Cut off point of extra bottom bar of continuous beam at support .

[Penpendrum]

I want to verify if my judgement (as shown in figure below) is correct. .

We know that the extra top bars of a continuous beam would terminate at the middle third from the support . and the extra bottom bars at midspan would terminate usually at quarter length of the beam.. but what about the extra bottom bars at the support ? is the figure above correct? is there any section of the aci code that would verify this.. will the cut off point be the same for tie beams, grade beam , 2nd storey and above beam?

 

[KootK]

It's important to remember that the code cut-off points are prescriptive minimums only. Your cut off points need to correspond to the actual inflection points and distribution of moment along the span. If you've got serious bottom steel at your supports, it suggests that your beam may be part of a moment frame. And that means inflection points near to mid-span. Grade beams might be a little different if your reinforcing for a soil upheaval load.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Penpendrum]

Yes!! KootK.

the beam is indeed a part of a frame and it is a SMF. . .so where would you suggest to cut of the bottom bar? if the inflection point due to lateral load like earth quake would cause the inflection point to move near mid span (the moment diagram would be like a shear diagram ).

 

[KootK]

. . .so where would you suggest to cut of the bottom bar?

I don't think that there's a rule of thumb that would be universally applicable Penpendrum. I'd analyze the beam, locate the inflection points, and plan any cutoffs around those locations. As I'm sure you know, ACI318 has a lot of prescriptive requirements for rebar detailing in special moment frames (Link). When all is said and done, I find that you don't have all that many choices left to you as a designer other than rebar quantity. And you usually end up with top and bottom steel running the entire span.

Link

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[JAE]

Keep in mind also that there is a difference between "cut off points" and splice locations.

For a typical beam - you'd have all sorts of bottom bars for gravity loading and your through-column bottom bars used for the wind moment would probably simply splice with certain gravity bottom bars. Splices can be anywhere but usually preferred at locations of minimum stress where possible.

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