how to analyse this piled slab system?

[ukengineer58]

Hi, I have, by neccessity a very deep piled foundation. Considering the plan size it covers a large area and pick up numerous columns and storage vessels and the like. I planned to design it, due to the depth as a strut and tie system. The other advantage of which is I could limit the top steel provided as based on min steel percentages for tension zones there would be a lot of steel which is probably not actually needed. How would you analyse this as there is a lot of interaction from several directions into the same region. Normally for a thinner piled raft I would do an FEM plate in Stadd and analyse that way but obviously that would form hogging regions and would need RC in the top, in theory needing more steel due the depth. I am not sure if its the most approarpraite anyway as the depth is +2m. One thought I had would be to use a FEM plate to get all my moments (including hogging) then in any region add the sagging to the adjacent hogging over the piles to get back to wl^2/8 or a simple support and provide design bottom steel only for that moment. Provide only steel to limit cracking in the top surface.

Any thoughts from you guys out there? I am not really after general comments about temp. control steel for large pours etc as I can deal with that seperatly, what I am after is any thoughts on the strength design of the section. As there a no limits on cracking of surface (a topping will be applied) and it will be largely covered over I might even get away without any steel in some areas.