bundled bars aci-02 7.6.6.2 requirement to enclose within s

huangyhg

bundled bars aci-02 7.6.6.2 requirement to enclose within st
i am trying to find an explanation to the aci-02 (99 also) 7.6.6.2 that reads "bundled bars shall be enclosed within stirrups or ties."  is this a compression provision to prevent the bars from buckling or a flexural provision to confine the concrete?  i am designing a 7'0" deep pile cap with bundled bars and am trying to figure out the background for this to try to find the best solution for a pile cap.  
thanks
my understanding is that the stirrup/tie requirement has to do with concrete confinement -
the pile cap is 7 feet deep!! i have designed hundreds of pile caps nothing at 7 feet. i am interested to know what are you holding up? the ties are meant to hold the large bundled bars in "place" assuming shear does not warrant another type of tie design. good luck.
the pile cap is the foundation for all the shear walls of a 30-story tower in florida. 7'0" is not uncommon for buildings this size.  it transfers all the lateral and gravity loads from the shear walls to the piles.
is the structure in the pensacola area?
i am still having problems seeing the real need for the ties.  say i cut a section through my pile-cap, it would be a 20'0" by 7'0" section;  the bundled bars in the middle are fully confined by a huge mass of concrete, would two vertical bars at the edges serve the purpose to enclose the entire section?
the building is in st. petersburg, fl.
i would expect the stirrups/ties to be transverse to the bundled bars - not parallel to them.
i live in new jersey, but in 1992 i was a consultant for the oceania ii built in sunny isles, florida. as i recall it was about a 30 story tower with pile caps ranging from from 2 to 13 piles in the caps. i do not recall ever doing a 7 foot thick cap. 5 foot was the max and some piles were battered to resist lateral forces. you have a unique cap with bundled bars and probably should divide the ties-up equally across at a nominal width and vertically do the same. one tie as you suggest would be useless as you are indicating. good luck.
i look at aci 318
7.6.6.2   bundled bars shall be enclosed within stirrups or ties.
r7.6.6 ---bundled bars
sentence 2. bundled bars shall be tied, wired, or otherwise fastened together to ensure remaining in position whether vertical or horizontal.
it looks like there are two requirements. both to tie the individual bundle tightly to act as a unit and maintain the geometry you intend. and also to enclose the bundled steel as you would typical long beam bars with stirrups or with ties as in a column.
as to the effect this tying may produce....
look how the bridge columns failed in the loma prieta earthquake. the ties act like a cage to contain the concrete after failure to maintain a semblence of integrity after failure allowing for safety. the bars in compression act like independent columns. the ties then act to limit the unbraced length of the steel rebar columns.  even though the calculations don't reflect this, that is another intent. beam bars in compression are similar. only, the requirement is specific for bundled bars. the bundled bars have their advantages, but a disadvantage may be their greater tendency to buckle compared to individual concrete encased bars.