Top Layer of Reinforcement for a Pile Cap or Spread Footing 1

[OKHawkeye]

I'd like to inquire as to what might be some of the considerations that other experienced Structural Engineers take into account when it comes to specifying the reinforcement for the top of a pile cap or a spread footing. Generally, pile caps can get rather thick due to the particularly high shear load that is inherent with them; this can also be true for some spread footings that must resist abnormally high shear loads.

Specifically, I'd wondering:

1. Is there an overall thickness (or effective depth) at which you feel that a two-way layer of reinforcement is required near the top of such an element, in addition to the two-way layer near the bottom?

2. What are some of the parameters you use in designing such reinforcement? For example, do you design such steel in conformance with, say, ACI 318-11, Sect. 7.12 (Shrinkage and temperature reinforcement) or perhaps Sect. 10.5 (Minimum reinforcement of flexural members)?

3. Do you "mirror" the two-way layer of reinforcement near the bottom of the pile cap (or spread footing) and simply duplicate it and place it near the top? Do you maybe use a certain "fraction" or "percentage" of the two-way layer near the bottom for the two-way layer near the top?

4. Finally, what considerations do you take into account when designing/specifying the steel "lifts" (i.e., vertical bars) that will be needed to form the vertical members of the "cage" of the pile cap/spread footing reinforcement?

I know I'm asking a lot, but I'm just curious as to what others have experienced, or if anyone else had any thoughts on the matter. Thanks very much!

 

[hokie66]

1) No
2) It depends. Temperature and shrinkage doesn't enter into it. If flexural reinforcement is required, then it should be designed in the normal manner.
3) No, if top bars are not required flexurally, leave them out. They don't do anything and often lead to plastic settlement cracking.
4) Use standees (bent bars) to support a top mat if the top bars are required flexurally. In some cases, stirrups may be required for shear, and these will support top bars.

 

[KootK]

Great question. I wonder about this often myself.

1) No, not for run of the mill spead footings. I've known colleagues that do however and the bar seems to be somewhere in the 900 mm range.

2) If there will be bonfide tension in the top of the footing, then I'll provide at least T&S reinforcement simply as a code compliant means of satisfying crack control requirements.

3) Once I've decided to use top steel, I usually provide the max according to flexural demand, crack control, or 1/3 the bottom steel. The 1/3 business is just my own judgement and I don't always do it.

4) Same as Hokie.

There are two common situations where I'll provide a "feel good" top mat:

1) when I feel that the stresses are difficult to predict. Footings below shear wall shafts is a frequently encountered example for me.

2) Any situation where a significant uplift force will be applied to a footing, even if it doesn't create net uplift. I do this to enhance uplift rebar anchorage.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[OKHawkeye]

Awesome! You both have not only confirmed my thoughts on the matter but have also given me some additional considerations on which to reflect. I appreciate your candor and your generous help.

Thanks very much to you both for taking the time to respond. I'm a bit new at this website and I hope to use it, in part at least, in such a way as to elicit the opinions of other Structural Engineers on uncertain and/or challenging matters.

 

[Guastavino]

My thoughts:

1. Just depends on the loads. With uplift, sometimes, without uplift rarely.
2. I check it for flexure for uplift loads but I don't use Asmin for flexure, I use Temp/shrinkage min. area.
3. I don't mirror because my goal is to keep the cost down. Just about anyone can provide a sufficient footing for a building column, engineers just do it efficiently and cost effectively. I just see what is needed and provide that.
4. I leave that to the rebar fabricator. I've never had a problem letting them spec standzees, etc.

With that said, the company I used to work for mirrored the top whenever there is uplift. I think that is overly conservative. In fact, in you have RISA-Foundation, you get a nice check for uplift on spread footings that tells you the Mcr moment and checks that versus the moment due to uplift (although I think it disclaimers that). Some engineers aren't comfortable using unreinforced concrete to take negative moment in footings, I'm just not one of them (as long as Phi*Mcr is OK). With that said, when footings with net uplift get to be 9'x9' or more, then I just add some top bars even if the Mcr moment is OK. Belt and suspenders based on the size and what not.

The reason I'm not really conservative with footings is because A) I don't believe they will ever see the calculated loads, and B) even if they do, I have never heard of a failure of a footing based on negative bending. I'm sure it's happened, but I concern myself more with a Factor of safety in the overturning mass, versus the flexure for uplift.

 

[OKHawkeye]

Thanks njlutzwe!

You touched on a couple of points I'll consider more closely in the future. I've rarely needed to design pile caps, which is what led me to inquire in the first place.

I very much appreciate your help!

 

[hokie66]

Pile caps are different from spread footings. They are more like deep beams, and these days are usually designed using strut and tie analysis (truss analogy). Depending on how many piles support the pile cap, there is often need for top reinforcement.

 

[hokie66]

2) njlutzwe, I thought that the new ACI code had taken out the illogical relationship between flexural and T&S reinforcement.

 

[MacGruber22]

hokie - yes. In 318-14, that has been removed.

"It is imperative Cunth doesn't get his hands on those codes."

 

[Ron]

1. No...depends on load and particularly, reversals.
2. Usually only a flexural consideration in load reversals
3. Usually yes (mirroring the worst case), only to decrease the brain damage in the field
4. I prefer a "cage" configuration, but rebar fabricator often provides two mats tied together with verticals in the field.

Most of the footings I design are special purpose with significant uplift/overturning.

 

[OKHawkeye]

"...brain damage in the field...", I like that (ha)! That's a very good point, Ron, thanks.

May I ask why you prefer the 'cage' configuration as opposed to, say, the standees?

I confess that my pile cap experience is a bit limited and I'm just curious as to what others have encountered.

Thanks again!

 

[TLHS]

It's not in any code that I'm aware of, but along with some of the other cases above, I'll provide it in zones where seismic controls design. You're generally assuming R factors and some amount of rocking in your footing. So even when your design loads don't show uplift the *actual* seismic event might involve your footing pushing up against the soil. I would prefer to have some reinforcing to hold cracking together with that motion, even if the concrete tension capacity calcs out. I don't want progressive cracking during the earthquake.

 

[hokie66]

I agree with TLHS for seismically active areas. I think flexural reinforcement is always warranted in such locations.

 

[KootK]

@TLHS/Hokie66: you're speaking of foundations that are part of the designated lateral system, right? Not just any old gravity footing? Granted, there are some blurred lines there.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[hokie66]

No, I mean any old footing in seismic areas. All sorts of things can happen in earthquakes, and you want footings to remain intact. Footings can see uplift in earthquakes which never occur due to other types of loading.

 

[KootK]

No, I mean any old footing in seismic areas.

We have divergent opinions here. I don't include feel good top steel in my seismic area foundations unless they're part of the designated lateral system.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[hokie66]

To my mind, it is just common sense. Like putting bottom bars in flat slab drop panels for seismic integrity.

 

[KootK]

To my mind, it is just common sense. Like putting bottom bars in flat slab drop panels for seismic integrity.

So what, now you find my views nonsensical? Why not express your true feelings and just accuse me of being downright Coloradan. Heaven forbid.

In my neck of the woods, the bars in the bottom of drop panels are there as a second line of defense for punching shear. So that represents a belt and suspender system for a load that is definitely present and may be amplified under seismic action. To use the same logic for a load that has a high probability of not being there at all is an altogether different kettle of fish.

Let's consider a gravity column in a multi-story building supported by a pad footing. There would be no need for top steel in the footing unless the gravity column is actually in tension. And it would take a fair bit for a gravity only column to end up in tension.

Now lets say that our gravity column did somehow end up in tension. Would the columns be designed with mechanical couplers on the rebar? Would the slabs be designed to have the columns hanging from then rather than supported by them? Would we design the slabs for upside down, column punching shear? If we follow that load path through logically, where would one stop?

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[OKHawkeye]

I genuinely appreciate everyone's responses! It's a great comfort to know that differing opinions can be offered so freely, respectfully, and professionally.

Like so many other aspects of our chosen profession, there are a lot of things to consider in this matter - and I'll certainly be doing so in the future.

(P.S. You gotta love 'seismic.' It makes our engineering world so interesting - and unnerving! lol)

Thanks everyone!

 

[TLHS]

I often work in areas where the short period mapped spectral acceleration is around or greater than one gravity. Your reduced loads are obviously smaller, but under these conditions all sorts of things could end up uplifting or rotating during the rocking process. In a lot of cases I'm going to spend a few bucks putting in a top mat to ensure I don't end up with a a lad reversal moment failure that would kill the gravity capacity of the footing.