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Wood piling falsework

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I have a design task of designing formwork/falsework for a marine mooring dolphin’s concrete cap.  The cap is 20’-0” square by 5’-0” thick and constructed of concrete.  The contractor is permitted to use up to 12 wood piles to shore the formwork.  The contractor has developed a layout scheme for these wood piles to be placed without conflict of the neighboring permanent steel pipe piles.  The wood piles carry loads of between 35k-50k each (worst case) among wood piles in their own group.  The contactor states that they will obtain 90’ long wood piles with a tip diameter of about 8” and a butt diameter of 15”-18”.  The piles will be 57’-0” in length from the seafloor (river) mudline to underside of formwork (yikes!).  With some exception, I would expect the remaining length of the 90-footers to be driven into the soil (approx. 30’).  Does anybody have experience with the design of a similar system?  I’m concerned with the unbraced length and how to make these piles support their load without buckling.  We can install bracing in the upper 14 feet of the piles, but are below waterline from their.  Does anybody know if the critical section, as mentioned in the 1991 NDS Manual, is the location of the pile at the mudline?  How far does one need to have a pile embedded in soil before fixity (fixed end condition) can be assumed?  Any comments are appreciated.  Thanks in advance.

 

 

 

Bill S. Marczewski, P.E.