Design Considerations for Economical Formwork - Simplified Design of Reinforced Concrete Buildings - page 259

tile, etc. should be accomplished by adding concrete to the top surface of the slab rather than maintaining a
constant slab thickness and forming offsets in the bottom of the slab. Cross section (a) in Fig. 9-2 is less costly
to form than cross section (b).

h

(a)

h

h

(b)

Figure 9-2 Depressions in Slabs

When drop panels are used in two-way systems, the total depth of the drop h 1 should be set equal to the actual
nominal lumber dimension plus 3 / 4 -in. for plyform (see Fig. 9-3). Table 9-2 lists values for the depth h 1 based
on common nominal lumber sizes. As noted above, designs which depart from standard lumber dimensions are
expensive.

Slab

h 1

Plyform

Drop

16'-6” or greater

*Keep drop dimensions constant

Figure 9-3 Formwork for Drop Panels

Table 9-2 Drop Panel Depth, h 1

Nominal lumber
size

Actual lumber
size (in.)

Plyform
thickness (in.)

h 1
(in.)

2X 1½ ¾ 2¼

4X 3½ ¾ 4¼

6X 5½ ¾ 6¼

8X 7¼ ¾ 8