1% Minimum in Concrete Columns? What is the theory? 5

[seattlemike]

Hello fellow engineers,

Question 1 of 2:

Our ACI code specifies a minimum 1% steel in columns so that we don't have catastrophic failures such as in the Pakistan earthquake last year. I believe the minimum 1% is in there to provide 3 fold protection:

1) Ductility in the ends and joints, where rotation & hinges could occur.
2) 12% reserve axial strength if the concrete deteriorates at a joint.
3) Reserve moment capacity for unpredicted lateral forces.

Do you agree? Are there other reasons?

Question 2 of 2:

"Foundations" only require 0.5% steel, but if say a column continues down to a deeper foundation pad, is it correct to say that the column above grade is a "column" while the column below grade is a "foundation." Obvious, the soil does provide some confinement, and if it suddenly lost it's strength due to liquification, you'd probably have much bigger problems. And besides, you're not supposed to have hinging in your foundations anyway.

What do you think?

 

[Taro]

1. Check out the ACI commentary for a discussion of the rationale for minimum reinforcing. Creep and shrinkage resistance is a main reason.

2. The interface between the column and the foundation only requires 0.5% steel. If the column continues below grade, it still needs 1%.

 

[jike]

I had the same question many years ago. I looked up the original research that was done in the 30's, if I remember correctly. As Taro says it has to do with the load creeping out of the concrete into the resteel. At the minimum recommended percentages, the longitudinal steel actually reaches or comes very close to yield.

It is advisable to continue the column steel all the way to the footing.

 

[JAE]

And 1.0 is a NICE....ROUND.....NUMBER....

 

[cap4000]

Based on my 1995 ACI 318 Section 10.8.4 if the column section is "oversized" you could reduce the reinforcing to 0.5%. I never exactly understood what oversized meant. My Ram Advanse program flags 1% as a min. Nice thread.

 

[miecz]

According to the commentary to ACI 9.2, "oversized" means every column larger than neccesary.

Say you need a 14"x14" column with 4#7 to carry an axial force, but the architect requires a 24"x24" column. Your minimum steel is based on 1% of the 14x14, and .5% of the 24x24, or 2.88in2.

 

[miecz]

Oops, I meqant the commentary to 10.8