Laterally loaded concrete pile

RESENG2 · Apr 4, 2007

I posted this in the concrete pile forum, but not sure how many structural users make it over to that forum?? Any help is greatly appreciated

2 story house on the beach. 120/C/Enclosed.
Finish Floor @ 18' (based on Base Flood Elevation)
Existing grade @ 9'
Scour to 5.5'
Using the Coastal Construction Manual, point if fixity is 8.5' below the localized pile scour (2'-8" based on 2x the diameter of a 16" pile).
So that leaves me with a 23'-3" cantilevered pile. I ran my design and I am coming up with a 24" diameter pile w/ (12) #7's.
The builder I am doing this for built a house a few houses down (same grade/scour/etc.) and the previous engineer specified a 14" dia. pile with (6) #6 verticals.
So now I'm scratching my head. Either the previous engineer did not design for any flood loading which is much greater that the lateral wind load, or he assumed the column to be fixed at the top or even fixed at the scour depth (thus the column being ~11' shorter).
My question is, am I correct to assume the column is not fixed at the top? The floor system will tie in ~50 piles supporting the floor beams.
I DON'T WANT TO BE THE "OVERKILL ENGINEER"!!
Any suggestions would be greatly appreciated. Heck, requests of consultation fees for quality feedback would even be considered.

FM1 · Apr 4, 2007

How are the piles connected to the "floor system"?
If you are developing the pile steel into, say, grade beams and, the beams are sized to carry any moment transfered from the piles ( a concrete frame, basically) then, the pile heads are FIXED. They are free to "translate" (all of them, like columns in a story) but not free to rotate. A fixed head.

Otherwise, from what you have explained, you are designing the piles for their tributory loads (axial and lateral) and with due regard to what has been prescribed as the point of fixity in the absence of any nonlinear analysis, which is the logical approach.

FM1 · Apr 4, 2007

I have to add an explanation to the first paragraph of my post;

the pile heads' lack of rotation (or amount of rotation) if the system is solved as a frame, will depend on the relative stiffness of the members connected at each pile head.

It would, still, result in smaller size piles compared with free head condition.

RESENG2 · Apr 4, 2007

Thanks FM1,
I have been buried in the NDS for far too long.
I need to determine the effective length of the column based on the relative stiffness of the column and intersecting grade beams. Then size the column fixed top and bottom with that adjusted length.
How do the load positions move when the column length is adjusted (I'm assuming proportionally?).
Thanks again

sundale · Apr 5, 2007

I would run a FEA model with equivalent lateral soil springs (horizontal subgrade modulus). The bending moments in the pile and the head deflection are a function of of the soil stiffness, the cracked stiffness of the pile and the BC's (i.e. fixed head or pinned head).

The soil stiffness is non-linear (P-y curve stuff) but you are likely not going to allow enough deflection to sweat this too much. Doing a non-linear FEA on soils properties known to ONE significant figure is putting lipstick on a pig.

Get some help from your geotechnical engineer with this. In the soils reports I see, in order to ignore group action effects, piles need to be placed 8 diameters apart.

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