Simplified Design for Beams and One-Way Slabs - Simplified Design of Reinforced Concrete Buildings - page 69

3.6.2 Selection of Stirrups for Economy

Selection of stirrup size and spacing for overall cost savings requires consideration of both design time and
fabrication and placing costs. An exact solution with an intricate stirrup layout closely following the variation
in the shear diagram is not a cost-effective solution. Design time is more cost-effective when a quick, more
conservative analysis is utilized. Small stirrup sizes at close spacings require disproportionately high costs in
labor for fabrication and placement. Minimum cost solutions for simple placing should be limited to three
spacings: the first stirrup located at 2 in. from the face of the support (as a minimum clearance), an
intermediate spacing, and finally, a maximum spacing at the code limit of d/2. Larger size stirrups at wider
spacings are more cost-effective (e.g., using No.4 for No.3 at double spacing, and No.5 and No.4 at 1.5 spacing)
if it is possible to use them within the spacing limitations of d/2 and d/4.

In order to adequately develop the stirrups, the following requirements must all be satisfied (ACI 12.13):
(1) stirrups shall be carried as close to the compression and tension surfaces of the member as cover
requirements permit, (2) for No.5 stirrups and smaller, a standard stirrup hook (as defined in ACI 7.1.3) shall
be provided around longitudinal reinforcement, and (3) each bend in the continuous portion of the stirrup must
enclose a longitudinal bar. To allow for bend radii at corners of U stirrups, the minimum beam widths given in
Table 3-10 should be provided.

Table 3-10 Minimum Beam Widths for Stirrups

Stirrup size

Minimum beam width
(b w )

#3 10 in.

#4 12 in.

#5 14 in.

Note that either the No.3 or the No.4 stirrup in the example of Fig. 3-8 can be placed in the 12 in. wide beam.

3.7 DESIGN FOR TORSION

For simplified torsion design for spandrel beams, where the torsional loading is from an integral slab, two
options are possible:

(1) Size the spandrel beams so that torsion effects can be neglected (ACI 11.5.1), or

(2) Provide torsion reinforcement for a prescribed torque corresponding to the cracking torsional
moment (ACI 11.5.2.2)