18.10.6 Boundary Elements of Special Structural Walls
(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 22.214.171.124(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 126.96.36.199(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 188.8.131.52(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 184.108.40.206(a) through (e) and 220.127.116.11, except the value hx in 18.104.22.168 shall not exceed the lesser of 14 in. and two-thirds of the boundary element thickness, and the transverse reinforcement spacing limit of 22.214.171.124(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 126.96.36.199(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 188.8.131.52, 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 184.108.40.206.
(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 220.127.116.11(a) through (e) over the distance calculated in accordance with 18.104.22.168(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.