Long, Wide Mat Slab Reinforcing 2

[Boiler106]

I have a 24 inch thick mat slab stretching about 300ft x 225ft which will support a tall rack system building and act as the slab on grade in the southeast US. The mat foundation is completely enclosed.

I am analyzing and designing with RAM concept, and have coordinated subgrade modulii with the geotechnical engineer.

I'm trying to address cracking in the mat, which must remain continuous.

Per ACI 13.3.3.4, minimum reinforcement should be 0.0018, and on both faces per 8.6.1.1 which is about #7 @12".

However, per ACI 360, i should provide 0.005 in the top layer for slabs on grade to combat cracking. For a 24" slab, that's approx #8 @6".

Does anyone have a reference or guidance for this type of structure? Can construction joints and max distances with minimum cure times be specified?

 

[jayrod12]

I'm more partial to the 0.005 if you can't have control joints. I feel the minimum reinforcing is just intended to keep the crack size small and assumes you're providing adequate control joints.

 

[r13]

Note the minimum T/S steel specified by ACI318 applies to structure elements above the ground, ACI 360 is specific to SOG.

 

[JoshPlumSE]

Note the minimum T/S steel specified by ACI318 applies to structure elements above the ground, ACI 360 is specific to SOG.

Not sure that's really true. ACI 318 has chapter 13 which directly addresses foundations. ACI 360 is more of a "guide" or "report" that will help with slab design, but doesn't have the "codified" requirements that ACI-318 does.

That being said, pointing the OP towards ACI 360 is a great idea. It has chapters on crack width control and joints. Very useful for this topic.

 

[r13]

Not sure that's really true. ACI 318 has chapter 13 which directly addresses foundations. ACI 360 is more of a "guide" or "report" that will help with slab design, but doesn't have the "codified" requirements that ACI-318 does.

Josh, it is the fact. Below are the provisions of ACI318-14 regarding T/S steel. Let me know, if you find something else that differs.

 

[rowingengineer]

Does aci 318 have a crack control area for constraint slabs, I would have expected higher requirements for this type of slab.

 

[Boiler106]

it seems that my client has two options: accept the slab as a foundation with cracking based on 0.0018bh reinf or pay for reduced cracking with 0.005bh reinf.

As a rack system with cranes running on rails, the cracking may not be an issue since there wont be vehicles travelling directly on the mat slab itself.

 

[rowingengineer]

For option 1. I would add in saw cuts at least. The two can be continuous if required.

 

[dik]

one continuous pour? no construction joints (50'x50' panels)? can it be post-tensioned? shrinkage compensating concrete? higher strength concrete? Really low slump concrete with SuperP? seems like a lot of rebar. Can you use #11 bars? What is the least costly rebar arrangement for size and spacing?

Are you sure of the thickness? I've designed 747 runways that were of that magnitude.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[r13]

I don't know the latest ACI318 has provision for crack width control as the old, but ACI350 shall cover. The recommendation is usually goes for smaller bars at a closer spacing to limit the width of crack - many fine cracks rather than one wide open crack.

 

[JoshPlumSE]

All -

There is lots of confusion here.... Let's be clear about a few things:
1) ACI 318 100% does apply to Structural mat foundations. When it says "slabs on ground" in the commentary it is referring to things like paving of your patio. You walk on it, but nothing else... No columns or walls are supported by it. Therefore, it is non-structural.

2) ACI 360 is a REPORT, it is not a code. It's full name is "Guide to the Design of Structural Slabs on Grade". It has excellent suggestions and practical advice on slabs. But, it does not qualify as code requirements. Think of it like an AISC Design Guide, but a little more formalized. I think this is probably where the OP should turn to for the most practical advice on his situation. I don't have a copy of this report right now. So, I cannot quote from it for him. That being said, if I remember correctly there are sections that will help him with determining how to space his control joints or such and handle cracking and such.

3) ACI 350 is mainly for sewage treatment or waste treatment facilities (as I understand it). It's full name is "Code Requirements for Environmental Engineering Concrete Structures". It is definitely a code and it does have really good information on crack control. But, it's not specifically related to structural Mat foundations.

I'm not sure what exactly the OP meant by "I'm trying to address cracking in the mat, which must remain continuous". It might mean that he's not allowed to have any control joints in the slab. That would be pretty rare. We're just trying to force the cracking to occur at these spots in the slab. The slab is still considered to be continuous.

 

[dik]

Thanks Josh... pretty much my understanding...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[Boiler106]

The slab is 24" thick to accommodate the rack manufacturer's anchors. it is a minimum thickness set forth by the manufacturer. This is a LARGE rack system.

The mat MUST be continuous, free of control joints, as dictated by the manufacturer. This mat serves as the building foundation and ground level slab.

Again, NO movement/control/contraction joints are permitted.

These two requirements come from two different competing manufacturers

Beyond reinforcing, can construction joints help in this situation? Perhaps, it will limit plastic shrinkage but i think given that 'checkerboard' placement has gone out of style, this may not be very helpful.

 

[dik]

bars at 12", to me, are close. bars @6", to me, are silly.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[r13]

One thing I can agree that, if the mat foundation supports column, thus required for structural equilibrium, then it can't be called a slab on grade. Though ACI360 and 350 are not the code, but they were prepared and referred by ACI as supplements to the code for good practices. As a practical engineer, we shall evaluate the situation, and determine either stick to the rigid code, or use the supplements that make better sense, and no less stringent than the code. You, make the call.

BTW, ACI350 is for environmental and hydraulic structures that require tighter crack control, I think it is better reference if crack is your concern. Or you might consider use leaner mix, if strength permits; use low heat of hydration cement; add fibers...many ways for crack control that are not explicitly addressed by the code, but the reports.

 

[dik]

Then the owner pays for what he gets... just a matter of determining the minimum cost for what he just bought... concrete properties, rebar quantity, etc.

Is there an alternative racking company he can use that might be less onorous? I'm surprised it doesn't have to be superflat... I've done large cargo terminal buildings with substantial 40' racks with far less requirements... actually used PCA's Airport program for designing the slab for the rack loads. Can you provide me with a magnitude (+/-) and spacing of the rack loads?

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[r13]

For industrial projects, rebar and concrete rarely ranked high in the equation of main cost concerns compared to other issues.

Strictly speaking, if you treat this mat foundation as structural element, then you shall provide the minimum steel according to the requirements for flexural members, T/S alone won't cut.

 

[dauwerda]

if you treat this mat foundation as structural element, then you shall provide the minimum steel according to the requirements for flexural members, T/S won't cut.

Unless of course the T/S also meets the requirements for flexural strength.

It sounds to me like you have a structural mat foundation and it needs to be designed (at a minimum) to ACI 318.
Your concrete will absolutely crack. But the reinforcement will prevent any normal detrimental issues with those cracks. If, you feel that you need better crack control than you get with typical structural concrete, then you can look to the other codes/guides mentioned for guidance, there is nothing wrong with going beyond the minimum requirements of the code.

The scope of ACI 360 state this,
"This guide covers the design of slabs-on-ground for loads
from material stored directly on the slab, storage rack loads,
and static and dynamic loads associated with equipment and
vehicles. Other loads, such as loads on the roof transferred
through dual-purpose rack systems, are also mentioned.

In addition to design, this guide discusses soil-support​

systems; shrinkage and temperature effects; cracking, curling
or warping; and other concerns affecting slab design. Although
the same general principles are applicable, this guide does not
specifically address the design of roadway pavements, airport
pavements, parking lots, and mat foundations."

Based on the above scope, it sounds like this would be a great document to use for guidance for your situation to ensure the performance required by the manufacturers is met.

 

[r13]

Unless of course the T/S also meets the requirements for flexural strength.

Noted, and corrected. Thanks.