3 pile, pile cap 1

[Lytnur]

I’ve noticed engineers design 3 pile, pile caps simplified as a rectangle cap with orthogonal reinforcement how is this justified?

 

[BridgeSmith]

Please post an example detail. I have no idea what the configuration is that you're questioning.

 

[canwesteng]

Same way orthogonal reinforcing is justified in any two way slab or strut and tie model.

 

[Lomarandil]

In markets where labor is expensive, the material savings of a triangular pile cap and rebar layout does not outweigh the extra complexity of building and forming that triangular cap.

 

[HTURKAK]

I’ve noticed engineers design 3 pile, pile caps simplified as a rectangle cap with orthogonal reinforcement how is this justified?

Orthogonal reinf. will not be effective when compared with triangular form. If you write ( pile cap for three piles ) and search the web , one of the outcomes;


Besides main diagonal reinf.Provide minimum orthogonal cage reinf. for all faces .

 

[BridgeSmith]

Ah, now I get what the OP was talking about.

I suppose the reinforcement going directly between the piles would be more efficient in material, but as Lomarandil noted, material cost is only a part of the cost, and in many markets, a small part of the total cost. That balance varies by location and also by when the work was/is done. We have many bridges built 40 or 50 years ago using thin (5/16") steel plates for the webs and vertical stiffeners every 3' or so. Today, that would make the girders twice as expensive as using a 1/2" or 5/8" web and no stiffeners.

 

[dik]

with orthogonal reinforcement how is this justified?

If a slab has the same moment capacity in both orthogonal directions, it has the same moment capacity in all directions.

 

[canwesteng]

In markets where labor is expensive, the material savings of a triangular pile cap and rebar layout does not outweigh the extra complexity of building and forming that triangular cap.

The 3 pile layout is still common though, and rebar is detailed without going diagonally generally.

 

[EngDM]

If a slab has the same moment capacity in both orthogonal directions, it has the same moment capacity in all directions.

Curious how this works. If the reinforcing steel provides the tensile resistance and the rectangular stress block the compressive, how can the slab be the same moment capacity on a 45° where there effectively is no tensile strand parallel to that span direction?

 

[ANE91]

how can the slab be the same moment capacity on a 45° where there effectively is no tensile strand parallel to that span direction?

There is no orthogonal tensile strand, no, but there are vector components of the two strands in place.

 

[EngDM]

There is no orthogonal tensile strand, no, but there are vector components of the two strands in place.

Interesting. So the strands on 45° in both directions essentially create a resultant of resistance?

 

[HTURKAK]

If a slab has the same moment capacity in both orthogonal directions, it has the same moment capacity in all directions.

A pink star for this post. This is true and literally means the most effective reinforcement is the vertical ones to the yield lines.

 

[Tomfh]

Interesting. So the strands on 45° in both directions essentially create a resultant of resistance?

Yes. The same way a regular slab works in 360 degrees, not just in the reinforcement direction.

It’s can however be more efficient to have reinforcement aligned. If you look at plates from 100 years ago they often have triangular grid pattern or similar; with bars oriented at 45 or 60 degrees across whole plate, to provide extra efficiency. In the old days material was more expensive and labor was cheaper.

 

[KootK]

I’ve noticed engineers design 3 pile, pile caps simplified as a rectangle cap with orthogonal reinforcement how is this justified?

I'm not sure that it is justified. Yeah, sure, you can get your moment capacity to calc out based on the summation of vector components etc. But that's not the entirety of the design and, in my opinion, not even the most important part. At normal pile cap proportions, a three pile cap will behave as a deep beam element (three beams). As such, things like the proper anchorage of the primary tension reinforcement over the piles will be paramount. And a uniform, orthogonal layout is unlikely to have this without special attention to detail. In this sense, at normal proportions, I would say that a three pile cap is not just a slab.

I feel that the three pile caps being rationalized as rectangular caps with orthogonal reinforcement is mostly a product of FEM jockeys falling in love with their stress fields and forgetting the fundamentals (if indeed they ever knew them).

 

[KootK]

So the strands on 45° in both directions essentially create a resultant of resistance?

They do. But, as a I mentioned above, to assume that to be the end of the story is to completely forget that concrete design is all about the detailing. Very often, these vector summation exercises will fall apart in the execution of the detailing.

 

[EngDM]

They do. But, as a I mentioned above, to assume that to be the end of the story is to completely forget that concrete design is all about the detailing. Very often, these vector summation exercises will fall apart in the execution of the detailing.

I meant more for slabs and to further my understanding, I hadn't thought of it like that.

For pile caps I hook the bars always, lock that sucker in even if it may not be explicitly required.

 

[KootK]

I meant more for slabs and to further my understanding, I hadn't thought of it like that.

I know. I was mostly just using your comment as a jumping off point to critique the method in general. You sounded like a kid with a new toy on Christmas morning. I wanted to make it clear that even a new toy can put your eye out if you don't wield it with respect for its limitations.

For pile caps I hook the bars always, lock that sucker in even if it may not be explicitly required

Another jumping off point... Sure, hook them bars if you like. But if those bars are too far away from the piles laterally, the hooking won't do all that much good.