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?
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
So is there any possible way to get enveloped Reaction results for point springs for multiple load combinations?
No. If it's a lot of data you could export out the standard spring results and use Excel to help you locate min and max values.
I've gone this route and designed a mat on piles with springs in the R & S directions above each pile mid cap.
My columns, 18" diameter and 20" diameter, ended up being fairly long.
When I read the "column below" reaction, is that the reaction at the bottom of the column/pile?
Meaning I have to take into account the column/pile self weight?
If it is the reaction at the bottom of the pile/column, does this column self weight get multiplied by factors in combinations like 0.6D+0.6W?
How about the mat foundation/pile cap self weight? I suppose this is included in the column below reaction. Is this weight also multiplied by a factor with the rest of the dead load in load combinations?