40MPa column on 25MPa Base

[sybie99]

It is quite normal to use a lower grade/class concrete in foundations, such as 25MPa. But for columns often higher strength concrete is used such as 40MPa.

My question is, if you have a 40MPa column supported on a 25MPa spread footing, will it be okay.

According to BS 8110 you can use 0.67 of your concrete strength in direct bearing, so I would reason that as long as your can carry the full column load on the bearing area multiplied by 67% of the concrete strength you are fine.

Any thoughts?

 

[asixth]

I do it often. I normally go a footing with a strength grade lower than the vertical bearing element.

 

[hokie66]

Concrete codes allow for this in bearing on footings and in transferring load through floor slabs. I think the provisions are slightly different in different codes, but the principle is that the concrete on which the column bears is restrained, thus capable of carrying higher stress. If the difference gets too severe, sometimes "puddling" of higher strength concrete is done in the floors in the vicinity of the high strength columns.