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.

 

[par060]

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...

 

[Robbiee]

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]

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

 

[ron9876]

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).

 

[StructocivilEngr]

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.

 

[ron9876]

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.

 

[escrowe]

I suggest reviewing these lectures for geotechnical concerns.

http://web.eng.fiu.edu/~prieto/geo2/Lecture10-Combined-Footings.pdf

http://web.eng.fiu.edu/~prieto/geo2/Lecture11-Mat-Foundations.pdf

And please remember to thank Dr. Prieto.

 

[ron9876]

Nice article.

 

[concretedoc2]

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.