Bearing strength of concrete VS Compressive strength of concrete 2

[Ahmed A. Alamin]

I'm having trouble differentiating between bearing strength of concrete and compressive strength of concrete, why bearing strength is used istead in some design situatio, for example sizing base plates resting on concrete.. Eurocode-2 gives a values of "2/3 fcd" to bearing strength of cincrete...fcd= characteristic design compressive strength"

 

[jdonville]

American code gives a nominal value of compressive bearing as Pn=0.65 * 0.85 * f'c * A1, where f'c is the design unconfined compressive strength and A1 is direct bearing area. This can be increased if there is a larger "supporting area" A2 below the bearing area by min( 2, (A2 / A1)^0.5 ).

Bearing strength is normally evaluated for conditions where the concrete supports a load from a distinctly different member or element at a well-defined bearing area and location. Compressive strength is a design parameter/intrinsic property of the concrete itself.

 

[Ahmed A. Alamin]

jdonville, What will happen if the bearing strength of concrere is exceeded? Will concrete fail in compression, if so, why don't we use compressive strength...

 

[BridgeSmith]

The design compressive strength of the concrete is its unconfined compression strength. The full bearing capacity is higher because it is the confined compression strength.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[JoelTXCive]

I'll take a stab at this........

The compressive strength of concrete (fc') is a nominal material value. If you were to actually break 100 cylinders of same strength concrete though, you would get a decent amount of variability. This variability comes from different aggregate size, different w/c ratios, and other parameters.

Since you have high material variability, and a small localized load application; the available compressive strength is lowered significantly for bearing pressures. That ensures you stay well below the concrete crushing limit state.

 

[BridgeSmith]

I have to disagree with you, Joel. If there is adequate confinement of the area where the bearing pressure is applied to the concrete, an increase factor is applied to the bearing capacity. In AASHTO, it's called the "confinement modification factor" and it has a value up to 2, depending on how much bigger the concrete pedestal is than the area that the pressure is applied to. IOW, when confined, the capacity of the concrete to support the pressure from a bearing device can be doubled over what it would be for one where the pedestal is only the size of bearing device it supports (no confinement by additional concrete).

After rereading the question and your response, Joel, I may not really be disagreeing with you; we may just be talking about different things. I believe the 2/3 factor from the Eurocode referenced in the OP is a reduction factor that accounts for variability in the actual concrete strength, which I think is what you were getting at. However, that's not a distinction between compression strength and bearing capacity, since similar reductions are used for design compressive strength.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[Ahmed A. Alamin]

I think the variabilty of actual concrete is already taken into account by considering the characteristic strength of concrete fck (the strength below which no more that 5% of test results fall assuming normally distributed test data) and applying the partial safety factor of concrete 1.5; fck/1.5 = fcd ....however to get the bearing strength fcd is further reduced by 2/3