#8 rebar equivalant to how many #4's?

[Brem]

I am told that to match our local code of one #8 bar top and bottom of footing walls one has to use 4 x #4 bars top and bottom. [Redwood Shores CA, reclaimed marshland]We only have 2x #4's top and bottom with much sagging and cracked wall problems. Could somebody please verify this for me? Thanks.

 

[rapt]

Brem,

This sounds like pretty basic engineering and mathematics.

A #4 bar is approximately half the diameter of a #8 bar. It is therefore approximately 1/4 the area of a #8 bar.

Based on this it is fairly obvious that 4 #4 would be needed to give the same area of reinforcement as 1 #8 bar.

 

[3dboy]

As of 1 #4 bar, .2
As of 1 #8 bar. .79

therefore 4 #4s = 1 #8

 

[StephenA]

A slight technicality but assuming the same minimum cover to the rebar 4 No. #4 bars would give you greater moment capacity as the lever arm would be greater.

StephenA