Eccentric Wall on Pile Cap

[JNEnginr]

I have a reinforced concrete wall being constructed on a pile cap, but due to an adjacent building, i have to move my cap and piles over (see attached image) When i run analysis in RAM concept, im getting a huge downward force on the first row of piles (leftmost), and uplift on the right piles, as would be expected for a cantilevered beam (see attached). My question is, is this the right way to analyze this? You can see that i want to use grade beams off of the grade beam and to the right to stop this cap from rotating. If i do that, should the way im looking at this change?

If you were looking at this, would you go about solving it a simpler way? I feel like sometimes i get lost in the World of RAM Products and miss the Keep It Simple mantra. Help/Criticism is much appreciated.

 

[JNEnginr]

Here's the other image

 

[hokie66]

I would do it by hand as a sanity check. But it sounds like your program is leading you in the right direction. The piles closest to the wall will certainly carry the most load, even if you use rectifying beams to limit the rotation. Is it possible to just use one row of piles, of higher capacity? In conjunction with the rectifying beams, of course. The closer the piles are to the load, the better.

 

[mtu1972]

Before computers we may have solved this using the grade beams as cantilevers over (or through) the pile cap to pick up the wall. Then the pile cap would provide the support for the cantilever beams. This would result in the center of the pile group supporting the vertical reaction. A variation of what was called a "pump-handle" footing.

I realize that computers have made our lives better, but sometimes things like this are troublesome. I would have deep concerns with putting in piles and having half of them take uplift loads and the other half having to support an exaggerated vertical downward force.

I'm not saying that the hand solution is better, but I am suggesting that maybe a more encompassing computer model (including the grade beams, etc.) will provide a result that is more compatible with the hand calc described above.

gjc

 

[JNEnginr]

As far as proximity to the adjacent building wall, I was under the impression that it was difficult to drive a pile any closer than the 2'-6" to 3'-0" range (to center of pile) Anyone have any say on that?

 

[KootK]

3' is the "safe" number that I've been quoted locally. Much depends on the rig, the pile type, and the pile depth. And a bunch of other stuff no doubt.

Will this wall be attracting any building shear?

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.

 

[KootK]

And that's centreline.

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.

 

[KootK]

Is it possible to just use one row of piles

I think that this is the answer. Ditching the second row of piles should reduce the load on the first row. As such, there's no doubt that this is possible.

I'm always skeptical of having tension piles right next to compression piles. It seems to me that, end bearing aside, doing so would require using the tension pile to pull upwards against some of the same soil that the compression piles are pushing down on. And nothing that I've seen in geotechnical reports leads me to believe that this condition is commonly considered.

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.

 

[JNEnginr]

KootK,

Yes, that wall is a shear wall, so depending on the load case, a high concentration of load will be attracted to either the top or bottom pile. I agree with the one row of piles as well...better to up the capacity of the piles than create the uplift. I'll send grade beams back and resole the uplift more efficiently. What about a situation like this? (see attached) Would it be better to remove to of the piles (the top left and the bottom right) and send grade beam back and resolve that way? Concept analysis would show an uplift at the top right pile...i know i could probably support the pile cap with a single pile, but that just doesn't seem like the best idea. Thoughts?

I appreciate everyone's help thus far..

 

[KootK]

Yes, that wall is a shear wall, so depending on the load case, a high concentration of load will be attracted to either the top or bottom pile.

Read. My. Mind.

Would it be better to remove to of the piles (the top left and the bottom right) and send grade beam back and resolve that way?

Could do. It may be more economical this way.

1) Delete the diagonal grade beam.
2) Delete the pile cap.
3) Delete the two upper piles.
4) Use the EW grade beam to support the load using two piles closest to the column.
5) Use the NS grade beam to resolve any minor eccentricity, and to hold up cladding etc.

I'm assuming that the EW and NS grade beams will be required for practical reasons, no matter what path you take.

With all of these scenarios, keep a close eye on punching shear and rebar detailing below the columns. You seem pretty savvy with this though so I'm probably wasting my breath.

Regarding RAM, how does it treat the piles? As a rigid vertical restraint? Or as vertical springs of some sort?


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.

 

[JNEnginr]

I like that idea alot. Make sense and is simple.

Thanks for the heads up on the detailing, should be good there.

Currently in RAM its modeled as a 5ft tall column, fixed at the far end, so rigid vertical.

Not to beat a dead horse, but I'd be treating the two piles as a single support, correct? Not as 1 compression, 1 tension pile. If i left the two pile in place as currently drawn, i pretty much have the same scenario as my original pile cap question. So I'd probably want to take that bottom right pile and move it to the right more if i was really worried about the uplift, but then again, now I'm back to supporting the column on a single pile which worries me.

I feel like I'm overthinking this now.

 

[KootK]

I'd be treating the two piles as a single support, correct? Not as 1 compression, 1 tension pile.

Yesir. As hokie66 said, the closer to the load, the better.

I feel like I'm overthinking this now.

There's no such thing. We think until we're confident in our understanding, then we execute. Anything less would be remiss.

I think that the model may actually be leading you astray. I think that it's overestimating the stiffness of the piles, particularly in tension, and causing the stiffness of your rectifying beams to be neutered.

One nice feature of piles is that their capacity is kind of plastic in nature. If you get those crazy high compression and tension forces, give in the system will just cause it to migrate towards the assumptions that we're making here. Grade beam stiffness is your friend here.

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.

 

[JNEnginr]

Gotcha. Thanks for the insight, I'm feeling better now.

 

[JNEnginr]

One more question,


Based on experience, whats the closest that two piles can be driven next to each other? Im sure it depends on the pile diameter, so lets say I'm using 8.625" piles. Thoughts?

 

[KootK]

My experience with driven piles is limited. I think that you could install them pretty close together so long as they weren't touching once out of plumb tolerances were considered. For the CIP piles that I usually deal in, group action reduction factors start to kick in below a center to center spacing of 3d. Ultimately, you'll want to ask your geotechnical engineer.

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.

 

[JNEnginr]

I did just that and she said approximately 2ft o.c. is what she sees for steel pipe piles with an 8.625" diameter, just FYI in case you were interested.


Thanks again

 

[KootK]

I'm interested, particularly since 2'/8.625" ~ 3. Thanks for reporting back.

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.