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Pile Cap Reinforcing

Pile Cap Reinforcing

Pile Cap Reinforcing

(OP)
I have been consulted to review the following situation.  Here is what I have.  A square pile cap 8'-6" x 8'-6".  The piles are on a 4'-6" square pattern with 2' from pile to edge and 18" round concrete piles extending 4" into the cap. The pile cap is 3' thick with reinforcing 2" clear above the top of the piles.  Apparently, the bar detailer did not pick up on the 180 degree hooks for the reinforcing which is 9-#8 ea. way.  These caps were poured with this "straight" reinforcing.  I have a set of plans and am just beginning to chase down the loads that are involved. W14 steel columns bear directly (grout) on top of the cap.  I am at a loss as to how this slipped through the cracks, and the several sets of eyes that should have caught this.  Is there any remedy other than tearing these out and replacing?  I should add that 6-#8 and 1-#9 extend from the piles to near the top of the cap.

RE: Pile Cap Reinforcing

You may not need the hooks.  From the information you gave, it seems as though you have enough development length for a straight bar.  

RE: Pile Cap Reinforcing

I think you may have a problem.

Pile caps are not designed as flexure members, but as a 'tie-and-strut' model. The bars need to be fully developed by the centerline of the pile, not the centerline of the cap. With 2'-0" from edge of cap to the centerline of the pile, minus 3" from edge of concrete to tip of bar leaves a 1'-9" development length. Not enough to develop a #8.

RE: Pile Cap Reinforcing

9#8 seems high for an 18" conc pile.  I've often used a single 20M (like a #6) for 18" and 15M for 16" unless there is flexure involved.  If the load is axial compression only, the concrete bearing at the pile/cap interface should be adequate to transfer the load.

Dik

RE: Pile Cap Reinforcing

All buildings have to resist lateral load.  If a pile foundation is used, each pile in the pile caps that resist the lateral load will have moment in them, due to that lateral load.

RE: Pile Cap Reinforcing

The pile cap had 9-#8 each way, at what location?  I am visualizing reinforcement in two mats top and bottom, but that is not exactly what you said.

At any rate, the only fix I can think of is to convince yourself you don't need the 180 degree bends.  If signicant bending or tension is being transferred from the column, it might be hard to convice yourself of that.  Compression only?  That might be easier to swallow.  Maybe not all will have to be replaced?

RE: Pile Cap Reinforcing

lkjh345 is correct.  

RE: Pile Cap Reinforcing

Sorry... cap rebar not pile rebar...

RE: Pile Cap Reinforcing

If the ends of the #8 bars are within two inches clear of the side walls, then chipping in 4-5" will expose enough bars to weld a cross bar for a mechanical development of the existing bars.  Cast back a repair cover and leave the majority of the pile cap intact.

RE: Pile Cap Reinforcing

(OP)
Thanks for the prompt replies and advice.
civilperson:  the plans (who looks at those anyway?) call for 2.5" clear.  I am leaning toward a solution such as this.

RE: Pile Cap Reinforcing

what's the moment at the face of the column, and is there enough development to that point?  it seems that there is plenty to develop the bars.  i usually put 90 degree hooks for added safety, but for the most part i've seen straight bars working just fine.

RE: Pile Cap Reinforcing

isn't strut and tie for a deep beam shear check vs. just a flexural check?  someone correct me if i'm wrong...but i'd venture to say those straight bars are there for flexure and not anything else.  

RE: Pile Cap Reinforcing

You may be able to get it to work without resorting to the strut and tie model. I have an old CRSI handbook which has a good discussion on pile caps and their details show no hooked ends on the bars.

RE: Pile Cap Reinforcing

swivel63 - strut and tie is for deep beam type action where there is more of a likelyhood to develope a compressive strut between the load and the support and a tension tie between the supports than it is for the member to actually bend and "flex".  Strut and tie action will normally begin to control if the angle between the applied load and the support is more than about 30 degrees.  In this case, with a 3' thick footing and the pile center (support) located 2'-3" from the center of the applied load, the angle is greater than 45 degrees and compressive struts and tension tie action will most certainly predominate over flexure.  When this occurs, as lkjh345 pointed out, the tie steel must be fully developed at the support (which in this case is the center of the pile).  CRSI discusses this in their pile cap design provisions and required hooks at the ends of the reinforcing for all configurations where only one pile row is located beyond the point of load application ( so 3 and 4 pile configurations) to prevent this type of "tied-arch" failure.  

RE: Pile Cap Reinforcing

I agree that it's the strut and tie action that must be checked.
Your only hope is if the bottom mat is over reinforced so that you don't require the full development length.

RE: Pile Cap Reinforcing

Pile caps are routinely designed as flexural members and checked for beam and punching shear.  

You can check any reference you'd like on this but the following will bear this out:

Foundation Engineering by Peck, Hanson and Thornburn

Principles of Foundation Engineering by Das,

Foundation Analysis and Design by Bowles.

Most works on this have ratios of distance from face of the column to the pile divided by the depth of the footing to determine if the member can be treated as a flexural member.  If that ratio is too large, the pile become ineffective in resisting overturning loads; too small the footing is more or less designed only for vertical loads.

Strut and tie as noted by willisV is for deep concrete members.  Although it could likely be used here, the pile cap is likely to be too short.

Regards,
Qshake

Eng-Tips Forums:Real Solutions for Real Problems Really Quick.

RE: Pile Cap Reinforcing

I agree that pile caps larger than 3-4 piles are designed as flexural members and rightly so.  However for small 3-4 group pile caps supporting a "gravity-only" column, the design should be performed with the tied-arch method per CRSI (which in general provides steel requirements that agree pretty closely with a flexural check however it requires the tie steel to be devloped at the pile centerlines as opposed to the face of column).

RE: Pile Cap Reinforcing

Pour another layer of concrete on top of the existing pile cap. Make them composite by roughening the surface and/or install dowels. Let the column sit on top of the new concrete. The tensile force in the "tie" is smaller with a thicker pile cap so that the required development length can be reduced.

You can also embed a structural steel mat in the new concrete to transfer (at least part of) the load directly to the piles.

RE: Pile Cap Reinforcing

According to Figure 17-14 of PCA's Notes on ACI 318-02, the development length doesn't begin at the centerline of the pile, but at the "extended nodal zone".  If this extended nodal zone were long enough, then there would be enough development.  Trouble is, the zone is dependent on the width of the strut, and PCA calculates the width of the strut as though the strut were stressed to the limit, while the surrounding concrete is not stressed at all.

It doesn't make a lot of sense.  If the concrete that makes up the "strut" is in compression, then so is the surrounding concrete. Ignoring this obvious fact, and assigning all of the compression to a tiny strut, the extended nodal zone shrinks to nothing, and the tie must be developed beyond the pile.  If the width of the strut could be chosen at some realistic dimension, like 1/3 of the footing depth, then the extended nodal zone would increase, and the development length would make more sense.

RE: Pile Cap Reinforcing

jmiec - traditionally CRSI has not used the formalized "Strut and Tie" method in ACI which you are referring to (which wasn't codified until fairly reasontly) but rather a simple tied-arch analogy assuming that the bars must be developed to the required amount by the centroid of the pile (see Figures 13-12 and 13-13 and the associated discussion on p13-22 of CRSI-02).  

RE: Pile Cap Reinforcing

Is the column at the center of the pile group?

If it is, the direction of the tie is not perpendicular to conrete edge. More room is availble for the development length.

In addition, I would check As,required/As,provided, as apsix mentioned (this may not be recommended in some cases, e.g., in seismic design). If the tie is in an angle with the rebars, #8 in two directionis will be involved, As,required is even smaller.

Another thing to check is the lateral capacity of the pile as a bottom safe guard.

RE: Pile Cap Reinforcing

(OP)
Thanks for all of the responses.  The column is a steel column sitting directly in the middle of the cap.  We have decided to add additional concrete at the sides and above to "cap" the pile cap. We will drill and epoxy additional bars at the sides.  Thanks again for an interesting discussion.

RE: Pile Cap Reinforcing

I would like to support Qshake on this one.
One an observational note it has only been recently I have seen hook bars in pile caps. They are more difficult to install and as Qshake pointed out do little to add capacity.
I would be careful in capping the pile cap. Be certian you can transfer the shear and tension, and that the anchors are properly installed. If the cap is suseptible to freeze thaw, besure the joint will exclude water.
My concern here would be that you would take an acceptable foundation and through the course of repair, create a problem.

RE: Pile Cap Reinforcing

geez, i guess it really does show that if you give 20 engineers the same problem, you'll get 20 different answers.

RE: Pile Cap Reinforcing

WillisV-

I see that CRSI does not include the "nodal zone" concept.  CRSI (I'm looking at the 1992 Handbook) makes some vague statements about the cross bars and compressive reactions contributing to confinement, but then doesn't modify the development length to account for these effects.

I'm coming late to the strut and tie table, and it seems that the approach is not precisely agreed upon.  My point is that strut and tie begins with a fictional assumption (isolated tiny struts) that leads to a requirement that doesn't feel right (ties must be developed beyond the pile).  The approach is surely conservative, but in this case, it seems too conservative.

If one includes the nodal zone concept and widens the strut width, then the development requirement seems more realistic.

RE: Pile Cap Reinforcing

jmiec - I agree that the strut and tie design procedure is a bit in flux and is open to some interpretation and judgement.  I also agree that it is a more refined procedure than that used by CRSI.  My point was simply to note that CRSI had been using their own simplified tied-arch model for years for these specific pile caps that required development at the pile.  

RE: Pile Cap Reinforcing

Check ACI318R-05 Appendix A, the development of the tie rebars is more the the distance from center of the pile to edge of pile cap. The anchorage seems Ok therefore.

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