Soil Pressure Under Spread Footings 1

[WiSEiwish]

Hello,

I am designing spread footings for the first time in my career. I understand that the area of the footing is determined by the soil pressure at service loads. Is the load to be used, then, simply a result of running the ASD load combinations, or is there something more to it?

Thanks.

 

[JAE]

You just run through all the ASD load combinations to get the various conditions of stress under the footing.

Some of the load combinations can result in uniform pressure under the footing.
Some can result in non-uniform loading (i.e. a triangular stress distribution across the footing bottom).

Depends on how the structure above is designed, eccentricity of the load on the footings, etc.

When designing the actual concrete footing itself to check for shear capacity, flexural bending capacity in the concrete, etc. then you can use either ASD or convert to LRFD combinations.
In my neck of the woods the LRFD is used for the concrete design.

 

[WiSEiwish]

Thanks. When I think of service loads my mind does not go to combinations, but it did not seem right to, say, design for the whole roof live load in combination with others. The footing strength design will definitely be LRFD.

 

[hokie66]

Why is that? Ast = M/Fs/jd has always been good enough for me.

 

[WiSEiwish]

Not a sleight to ASD but just a comment that I will be using LRFD.