18.7 Columns of Special Moment Frames
(a) The shortest cross-sectional dimension, measured on a straight line passing through the geometric centroid, shall be at least 12 in.
(b) The ratio of the shortest cross-sectional dimension to the perpendicular dimension shall be at least 0.4.
|∑ Mnc≥ (6/5)∑Mnb||(126.96.36.199)|
∑Mnc is sum of nominal flexural strengths of columns framing into the joint, evaluated at the faces of the joint. Column flexural strength shall be calculated for the factored axial force, consistent with the direction of the lateral forces considered, resulting in the lowest flexural strength.
∑Mnb is sum of nominal flexural strengths of the beams framing into the joint, evaluated at the faces of the joint. In T-beam construction, where the slab is in tension under moments at the face of the joint, slab reinforcement within an effective slab width defined in accordance with 6.3.2 shall be assumed to contribute to Mnb if the slab reinforcement is developed at the critical section for flexure.
Flexural strengths shall be summed such that the column moments oppose the beam moments. Equation (188.8.131.52) shall be satisfied for beam moments acting in both directions in the vertical plane of the frame considered.
(b) One-sixth of the clear span of the column
(c) 18 in.
(c) Crossties of the same or smaller bar size as the hoops shall be permitted, subject to the limitation of 184.108.40.206. Consecutive crossties shall be alternated end for end along the longitudinal reinforcement and around the perimeter of the cross section.
(e) Reinforcement shall be arranged such that the spacing hx of longitudinal bars laterally supported by the corner of a crosstie or hoop leg shall not exceed 14 in. around the perimeter of the column.
(f) Where Pu > 0.3Agf'c or f'c > 10,000 psi in columns with rectilinear hoops, every longitudinal bar or bundle of bars around the perimeter of the column core shall have lateral support provided by the corner of a hoop or by a seismic hook, and the value of hx shall not exceed 8 in. Pu shall be the largest value in compression consistent with factored load combinations including E.
(a) One-fourth of the minimum column dimension
(b) Six times the diameter of the smallest longitudinal bar
(c) so, as calculated by:
The value of so from Eq. (220.127.116.11) shall not exceed 6 in. and need not be taken less than 4 in.
The concrete strength factor kf and confinement effectiveness factor kn are calculated according to Eq. (18.104.22.168a) and (22.214.171.124b).
|Transverse reinforcement||Conditions||Applicable expressions|
|Ash/sbc for rectilinear hoop||Pu ≤ 0.3Agfc' and fc' ≤ 10,000 psi||Greater of (a) and (b)|
|Pu > 0.3Agfc' or fc' > 10,000 psi||Greatest of (a), (b), and (c)|
|ρs for spiral or circular hoop||Pu ≤ 0.3Agfc' and fc' ≤ 10,000 psi||Greater of (d) and (e)|
|Pu > 0.3Agfc' or fc' > 10,000 psi||Greatest of (d), (e), and (f)|
(a) Transverse reinforcement required by 126.96.36.199 through 188.8.131.52 shall be provided over the full height at all levels beneath the discontinuity if the factored axial compressive force in these columns, related to earthquake effect, exceeds Agfc'/10. Where design forces have been magnified to account for the overstrength of the vertical elements of the seismic-force-resisting system, the limit of Agfc'/10 shall be increased to Agfc'/4.
(b) Transverse reinforcement shall extend into the discontinued member at least ℓd of the largest longitudinal column bar, where ℓd is in accordance with 18.8.5. Where the lower end of the column terminates on a wall, the required transverse reinforcement shall extend into the wall at least ℓd of the largest longitudinal column bar at the point of termination. Where the column terminates on a footing or mat, the required transverse reinforcement shall extend at least 12 in. into the footing or mat.
(a) The earthquake-induced shear force, calculated in accordance with 184.108.40.206, is at least one-half of the maximum required shear strength within ℓo.
(b) The factored axial compressive force Pu including earthquake effects is less than Agfc'/20.