modeling mat foundation

huangyhg

modeling mat foundation
hi all
  i use some fem programms for solving mat foundation by making plate (shell elements)and assigining k spring.
now i have a rc mat foundation under it there is pc with thickness 60 cm and the mat thickness is 150 cm.
i decided to model only the rc mat but i want to know how can i get the value of k srping taking in consideration the existance of pc between the mat and the soil?
                                       thanks all

by k, i assume you mean the stiffness. the way it works in the fem product i use (staad.pro) is something called an autoamtic spring support generator for elastic mats where you provide something called a subgrade modulus (i had asked a question about this very thing a few months back). this constant is in units force/area/length. staad will multiply this constant by the influence area of each node/plate you want the mat to reside under to come up with the k value for each node. the process takes about 2 minutes. it was a huge time saver. you can also get the base pressure diagram after that.
dawn836, your problem is different. fleisher misunderstood you. for your particular case, you should keep in mind, the way, value of "k" is obtained. it is obtained by plate load test (i rely on plate load test for larger jobs). since the extent of pc is very finite, we can not consider it as a soil layer. presence of this pc layer will not change the soil behavior. i would suggest to ignore pc. take the value of "k" for the soil only.
thank you msajjadh and thanks fleisher for his care for my question althought he realy misunderstood the question
if i do not have plate loading test in soil report. would i use the allowable bearing capacity multiplied by maximum allowable settelment to get the modulus of sub-grade?
for detailed description consult j. e. bowles "foundation analysis and design" page 501. there, you will find a table for the range of k values. use the average value for the specific soil condition applicable to you.
thanks for appreciation.
ask your geotechnical consultant.fff">  i cannot imagine taking the time to perform a detailed computer analysis (staad pro) yet making a wild guess about the soil behavior - a potentially critical component in the analysis. and if you are concerned about deflections, you need to get really good geotechnical information.
don't guess.  the reputation you ruin will likely be your own.