bottom reinforcement anchorage3 into support

[chekre]

Good day,

Regarding the anchorage of positive moment reinforcement, aci318-11 states that u need to enter the support by 150mm and the bar diameter should be small in order to satisfy an equation ( section 12.11.3) (ld <Mn/Vu + la). This option is disregarded if the reinforcement is anchored with a standard hook beyond the center of the support.

Do u usually satisfy that condition or do u just anchor the bottom reinforcement using a standard hook ?

Same thing applies for the 2-way slabs.
in columns strips, bottom reinforcement should be anchored by a standard hook at the slab edge while in the middle strips, 150mm is enough. I think it is too complicated to be done and to verify on site so i ask for all the bottom reinforcement to be anchored by a standard hook.
what do u think ?

 

[JAE]

I've never used a hook for bottom bars unless it is an end span where I might hook the bars if in a moment frame.
Chapter 7 has additional requirements for SOME of the bottom bars depending on certain factors (continuity bars). For those continuity bars I do hook them all in the end spans.

Otherwise I usually go with the 6" (150mm).

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[chekre]

thanks JAE. i was talking of course for the end spans. when u go for the 150mm, do u check the condition stated in section 12.11.3 because it states that the diameter should be small in order to respect the above condition ?

 

[JAE]

To be honest I've not used that check too much - per my original concrete design mentors who told me they never really considered it. I also had a professor once shake his head at that requirement in that it didn't, in his view, usually control anyway.



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[KootK]

In my opinion, this check doesn't apply at supports in most monolithic CIP construction scenarios (deep beams excepted). My reasoning is that, designer modelling simplifications aside, very few monolithic CIP supports are truly pinned.

As soon as the negative moment pushes the point of inflection a foot or two into the span, the support check becomes irrelevant and the requirement only needs to be satisfied at the point of inflection which is generally easy to do.

In my mind, there's little sense compromising constuctability to quickly tension develop support bars that are in compression. I suppose that one could argue that the designer's detailing ought to be consistent with their analysis assumptions at faux pins. That's a whole separate kettle of fish however.

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.

 

[chekre]

Kootk, u are saying this (very few monolithic supports are truly pinned) because your are assuming some top reinforcement over the support.

However, if your support has a small width and the top reinforcement can not be anchored since it wont be developed, then your support will be fully pinned.

I think there are some topics in the code which results in congested reinforcement areas but when u the client has hired a sub consultant, you are obliged to satisfy the code requirements.

For example, regarding the 2 way slab, any ideas why the top reinforcement extension should have a length equal the third of the biggest span from each side and not rely on the FEM method even if it results in smaller bars extensions.

 

[KootK]

Kootk, u are saying this (very few monolithic supports are truly pinned) because your are assuming some top reinforcement over the support.

Correct. As far as I'm concerned, any proper design is going to include top steel over the supports.

However, if your support has a small width and the top reinforcement can not be anchored since it wont be developed, then your support will be fully pinned.

Sure, if your case is one of the rare, truly pinned supports, then the bottom steel anchorage checks at the support would apply. The overwhelming majority of the one and two way CIP slabs that I deal with are monolithically connected to CIP beams, walls, or columns. The only exceptions that come to mind are the odd case where slab end spans are supported on steel or masonry, mostly in renovation applications.

I think there are some topics in the code which results in congested reinforcement areas but when u the client has hired a sub consultant, you are obliged to satisfy the code requirements.

Yes, we are always required to satisfy code requirements. My point is that the code requirements regarding positive steel anchorage at supports do not apply to the vast majority of supports. This is true, at least, in my practice.

For example, regarding the 2 way slab, any ideas why the top reinforcement extension should have a length equal the third of the biggest span from each side and not rely on the FEM method even if it results in smaller bars extensions.

As luck would have it, I now exactly why this is the case:

1) Designer sanity. Making every run of bars it's exact FEM required length is tedious.
2) Engineering judgment. FEM results are an answer not the answer.
3) Constructability. Assembling and inspecting a slab where every run of bars is different is difficult and error prone.



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.

 

[chekre]

i totally agree with u. But cost wise, lets say if u have a span of 10meters followed by a span of 6meters, and while the FEM will show for example that u need additional top reinforcement over a small length, Chapter 13 says that the minimum bar extension will be 10x0.3x2 + the the width of the support which will result in a longer required bar then required.

 

[KootK]

The prescribed bar lengths are only mandatory for the direct design method which would be invalid, or close to it, for the span lengths that you've proposed. For all other methods designers at free to specify bar extensions as they see fit, including unsymmetrical layouts as you've proposed.

That said, unsymmetrical top bar mats are a QC nightmare. They will occasionally be installed backwards or symmetrically. In the situation that you've described, there may well be top steel demand over the entire 6m span. This is particularly true when unbalanced live load is a consideration. A practical top mat may well be a symmetrical one that spans over the entire 6 m span.

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.

 

[chekre]

can u indicate please the section where the ACI states that the bar extension of figure 13.3.8 are only for the direct design method ?

What i usually do is to take these bar extensions as a minimum and check using the finite element method that the required reinforcement falls within the required range and if not, if would increase my bar length.

I would be grateful if u could indicate where did u find the clause saying that the bar extensions need not to be followed unless we are designing the slab the Direct Design Method .
thank u

 

[KootK]

can u indicate please the section where the ACI states that the bar extension of figure 13.3.8 are only for the direct design method ?

No. I dug around and came to the conclusion that I've been wrong about this. ACI figure 13.3.8 needs to be adhered to for all methods it seems. Good thing that's what I do.

What i usually do is to take these bar extensions as a minimum and check using the finite element method that the required reinforcement falls within the required range and if not, if would increase my bar length.

Me too.

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.

 

[chekre]

great. The only place where i don't respect these extensions is when i have a slab with some irregular grids ( no clear span can be defined) or a slab having 2 way beams. in that case, i rely on the FEM method and increase by ld my tension bar.

I see that u are very active Mr kootk. Very good job and answers ! keep it up ! cheers