Flexural bars extension aci 318-11

[structural87]

Hello i have 2 questions regarding flexural bars extension according to aci318
1- does the slab bottom reinforcement needs to be terminated by a hook at the discontinious edges ? I would vote for 150mm bar extension into support is enough but there is this section which puts limit on the bar diameter (section 12.11.3- ld<Mn/Vu+la)  unless reinforcement is terminated by a standard hook,
 Therefore i have some colleagues which specify a hook for bottom bars anchorage in order to escape  this verification.

2- i usually specify a mesh reinforcement in the upper part of the slab and add reinforcement where needed. Does this additional reinforcement needs to be terminated beyond the point where it is no longer required a distance of d or ld ? Section 12.10 indicates d in the figure of continuous beam. Same thing applies to slab. Please confirm.

Thanks for the help.

 

[KootK]

1) Depends what model you're using for the connection. If you're calling the slab pinned at the discontinuous edge then, yes, it needs to be developed per 12.11.3 or have a standard hook. If you're calling the slab fixed to a wall etc, then you have a demand for top steel and the extra anchorage for the bottom steel is not required. I generally go with the later approach as a) I think that it's a better reflection of reality and b) things get a little busy and weird looking with both the top and bottom bars hooked at the edge.

2) Yes, the additional bars would still need to be extended as you've described. The extensions are about the shifts that occur in the location of rebar demand demand. The fact that you have other rebar in the area doesn't come into play unless, I suppose, excess rebar has been provided in the mesh beyond that theoretically required at the location of concern.



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.

 

[structural87]

1- clear. Thank you

2- let me rephrase please. Regarding the point 2 the other rebar in the area are extended beyond the point of inflection a distance of d, 12db and ln/16 (whichever is greater) while the additional reinforcement are stopped after a distance equal to d from the location where the mesh bars are enough to resist the existing bending moment. Approved ?

 

[KootK]

I see. That's a little more nuanced. I still think that whatever extension rules apply to the mesh bars should also apply to the additional bars. I've kind of lost track of the intent of the various rules but, If I remember correctly:

1) d; about tension shift; would apply to additional bars as it does to other bars.

2) 12db; about starter anchorage; would apply equally to additional bars as to other bars.

3) Ln/16; about our approximations in loading and knowing the locations of the inflection points; would apply equally to additional bars as other bars.

So yeah, same answer. But, now, my answer represents my personal interpretation of things.

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.