Bearing strength and compressive strength of concrete

[novembertango88]

Hello all

Can someone please explain the difference between bearing strength and compressive strength?
I just don't get it.
In the British code BS 8110 the bearing strength is 0.4*comp. strength and I use it for checking under the compression zone for base plates etc but I just don't get why I can't use the comp. strength.

Thanks!

 

[KootK]

Over here in North America, I believe that the difference simply comes down to:

1) Factors of safety and;

2) Possible benefits of confinement.

 

[dik]

In Canada, we go quite a bit beyond 0.4 f'c (nearly 1.5x) providing additional bearing capacity for confinement as Koot mentioned.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[w-streng]

A coefficient of 0.6 takes into account the difference between lab test and site strength. Then the materials safety factor for concrete in bearing is 1.5
So 0.6 / 1.5 = 0.4

 

[rscassar]

It differs with application. Plain concrete generally the lowest 0.4f'c (0.6f'c in my jurisdiction). Columns will be around 0.8f'c and well confined zones such as column thru floor slabs and confined bearing areas in reinforced concrete would be as high 1.33f'c.

 

[HTURKAK]

The subject bearing stress is for RC elements supported on corbel etc.. I have copy and pasted the relevant section of BS 8110 1997.

5.2.3.4 Design ultimate bearing stress
The design ultimate bearing stress is based on the weaker of the bearing surfaces and has the following
value:
a) for dry bearing on concrete: 0.4fcu; (A bearing without intermediate padding )
b) for bedded bearing on concrete: 0.6fcu; ( if padding material is cement mortar )
c) for contact face of a steel bearing plate cast into a member or support with each dimension not
exceeding 40 % of the corresponding concrete dimension: 0.8fcu.
Bearings using flexible padding may be designed using stresses intermediate between those for dry and for
bedded bearings.
NOTE 1 These values include an allowance for γm = 1.5.



I looked ACI 318 -08

10.14.1 — Design bearing strength of concrete shall not exceed φ(0.85fc′A1), except when the supporting
surface is wider on all sides than the loaded area, then the design bearing strength of the loaded area shall be
permitted to be multiplied by (√A2/A1 ) but by not more than 2.

If , φ is the strength reduction factor, in this case 0.65. Thus, φ*0.85fc′ will be approximately equal to 0.44*fck

 

[novembertango88]

Thanks all. I just don’t understand why if you crushed a cube of the same concrete you would reach somewhere around 1.0fcu but if you’re checking a base plate say then the code limits you to 40% of that. In my mind a cube should be weaker as there is less confinement.

 

[Tomfh]

Is there any enhancement factor in the British code?

In the Australian code if the bearing is well confined (Eg a baseplate far from any edges) then you can double the compressive strength. You still have to factor it down, but it’s a lot better then 0.4fc

 

[HTURKAK]

Your interpretation is not true...
If the subject is the bearing pressure under the base plate;

- ACI allows up to 0.85fc′A1*sqrt(A2/A1) ≤ 2.0

- EC-3 allows stress concentration factor kj=5.0 for a square base plate .. that is , concrete bearing strength fj=2/3*5*fcd = 3,33 fcd.

The ultimate bearing stress 0.4fcu is for dry bearing on concrete ( e.g. beam supported on corbel etc)...

Ihope this respond answers your question..