Design the Column footing joint 1

[XinLok]

The maximum bearing load on the bottom of the column (ACI Code Table 22.8.3.2) = ∅ x 0.85 x f'c A1.where A1 is the area of the contact surface between the column and the footing.
When the contact-supporting surface on the footing is wider on all sides than the loaded area, the maximum bearing load on the top of the footing may be taken as
0.85 x ∅ x f'c x A1 x sqrt(A2 / A1)2 , but not more than 1.7 x ∅ x f'c x A1.

Can anyone explain this please?

 

[HTURKAK]

Sometimes 'One Picture is Worth a Thousand Words ' The bearing strength varies if the bearing area is confined or not .
The figure is copy and paste from Reinforced Concrete_ Mechanics and Design ( By James K. Wight )

He is like a man building a house, who dug deep and laid the foundation on the rock. And when the flood arose, the stream beat vehemently against that house, and could not shake it, for it was founded on the rock..

Luke 6:48

 

[XinLok]

Thanks for the reply.
I am not able to identify when to use the value of ∅ x 0.85 x f'c A1 and when to use 0.85 x ∅ x f'c x A1 x sqrt(A2 / A1)2.

What does mean "When the contact-supporting surface on the footing is wider on all sides than the loaded area"

 

[HTURKAK]

Pls look 22.8.3.2 and R22.8.3.2.
...Where the supporting area is wider than the loaded area on all sides, the surrounding concrete confines the bearing area , resulting in an increase in bearing strength. No minimum depth is given for the support, which will most likely be controlled by the punching shear requirements of 22.6. A1 is the loaded area but not greater than the bearing plate
or bearing cross-sectional area ...

The figure that i have posted defines A1 and A2 . A1 is the contact , loaded area , and A2 supporting area.



He is like a man building a house, who dug deep and laid the foundation on the rock. And when the flood arose, the stream beat vehemently against that house, and could not shake it, for it was founded on the rock..

Luke 6:48

 

[XinLok]

Can anyone explain please what does mean: when supporting sides is wider on all sides than the loaded area?

 

[lexpatrie]

Xin - For example a 22" x 22" concrete column landing on a 5' x 5' concrete footing. The concrete footing is larger on all sides, provided the column placed far enough from the edges.

 

[XinLok]

Thanks Lexpatrie, the reason why I am asking is that in the example which you provided above, we should use 0.85 x ∅ x f'c x A1 x sqrt(A2 / A1)2 instead of ∅ x 0.85 x f'c A1.
but in one of the example which I saw, The allowable bearing on the base of the column used was ∅ x 0.85 x f'c A1

 

[Celt83]

When you check column/foundation joints for bearing there are two checks:

- column bearing on foundation where typically the foundation F’c is lower than the column but the foundation is wider so you get the benefit of the A2/A1 component.

- foundation bearing against the column here you use the column F’c but you do not get the A2/A1 component.