I'm currently designing a RC Pile supported foundation. I'm designing it as a elevated slab w/ columns equivalent to the piles provided by the geotech.
I have design strips spanning both latitude and longitude b/t piles and designated a 2-way system. I had a couple questions concerning this design.
1. In a few cases a column load is directly in the middle of 4 piles. I was expecting the 4 strips (2 lat, 2 long) to split this load equally. Instead it looks like the 2 lat split the load fully, then the 2 long strips split the load. This doesn't seem to exhibit a 2-way slab behavior as I was expecting. Can you further expand on why this is so? Do the long. and lat. strips not act simultaneously? If not, how would I model the design strips to have the slab behave like a true 2-way slab?
2. When would you utilize a 1-way slab instead of a 2-way slab and how would model the design strips in order to do so?
Design strips never affect analytical behavior, other then a small effect from regulating the mesh.
The program integrates the nodal forces of the elements cut by the sections of the strips and then determines the reinforcement required for that resultant force. If you are getting one-way or two-way behavior, that is a consequence of the mesh input, the element behavior, and the load placement, but not the strips. Refer to the program manual, section "55.8 Design strip and design section forces" for more details.
For what you describe, make sure the column above and the point load are modeled as intended and that the mesh in this region is OK.
My advice on behavior is to use two-way slab behavior unless you have a good reason to force the program into modified behavior. Slab and beam systems, for example, are good for beams in one direction and one-way slabs in the other.
Ok, thanks for the response. I have another follow up question concerning design strips.
For my scenario piles are located relatively close at 4'-5' center to spacing. With a 4' pile cap thickness I've elected to use full width design strips as using a center strip/column strip of only 1-'2' seems over conservative. From my understanding using the full width instead of column strips only changes the effective width from which the program calculates the resultant force and capacity. Is this correct?
Correct, use a full width strip for such a thing.
Be aware, the program uses plate analysis theory to design the slab. Using strut & tie approaches might be better for such a thick foundation, but Ram Concept cannot do that. Also, for one way shear checks we will check this at the sections of the strips, i.e. at the face of the support, which is quite conservative for such a thick foundation.
Is plate theory just more conservative for thick foundations or just not as accurate typically?
I would say that plate analysis, which ignores arching or truss action, is usually conservative for such things.