raft mat foundation design

huangyhg

raft/ mat foundation design
i would like that any experienced practicing structural engineer may please describe the steps involved and method of designing of raft or mat foundation, describing any useful hints and valuable information.
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do you have access to a computer program that can do a finite element analysis?  if so, you can model the mat as a concrete plate, subdivided into finite elements, on soil springs.  check service load soil bearing pressures against the allowable soil bearing pressure, and design the mat for factored pressures.
if you design this by hand, you must assume the mat is infinitely rigid.  first, you calculate the soil bearing pressure at all four corners of the mat (check these against the allowable soil bearing pressure).  then, using factored soil bearing pressures, design the concrete mat as a two way flat slab.
daveatkins
thankz, dave atkins for the valuable information. i would like to further know that the bearing pressure at the four corners would not be the same due to eccentricity of loading. do we assume the loads to be concentric! if not, then flat plate would have to be designed for linearly varying pressure. what would you like to say! and other structural engineers are also requested to explain this if they like.
this is interesting.. i have done mat foundation on piles using finite element and it's pretty easy but on soil
i wonder what one will use as spring constant for bearing pressure.. i am assuming one will use rigid approach to get the spring constant.. then finite element??
what the heck is the finite element good for if the bearing pressure was computed using rigid approach..

if you are talking about a true mat foundation you have can't assume uniform pressure distribution. will need fem analysis.
....my text book has an example without using fem....sum up loads determine e...see if its greater than b/6....seems straight forward.
bowles and das both say that assuming a rigid mat foundation is perfectly reasonable.  
as par060 said, you don't assume the loads are concentric.  you find e in each direction, determine if you are inside the kern limit, and design for p and m appropriately.
daveatkins
so you get bearing pressure using "rigid" approach..
then apply it to the plates and design the footing as fem..
hmmm...  
no, if you have a computer program that can do fem, just put soil springs under the mat and let the computer calculate the bearing pressures.  the only reason to assume the mat is rigid is if you are designing by hand.
daveatkins
dave atkins, i didnt understand.. what is the value of soil spring?
i thought the bearing pressure is the one that you should use for fem as soil spring value for computer program..
i guess other engineers here will agree with me?