Pile Cap Reinforcing

[slickdeals]

http://www.universityclubtower.com/progress/photo_gallery/2005_2.cfm

In the above link, scroll down to the reinforcing detail for the pile cap. It consists of an "X" shaped reinforcing band over piles. I must admit, I haven't see such a detail before. It makes sense in a way that the piles have highest loading when the tower crane is usually at 45 degrees.

Is there an advantage to reinforcing it this way compared to the traditional orthogonal layout?

It’s no trick to get the answers when you have all the data. The trick is to get the answers when you only have half the data and half that is wrong and you don’t know which half - LORD KELVIN

 

[derim01]

btw this is mat foundation, not pile cap...

 

[ishvaaag]

I think to have made X and radial (at 120º) reinforcement schemes for some columns on 4 and 3 piles, not for cranes like here; alternative designs (mainly a perimetral belt of reinforcement linking the piles) were also considered and maybe used even more for such pìle caps. I don't remember if the reason was on economy or what, but merely making the estimate of the weights following the required reinforcements from trigonometry and minimum steel should give a quick answer.

Most likely for my problem, the preference of those using the radial schemes was caused to ensure significant steel was under the applied load, then making less likely a punching failure. However strut and tie systems are also viable with the belts following the perimeter piles. Most surely those using the radial reinforcement schemes disliked the reinforcement laid in such way since some pyramidal chunk of concrete could be ejected even with a satisfactory strut and tie system at work if not enough complementary reinforcement was placed in the pile cap.

The situation in your photo may follow the same kind of reasoning and as you seem to think, lead to the better strength and stability when the pile (or even soil when no pile cap) response has to concentrate towards some tip of the pile cap.

 

[hokie66]

I thought it was a pile cap as well, but can't see the reinforcement from piles in the corners extending up into the cap. Hard to tell what might be under there. Even more curious is how there is a pad under each of the buried tower legs, and the bottom diagonal bars go through this pad (I think). If it is just a footing without piles, it looks small. Hope they tie it to the building often.

By the way, does that link work for you guys? I had to copy and paste it into the browser. Links don't work for me in the body of the posts, but nobody else seems to be complaining.

 

[derim01]

There are no piles.
These photos are showing only small pads as temporary support:

http://www.universityclubtower.com/progress/photo_gallery/2005_1.cfm

 

[hokie66]

With no piles, this is just a pad footing, and the diagonal configuration of the reinforcement is even more curious, even weird.

 

[slickdeals]

There has to be piles on this job. Hand dug caissons?

http://www.universityclubtower.com/progress/photo_gallery/2005_3.cfm

It’s no trick to get the answers when you have all the data. The trick is to get the answers when you only have half the data and half that is wrong and you don’t know which half - LORD KELVIN

 

[hokie66]

Bored piles, I think. Those big ones are under the core. It appears from later photos that the tower crane was not used in the early stages, but rather mobile cranes were used for the core construction. The crane was braced off the building when the building advanced far enough, thus the building resisted lateral and overturning of the crane, and the relatively small footing just had to carry the crane gravity loading. Still a strange reinforcement arrangement in the footing.

 

[Teguci]

Rebar hooks at the end of a mat don't make sense. Spacing of stirrups doesn't make sense for a mat (tight spacing near loads not equal). 45 deg rebar pattern doesn't make sense for a mat. The big rock with rebar sticking out that they chipping away with a chipping hammer (series 1 photo) doesn't make sense for a mat.

I like the 45 degree pattern. With a fully loaded pile, you'd expect to get crack perpendicular to the reinf. If two piles are loaded, you get two cages activated at a smaller moment arm.

 

[slickdeals]

Teguci,
If you think it is a pile cap, the you should have hooked bars at the end, beyond the pile. You always have hooked bars in pile caps.

It’s no trick to get the answers when you have all the data. The trick is to get the answers when you only have half the data and half that is wrong and you don’t know which half - LORD KELVIN

 

[BigInch]

I'm surprised they didn't use an X-shaped form as well.

What would you be doing, if you knew that you could not fail? Ans. Bonds and derivative brokering.

 

[Teguci]

Slick,
Thats my point. Its a pile cap not a mat (should have used spread footing term I guess).

 

[TLHS]

While this isn't a pile cap, I've often used similar arrangements for four pile caps that sees anything other than primarily vertical loads. It's something that gives you a very direct load path and results in an easy strut and tie analysis.

I'll normally take a standard orthangonal cage to hit a minimum level of shrinkage steel, though, and overlay the necessary diagonal bars on top. It's not normally too many.

 

[BigInch]

Doesn't it tend to overly concentrate two way bending moment resistance at the center, rather than spread it uniformly through the mat?

What would you be doing, if you knew that you could not fail? Ans. Bonds and derivative brokering.

 

[peal724]

i've designed several pile caps for tower cranes and the overturning moment always put tension into the piles. i can't imagine that size base working as just a mat (spread) footing.

also, i've always put the pile cap reinf in an orthagonal pattern with additional bars at the pedastals. The only reason I see to detail the rebar in this manner is if there were only 4 large diameter caissons as the deep foundations.

 

[hokie66]

If you look closely at the first photo in the 4 June 06:09 photos linked by derim01, you can see the bored piles. The one at the bottom shows quite clearly, and the workmen at the top are chipping away at a pile which has been cast too high. However, you would have to question the development of the tensile capacity of the piles, as the excavator looks to have done a good job of mangling the pile reinforcement extending into the pile cap, and the beam cages appear to stop a fair distance away from the corners. Oh well, I suppose if the crane had fallen over, we would have heard about it.