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Raft/ mat foundation design
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Source：Internet Author：Unknow Pubdate：2008-10-29
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StructocivilEngr (Civil/Environme)
26 Oct 08 8:49
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 (Structural)
26 Oct 08 14:18
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
字串6
StructocivilEngr (Civil/Environme)
27 Oct 08 8:27
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 (Structural)
27 Oct 08 9:45
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.. 字串2
ron9876 (Structural)
27 Oct 08 11:33
If you are talking about a true mat foundation you have can't assume uniform pressure distribution. Will need FEM analysis.
par060 (Structural)
27 Oct 08 11:55
....my text book has an example without using FEM....sum up loads determine e...see if its greater than b/6....seems straight forward.
StructuralEIT (Structural)
27 Oct 08 12:02
Bowles and Das both say that assuming a rigid mat foundation is perfectly reasonable.
DaveAtkins (Structural)
27 Oct 08 13:28
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
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edwin17th17 (Structural)
27 Oct 08 15:44
so you get bearing pressure using "rigid" approach..
then apply it to the plates and design the footing as FEM..
hmmm...
DaveAtkins (Structural)
27 Oct 08 15:46
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 (Structural)
27 Oct 08 15:51
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? 字串3
edwin17th17 (Structural)
27 Oct 08 16:21
nevermind.. i think i got it.. =)
cessna98j (Civil/Environme)
27 Oct 08 18:43
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 (Civil/Environme)
28 Oct 08 12:45
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.
字串9
mudflaps (Structural)
28 Oct 08 13:13
FEM has it's uses, but this sure isn't one of them.
Old CA SE
par060 (Structural)
28 Oct 08 13:38
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 (Structural)
28 Oct 08 15:18
"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
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StructocivilEngr (Civil/Environme)
29 Oct 08 8:32
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 (Structural)
29 Oct 08 9:00
I found this helpful.
http://www.vulcanhammer.net/utc/ence461/461-sl9.pdf
herewegothen (Structural)
29 Oct 08 9:56
design for the actual bearing pressures. Designing for max throughout wont be correct. think alternate spans in a suspended slab.
par060 (Structural)
29 Oct 08 10:41
字串2
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...
Ailmar (Structural)
29 Oct 08 10:59
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?
par060 (Structural)
29 Oct 08 11:15
....2P/xw....where x=3((L//2)-e)....
ron9876 (Structural)
29 Oct 08 12:06
字串7
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).
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