Simplified Design for Two-Way Slabs - Simplified Design of Reinforced Concrete Buildings - page 123

At an exterior column, the total exterior negative moment from the slab system is transferred directly to the
column. Due to the approximate nature of the moment coefficients of the Direct Design Method, it seems
unwarranted to consider the change in moment from face of support to centerline of support; use of the
exterior negative slab moment directly would usually be accurate enough.

Columns above and below the slab must resist portions of the support moment based on the relative
column stiffnesses (generally, in proportion to column lengths above and below the slab). Again, due to the
approximate nature of the moment coefficients, the refinement of considering the change in moment from
centerline of slab to top or bottom of slab seems unwarranted.

4.6 REINFORCEMENT DETAILING

In computing required steel areas and selecting bar sizes, the following will ensure conformance to the Code
and a practical design.

1. Minimum reinforcement area = 0.0018 bh (b = slab width, h = total thickness) for Grade 60 bars for either
top or bottom steel. These minima apply separately in each direction (ACI 13.3.1).

2. Maximum bar spacing is 2h, but not more than 18 in. (ACI 13.3.2).

3. Maximum top bar spacing at all interior locations subject to construction traffic should be limited. Not less
than No. 4 @ 12 in. is recommended to provide adequate rigidity and to avoid displacement of top bars with
standard bar support layouts under ordinary foot traffic.

4. Maximum ρ max is recommended to provide deformability,

to avoid overly flexible systems subject to objectionable vibration or deflection, and for a practical balance
to achieve overall economy of materials, construction and design time.

ρ = A s /bd is limited to 0.0206 however, ρ ≤ 0.50

5. Generally, the largest size of bars that will satisfy the maximum limits on spacing will provide overall
economy. Critical dimensions that limit size are the thickness of slab available for hooks and the distances
from the critical design sections to edges of slab.

4.7 EXAMPLES: SIMPLIFIED DESIGN FOR TWO-WAY SLABS

The following two examples illustrate the use of the simplified design data presented in this chapter for the
analysis and design of two-way slab systems. The two-way slab system for Building #2 is used to illustrate the
simplified design.