mat foundation supported on piles

huangyhg

mat foundation supported on piles
when modeling a mat foundation supported by piles, is it common practice to modify the pile spring constant such that all pile share the load without exceeding their allowable values. this analysis is being performed in ram concept, so the program is taking into account the relative stiffness of the mat. the piles are input as point springs.  i would not imagine that the spring constant for a particular pile type would change.  any and all thoughts would be appreciated.
i am in the process of modelling a piled substructure with springs to represent the soil behaviour.  i will be comparing results of this computer model to a mathematical model such as broms' method for added peace of mind of the results.
1. the spring stiffness will depend on the modulus of subgrade reaction which is most accuratley determined by load tests. it is unlikely that you will have load testing done and therefore must make an estimation on the modulus of subgrade reaction. any assumption is really valid, the modulus of subgrade reaction increases with depth, this shouldn't be an issue because you are designing a mat foundation.
2. if your assesment of spring stiffness is between half and double the correct value than your distribution of forces will be close enough to the correct answer. incorrect assumptions of spring stiffness will only affect the soil settlement that is calculated.
a good reference to this theory is bowles.
if anyone else has tips out there on how to model soil as an elastic spring then please share.
go the hawks.  
if you are using ram concept i ususally use columns below to represent the piles that way it will check punching shear. i use the same e and a as the piles are going to be and adjust the length to equal the spring stiffness for the pile.