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
RE: Pile Cap Reinforcing
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
Dik
RE: Pile Cap Reinforcing
RE: Pile Cap Reinforcing
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
RE: Pile Cap Reinforcing
RE: Pile Cap Reinforcing
RE: Pile Cap Reinforcing
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
RE: Pile Cap Reinforcing
RE: Pile Cap Reinforcing
RE: Pile Cap Reinforcing
RE: Pile Cap Reinforcing
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
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
RE: Pile Cap Reinforcing
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
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
RE: Pile Cap Reinforcing
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
RE: Pile Cap Reinforcing
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
RE: Pile Cap Reinforcing
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
RE: Pile Cap Reinforcing