Joel, in the 2019 version of 318 they seem to have increased the development of headed bars substantially if my memory serves me correctly.
I'm also not aware of any codified advice on taking the straight development around a corner. I do believe it's been discussed here before, so have a search.
No, because the vertical leg is not quite effective in developing required strength, many studies and tests have been done on this subject. Use smaller bars to reduce Ldh. Ld is the basic development length for straight bars only.
retired, it looks like the sketch says "<L[sub]dh[/sub]", so the OP doesn't have enough room to make the full anchorage.
You can't take straight development around a corner. To get tension in the bar around the bend, the concrete at the corner would have to crush and the bar would have to start pulling out to engage that leg. The leg is there to provide that area of higher resistance at the bend.
In ACI 318-05 I believe there was a provision allowing you to reduce the development length by a factor of A[sub]s(provided)[/sub]/A[sub]s(required)[/sub]. I'm not sure if that's still buried in there somewhere.
Can you reduce your bar size and use more bars? Perhaps widen the beam to make room for them - that's probably more palatable than enlarging the column.
Seems that the responses have been in agreement that Ldh should be met. I always thought that extending the tail would compensate for it. Thanks for the responses.
The effect of the hook is accounted for in the required development length for hooks. AASHTO has modification factors for confinement that reduces l[sub]dh[/sub] if stirrups, ties, or cover at least 2" is provided.
Rod Smith, P.E., The artist formerly known as HotRod10
I remember positive confinement over hook tail is a important factor in determining its effectiveness. Lack of confinement, tests have shown the bar can be straightly pull out before reaches its design strength, the Ldh is the minimum length required to develop such confinement. Recent development might have incorporate some other mechanism as pointed out by BridgeSmith, and permitted by AASHTO. I wonder ACI has similar clause though.
Theoretically you can, but how much to reduce? By direct proportion or any verified method? If you don't need that much reinforcement, reduce the As and select smaller bars that meet both requirement is the most straightforward solution.
What you are proposing on the diagram is not valid. If you don't have the hook development length, you can't make the hook larger to compensate and call it a day; the concrete will crush in the bend or the hook may pull out.
That being said, (if you go by ACI 318-19 only; previous editions don't apply), you could technically model that as a curved bar node with a strut and tie diagram and providing full splice development below the hook. It may or may not check out that way (depending on the rebar bend radius, etc). You'd need to check the strut and tie diagram for all possible load combinations.
I don't recommend that, though. In my opinion, you should either switch to a smaller bar or, alternatively, add more steel and use the A[sub]s,provided[/sub] vs A[sub]s,required[/sub] provision to shorten the development length (assuming seismic is not a concern).
The basic development length for a hooked bar (l[sub]hb[/sub]) may be modified by reductions for confinement (concrete end cover or confining reinforcement) and reduced demand (As required / As provided) to get the required development length (l[sub]dh[/sub]).
Edit: The minimum l[sub]dh[/sub] is subject to any applicable seismic provisions, as Michael Chen mentioned.
Rod Smith, P.E., The artist formerly known as HotRod10
Development length of standard hooks in tension (R25.4.3)
Study of failures of hooked bars indicates that splitting of the concrete cover in the plane of the hook is the primary cause of failure and that splitting originates at the inside of the hook where local stress concentration is very high. Thus, hook development is a direct function of bar diameter db, which governs the magnitude of compressive stresses on the inside of the hook.
Development length (Ldh) of a standard hook in tension is greater of 8db, 6 inches and the equation below:
[(fyψeψcψr)/50λ√fc']d[sub]b[/sub]
Where:
fy: specified yield strength of rebar in MPa
fc’: specified concrete compressive strength in MPa
db: diameter of the bar For the modification factors below, refer to table 25.4.3.2 on ACI 318-14 code:
ψe (psi sub e): development length modification factor
ψc (psi sub c): development length modification factor based on the cover
ψr (psi sub r): development length modification factor based on confining reinforcement
λ (Lambaba): modification factor base on reinforcement coating
Looking outside ACI318, some codes like Eurocode 1992-1-1 (clause 8.3) and the old BS8110, allow the actual length to be used for development as long as the bend radius is increased sufficiently to reduce the compression problems in the bend.
A 180 degree hook in a beam column joint performs quite poorly, so is not allowed in some standards. It's to do with anchoring of the diagonal joint force by the vertical leg of the 90 degree hook, and it forming a lap of sorts with the vertical column bars. With a 180 degree hook you don't achieve this load path.
You know your reasoning is weak without backups, and you know too that nobody has done that on a beam-column joint, and no one will follow the suggestion, as there is no backup for this application also. But, just having a little sense of humor, then one can get it
