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Why Different Concrete Strength for Footings Vs. Walls Vs. Slabs 1

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learning2geotech

Civil/Environmental
Apr 4, 2019
38
Usually on a structural drawing's specifications, the concrete strength for footings are lower than the concrete strength for walls and slabs. Why?
 
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It depends where in the world you are practicing. It has to do with the thickness of the members. Footings tend to be thicker
than walls and slabs (due to punching etc) and do not necessarily benefit from increased concrete strength that much.
 
In addition to SteynvW's comments, the stress mode also dictates. For instance, footings are typically fully ground supported and are generally not loaded so as to create a lot of flexure or bending moment. Their stress mode is mostly shear. Similar for slabs on grade, though if subjected to traffic (forklifts, etc.), flexure becomes important as well.

Vertical members are usually long and slender, thus susceptible to bending, eccentricities and buckling. For each of these, the concrete strength comes into play.





















 
Cost, higher strength concrete costs more so isn't usually used used were higher compressive strength is required or is more efficient. Adding extra concrete to a deck would result in larger beams and substructure units all the way down the load path, its more cost effective to bump the compressive strength. Adding extra concrete and weight to a footing or column isn't usually that big a deal as you only affect the dead load of that member on the ground or piles.

that and as others have said, compressive strength plays a larger role in efficiency with flexural members than compressive members.
 
I don't know who made the drawing, throughout my practice time, I only specify two different grade concrete, one for mud slab, or anywhere a lean concrete is suffice, and one for all others. There is no good reason for specifying different grade concrete to different structures. Also, in these days, if you order three or more concrete with different strength, you may end up receiving only the highest grade concrete anyway. Note, the above is true for normal weight concrete with a 28 days compressive strength of 5000 psi and less.
 

I have observed in most of the cases the concrete grade of foundation and columns will be more than slabs and beams please note that this is not the general rule that's why I said in most of the cases
The reason number one is foundations and columns are compression members whereas beams and slabs are tensile members
the reason number two is the load shall be transferred from slabs to beams, beams to columns and columns to foundations.
The third reason is its economical to reduce the grade of concrete for slabs and beams.

 
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