Development of Bundled Bars

[preynolds]

I have been looking at ACI 12.4 regarding development length of bundled bars. I am a little confused on the proper way to figure this. If I am looking at a 3-bar bundle consisting of #9 bars: ACI 12.4.1 says to increase the individual bar ld by 20%, which would equal 1.2ld of a single #9. However, 12.4.2 says to use an equivalent diameter (in this case, 1.95&quot to figure the factors in ACI 12.2. That being said, does the 1.95" only pertain to the factors in the equation or does it also apply to "db"?

My interpretation: use db = #9 bar but use value of 1.95" when calculating the factors and cover requirements in the ld equations. Then, the increase of 20% would be used.

Is this correct?

 

[ishvaaag]

What Nawy has to say is:

"... although splice and development lengths of bundled bars are based on the diameter of individual bars increased by 20% or 33% as applicable, it is necessary to use an equivalent diameter of the entire bundle derived from the equivalent total area of bars when determining the factors that consider cover and clear spacing and represent the tendency of concrete to split."

Reinforced concrete, a fundamental approach, 3d ed.
Edward G. Nawy
Prentice Hall
p. 425

so it looks quite what you are saying. In any case if you can take a look to a CRSI table in accord to the current code.