Fixed Headed Auger Cast Piles - Embedment into the Pile Cap

[Armar007]

(Reposting in the correct Forum)
Hello,
I am structural engineer designing AC Piles for the first time. I have a situation where I am using 2- 18" dia. ACPs (w/ Pile cap) to resist moment and shear in both orthogonal directions. (Pile cap is supporting 45' tall H-Frames for a flare line). I don't see much of a problem along the two piles, but in the direction perpendicular to H=Frame (and two piles), I have substantial moment to resist.
The shear and moment values in the pile are within allowable limits. However, one of the senior engineers is insisting that in this situation, pile must be embedded in the pile cap by about 1'. I have never seen so much embedment before. I have seen 4" embedment on most and 6" embedment on some jobs. I tried looking for any guideline that dictates or gives guidance on the amount of embedment but to no success.
I would greatly appreciate if any of you experienced and knowledgeable folks here have any input or a pointer to any document that might have this info.
Thank you in advance.

Regards,
-Arvind

 

[ron9876]

How large is the gravity load. Normally the building code requires a minimum of three piles or restraint in both directions. Sometimes I accomplish this with two piles plus a grade beam.

I normally embed the tops of the piles 4" into the pilecap. The real trick is developing the pile reinforcement into the pilecap since you normally wouldn't use hooked bars.

 

[Armar007]

Unfortunately grade beam is not an option since the pipe supports are 50' apart and pile moment capacity is the only moment resisting mechanism.

 

[wannabeSE]

Often times the longitudinal pile reinforcing extends into the pile cap. Once in a while, the pile cap is thick enough to fully develop these bars. If not, standard hooks are used. I have also seen terminators or other proprietary products used in lieu of hooks to avoid congestion problems.

The title of your post says the pile has a fixed head. How are you providing fixity perpendicular to the frame? What is keeping the pile cap from rotating?

 

[Armar007]

@wannabeSE
I am extending all longitudinal rebars (8~ #8) into the pile cap and have full development length (achieved by terminators).
The only fixity perpendicular to the frame is provided by the flexural stiffness of the piles. I understand that the pile cap will rotate and this rotation will increase the H frame deflection, but I think this total deflection is still within acceptable limits.
I thought the bending and shear from the pile cap to piles will be transferred through rebars (just like beam framing into the column) and the embedement length (4" or 6") was for accommodating construction tolerances in pile cutoff elevations.

 

[ron9876]

My understanding is that the reason that the code requires three piles is because of the flexibility of the piles in bending. Also the piles are typically misplaced somewhat such that there is an eccentricity between the applied vertical load and the centroid of the pile group.

What is the magnitude of the reactions that you are trying to resist?

 

[Gumpmaster]

I think if you can develop your fixed moment with your reinforcing, then no need to embed the pile at all really.

There's an article from the PCI journal "Behavior of Precast, Prestressed Concrete Pile to Cast-in-place Pile Cap Connections" that has good discussion on what it takes to develop moment at the pile cap. Even though you have auger cast piles, the concepts are the same.

 

[TXStructural]

You are calling them auger cast piles, and you mention that there are two 18 inch piles, and 8 bars. I would call these drilled piers unless the bars and grout are inserted/injected through a continuous flight auger. The difference is that a multi-bar pier can take substantially more moment than a small diameter auger cast with a single bar.
I would make the pile cap stiff enough so that it does not rely upon the piers for moment resistance and would fix the tops of the piles to reduce their deflection.

Are you using some thing like L-Pile to computer lateral loading capacity and deflection of the piers?

 

[slickdeals]

A few questions/comments:
1. Has the geotech provided you the flexural length of the pile, i.e., point of fixity at a certain depth below the ground?
2. Not sure how you can make get away from the piers not having to carry the moment in the perpendicular direction, regardless of how stiff you make the pile cap.
3. How sensitive is your H-frame to tip deflections? TX brings up a valid point about the lateral capacity and deflection. You probably assumed a fixed base for your deflections, but in reality the deflections will be more.

 

[slickdeals]

Look in IBC Section 1808.2.5 regarding stability requirements.

 

[UcfSE]

FDOT has some rules they use for pile embedment into a cap and considering fixed or pinned condition.

http://www.dot.state.fl.us/structur...07July/DesignGuidelines/SDG3.5DrivenPiles.htm

 

[FixedEarth]

Your analysis has to match the foundation detailing. If you assume a free headed connection, you don't need much embedment of the pier into the pier cap, and you can assign axial, shear and moment at the top of the pier. Then look at your induced deflection and passive pressures. You will still need to provide hooked "butcher" type connection into the cap.

However, if you embed the pier 18 to 24 inches into the cap, then you created a fixed head connection and it will be prevented from rotation. So you can only apply axial and shear (no moment), then check your deflection. Kind of like a diving board analogy.

It is more conservative to assume a free headed connection and detail it that way. I use Lateral Foundation software to check these things since it will take both the soils values and structural loads. The program will give you maximum moment & its location, so there is no need to assume point of fixity.

You may decide to get the resultant moment and use that value in your pier analysis since you have a round section.