column strip widths - direct design method - limit to drop widths for negative moment design????

[structineer]

I have a flat slab with 8'x8' drops. The column spacing is 30'x20' If I follow the CRSI examples in chapter 10, they use a column strip width equal to the lesser of l1/2 and l2/2. If I follow this, I will have a column strip width of 10', which is greater than my drop width. I was thinking that I would need to limit my column strip width to 8' when considering the negative moment reinforcing (located in the top of the slab at the supports). Otherwise, how could I use the d that I get from the total drop thickness outside of the limits of the drop where the slab is thinner? This is exactly what CRSI uses.....a column strip that is wider than the drop, but also uses the greater "d" dimension that only occurs at the drop. This is puzzling to me.

 

[JAE]

I would think you'd use:
1. The larger "d"
2. The full column strip width (for As determination)
3. The drop width for your compression block width.

 

[structineer]

I would agree with you, however what I'm finding is that they use the full width of the column strip for "b" and the larger "d" as well. This doesn't make sense.

Here is an example. Example 2 starting on slide 40 follows the same method as CRSI.

http://www.powershow.com/view/a095c...Floor_Slab_System_powerpoint_ppt_presentation

 

[BAretired]

I agree with JAE. Use a column strip of 15' in one direction and 10' in the other. There is no problem using the larger 'd'. It is what you would do for a Tee Beam in the negative moment region.

BA

 

[dik]

Your column strip width is 10' and your drop panel width is 8'... centred on your gridline... Is that correct?

Dik

 

[structineer]

Dik,

Yes, that is correct.

 

[rapt]

Agree with JAE. Treat it as a T section over the support using the effective depth of the drop panel. As long as the drop panel length and width are a minumum of L/6 each side of the column in each direction. For normal section shapes this normally means that the "flange" overhang on each side is only about 4Ds which is similar to a T beam flange.

British code actually requires that the column strip width is the same as the drop panel width, but the momeent in the column strip is reduced to match the reduced width. I do not think this is logical and use of the normal column strip width with the full column strip moment is logical as long as the column strip width rules are followed.