Offshore batter pile systeme

[Cos0Sin90]

Hello,

Has anyone here designed an offshore platform? Are there any design examples available?

I have this deep pile cap (1.2m deep) on 4 batter piles at 1.8m spacing that is subject to lateral load from lines.
I am wondering:
- Do you assume full fixity between piles and pile cap and design them accordingly?
- How do you design the pile cap? I have run a frame analysis. Do I need to design the pile cap to the resultant shear that I get from this analysis, or should I ignore it considering the pile cap is deep beam, so the stress becomes axial force going directly to piles? (I know how to design a normal pile cap, but this is different)

 

[Sam_G]

Full fixity should be used at the top of piles as the piles will be fully embedded into the pile cap. As for the pile cap design the most accurate way is to design it using finite element software and modelling the pile cap with brick elements. Shell elements or beam grillages are not as accurate for such a deep beam depth to span (1.2m depth vs 1.8m span) However the pile cap is so thick and the span so small the resultant forces in the pile cap should be minimal and it likely minimum reinforcement will govern in any case.

 

[Cos0Sin90]

Thank you, Sam.

While I am not familiar with brick elements, I do agree that it should be modelled as brick element. Just wondering what should I compare with after getting the von mises stress? It should be compared to the compressive stress instead of shear stress correct?
Also, as the lateral load is applied at the top, while the reinforcement from piles does not project all the way to the top where load is applied (only 800-1000 projection with hooks in the cap, with steel pile it is even less). How do you check if the "tie" (from the top of the pile to the position of the load at the top level of the cap) is ok under this lateral load without vertical reinforcement in the cap?

Is there any simply way of doing this? I have found any good example regarding this yet.

 

[Lomarandil]

Maybe I'm missing part of the picture, but I don't see why FEA is required. Shouldn't standard Strut and Tie methods work? (Probably starting from the geometry of a CSRI standard pile cap design)

 

[TTomaz]

Hi, I understand we are discussing steel pile /pile caps.

* For proper shear and moment transfer it is recommended full fixity. Generally offshore platforms have considerable horizontal loadings (wave loads/currents/mooring loads/seism?), so some caution should be given to this moment transfer in the pile / pile cap or slab connection.
* The analysis should be driven by the geometry of the elements under consideration. In cases where the pile cap design involves a beam system, a straightforward frame analysis is typically adequate. You can also "extrude" 1D-frame analysis of a beam to a shell analysis using FEA software "extrude" commands if you want a more detailed analysis.
Conversely, when dealing with concrete slabs, employing shell elements and conducting a standard concrete shell analysis is recommended. 3D-solid analysis are generally more complex and time-consuming.