Column/Cap Rebar Development Length

[MoonBridge]

I am verifying the required development length for a cap/column connection point. And I am curious about how I would calculate the reinforcement confinement factor to reduce the development length. My understanding is that the reinforcement of interest would be the length of column reinforcement being developed into the bent cap. This means that the confinement factor would care about the reinforcement within the bent cap potentially confining the longitudinal column bars. Can I consider the cap longitudinal reinforcement to confine the embedded bars? Or would I ignore this factor for this scenario?

Here are some quick sketches to hopefully help better illustrate what I am asking.




I don't imagine the cap reinforcement to be doing any meaningful confinement. So it is my understanding I would then have to use the basic development length. What about in a scenario of a rectangular column as shown below?

Part of my confusion comes from AASHTO showing the confining column reinforcement in Figure C5.10.8.2.1c-1.

 

[BridgeSmith]

Not sure about how to interpret 'the letter' of the spec, but considering that the confinement factor is based on a concrete splitting failure, and the bars that are highly stressed in tension are the ones near the center of the cap width, where they have plenty of concrete cover and splitting of the concrete is not going to happen, I'd say you could realistically use the maximum reduction allowed for the development length.

That said, theoretically, the bars nearest the side face would have a the confinement factor calculated using the cover to the side face and the longitudinal reinforcement in the side face (AKA the skin reinforcement) of the cap to calculate the confinement factor.

We haven't had much problem with that because we always make the cap width 6" more than the column diameter to make construction easier, so ours have plenty of concrete cover, even for the ones closest to the side face.

 

[BridgeSmith]

For the column bars closest to the side faces, you can reduce the development length using lambda er (excess reinforcement adjustment). The rest of the bars will have cb (Eq. 5.10.8.2.1-2) large enough that the ktr (transverse reinforcement index) doesn't matter - lambda rc will be at the minimum value (0.4) already.

 

[BridgeSmith]

For the rectangular column, lambda rc would be calculated using longitudinal skin reinforcement and the distance from the vertical bars being developed to the side face.