Drop Panel Width

[BOCH]

Hello guys,
I have a question regarding drop panel size in flat slabs, ACI code mentioned that when using drop panel to reduce the amount of negative moment over a column, a drop panel shall extend in each direction from the centerline of support a distance not less than 1/6 the span length measured from center to center of support in that direction. Hence, in PCA notes on ACI, it is mentionned that the width of drop panel shall not be less than l/3 (refer to the attachement).
These two provisions are making conflict:
- The drop panel should be centered to the column with a width of lx/3 ? and which lx should be taken if we have different spans in x direction, and same for y direction ??
- Or it can be excentrique with a width of lx1/6 + lx2/6 ??
Thank you.

 

[hokie66]

It is just an arbitrary dimension that has been found by experience to work well. Don't make drop panels eccentric...your genius builder will get it around the wrong way. In most cases, making them 1/3 of the longer span makes sense, but if there are irregular spans, commmon sense application of this general rule is required.

 

[JoshPlumSE]

PCA notes is just trying to make the code language clearer. They must be assuming the same span in each direction. Or, they are saying L/3 of the average span in that direction. Either way, your equation L = lx1/6 + lx2/6 would be the proper/ acceptable interpretation in my mind.

 

[rapt]

For each direction, I would use the average of the spans either side of the column in that direction. And if they are relatively close for all columns, use the largest value for the floor for all columns.

As Hokie said. if you make them different some idiot will put the wrong one in the wrong place!

 

[TXStructural]

You are talking about an irrelevantly small amount of concrete to have the potential problems noted by previous posters. Use a square drop that is l/3 of the longest adjacent span in either direction. You should probably be limiting the variation in span to reasonable numbers anyway. Most problems are avoided by keeping spans within 20% of each other.