18.10 Special Structural Walls
(b) At locations where yielding of longitudinal reinforcement is likely to occur as a result of lateral displacements, development lengths of longitudinal reinforcement shall be 1.25 times the values calculated for fy in tension
(a) Compression zones shall be reinforced with special boundary elements where
and c corresponds to the largest neutral axis depth calculated for the factored axial force and nominal moment strength consistent with the direction of the design displacement δu. Ratio δu/hw shall not be taken less than 0.005.
(b) Where special boundary elements are required by (a), the special boundary element transverse reinforcement shall extend vertically above and below the critical section at least the greater of ℓw and Mu/4Vu, except as permitted in 126.96.36.199(g).
(a) The boundary element shall extend horizontally from the extreme compression fiber a distance at least the greater of c — 0.1ℓw and c/2, where c is the largest neutral axis depth calculated for the factored axial force and nominal moment strength consistent with δu.
(b) Width of the flexural compression zone, b, over the horizontal distance calculated by 188.8.131.52(a), including flange if present, shall be at least hu/16.
(c) For walls or wall piers with hw/ℓw ≥ 2.0 that are effectively continuous from the base of structure to top of wall, designed to have a single critical section for flexure and axial loads, and with c/ℓw ≥ 3/8, width of the flexural compression zone b over the length calculated in 184.108.40.206(a) shall be greater than or equal to 12 in.
(d) In flanged sections, the boundary element shall include the effective flange width in compression and shall extend at least 12 in. into the web.
(e) The boundary element transverse reinforcement shall satisfy 220.127.116.11(a) through (e) and 18.104.22.168, except the value hx in 22.214.171.124 shall not exceed the lesser of 14 in. and two-thirds of the boundary element thickness, and the transverse reinforcement spacing limit of 126.96.36.199(a) shall be one-third of the least dimension of the boundary element.
(f) The amount of transverse reinforcement shall be in accordance with Table 188.8.131.52(f).
|Transverse reinforcement||Applicable expressions|
|Ash/sbc for rectilinear hoop||Greater of||(a)|
|ρs for spiral or circular hoop||Greater of||(c)|
(g) Where the critical section occurs at the wall base, the boundary element transverse reinforcement at the wall base shall extend into the support at least ℓd, in accordance with 184.108.40.206, of the largest longitudinal reinforcement in the special boundary element. Where the special boundary element terminates on a footing, mat, or pile cap, special boundary element transverse reinforcement shall extend at least 12 in. into the footing, mat, or pile cap, unless a greater extension is required by 220.127.116.11.
(h) Horizontal reinforcement in the wall web shall extend to within 6 in. of the end of the wall. Reinforcement shall be anchored to develop fy within the confined core of the boundary element using standard hooks or heads. Where the confined boundary element has sufficient length to develop the horizontal web reinforcement, and Asfy/s of the horizontal web reinforcement does not exceed Asfyt/s of the boundary element transverse reinforcement parallel to the horizontal web reinforcement, it shall be permitted to terminate the horizontal web reinforcement without a standard hook or head.
(a) If the longitudinal reinforcement ratio at the wall boundary exceeds 400/fy, boundary transverse reinforcement shall satisfy 18.104.22.168(a) through (e) over the distance calculated in accordance with 22.214.171.124(a). The longitudinal spacing of transverse reinforcement at the wall boundary shall not exceed the lesser of 8 in. and 8db of the smallest primary flexural reinforcing bars, except the spacing shall not exceed the lesser of 6 in. and 6db within a distance equal to the greater of ℓw and Mu/4Vu above and below critical sections where yielding of longitudinal reinforcement is likely to occur as a result of inelastic lateral displacements.
(b) Except where Vu in the plane of the wall is less than , horizontal reinforcement terminating at the edges of structural walls without boundary elements shall have a standard hook engaging the edge reinforcement or the edge reinforcement shall be enclosed in U-stirrups having the same size and spacing as, and spliced to, the horizontal reinforcement.
(a) Vn shall be calculated by
where α is the angle between the diagonal bars and the longitudinal axis of the coupling beam.
(b) Each group of diagonal bars shall consist of a minimum of four bars provided in two or more layers. The diagonal bars shall be embedded into the wall at least 1.25 times the development length for fy in tension.
(c) Each group of diagonal bars shall be enclosed by rectilinear transverse reinforcement having out-to-out dimensions of at least bw/2 in the direction parallel to bw and bw/5 along the other sides, where bw is the web width of the coupling beam. The transverse reinforcement shall be in accordance with 126.96.36.199(a) through (e), with Ash not less than the greater of (i) and (ii):
For the purpose of calculating Ag, the concrete cover in 20.6.1 shall be assumed on all four sides of each group of diagonal bars. The transverse reinforcement shall have spacing measured parallel to the diagonal bars satisfying 188.8.131.52(c) and not exceeding 6db of the smallest diagonal bars, and shall have spacing of crossties or legs of hoops measured perpendicular to the diagonal bars not exceeding 14 in. The transverse reinforcement shall continue through the intersection of the diagonal bars. At the intersection, it is permitted to modify the arrangement of the transverse reinforcement provided the spacing and volume ratio requirements are satisfied. Additional longitudinal and transverse reinforcement shall be distributed around the beam perimeter with total area in each direction of at least 0.002bws and spacing not exceeding 12 in.
Longitudinal spacing of transverse reinforcement shall not exceed the lesser of 6 in. and 6db of the smallest diagonal bars. Spacing of crossties or legs of hoops both vertically and horizontally in the plane of the beam cross section shall not exceed 8 in. Each crosstie and each hoop leg shall engage a longitudinal bar of equal or greater diameter. It shall be permitted to configure hoops as specified in 184.108.40.206.