Pile Cap Reinforcing 1

[dylansdad]

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.

 

[miecz]

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

 

[lkjh345]

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.

 

[dik]

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

 

[twinnell]

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.

 

[structuralaggie]

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?

 

[WillisV]

lkjh345 is correct.

 

[dik]

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

 

[civilperson]

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.

 

[dylansdad]

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.

 

[swivel63]

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.

 

[swivel63]

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.

 

[JLNJ]

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.

 

[WillisV]

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.

 

[apsix]

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.

 

[Qshake]

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.

 

[WillisV]

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).

 

[swmiami]

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.

 

[miecz]

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.

 

[WillisV]

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).