Cb in rebar development length

[windrunner888]

I am a little bit stuck on one of my project. The project is adding new post-installed rebars on existing round pier. When calculating its development length, what is cb? The definition of cb in ACI is the lesser of side cover (not governed in my case) and spacing of the bars being developed. But should it be the spacing of new bars and existing bars instead? Using the spacing of new bars gives me 5’ of development length, while spacing of new to existing bars gives me 18’.

The way I am thinking is that it should be like spliced bars, where the cb is taking the spacing of bar being spliced.

Any thoughts? Thanks in advance.

 

[driftLimiter]

Can we step back up a minute. How exactly are you planning to post-install rebar? It seems like this would be the most important factor for you to determine the development length. All things left unconsidered, It would be the spacing of all the longitudinal reinforcement crossing a given potential splitting plane. I think also perhaps you have a unit issue 5' = 5 feet and 18' = 18 feet which are both quite high. Also if you mean inches, there are code required minimum lengths to review.

 

[KootK]

1) What size bars are you using?

2) What are you doing here? Are you casting a new column on top of an existing one?

3) Is it your intent to lap splice the new bars with the old?

If you can spare the time to post a sketch of your situation, I expect that it will greatly improve the quality of the responses you get to your question.

 

[windrunner888]

Sorry for the confusion. I meant drilled pier instead of column. Numbers are correct, 18’ is I use cb as the distance between new and existing rebars.

The intention is to add bending moment capacity for the pier. The new rebar size are 11 and the existing ones are 8. Here is a sketch (hope I post it correctly). I don’t think we will align both regards perfectly as shown in 90 degree direction but I will take the worst case, leaving a clear distance between them as 0.3”.

 

[BridgeSmith]

If the new bars are within 6" (at least that is the limit for a "non-contact lap" in AASHTO) of the existing bars, they are lapped rather than being developed.

Anyway, AASHTO has an upper limit on the adjustment factor for cover and spacing; doesn't ACI, also?

Rod Smith, P.E., The artist formerly known as HotRod10

 

[jayrod12]

I don't think this is the best way to add bending capacity to the column, because as you're finding you'll need extreme embedment depths to develop the reinforcing beyond the point it is required. A better solution would be FRP wrap or external steel, or a concrete sleeve. You can barely drill 5 feet, much less 18 feet into a drilled pier. Why do you need additional moment capacity of an existing drilled pier?

 

[KootK]

1) If this is the addition of flexural capacity, then it is decidedly not a lap splice situation. So we don't need to concern ourselves with that per se.

2) I don't believe that one needs to develop the bars past the point where they are needed if they consider the bars to pick up tensile capacity linearly between where the bars are terminated and where full/partial development is achieved. Going Ld past the theoretical cutoff point is a convenience that we use in situations where more detailed study isn't worth the effort in my opinion.

3) I would still extend the reinforcing past the theoretical cutoff point by the shear depth of the pier. And one might argue that even this is not necessary if you're confident that the pier will not develop shear cracking.

 

[KootK]

Ultimately, I suspect that we'll be able to do a better job of helping you with this if you tell us a bit about your loading condition.