Simple question in reagrd to different slab thickness 2

[FreshMan2020]

Hi Fellows,

I have a question in relation to flat slab design of a job I am doing now.

I am trying to use different slab thickness for one floor
Please see my sketch below. (red lines denote walls under and external walls not shown)

I am newbie and need your thoughts on this. Is it a common way to have different thickness for flat slab (not step down but just different thickness). If so, is the detailing above seem fine and is it a must to arrange a beam over the opening and what do I do if I don't? What else that I need to pay attention to?
Thank you.

 

[KootK]

My attempt at a force flow diagrams for the joint.

 

[hokie66]

KootK,

But is that a real concern? This is all in a flexural zone, not something like a corbel. These abrupt depth changes are done every day without any special detailing. If there is a step both top and bottom as in Celt83's reference above, that is a different story.

It is the top reinforcement that ties everything together, not "some kind of concrete tension".

 

[r13]

The shear path makes sense though, as the compressive force from the thinner slab can be quite high.

 

[KootK]

But is that a real concern?

Nope, just trying to figure out how it works and speak to Hardbutmild's valid concern expressed previously.

These abrupt depth changes are done every day without any special detailing.

1) I know, I've got the same standard details as everybody else.

2) That no special detailing is required is precisely what I think my model show. Short of vertical reinforcing, which I'm not advocating, all the bottom reinforcement needs to do is anchor the last shear struts coming in from each side. All is right with the world.

It is the top reinforcement that ties everything together, not "some kind of concrete tension".

I'm not convinced that's the end of it. I think there's something going on in the neighborhood of the yellow rectangle below. Localized panel zone shear, some concrete in tension... something. You don't ever make a concrete moment joint and just magically have no other considerations besides flexure to worry about. This just works with out reinforcing or special checking because the demand is not localized or intense.

 

[hardbutmild]

You're right, no vertical bars are needed if the change of slab height is small, since concrete alone is able to resist this tension. Plates are good at this. My bad, I was for some reason thinking of a more pronounced height change. I believe that in such a case vertical bars should be placed.

 

[KootK]

I agree with your previous comment, before the retraction hardbutmild.

I attempted to reconfigure things to eliminate the bursting tendency that your model showed. I was able to manage that on the bottom of the slab but the strut that I've labelled (1) below, still creates a similar bursting tendency at the top of the slab, just as it did in your sketch. Something's gotta resist that and it 'aint rebar. The strut below and to the right of (1) is just your usual shear strut coming to ground. (1) is something additional... I think.

 

[hardbutmild]

Sure, I agree with you that it's concrete tension strength that's keeping it from bursting. Since in plates struts can go in the third dimension (out of paper) I think it's not as pronounced and top reinforcement in the other direction is partially activated. It's true that some tension stresses develop, but they are probably low in standard situations. That's why I said that I agree with both of you, there is some tension theoretically, but plate probably disperses it nicely.

 

[Tomfh]

How is the internal tension (aka shear force) worse in the thick zone?

Surely the thinner bit will break first?

 

[hokie66]

Tomfh,

Not as much, so I fail to see that argument.

 

[KootK]

I'm not saying that it's a problem. I'm simply trying to sort out a plausible mechanism to resolve the issue of the bursting force tendency that was raised previously: where it goes and what form it might take. A thing doesn't have to be a critical failure mode to be worth thinking about.

 

[Tomfh]

Do you think it could be worse/weaker than if the thin slab continued through?

 

[KootK]

I have no idea as I don't really know how to evaluate the phenomenon yet.

 

[r13]

For interior support at balanced condition, the thinner slab will have to resist the same loads as the thicker slab. If both works, one would think way not use thinner slab throughout.

 

[KootK]

Agreed, I've been mostly contemplating the effect on the thicker slab on the assumption that it's thicker for a reason.

 

[hokie66]

retired13,

It is not balanced. Look at the plan.

 

[r13]

hokie66,

Yeah, I missed that. Thanks.