# 18.10 Special Structural Walls

**ρ**and

_{ℓ}**ρ**, for structural walls shall be at least 0.0025, except that if

_{t}**does not exceed ,**

*V*_{u}**ρ**and

_{ℓ}**ρ**shall be permitted to be reduced to the values in 11.6. Reinforcement spacing each way in structural walls shall not exceed 18 in. Reinforcement contributing to

_{t}**shall be continuous and shall be distributed across the shear plane.**

*V*_{n}**, in which**

*h*/_{w}*ℓ*≥ 2.0_{w}**and**

*h*_{w}**refer to height and length of entire wall, respectively.**

*ℓ*_{w}**in tension in accordance with 25.4, 25.5, and (a) through (c):**

*f*_{y}(a) Longitudinal reinforcement shall extend beyond the point at which it is no longer required to resist flexure by least **0.8 ℓ_{w}**, except at the top of a wall

(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 ** f_{y}** in tension

(c) Mechanical splices of reinforcement shall conform to 18.2.7 and welded splices of reinforcement shall conform to 18.2.8

**shall be obtained from the lateral load analysis in accordance with the factored load combinations.**

*V*_{u}*V*of structural walls shall not exceed:

_{n}(18.10.4.1) |

where the coefficient **α _{c}** is 3.0 for

**, is 2.0 for**

*h*/_{w}*ℓ*≤ 1.5_{w}**, and varies linearly between 3.0 and 2.0 for**

*h*/_{w}*ℓ*≥ 2.0_{w}**between 1.5 and 2.0.**

*h*/_{w}*ℓ*_{w}**used to calculate**

*h*/_{w}*ℓ*_{w}**for segments of a wall shall be the greater of the ratios for the entire wall and the segment of wall considered.**

*V*_{n}**does not exceed 2.0, reinforcement ratio**

*h*/_{w}*ℓ*_{w}**ρ**shall be at least the reinforcement ratio

_{ℓ}**ρ**.

_{t}**shall not be taken greater than , where**

*V*_{n}**is the gross area of concrete bounded by web thickness and length of section. For any one of the individual vertical wall segments,**

*A*_{cv}**shall not be taken greater than , where**

*V*_{n}**is the area of concrete section of the individual vertical wall segment considered.**

*A*_{cw}**shall not be taken greater than , where**

*V*_{n}**is the area of concrete section of a horizontal wall segment or coupling beam.**

*A*_{cw}**that are effectively continuous from the base of structure to top of wall and are designed to have a single critical section for flexure and axial loads shall satisfy (a) and (b), or shall be designed by 18.10.6.3:**

*h*/_{w}*ℓ*≥ 2.0_{w}(a) Compression zones shall be reinforced with special boundary elements where

(18.10.6.2) |

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

**δ**. Ratio

_{u}**δ**shall not be taken less than 0.005.

_{u}/*h*_{w}(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 ** ℓ_{w}** and

**, except as permitted in 18.10.6.4(g).**

*M*/4_{u}*V*_{u}**, exceeds**

*E***0.2**. The special boundary element shall be permitted to be discontinued where the calculated compressive stress is less than

*f*'_{c}**0.15**. Stresses shall be calculated for the factored loads using a linearly elastic model and gross section properties. For walls with flanges, an effective flange width as given in 18.10.5.2 shall be used.

*f*'_{c}(a) The boundary element shall extend horizontally from the extreme compression fiber a distance at least the greater of ** c — 0.1ℓ_{w}** and

**, where**

*c*/2**is the largest neutral axis depth calculated for the factored axial force and nominal moment strength consistent with**

*c***δ**.

_{u}(b) Width of the flexural compression zone, ** b**, over the horizontal distance calculated by 18.10.6.4(a), including flange if present, shall be at least

*h*/16._{u}(c) For walls or wall piers with ** h_{w}/ℓ_{w} ≥ 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

**, width of the flexural compression zone**

*c*/*ℓ*≥ 3/8_{w}**over the length calculated in 18.10.6.4(a) shall be greater than or equal to 12 in.**

*b*(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 18.7.5.2(a) through (e) and 18.7.5.3, except the value ** h_{x}** in 18.7.5.2 shall not exceed the lesser of 14 in. and two-thirds of the boundary element thickness, and the transverse reinforcement spacing limit of 18.7.5.3(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 18.10.6.4(f).

**Table 18.10.6.4(f)—Transverse reinforcement for special boundary elements**

Transverse reinforcement | Applicable expressions | ||
---|---|---|---|

A/_{sh}sb for rectilinear hoop_{c} | Greater of | (a) | |

(b) | |||

ρ_{s} for spiral or circular hoop | Greater of | (c) | |

(d) |

(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 ** ℓ_{d}**, in accordance with 18.10.2.3, 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 18.13.2.3.

(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 ** f_{y}** 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

**of the horizontal web reinforcement does not exceed**

*A*/_{s}f_{y}*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*/_{s}f_{yt}*s*(a) If the longitudinal reinforcement ratio at the wall boundary exceeds **400/ f_{y}**, boundary transverse reinforcement shall satisfy 18.7.5.2(a) through (e) over the distance calculated in accordance with 18.10.6.4(a). The longitudinal spacing of transverse reinforcement at the wall boundary shall not exceed the lesser of 8 in. and

**8**of the smallest primary flexural reinforcing bars, except the spacing shall not exceed the lesser of 6 in. and

*d*_{b}**6**within a distance equal to the greater of

*d*_{b}**and**

*ℓ*_{w}**above and below critical sections where yielding of longitudinal reinforcement is likely to occur as a result of inelastic lateral displacements.**

*M*/4_{u}*V*_{u}(b) Except where ** V_{u}** 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.

**(**and with

*ℓ*/_{n}*h*) < 2

**V**_{u}≥**4λ √f'****shall be reinforced with two intersecting groups of diagonally placed bars symmetrical about the midspan, unless it can be shown that loss of stiffness and strength of the coupling beams will not impair the vertical load-carrying ability of the structure, the egress from the structure, or the integrity of nonstructural components and their connections to the structure.**

_{c}A_{cw}(a) ** V_{n}** shall be calculated by

(18.10.7.4) |

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 ** f_{y}** in tension.

(c) Each group of diagonal bars shall be enclosed by rectilinear transverse reinforcement having out-to-out dimensions of at least ** b_{w}/2** in the direction parallel to

**and**

*b*_{w}**along the other sides, where**

*b*/5_{w}**is the web width of the coupling beam. The transverse reinforcement shall be in accordance with 18.7.5.2(a) through (e), with**

*b*_{w}**not less than the greater of (i) and (ii):**

*A*_{sh}(i)

(ii)

For the purpose of calculating ** A_{g}**, 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 18.7.5.3(c) and not exceeding

**6**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

*d*_{b}**0.002**and spacing not exceeding 12 in.

*b*_{w}s(d) Transverse reinforcement shall be provided for the entire beam cross section in accordance with 18.7.5.2(a) through (e) with ** A_{sh}** not less than the greater of (i) and (ii):

(i)

(ii)

Longitudinal spacing of transverse reinforcement shall not exceed the lesser of 6 in. and **6 d_{b}** 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 18.6.4.3.

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