Raft/ mat foundation design 6

[StructocivilEngr]

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.

 

[DaveAtkins]

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

 

[StructocivilEngr]

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.

 

[edwin17th17]

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..

 

[ron9876]

If you are talking about a true mat foundation you have can't assume uniform pressure distribution. Will need FEM analysis.

 

[par060]

....my text book has an example without using FEM....sum up loads determine e...see if its greater than b/6....seems straight forward.

 

[Lion06]

Bowles and Das both say that assuming a rigid mat foundation is perfectly reasonable.

 

[DaveAtkins]

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

 

[edwin17th17]

so you get bearing pressure using "rigid" approach..
then apply it to the plates and design the footing as FEM..
hmmm...

 

[DaveAtkins]

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

 

[edwin17th17]

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?

 

[edwin17th17]

nevermind.. i think i got it.. =)

 

[cessna98j]

The soil spring constant is a function of the subgrade modulus. We typically use RISA for FEM analysis of mat foundations where you can put in a subgrade modulus directly and it calculates the spring constant for the soil springs.

 

[StructocivilEngr]

Thankz to u all for comments,as suggested by Dave Atkins & par060, finding e and design mat for P & M means that the pressure would be different at the four corners of mat, and is it then required to design the mat for uniformly varying pressure!
Alternatively is it acceptable under any code or rules to assume that the bearing pressure under neath the foundation is uniform regardless of wether the load is eccentric or concentric.

 

[mudflaps]

FEM has it's uses, but this sure isn't one of them.

Old CA SE

 

[par060]

you could determine the maximum bearing pressure and design the entire mat for that pressure....or use the allowable bearing pressure for the entire mat.....you could make an arguement that either of these routes are within reason

 

[DaveAtkins]

"Alternatively is it acceptable under any code or rules to assume that the bearing pressure under neath the foundation is uniform regardless of wether the load is eccentric or concentric."

Not true. The bearing pressure under the mat will NOT be uniform unless the loads are concentric. This is true whether you use FEM or design by hand. Find the bearing pressure correctly, and design for that.

DaveAtkins

 

[StructocivilEngr]

David Atkins, if we find bearing pressure correctly and then use its max value under the foundation to design the entire mat considering uniform bearing pressure (selecting max.value of pressure) as suggested by par060, would it be right. par060 is requested for any reference for above practice please.

 

[McQSE]

I found this helpful.

http://www.vulcanhammer.net/utc/ence461/461-sl9.pdf

 

[oneintheeye]

design for the actual bearing pressures. Designing for max throughout wont be correct. think alternate spans in a suspended slab.