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continuous cut-off bar
- Thread starter pattontom
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What I was saying was this. Do you use L/3 as in:
or L/4 as in:
I was talking about the continuous cut-off bar or the development length. Locally. I saw different group using L/3 and some L/4. Which do you use?
Please reply asap because the plan has to be signed and sealed and I'm deciding whether L/3 or L/4. Thanks.
or L/4 as in:
I was talking about the continuous cut-off bar or the development length. Locally. I saw different group using L/3 and some L/4. Which do you use?
Please reply asap because the plan has to be signed and sealed and I'm deciding whether L/3 or L/4. Thanks.
The distance is up to you but if you cut off bars and still have negative bending you would need additional, continuous bars as well.
The lower drawing you post shows these continuous bars that extend past the cut-off bars. The L/4 or L/3 just affects the areas of steel you need for each.
The lower drawing you post shows these continuous bars that extend past the cut-off bars. The L/4 or L/3 just affects the areas of steel you need for each.
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- #6
Ba, it's purely beams above the columns.
Bpstruct, the lower drawing shows both exterior end and interior continious.. the interior is where there is continuous bar just like the top drawing. The standard is that L/4 is for exterior end. But for interior ends that need continuous bar. I think standard is L/3. I wonder if those L/4 used in interior continious bar drawings are just mistakes (the lower drawing is taken from another engineer plan). The top drawing from the ACI.
So guys. What do you actually use... L/4 or L/3 in interior continuous bar cut off. Again. I'm not talking of the exterior one which needs L/4. But the interior, hope you understand what I'm saying.
There is no "standard".
The distance to cutoff of SOME of the top bars is purely up to you as the designer.
You can do either L/3 or L/4 or L/whatever as long as you meet the required moment capacity at each section along the beam.
The cutoff bars must extend BEYOND the point where they are no longer needed per code (ACI 318 or whatever your governing code is).
For ACI 318, check out the diagram in Figure R12.10.2. which shows how partial length bars affect the area of steel required to be continuous.
Your top diagram doesn't work very well in my view as there are no continuous bars across the top.
The bottom diagram has the little hooks at the cutoff ends. I've NEVER seen this done this way - only straight bars or the older (not used anymore) bent down "truss" or "crank" bars which your diagrams don't show.
If you don't know how to do this find an engineer who can teach you.
The distance to cutoff of SOME of the top bars is purely up to you as the designer.
You can do either L/3 or L/4 or L/whatever as long as you meet the required moment capacity at each section along the beam.
The cutoff bars must extend BEYOND the point where they are no longer needed per code (ACI 318 or whatever your governing code is).
For ACI 318, check out the diagram in Figure R12.10.2. which shows how partial length bars affect the area of steel required to be continuous.
Your top diagram doesn't work very well in my view as there are no continuous bars across the top.
The bottom diagram has the little hooks at the cutoff ends. I've NEVER seen this done this way - only straight bars or the older (not used anymore) bent down "truss" or "crank" bars which your diagrams don't show.
If you don't know how to do this find an engineer who can teach you.
structSU10
Structural
If I had a lot of beams I would probably use a elevation of beam conditions with a schedule having a dimension string until developement length starts, say le, then the developement length dimension string labeled ld. I then would also have a table of developement lengths for the bars based on f`c, with a table for 2-1/2" cover or more, and less than 2-1/2" cover, that have ld, 1.3 ld, 2.0 ld, and ldh.
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- #10
check out figure 2 in "Details and Detailing of Concrete Reinforcement
(ACI 315-99)"
Is there provision in ACI which says L/4 can be used? Some local engineers I asked said L/4 is just wrong in continuous cut-off as it is only for exterior end.. while some engineers use it.. perhaps unaware where they got the diagram.. or just typo in their typical diagram labels. Our local code doesn't state or give details about it because we copy the United States ACI code but only partials of it to minimize thick book so many details are omitted.
(ACI 315-99)"
Is there provision in ACI which says L/4 can be used? Some local engineers I asked said L/4 is just wrong in continuous cut-off as it is only for exterior end.. while some engineers use it.. perhaps unaware where they got the diagram.. or just typo in their typical diagram labels. Our local code doesn't state or give details about it because we copy the United States ACI code but only partials of it to minimize thick book so many details are omitted.
As some engineers above have explained, it is up to you to choose either L/3 or L/4, however, ensure that the development length ld is satisfied. So if your L/4 does not satisfy the ld requirement, use L/3. Actually, the L/3 or L/4 is to make the bar cutting easier to determine for the rebar workers, rather than giving an exact value for the cut-off bar. Check on ACI 318 diagram as JAE mentioned.
pattontom,
Every situation is different. The cutoff position is dependent on the load pattern, relative span lenggths, bar diameter, moment diagram, member depth etc.
You must develop the reinforcement required at each point and most codes require an offset of the bending diagram to determine where the bars can be terminated. This means that the reinforcement required at any point and the point of contraflexure are offset normally by D or d in the direction of reducing moment and then the reinforcement must provide necessary developemnt from that offset point.
Every situation is different. The cutoff position is dependent on the load pattern, relative span lenggths, bar diameter, moment diagram, member depth etc.
You must develop the reinforcement required at each point and most codes require an offset of the bending diagram to determine where the bars can be terminated. This means that the reinforcement required at any point and the point of contraflexure are offset normally by D or d in the direction of reducing moment and then the reinforcement must provide necessary developemnt from that offset point.
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