anchorage of bottom bars in one-way slab, at exterior support beam

[ajk1]

I am reviewing our typical detail sheet for one-way reinforced concrete floor/roof slabs.
Regarding the length that the bottom bars should be specified on the typical detail to extend into the supporting beam at the exterior support, what should this be?

150 mm?
or
to within 75 mm of the exterior face?
or
to the centreline of the support
or
to some distance past the centreline, to satisfy CSA A23.3 Standard, equation 12-6 (see attached) and below:

ld ≤ Mr/Vf + la

 

[KootK]

I vote 150 mm. With regard to equation 12-6, it's a bit of an odd thing logically. If one considers the exterior support a pinned support, then you're pretty much stuck having to extend the bars past beam CL one way or another, which is rarely done. The usual, modern story to tell is that you do have some fixity at the support owing to the torsional stiffness of the beam and the presence of the "detailing" top steel that you threw in at the slab edge. Therefore it's not a simply supported slab even if you said it was when you designed the slab itself. It's a good reason to put in a reasonable quantity and length of top steel at the support in my opinion.

 

[KootK]

So when do I get my copy of these kick-ass, new and improved standard details we've been working on? There'as a steak sandwich with your name on it...

 

[ajk1]

LOL. If it were up to me, I would give them to everyone. The best would be if all structural engineers used the exact same typical details. I would take it up with management if I can post it on the internet, but I think I know the answer. At one time it was suggested in our company that we try to get our competitors together to agree on and come up with a set of typical details but that went nowhere. Each company has its own way of doing things so a common set of details would mean some would have to change their procedures and way of presenting the design on the drawings, etc.

Currently our detail shows 150 mm extension of bottom bars into beam support at exterior beam. That agrees with your recommendation, so I will leave that be.

You raise a good point about restraint and how it affects bottom bar extension over support. This suggests to me that we need another typical detail for the case of bearing on masonry wall where there is no moment restraint.

 

[KootK]

I would take it up with management if I can post it on the internet, but I think I know the answer.

I was just kidding as I know the answer too. I do a lot of 3rd party peer review work so, as you can imagine, my library of pilfered standard details is pretty deep. And I love it dearly.

 

[TLHS]

I really like the Eurocode detailing guide. Potentially if you used it right, it would be like everyone using the same standard details. I have no idea if that's what actually happens in practice though.

 

[ajk1]

to TLHS - I am not familiar with that but sounds interesting provided it is not overly complex for the type of structures that we do.