Minimum Flexural Reinforcement ACI 318-19 8.6.1.1 for a very thick slab

[oengineer]

I have a foundation slab that is 4'-11 thick (the total dimensions are 14 ft x 14 ft x 4'-11" thick) & I am wondering if Minimum Flexural Reinforcement ACI 318-19 8.6.1.1 is applicable to this foundation. The applied vertical force is 100 kips & an applied moment of 295 k-ft. This is an equipment foundation.

Would it be accurate to consider Minimum Flexural Reinforcement ACI 318-19 8.6.1.1 or just insure that the shrinkage & temperature reinforcement requirements are met? The allowable soil bearing is 4 ksf.

Comments/suggestions are appreciated.

 

[slickdeals]

Your loads don't seem right.

 

[JoshPlumSE]

So, there is a flaw (IMO) with the ACI code for minimum slab reinforcement when compared to beams.

When you have reinforcement in a slab, you are required to meet the same flexural minimums that were developed for beams. These insure that there is enough steel to avoid a brittle failure when the beam cracks. However, for beams the code allows you to alternately provide 4/3 of the amount of steel required based on demand moments.

This alternatively is NOT allowed for slabs. So, you have to provide a lot more steel than is necessary (IMO) for thick foundation slabs to resist the loads applied to them.

Also, the Temperature / Shrinkage requirement of 0.0018 must be applied to both the top and bottom layers if they both need reinforcement. Meaning, the total minimum slab reinforcement often becomes 0.0036.

 

[HTURKAK]

My past experience ,
I have provided minimum 0.35 % flexural reinf . ( both ways total ) and additional 0.2 % top and perimeter cage rein. for shrinkage and temp.

 

[rapt]

JoshPlum,

The 4/3 rule for beams is the anomaly and is not in any other code that I know of. It was in older Australian codes but was removed in 1988. It is not logical in limiting ductility.

If you are trying to ensure ductility, the tension strain in the reinforcement is the problem. But for some reason Design Codes do not use a stain limit for this. One day they might. Then the limits would vary depending on the ductility of the reinforcement.

The other anomaly with beams is that the top and bottom reinforcement minimums should be different as the T-section results in different effects for each face. Looking at it in terms of strain shows the problem with this. The old BS8110 minimums used to show this as they had different values top and bottom for beams.

 

[human909]

In the AS codes there is a clause regarding minimum steel requirements that states:



I've interpreted that for foundations like raft slabs that you don't need to meet minimum flexure steel requirements. Because they are statically indeterminant and brittle failure will not lead to collapse.

Naturally you still will need to meet minimum shrinkage & temperature reinforcement requirements.


I would encourage anybody, especially those familiar with AS3600 to chime in their agreement or disagreement with my interpretation here.

 

[CJLCivilStruct]

In the AS codes there is a clause regarding minimum steel requirements that states:

View attachment 8918

I've interpreted that for foundations like raft slabs that you don't need to meet minimum flexure steel requirements. Because they are statically indeterminant and brittle failure will not lead to collapse.

Naturally you still will need to meet minimum shrinkage & temperature reinforcement requirements.


I would encourage anybody, especially those familiar with AS3600 to chime in their agreement or disagreement with my interpretation here.

I must say i'm shooting from the hip here - and I don't have any experience with these codes but..

Minimum flexural steel requirements are based on the steel not yielding upon initial cracking of the concrete (which then may result in brittle failure).

So, if you detail a raft slab with less than minimal minimal steel requirements would the overall stiffness of the raft would be compromised, as every time there is a flexural demand greater than the uncracked moment capacity of the raft there is yield strain induced into the steel.

Without running the numbers which may say otherwise, wouldn't this negatively affect the performance of the raft?

Just a note - not trying to be argumentative, i'm genuinely curious of other peoples approach.

 

[JoshPlumSE]

CJLCivicStruct: Minimum flexural steel requirements are based on the steel not yielding upon initial cracking of the concrete (which then may result in brittle failure).

Close, but not quite. The minimum steel requirements merely ensure that the ultimate strength of the slab is greater than the cracking strength. So, if you were to go from an uncracked state to a cracked state you could get brittle failure. The steel would still yield, but it would be during failure.... The normal ductile failure occurs when the steel yields long before failure.... giving lots of warning that there may be a structural problem with the slab.

That being said, the more LIKELY situation with soil supported slabs is NOT one of catastrophic failure. You've got continuous support from the soil. When you get increased deflection, you have more soil resistance and load re-distribution. Plus, these slabs do tend to crack from temperature / shrinkage long before they experience their maximum loading. These cracks may not be large enough to be a structural problem. But, even those very thin cracks will be enough to prevent the type of sudden / brittle failure we're talking about.

In summary, I still don't believe there is any logical reason not to allow the 4/3 As_required liberalization that is allowed for beams. For ELEVATED slabs, or pile supported slabs, I'd be okay with not allowing the 4/3 liberalization. But, not for soil supported mat foundations. Though it doesn't matter what I think, the code writers decided they wanted to see double the amount of steel in slab foundations..... So, we have to put it in now.