Design Considerations for Economical Formwork - Simplified Design of Reinforced Concrete Buildings - page 260

Whenever possible, a minimum 16 ft (plus 6 in. minimum clearance) spacing between drop panel edges should
be used (see Fig. 9-3). Again, this permits the use of 16 ft long standard lumber without costly cutting of
material. For maximum economy, the plan dimensions of the drop panel should remain constant throughout the
entire project.

9.3.2 Joist Systems

Whenever possible, the joist depth and the spacing between joists should be based on standard form
dimensions (see Table 9-3).

The joist width should conform to the values given in Table 9-3 also. Variations in width mean more time for
interrupted labor, more time for accurate measurement between ribs, and more opportunities for jobsite error;
all of these add to the overall cost.

Table 9-3 Standard Form Dimensions for One-Way Joist Construction (in.)

Width Depth Flange width Width of joist

20 8, 10, 12 7/8, 2 ½ 5, 6

30 8, 10, 12, 14, 16, 20 7/8, 3 5, 6, 7

53 16, 20 3½ 7, 8, 9, 10

66 14, 16, 20 3 6, 7, 8, 9, 10

12
1

Flange width

Width

Width of joist=2 x flange width*

Width of joist

*Applies to flange widths > 7 / 8 in.

It is extremely cost-effective to specify a supporting beam with a depth equal to the depth of the joist. By doing
this, the bottom of the entire floor system can be formed in one horizontal plane. Additionally, installation costs
for utilities, partitions, and ceilings can all be reduced.

9.3.3 Beam-Supported Slab Systems

The most economical use of this relatively expensive system relies upon the principles of standardization and
repetition. Of primary importance is consistency in depth and of secondary importance is consistency in width.
These two concepts will mean a simplified design; less time spent interpreting plans and more time for field
crews to produce.