how to increase pile cap capacity

huangyhg

how to increase pile cap capacity?
i'm a structural engineer working on a one story pre-cast concrete parking deck project supported by h-piles. now that construction of the foundations is nearly complete, we've been asked if another level can be added (until this point we had been specifically directed not to design for additional floors).  among the many issues is the fact that even though we may have enough capacity in the piles themselves, the caps won't support the additional punching shear load.  there is only aboout a foot or less to work with from top of cap to top of floor slab.  i'm asking for ideas or experiences in dealing with this problem.
-jcali  
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the most obvious is increasing the concrete strength to increase punching shear.
the caps are already in place.
the punching shear is due to the upward force of the pile (each individually) transferring load through the cap to a singular wall or column.  if you have the vertical space, a steel bracket could be wrapped around the base of the column to create an extended footprint for the column, thus spreading out the column reaction from its initial cross sectional shape to that of a larger area....this might reduce the punching shear that would develop and the cap would become just a mass under compression.
sort of like steel columns where a singular base plate is not good enough and you use vertical, triangular stiffners around the column sides to reduce the base plate thickness.
jcali-
this is a new problem to me, but i have a few ideas.  there's a lot to consider in solving your issue but perhaps these initial thoughts are a start:
1.  increase the depth of the shear cone above each pile (and thus punching strength) by bonding (via vert dowels embedded in the existing cap and bonding agent) an additional layer of concrete above the cap. if the top of the existing cap is well roughened, no doubt that would help bond the two layers together. your vert dowels may need to be hooked to develop.
2.  another design approach could be to reinforce this new layer for both bending and shear forces almost as if it was a shallow conventional reinforced concrete beam.  you could easily add vertical stirrups in the new area, if needed, depending on your loads.
3. another idea might be to increase the bearing area of the piles on the underside of the cap by welding something to the steel at the interface between the cap concrete and the top of pile.  you'll have to dig to do it.  in essence you are making the shear cone wider.
4. can you drive more piles at this point near the existing caps and tie everything together with grade beams to increase capacity?
just a few thoughts.  good luck.


what about some type of external grillage?  
although the fondation has been placed, you may be able to increase your concrete shear strength by accounting for the actually concrete strength.  check you concrete breaks and/or core your existing pile cap for an in-place strength check.  aci 318 has provisions for checking in-place concrete strengths.
in-place foundation concrete strengths will often be higher due to the cooler, more humid surroundings in which they cure.
also, sharpen you pencil and check your punching shear calculations.  verify you have applied the correct load and phi factors, and that you have accounted for both concrete and reinforcing steel shear strengths permitted by the respective aci provisions.
good luck.