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!

 

[KootK]

Short period vertical accelerations are only about 1/5 of horizontal accelerations. So, even in a 1.0g environment, there's still little danger of structures "hopping around" and uplifting footings.

Most high seismic codes dictate that gravity systems be designed to accommodate the forces associated with the drift of the designated lateral system. And I would certainly expect this to apply to gravity column-footing assemblies as well. If such considerations created a demand for footing top steel, I would definitely provide it.

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.

 

[TLHS]

I'm not concerned about vertical accelerations, those generally don't seem to affect things all that much from a design standpoint. Since spread footing design R (or other reduction clauses) values in seismic cases are often justified in code based on their ability to 'rock', being sure that they can do so without cracking from moments induced by pushing up against the soil on top of them seems important. Add that to the fact that you might see some amount of uplift from the structure overturning moment even if it doesn't quite show up in the calcs and it seems like it should be a requirement.

 

[KootK]

Since spread footing design R (or other reduction clauses) values in seismic cases are often justified in code based on their ability to 'rock', being sure that they can do so without cracking from moments induced by pushing up against the soil on top of them seems important.

Sure but then these rocking footings are part of the designated lateral system, right? Like the pad beloow shaft wall?

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.