Reinforcing steel in the bottom of drop panels - is it required?

[cliff234]

I know this question has been asked before, but I would like to get some current feedback.

We are designing a heavily loaded 2-way slab and we are using drop panels to increase both the punching shear capacity and to get more negative moment capacity in the column strip over the columns. Our slab is 18" thick and our drop panel is 12" deep (30" total depth). We have a continuous mat of bottom bars in both directions in the slab. We have sufficient top steel in column strip over the column. We do not have any moment reversal in the slab at the column. (In other words, the slab will only see negative moment within the footprint of the drop panels.) Is there any reason for us to put bottom bars in the bottom of the drop panel? I have never put such steel in the bottom a drop panel, however I have never designed a slab with 12" thick drop panels. I have seen details from other engineers where they put each way bottom bars in the bottom of drop panels.

My gut is telling me that the 12" drop panel depth is of a size that would warrant each way bottom bars with vertical legs at each end turned up into the slab for crack control.

 

[Structcon]

I would use #4 U-bars at 12" o.c. each way at bottom - hook at sides of drop panel to top of slab. I know it's more than required, but I integrate the compression steel in the I effective calculation and the rho prime calc (equation 9-11) to get a reduction of around 10% in long term total load deflection.

 

[JAE]

I've never done it.

Check out Eng-Tips Forum's Policies here:
faq731-376

 

[steellion]

Reinforcement at the bottom of the drop caps would be for crack control only. In your case, a 30" thick drop cap is huge. Have you considered using shear reinforcement in the slab instead? You could fit a good-sized structural steel beam shearhead at the columns in an 18" thick slab.

 

[cliff234]

Thanks for the responses. We will be putting each way bottom bars in the bottom of the drop panels with end ends of the bars hooked up into the slab. We need the drop panels for negative moment capacity, so that's why we're using them (not just to get the punching shear capacity).

 

[hokie66]

The only reason I can think of for bottom steel in drop panels is to boost seismic resistance.

 

[msquared48]

I am in a high seismic zone. Wherever I have concrete, I have rebar.

Mike McCann, PE, SE (WA)

 

[rapt]

If you have a heavily loaded two way slab and your drop panels are sized correctly (L/6 in each direction each side of the column), then pattern loading will cause positive moments in your drop panels requiring bottom reinforcement. As long as compression reinforcement is not required, this reinforcement could be at the slab soffit level rather than the drop panel soffit, but then you will get no crack control!

 

[cliff234]

Thank you everyone for your responses. All good points. We will be putting each way bottom bars in the bottom of the drop panels for crack control. This is a low seismic area. Most of the load will be soil load.

 

[hokie66]

With mostly soil loading, I think you are wasting the steel. Crack control is not required when the bottom of the drop panel is always in compression.