Design Assumptions for Mat Foundation on Piles - RAM Concept

I'm designing a mat supported on piles in Concept. I exported the model and the loads from RSS, but now I'm trying to model the piles. In the past, I've used a point spring to represent the piles. My spring constant was always Ks=P/Delta, or the pile capacity divided by the expected settlement of the pile. However, I would like to use an equivalent column to be able to check for punching shear in my current model. To do so, I'm using the spring equations of Ks=AE/L and Ks=P/Delta. Since I have a pile size (18" dia), a pile capacity (400 kips), and expected settlement (<1/4"), I have a spring constant I can assume  for my model (400kips / 0.25in = 1,600Kips/in). To find an equivalent column, I believe I simply have to solve for L in the other spring equation. Since A is based on my pile diameter, and E on my concrete strength, solving for L should give me the column length representative of what my spring constant is. Running the numbers, I end up with an 18" dia round column with a length equal to 47.8ft. I then set the columns under the mat and run the model as if it was an elevated slab.

I believe I also have to make the columns "Compressible". Design strips will span between columns, similar to elevated slabs. 

Is this the right approach to model piles as columns?