In an earlier blog, New Confinement Requirements for Concrete Special Moment Frame Columns, we explained the new requirements for confinement of special moment frame columns in ACI 318-14 Section 18.7.5. For columns that are subject to factored axial force, P_u, exceeding 0.3A_g f’_c (A_g = gross cross-sectional area) and/or are made of concrete with specified compressive strength, f’_c, exceeding 10,000 psi, the required confinement over regions of potential plastic hinging (typically at the two ends) is now a function of the axial force. In this blog, we would like to present the following table in which we assess the impact of the changed requirements.

Bars larger than No. 6 in size are not very practical for use as transverse reinforcement. Also, the ensemble of one hoop and crossties in two orthogonal directions has a thickness of 2-1/4 in. for No. 6 bar size, which translates into a 1-3/4-in. clear spacing for a 4-in. center-to-center spacing. Thus, the above table shows the limitations on sustainable axial load as the specified compressive strength goes beyond 6 ksi. The limitations have become significantly more severe under ACI 318-14. It should be noted that ACI 318 does not allow P_u to exceed 0.8 (accidental eccentricity factor) x 0.65 (φ for columns with discrete transverse reinforcement) x P_o = 0.525 P_o, where

P_o = A_g f’_c + A_st (f_y – f’_c)

So 0.5 f’_c is an extremely high axial load level, which is unlikely to be encountered in special moment frame columns. Also, if one needs to go beyond the range of factored axial loads and concrete strengths that can be accommodated with No. 6 transverse reinforcement at a reasonable spacing, the most effective solution is to switch to transverse reinforcement with yield strength, f_yt, higher than 60 ksi. ACI 318 allows f_yt to be up to 100 ksi. The advantage to be gained from the use of 100-ksi yield strength bars is shown in the above table.

The following notation is used in the table:

A_ch = cross-sectional area of a member measured to the outside edges of transverse reinforcement

A_sh = total cross-sectional area of transverse reinforcement, including crossties, within spacing s and perpendicular to dimension bc

b_c = cross-sectional dimension of member core measured to the outside edges of the transverse reinforcement composing area Ash

f_yt = specified yield strength of transverse reinforcement

k_f = concrete strength factor

k_n = confinement effectiveness factor

n_l = number of longitudinal bars around the perimeter of a column core with rectilinear hoops that are laterally supported by the corner of hoops or by seismic hooks

s = center-to-center spacing of transverse reinforcement