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?
I assumed you were performing a 'Normal' analysis, not a Lateral Self Equilibrium analysis. Lateral SE is not appropriate here.
Even in a normal analysis, the structure is very flimsy when supported on tall columns (and instable if the columns are pinned). My instinct tells me to model the lateral resistance of each pile as a point spring, in the plan X and Y directions only, and modeled at slab mid-depth. This is the most realistic place to put the lateral resistance in my opinion. Using an area spring, or line springs on the edges, are the other options I can think of.
You can get the column below and point spring reactions through tables or plots for specific load combinations or rule sets, whatever you prefer.
Point spring reactions are currently only available in the "Standard" context because we don't envelope the point spring reactions. We only envelope things we design, like punching shear reactions for columns.
Consequently, you can plot or tabulate point spring reactions for load cases, or for single load combinations, but you can't plot point spring reactions for rule sets nor for load combinations that are Lateral Groups. Since your model is linear elastic (no area springs) you can manually combine the reactions from the various load cases, or you can alter the combinations to be in a singular form.
Hi Seth - assuming the soil bearing pressure is not negligible, can we set the columns under as described above in addition to an area springs and run as mat foundation rather than elevated slab?
You can combine area springs and columns below. The frequently asked question "Can Concept design for pile and mat (raft) action together?" on the web page below has a word of caution about this, however. If you are not certain of the soil properties, you may want to try a range of area spring constants to see how sensitive the column reactions are to changes in soil properties.
https://communities.bentley.com/products/ram-staad/w/structural_analysis_and_design__wiki/33842/modeling-mat-foundations-rafts