Corbel Design (Framing Bar)

[JohnRwals]

Hello!

I have a question about corbel reinforcement detail.
ACI 318 calls it 'framing bar' while BS calls it 'compression bar.'
Does ACI refer to how to calculate its area, reinforcement amount?
Regarding the end of framing bar, BS shows 'compression anchorage'
while ACI does not mention anything specifically.
Is how to extend the end of framing bar critical/important?
Is this embed length important?
Some detailed info is missing though this corbel design seems to be very simple.
Can I hear your opinion?

JRW

 

[bugbus]

Minimum diameter of the framing bar should be enough to anchor the stirrups, normally at least the same diameter as those.

If you're not relying on the framing bar to add compressive strength to the strut, I think it would be enough just to extend it a nominal distance into the core of the column and be done with it (like shown on the right).

 

[JohnRwals]

Compression can be transferred mainly thru concrete
and stirrups will confine compression strut according to strut-tie model.
I think these framing bars are only for framing stirrups (to make steel cage stable/easier).
I have sometimes seen very short two #3 straight rebars welded to bearing plate for precast concrete girders.
I guess this is a kind of another gray area.

JRW

 

[KootK]

I agree that, in the ACI context, the framing bars are just for constructability and stirrup anchoring.

Drifting into pedantry, however, I can see some logic in the BS approach. Consider:

1) If you put parallel bars in the compressions strut, creep ensure that they'll draw compression in excess of their relative stiffness within the cross section.

2) If the bars draw meaningful compression and are not compression developed, that sort implies that they fail in bond stress within the body of the column.