Structural Concrete - Hassoun

11.10 Rectangular Columns with Side Bars 377
K ∙ ft
Figure 11.11 Interaction diagram of the column section shown in Fig. 11.10.
two cases in the table represent the pure moment (PM) or beam-action case for φ = 0.9 and φ =
0.65 (M n = 391 K ⋅ ft). To be consistent with the design of beams due to bending moments, the ACI
Code allows the use of φ = 0.9 with pure moment, so φM n = 352 K ⋅ ft instead of 254.2 K ⋅ ft. Also
note that φ varies between 0.65 and 0.9 according to Eq. 11.2 for tied columns. Note that M n =
391.1 K ⋅ ft.
11.10 RECTANGULAR COLUMNS WITH SIDE BARS
In some column sections, the steel reinforcement bars are distributed around the four sides of the
column section. The side bars are those placed on the sides along the depth of the section in addition
to the tension and compression steel, A s and A ′ s, and can be denoted by A ss (Fig. 11.12). In this case,
the same procedure explained earlier can be applied, taking into consideration the strain variation
along the depth of the section and the relative force in each side bar either in the compression or
tension zone of the section. These are added to those of C c , C s ,andT to determine P n and Eq. 11.10
becomes
P n = C c + ΣC s − ΣT
(11.10a)
Example 11.7 explains this analysis. Note that if the side bars are located near the neutral axis
(Fig. 11.12b), the strains—and, consequently, the forces—in these bars are very small and can be
neglected. Those bars close to A s and A ′ s have appreciable force and increase the load capacity of
the section.