In an earlier blog, ACI 318-14 – Technical Changes, the significant technical changes from ACI 318-11 to ACI 318-14 were briefly described. In this blog, we elaborate on one of the more important changes – new requirements for confinement of special reinforced concrete columns (ACI 318-14 Section 18.7.5).

The dependence of the amount of required confinement on the magnitude of axial load imposed on a column has long been recognized by the Canadian and New Zealand codes, but was not reflected in ACI 318 through its 2011 edition. Also, longitudinal reinforcement that is well distributed and laterally supported around the perimeter of a column core provides more effective confinement than a cage with larger, widely-spaced longitudinal bars. Confinement effectiveness is a key parameter determining the behavior of confined concrete. ACI 318, through its 2011 edition, did not explicitly account for confinement effectiveness in determining the required amount of confinement.

In view of the above, confinement requirements for columns of concrete special moment frames (Section 18.7.5) with high axial load (P_u > 0.3A_gf’_c) or high concrete compressive strength (f’_c > 10,000 psi) are significantly different in ACI 318-14 (Figure 1).

Figure 1. Changes in ACI 318-14 for Confinement of Columns in Special Moment Frames

One important new requirement is found in Section 18.7.5.2 Item (f). In columns with P_u > 0.3A_gf’_c or f’_c > 10,000 psi, and with rectilinear hoops, every longitudinal bar or bundle of bars around the perimeter of the column core is required to have lateral support provided by the corner of a hoop or by a seismic hook, and the value of h_x cannot not exceed 8 in. (Figure 1), where P_u is the largest value in compression consistent with factored load combinations including E. In contrast, ACI 318-11 required only every other longitudinal bar to be laterally supported and a maximum permissible h_x to be 14 in. for all columns of special moment frames. Also, note a subtle difference in the way h_x is defined in the two editions of ACI 318. In the 2011 edition, h_x was the maximum center-to-center spacing of consecutive ties providing lateral support to the longitudinal bars. In the 2014 edition, however, it is the maximum center-to-center spacing of the laterally supported bars.

The other new requirement is in Section 18.7.5.4:

The amount of transverse reinforcement needs to in accordance with Table 18.7.5.4 (reproduced here as Table 1).

The concrete strength factor, k_f, and confinement effectiveness factor, k_n, are calculated by ACI 318-14 Equations 18.7.5.4a and b, respectively.

k_f = f’_c / 25,000 + 0.6 ≥ 1.0          (18.7.5.4a)

k_n = n_l / (n_l – 2)                             (18.7.5.4b)

where n_l is the number of longitudinal bars or bar bundles around the perimeter of a column core with rectilinear hoops that are laterally supported by the corner of hoops or by seismic hooks.

Equation (c) in ACI 318-14 Table 18.7.5.4 is new, so is the distinction between columns with P_u ≤ 0.3A_gf’_c and f’_c ≤ 10,000 psi and those with P_u > 0.3A_gf’_c or f’_c > 10,000 psi.

Table 1 (ACI 318-14 Table 18.7.5.4). Confinement of high-strength or highly-axially-loaded rectangular column of special moment frame

The changes discussed in this blog will come into play for projects with concrete special moment frames featuring columns having high axial load (P_u > 0.3A_gf’_c) or high concrete compressive strength (f’_c > 10,000 psi). We thought it important to bring this to the attention of designers.