ACI 318-08 Equation 12-1 Ktr Confinement Term

[sklev]

There is some discrepancy among long time engineers and new engineers in my office about development length:

The ACI 318-08 code states that equation 12-1 is the "general development lengt equation." In this equation there is a term of ((cb+Ktr)/db). The Ktr term is "a factor that represents the contribution of confining reinforcement ..." Section 12.2.3 allows for this term to be taken to 0. The cb term is a spacing and cover term defined in chapter 2 with no reference to confinement in the definition. And db is the diamter of the bar. The term ((cb+Ktr)/db) must have a value of at least 1.5 but must not exceed 2.5. I have used this equation many times and usually get a value for this term of greater than 2.5 so end up using 2.5 in the development length equation. HOWEVER, when using 2.5 in this equation, the development length ends up being about HALF the value of that when using the equations from the table in section 12.2.2. When the ((cb+Ktr)/db) term is taken to be the minimum value of 1.5, the value for development length ends up being the same that you would get if you used the table in section 12.2.2.

The question becomes: Can you use the "general" development length equation with the term ((cb+Ktr)/db) assuming Ktr to be zero when you don't have confining steel in a case such as a wall or slab? Is the Ktr term the confining term or is the ((cb+Ktr)/db) the confining term, even though it is not defined as such?

 

[ishvaaag]

From a lecture of the article and commentary,

((cb+Ktr)/db) is the confining term

you simply enter Ktr=0 where no stirrups confine the rebar in the development length. That the equation is applicable to your case without confining steel is seen in that Ktr is a factor that enters the equation with a value giving zero when no confining steel.

 

[Lion06]

The Ktr is the confining term. The cb is just the amount of clear cover or half the spacing to the next bar.

Also, I don't think that the value is required to equal at least 1.5. The commentary says that many common configurations result in a minimum of 1.5 and that is why 1.5 is assumed in the "other" cases of 12.2.2, but there is no code requirement that it be a minimum of 1.5.

The cb and Ktr both combine to determine whether the failure is a pullout failure or a splitting failure, but Ktr is the term that represents the contribution of the confining steel.

 

[ishvaaag]

I don't want to polemicize in what is just a semanctics thing, but the text clearly state the confining term being ((cb+Ktr)/db)

I quote from the code

"in which the confinement term (cb + Ktr )/db shall not..."

hence the confining term is what follows to the word "term".

It takes a value when there is confining steel, and other when not, or different amount. So the whole effect of there being or not confining steel is built in the overall term. Other thing is that within the term, the influence of confining steel gets appraised through the Ktr input data.

 

[Lion06]

You can call cb a confinement term, if you want, but the code doesn't refer to it that way. Ktr is the confining steel term, and the commentary is pretty clear on that.

I have my ACI 318-05 in front of me........... maybe it's different in '08.

 

[Teguci]

For most projects, I let the contractor use the detailing tables which use a 1.5 or 2 factor. When I come out to the site and find out the development lengths have been cut short, thats when I resort to the 2.5 factor. I have no problem using the 2.5 factor given cover only.

 

[ishvaaag]

Certainly, my quote, Lion, is from 08.