Mat/Slab Foundation Reinforcement per ACI 318-14 2

[engjg]

I know this has been discussed on here more than one once but I still have some questions...

One of the many changes in 318-14 included rewording and organization of minimum reinforcement requirements. Looking at 13.3.4.4 minimum flexural reinforcement for non-prestressed mat foundations shall be in accordance with 8.6.1.1 which makes no mention of the exception for 4/3 more than is required by analysis statement. R8.6.1.1 makes the point that in contrast to T&S steel this minimum should be placed at each tension face. So I interpret this as follows: even in a mat foundation which is lightly loaded but sees both positive and negative flexure one would have to provide 0.0018 x gross area at each face regardless of the demand. This would be in contrast to one providing 0.0018 x half the depth at the top & bot for T&S and checking if it is 4/3 more than required for flexure. Do you agree with this interpretation of ACE 318-14? Any strategies to reduce reinforcement staying within the code 318-14? Plain concrete mat foundation for flexure if it works (are there provisions that address this?) and then add T&S because you just want to? What about a single mat of steel in center for flexure(serving both pos & neg) and then provided t&s steel for half the depth at the top (this would be in the application for the top surface is an exposed floor surface.)

 

[slickdeals]

That is the correct interpretation for ACI 318-14.

 

[JoshPlumSE]

I would say that is correct with one slight caveat. When steel is required on both faces, the minimum is 0.0018 on each face.

If, for example, there is positive and negative flexure on the section, but the flexure causing tension in the top face is very, very slight.... would you have to provide top steel? There is an argument to be made that if the "plain concrete strength" of the section is sufficient to resist that moment then no steel is required. Hence, you would only have 0.0018 on the opposite face.

Now, KootK and I have argued over this once before and he made some good points about plain concrete strength. But, this 0.0018 reinforcement on both faces is so over-conservative / ridiculous for cases like these that I would still use the plain concrete strength as an acceptable way to avoid top steel in cases where it can be used.

 

[Agent666]

It's not only flexure that causes stresses though, restraint can lock in considerable tension forces which must be accounted for if you are considering departing from codified advice, restraint could be from restrained shrinkage, creep etc.

The intent for minimum steel provisions is to ensure if the member cracks that the reinforcement provided is sufficient to achieve a higher strength than the plain concrete stength, such that there is not a sudden change in capacity when the plain concrete strength is exceeded. I'd be very wary of departing from the minimum requirements as it serves a very important role in ensuring some robustness under unexpected forces that might occur in reality, but not be fully considered in design.

 

[KootK]

That is the correct interpretation for ACI 318-14.

I agree.

It's not only flexure that causes stresses though, restraint can lock in considerable tension forces which must be accounted for if you are considering departing from codified advice, restraint could be from restrained shrinkage, creep etc.

Agree X 10. As far as I'm concerned, if there's any significant plan extent to your mat slab, you can count on there being restraint cracking and thus zero plain concrete flexural capacity. Same for shear capacity in regions of meaningful flexure.

As a philosophical matter, I consider the most useful and important job of a code/standard to be this: to provide un-contestable backup to designers wishing to do intelligent things regardless of the numbers and in spite of the willingness of their market competitors to make compromises in order to hoard market share.

I see this, as that. I feel that the 0.0018 is just a reasonable lower limit on tension face reinforcing for any concrete resisting significant flexure. If ACI has tightened up the associated clause such that I no longer have to put client relationships at risk in order to enforce what I think is just good practice, I celebrate that. Big time. Spread the word.

 

[Agent666]

You also need to consider that the real longterm concrete strength can be up to 2 times the specified strength, this makes the issue even worse (more shrinkage and hence restraint, and higher plain concrete strength).

Though unrelated to slabs this aspect was a real issue in NZ in recent earthquakes in particular with singularly reinforced concrete walls, even at minimum steel, with higher real concrete strength, instead of distributed cracking to dissipate energy, a single crack formed and after a few cycles because the concrete strength was higher than the reinforced capacity the bars simply fractured due to low cycle fatigue instead of further cracks forming as intended. They recently improved our code by doubling the minimum reinforcement required within wall ends to address this issue (as well as heavily penalising the use of singularly reinforced walls).

 

[JoshPlumSE]

Note:

The "old interpretation" was that if you couldn't provide flexural minimum (which are considerably more than 0.0018) then you would have to provide 4/3* As_req'd. That's still allowed for flexural members, of course. However, it's definitely not allowed for slabs now.

 

[engjg]

Thanks all for the responses. JoshPlum what did you mean by "The "old interpretation" wasn't that if you could provide flexural minimum (which are considerably more than 0.0018) then you would have to provide 4/3* As_req'd."?

 

[JoshPlumSE]

Re-submitted / edited. It should have said:
"if you couldn't provide flexural minimum (which are considerably more than 0.0018) then you would have to provide 4/3* As_req'd."

The point being, that it's the flexural mins that are meant to ensure that the reinforced section strength is greater than the plain concrete cracking strength. But, the code gives you an out if you 33% more reinforcement than required by analysis. This is my big complaint with the newer code provisions. This out is allowed for flexural members, but inexplicably forbidden for slabs. My belief is that the code is really geared towards very thin elevated slabs rather than thick the mat foundations found in the heavy industrial projects that I've worked on.

 

[engjg]

That's what I thought you meant...and your last point is exactly what I am pondering...thanks

 

[Agent666]

In NZ code we have the same 4/3 requirement, however it is contingent on the detail not being part of the lateral load resisting system. Where the load cannot be higher than assessed for example gravity load cases then the minimum can be reduced, for seismic the load seen by the member can be the member capacity so obviously no reduction can apply as you'll never meet the 4/3 ratio.

 

[KootK]

My belief is that the code is really geared towards very thin elevated slabs rather than thick the mat foundations found in the heavy industrial projects that I've worked on.

I suspect that same.