Simplified Design for Beams and One-Way Slabs - Simplified Design of Reinforced Concrete Buildings - page 77

The above sizing equations are mixed units: M o ft-kips and section dimensions in inches.

Architectural or economic considerations may dictate a smaller spandrel size than that required to neglect
torsion effects. For a specific floor framing system, both architectural and economic aspects of a larger beam
size to neglect torsion versus a smaller beam size with torsion reinforcement (additional closed stirrups at close
spacing combined with longitudinal bars) must be evaluated. If a smaller spandrel with torsion reinforcement is
a more appropriate choice, Section 3.7.2 provides a simple method for the design of the torsion reinforcement.

3.7.1.1 Example: Beam sizing to Neglect Torsion

Determine a spandrel beam size to neglect torsion effects for Building #2, Alternate (1) - slab and column
framing with spandrel beams.

For N-S spandrels: ˜ 2 = 20 ft

˜ 1 = 24 ft
˜ n = 24 - (12 + 16)/(2 12) = 22.83 ft

w u = 1.2(136) + 1.6(50) = 243 psf

M o = q u ˜ 2 ˜ n 2 /8 = 0.243 20 22.832/8 = 317 ft-kips

For slab thickness h f = 8.5 in., for monolithic construction (ACI 13.2.4) the portion of slab considered in beam
design is the smaller of:

4h f
h - h f
For preliminary section calculations assume b f = 3h f = 3(8.5) = 25.5 in.

( )

( ) ( )

2

bh + 25.5 8.5

= 70(317)

( )

h + b + 25.5

Some possible combinations of b and h that will satisfy the requirement for neglecting torsion are shown in Table 3-13.

Clearly large beam sizes are required to neglect torsion effects. It would be more economical to select a smaller
beam and provide torsion reinforcement.