drilled pier boundry conditions

huangyhg

drilled pier boundry conditions
one of my current projects is a 16' x 44' mat slab on drilled piers supporting 3 12'dia. x 32' x 162k (full) fermentation tanks (beer!). i had a geotech bore and provide me with recommendations (600psf skin for piers, k=50pci for mat). in one of my geotechnical books i found a method for computing short and long term settlement for the 18" drilled piers. would you model these piers as springs or fixed? i would think some sort of spring would be correct but one of my co-workers dissagrees. my fea model for the slab is greatly affected by what i set as boundary conditions. thanks. this forum offers a wealth of knowledge and i will contribute to the best of my ability.
i would model the piers as pin supports.  i am not sure why the geotechnical engineer gave you a subgrade modulus for the mat--it doesn't make sense, since the mat is supported on piles and will not settle.
daveatkins
the piers are only along the 44' edges to help with om. would you ignore the soil underneath the slab and assume a clear span (14')? thanks.
you could model the piers as pins, and provide springs under the rest of the mat.  depending on how thick your mat is, the piers will probably still take most of the load.
daveatkins
you can also use the tried and true floor-ceiling method.
first analyze as fixed, then with springs - you know the answer is somewhere in between so if you design all components of your mat to meet both you are covered.  a bit conservative, i admit, but "when in doubt, make it stout".

thanks dave and jae. i checked it both ways and designed for the envelope. big, tall and heavy tanks in earthquake country. don't want to take any chances.   
locating the piles under the cg of the tanks would be more cost efficient.
this is my pier / mat arrangement.  
nice pun, jae.
don't spill the beer; agree with jae, make the lager stout.
stout is good, especially when you drink it.