Two Way Slabs With Drop Panels.

[Tinguindin]

I am using the Direct Design Method to determine the design moments for a Flat Slab with End Span Simply Supported on wall. My main concern is the detailing of the reinforcement. I am following ACI, Fig. 13.3.8 – and it appears that some areas of my slab will be unreinforced on the top, see attached sketch. Should I include at least minimum area of steel?

Thank you

 

[JoshPlumSE]

I don't have a ton of experience with elevated slabs. But, my take is the following (based on drawings that I have seen designed by others).

1) Since this is the positive moment region for the slab, the reinforcment is not required by analysis.

2) That being said, this is the portion of the slab that is easily exposed. Because of that, I believe a small ammount of reinforcement is typically extended over this region to control cracking and such. However, this is usually at a significantely wider spacing thann would exist in the negative moment regions.

 

[slickdeals]

Our typical practice is to have top bars in the column strips and no top bars in intersection of middle strips.

Usually we provide a 1/4 of the calculated top steel at column strip intersections in the column strip/middle strip intersection. These bars are then lapped over the column. So you get 1/4 + 1/4 = 1/2 and then we add the remaining bars as extra bars over the columns.

Hope that makes sense.

 

[steellion]

You should have a continuous layer of bottom reinforcement. There will likely be locations where the middle strips overlap where there is no top reinforcement and that is okay. That is part of what makes it an economical system.

 

[JedClampett]

It does seem strange until you make that leap and do a few of these. When they don't fall down, you'll feel better.
In addition to the advice above, don't forget that if you add top steel to just make yourself feel good, due to the thin sections in two-way construction, it's likely to make it harder to pour the concrete. So you haven't helped the structure, you've hurt it.

 

[dcarr82775]

I will often have a standard top and bottom mat of steel (#4 at 12" or 16"), and add bars over the columns and midspan as required. But you do not need top steel out at midspan.

 

[hokie66]

Flat slabs don't typically have top steel in the M-M strips, but watch the requirement for crack control steel based on the gross concrete area, particularly if your slab will get wet.

 

[JAE]

Agree with the others above about column strip top bars.