Eccentric load on pile cap

[longisland]

Hi all,
I received an as-built drawing of the piling work for my project. The pile in question is a 2 pile pile cap. To begin with, a 2 pile pile cap was designed to save cost & the piles are driven 6 inches off the center. this means there will be a moment induced due to this load eccentricity. How do I go about this problem?
the contractor gave me the as built only when the piling rig has left, so driving another pile to adjust the center of gravity of the pile cap is not an option.
the design load on each pile is 45 ton. the moment induced will therefore be 45tsf. I was thinking of using a tie beam between the pilecap in question & an adjacent pilecap
to counter the moment induced. the beam will act as a column to resist the compression from the induced moment M<=Fxl where F is the compression force & l is the distance between the c of g of the beam to the c of g of the pilecap. Is this approach on the right track?
the stump is 1 feet & the pile diameter is 8 inch diameter.
Any other way to overcome this problem?
thanx in advance

 

[Qshake]

Is this design-build? If not ask the contractor what he proposes should be done to correct his error (I'm assuming that there was some tolerance specified in the job special provision). Often times it will change the contractor's tune to find out how much a fix will cost him vs what it will cost to bring a crane and leads back to the job. And for a two pile bent its just small enough to make me wonder.

As for your fix, it grates me the wrong way to have to rely on anything designed afterward when it should have been built right the first time! However, the fix you propose doesn't seem out of the ordinary assuming you can develop the load to counter the moment.

BTW, how can you have a moment of 45tsf? That is a stress not a moment.

 

[longisland]

hi,
thanx for the early reply
yes, it's design & build. I just got bumped out from another D & B project because I was not CORPERATIVE by telling the contractor not to cut corners; I expect this one possess the same attitude, work ethics & wisdom as the previous one.Changing the contractor is beyond my limited training skill. Here's an interesting quote from my contractor&quot;Do you have any idea how much beer we can have if we don't do this or vice versa?&quot;
Sorry, my my mistake. Point load multiply eccenctricity equals moment. point load is 90 ton with 6&quot; eccentricity from pile cap C of G. That would be 45ft-tons. Stand corrected

 

[dlew]

Your proposed fix should work. We have use the same idea before, where for some reason the piles could not be centered under the load. The moment due to the eccentricity will distribute to the beam and the piles in direct proportion to their respective stiffnesses.
I would design the beam for its calculated moment and check each pile for the axial force combined with the moment it will carry.
AEF

 

[ishvaaag]

If I uderstand well you pretend to use and is correct a centering beam (viga centradora) to solve your problem of developed moment, just to oppose your load and small lever arm with that bigger arm and smaller change of load at some anchor point (next column). For wahtever the reason it has been common in the practice where I have worked to meet this problem, maybe 1 case every 40 or 50 pilecaps or even less. To be strict in demand would be OK, but the owner usually wants keep friends with the pile (in our case drilled) services provider, then the convenience of the centering beam solution.

 

[ronster]

In order to use a two pile cap, the pile cap must be braced in one direction. This is usually by the grade beam running over the top of the pile cap. If this is the case then the eccentricity can be resolved by the grade beam spanning an extra 6 inches and there will be no eccentric load on the piles. If there is no grade beam and it is an isolated 2 pile cap that was designed by someone else then I would look for better clients.