reinforcement in drop panels

[NS4U]

for a two way slab with drop panels...

At an interior column I have a drop panel for punching shear reasons. Do I/Should I provide reinforcement in the actual panel itself? If so, how much? Say the drop panel is 15" deep.

I saw a detail once that had something like 6-10' long #5 bars spanning each direction down in the panel. Not sure why.


This reinforcment would be in addition to the regular slab reinforcment, so it would not be required for flexural reasons.

Any thoughts?

 

[dougantholz]

We don't typically add reinforcement, but maybe we should. I've seen some cracks develop down there, so that would be the reason to reinforce - crack control.

 

[PanamaStrEng]

Is it effectively a drop panel as defined by ACI 318, or a column capital only required for shear considerations?
For drop panels you need to add it in case there are flexural stresses to handle. For example, could there be positive moments at supports due to seismic loads?. In Panama it is common practice to add something like #4@12" (sort of U shaped) in the bottom of drop pannels regardless. If required only for shear, I would not add anything.

 

[asixth]

I have never detailed a drop panel with reinforcement in the bottom of the panel. I have heavy top reinforcement for the negative moment over the support and I discontinue the bottom steel in the slab over the support.

I recently had a two-way flat slab design where I was relying on frame action of the column strip to resist lateral loads, however I didn't put reinforcement in the bottom of the drop panel as PanamaStrEng has indicated, however, this is a good point raised and I will bring it up with the seniors in my office.

 

[jike]

I have added bottom steel when I have used a non-conventional drop (deeper) so as to simply my forming and to combine capital and drop panel into one.