ACI Definition 
ACI 318-05 defines a drop panel as a "projection below the slab at least one quarter of the slab thickness beyond the drop."
- A drop panel must have a minimum thickness of at least that provided in the table here.
- ACI 13.2.5 mentions that "when used to reduce the amount of negative reinforcement over a column or minimum required slab thickness, a drop panel shall project below the slab at least one-quarter of the slab thickness beyond the drop and extend in each direction from the centerline of support a distance not less than one-sixth the span length measured from center-to-center of supports in that direction."
- To shorten this statement, if you're using a drop panel to reduce the amount of negative enforcement necessary you must: project the panel 1/4 of the slab thickness and extend the panel at least 1/6 the distance between supports.
- ACI 13.3.7 mentions that "in computing the required slab reinforcement, the thickness of the drop panel below the slab shall not be assumed to be greater than one-quarter the distance from the edge of drop panel to the face of column or column capital."
To be inserted...
- American Concrete Institute, "Building Code Requirements for Structural Concrete (ACI 318-05) and Commentary (ACI 318R-05)", 2005