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Raft/ mat foundation design
6

Raft/ mat foundation design

Raft/ mat foundation design

(OP)
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.

RE: Raft/ mat foundation design

2
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

RE: Raft/ mat foundation design

(OP)
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.

RE: Raft/ mat foundation design

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..
 

RE: Raft/ mat foundation design

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

RE: Raft/ mat foundation design

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

RE: Raft/ mat foundation design

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

RE: Raft/ mat foundation design

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

RE: Raft/ mat foundation design

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

RE: Raft/ mat foundation design

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

RE: Raft/ mat foundation design

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?



 

RE: Raft/ mat foundation design

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

RE: Raft/ mat foundation design

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.

RE: Raft/ mat foundation design

(OP)
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.

RE: Raft/ mat foundation design

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

Old CA SE

RE: Raft/ mat foundation design

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

RE: Raft/ mat foundation design

"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

RE: Raft/ mat foundation design

(OP)
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.

RE: Raft/ mat foundation design

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

RE: Raft/ mat foundation design

That is true....you may have cases where the spans are such that useing the max pressure(uniform load) can be unconservative....I am sitting here trying to think of a time thats happened to me and I can't.
The max pressure typically occurs at an edge in a ridgid foundation...and then that pressure decreases as a triangle load so spans are loaded differently...could this cause a greater moment than the max load?....maybe it could...

 

RE: Raft/ mat foundation design

DaveAtkins:
You said "determine if you are inside the Kern limit, and design for P and M appropriately."
what happens if you are NOT inside the kern limits?

RE: Raft/ mat foundation design

....2P/xw....where x=3((L//2)-e)....

RE: Raft/ mat foundation design

How thick is the mat compared to the slab span that you are referring to. Will you use a constant thickness or increase thickness at columns. If constant thickness this will increase concrete volume. Likely more cost effective to vary thickness and use more detailed analysis method. I believe that ACI says not to use equivalent frame for soil loading. If not sufficiently thick/stiff pressures will vary under the slab with increases near the columns(in addition to M/S).

RE: Raft/ mat foundation design

(OP)
Will ron9876 or any one refer the ACI code Clause No. that discourages the use of equivalent frame for soil loading. If it is correct then FEM is the only choice available.

RE: Raft/ mat foundation design

I don't see it in ACI 02. I am pretty sure that I have seen it before in older versions.

The reason that it wouldn't apply, in my opinion, is if there is relative vertical movement of columns the stresses developed due to the differential movement won't be accounted for. I guess if you feel that the mat is stiff enough to resist relative movement (which is the assumption that I apply for say a 5' thick shearwall pilecap) then maybe it would be okay. I think if you have say 4' thick areas under columns and 8"-12" thick between columns that there may be differential movement.

RE: Raft/ mat foundation design

Nice article.  

RE: Raft/ mat foundation design

I would suggest reviewing ACI 336 "Suggested Analysis and Design Procedures for Combined Footing and Mats".  I used to be a member of that committee and we provided typical design procedures and assumptions.  This would be a good place to start and then you can move from the references listed in that document.  

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