, read through hokie66's link and enjoy!

 

[WinelandV]

Yes with all this Covid time on our hands, by all means, read through hokie66's link and enjoy!

Or read it and become absolutely flabbergasted at ACI's inability to answer a direct question about what the intent and meaning of the words they wrote.

YMMV

 

[JoshPlumSE]

Excellent thread Hokie! Thanks for sharing.

Let's take a look at the 2014 version of ACI-318 to see if they have made this any more clear. I'd use the 2019 version. But, I don't have that one.
Chapter 7 (which is purely for slabs)

7.6: Reinforcement Limits:
7.6.1: Minimum flexural reinforcement in nonprestressed slabs
7.6.1.1: A minimum flexural reinforcement, As_min shall be provided in accordance with Table 7.6.1.1.
(Note: this shows 0.0018*60,000*Ag / Fy when the Fy of the reinf is 60 ksi or greater)​

That's it. Chapter 13 (on foundations) merely points us back to this section for our reinforcement limits. However, it is interesting to compare the above code sections to the equivalent sections for Beams.
Chapter 9 which is purely for beams.

9.6: Reinforcement Limits:
9.6.1: Minimum flexural reinforcement in nonprestressed slabs
9.6.1.1: A minimum flexural reinforcement, As_min shall be provided at every section where tension reinforcement is required by analysis.
9.6.1.2:As,min shall be the greter of (a) or (b) except as provided in 9.6.1.3....
(a) 3*sqrt(fc)*bw*d/fy
(b) 200*bw*d/fy
9.6.1.3: If As provided at every section is at least one-third greater than As required by analysis 9.6.1.1 and 9.6.1.2 need not be satisfied​

I read this as saying that 0.0018 is required at any face where flexural reinforcement is required in the slab. Some thoughts about this:
1) The ACI guys are likely thinking about elevated slabs, where the reinforcement is bent down at the transition between negative and positive flexure. So, 0.0018 per face isn't really a problem because it's mostly the same reinforcement.
2) This would make some sense in that allowing less than 0.0018, (even if it's 4/3*As_required) might be problematic for elevated slabs that are more prone to brittle failures.
3) Mat slab Foundation aren't directly considered. While the code provisions make little sense for foundations, they apply none-the-less.... at least until they change the code.
4) Personally, I think it's asinine that the 4/3*As_req liberalization is not currently allowed for structural Mat slab foundations or footings.

 

[r13]

3) Mat slab Foundation aren't directly considered. While the code provisions make little sense for foundations, they apply none-the-less....

Have you checked 13.3.4 for requirement on mat foundation? Also, you didn't address CH-8 in your evaluation, will that change anything? (I don't have the code, so depend on you to do the lift

Chapter 7—One‐Way Slabs
7.1—Scope
7.1.1—This chapter shall apply to the design of nonprestressed and prestressed slabs reinforced for flexure in one direction, including: (a) Solid slabs (b) Slabs cast on stay‐in‐place, noncomposite steel deck (c) Composite slabs of concrete elements constructed in separate placements but connected so that all elements resist loads as a unit (d) Precast, prestressed hollow‐core slabs.

Chapter 8—Two‐Way Slabs
8.1—Scope
8.1.1—This chapter shall apply to the design of nonprestressed and prestressed slabs reinforced for flexure in two directions, with or without beams between supports, including (a) through (d): (a) Solid slabs (b) Slabs cast on stay‐in‐place, noncomposite steel deck (c) Composite slabs of concrete elements constructed in separate placements but connected so that all elements resist loads as a unit (d) Two‐way joist systems in accordance with 8.8

Chapter 9—Beams
9.1—Scope
9.1.1—This chapter shall apply to the design of nonprestressed and prestressed beams, including: (a) Composite beams of concrete elements constructed in separate placements but connected so that all elements resist loads as a unit (b) One‐way joist systems in accordance with 9.8 (c) Deep beams in accordance with 9.9.

Chapter 13—Foundations
13.1—Scope
13.1.1—This chapter shall apply to the design of nonprestressed and prestressed foundations, including shallow foundations (a) through (e) and, where applicable, deep foundations (f) through (i): (a) Strip footings (b) Isolated footings (c) Combined footings (d) Mat foundations (e) Grade beams (f) Pile caps (g) Piles (h) Drilled piers (i) Caissons.

13.3.4—Two‐way combined footings and mats foundations

 

[JoshPlumSE]

Retired13 -

Sorry, Chapter 8 (two way slabs) is essentially identical. Though, that chapter emphasizes that the min flexural reinforcement Asmin shall be placed "near the tension face in the direction under consideration".

And, I did check chapter 13, it merely points us back towards chapters 7 and 8. Though there is the section about distribution of reinforcement in a rectangular footing. But, nothing that would help us soften the 0.0018 min flexure on any face that requires tensile reinforcement.

 

[hokie66]

Josh, why would you want to "soften" that requirement?

 

[JoshPlumSE]

Hokie -

If I've got 2*As_required in both the top and bottom of the slab then I'd prefer to use 0.0018 as temperature shrinkage split between the two faces. Instead of 0.0018 for each face. The 0.0018 flex minimum is requiring me to double the total amount of steel in my foundation.

 

[hokie66]

This thread is about a "mat foundation". I assume that means a thick slab of substantial dimensions, cast on the ground. If you want to control restraint shrinkage cracking in that, you need a lot more than 0.18% reinforement, more like 0.5% to 0.6%.

 

[JoshPlumSE]

If you want to control restraint shrinkage cracking in that, you need a lot more than 0.18% reinforement, more like 0.5% to 0.6%.

Really? Do you have any code references that say this?
Doesn't "mass concrete" usually only require 0.0018 over the outer 12 or so to control cracking? Not even on the gross area. It's been years since I looked at it though.

 

[Settingsun]

How good is it when As,min isn't just the minimum requirement of As? (Referring to Hokie's old discussion)

0.18% reinforcement gives steel yield capacity about 25% of concrete tensile capacity (order of magnitude due to wide variation). Real risk that whatever caused the concrete to crack just yields the reo too giving one wide crack instead of many narrow cracks.

Some ground slab guidance gives <0.1%. In that case, just leave it out. It's doing 0.1% of SFA. The close joint spacing is what saves the day.

 

[Lomarandil]

Just following Josh's take on ACI 318-14, here's the parallel text from 318-19.

Chapter 7 -- One way slabs
7.6.1 Minimum flexural reinforcement in nonprestressed slabs
7.6.1.1 A minimum area of flexural reinforcement, As,min, of 0.0018Ag shall be provided.
R7.6.1.1 The required area of ... minimum flexural reinforcement is the same as that required for shrinkage and temperature in 24.4.3.2 However, whereas shrinkage and temperature reinforcement is permitted to be distributed between the two faces of the slab... minimum flexural reinforcement should be placed as close as practicable to the face of the concrete in tension due to applied loads.

Chapter 8 -- Two way slabs
8.6.1 Minimum flexural reinforcement in nonprestressed slabs
8.6.1.1 A minimum area of flexural reinforcement, As,min, of 0.0018Ag, or as defined in 8.6.1.2 shall be provided near the tension face of the slab in the direction of the span under consideration.
R8.6.1.1 The required area of ... minimum flexural reinforcement is the same as that required for shrinkage and temperature in 24.4.3.2 However, whereas shrinkage and temperature reinforcement is permitted to be distributed between the two face of the slab... minimum flexural reinforcement should be placed as close as practicable to the face of the concrete in tension due to applied loads.

R8.6.1.1 then continues with a figure illustrating a reinforcing arranagement with cutoffs.
8.6.1.2 specifies a higher minimum limit for slabs with high levels of two-way shear stress (e.g. at a column).

Chapter 9 -- Beams
Apparently no change from 318-14

Chapter 13 -- Foundations, kicks:
One-way shallow foundations to the "applicable provisions of Chapter 7 and Chapter 9"
Two-way isolated footings to the "applicable provisions of Chapter 7 and Chapter 8"
Mat foundations and two-way combined footings to Chapter 8 (no direct design method, minimum reinforcing specifically per 8.6.1.1).

Which begs the question... which minimum steel provisions are applicable for one way shallow footings? Like a slab or like a beam?

----
just call me Lo.

 

[Settingsun]

Really? Do you have any code references that say this?

Not a code reference, but the Australian Standard has 0.175% minimum reinforcement for a 'minor' degree of crack control in restrained slabs (secondary direction). The same value applies for unrestrained slabs. The Concrete Institute of Australia however says this "provides very little control over cracking and the steel will usually yield at first cracking... it is difficult to imagine a situation where this would apply, except in an unrestrained direction or for a slab with closely spaced control joints that eliminate restraint".

P.S. I trust our CIA more than I trust yours.