where Acw is the area of concrete section of the individual pier considered.
7.7.4.5 For horizontal wall segments and coupling beams, Vn shall not be taken larger than 0.83 Acw ffffcccc'''' (10 Acw ffffcccc'''' ),
where Acw is the area of concrete section of a horizontal wall segment or coupling beam. Horizontal wall segment refers to a part of a wall bounded by openings or by an opening and an edge.
7.7.5 Design for Flexure and Axial Loads
7.7.5.1 Structural walls and portions of such walls subject to combined flexural and axial loads
shall be designed in accordance with ACI 318-2005 Sections 10.2 and 10.3 except that 10.3.6 and the nonlinear strain requirements of ACI 318-2005 Section 10.2.2 shall not apply. Concrete and developed longitudinal reinforcement within effective flange widths, boundary elements, and the wall web shall be considered effective. The effects of openings shall be considered.
7.7.5.2 Unless a more detailed analysis is performed, effective flange widths of flanged sections
shall extend from the face of the web a distance equal to the smaller of one-half the distance to an adjacent wall web and 25 percent of the total wall height.
7.7.6 Boundary Elements of Special Reinforced Concrete Structural Walls
7.7.6.1 The need for special boundary elements at the edges of structural walls shall be evaluated
in accordance with 7.7.6.2 or 7.7.6.3. The requirements of 7.7.6.4 and 7.7.6.5 also shall be satisfied.
7.7.6.2 This section applies to walls or wall piers that are effectively continuous from the base of
structure to top of wall and designed to have a single critical section for flexure and axial loads. Walls not satisfying these requirements shall be designed by 7.7.6.3.
) ) ) ) /h /h /h /h 600( 600(600( 600(δδδδ llll cccc w ww w uuuu w ww w ≥ (7.9)
c in Eq. (7.9) corresponds to the largest neutral axis depth calculated for the factored axial force and nominal moment strength consistent with the design displacement δu. Ratio δu /hw in Eq. 7.9 shall not be taken less than 0.007.
(b) Where special boundary elements are required by 7.7.6.2(a), the special boundary element reinforcement shall extend vertically from the critical section a distance not less than the larger of lw or Mu /4Vu.
7.7.6.3 Structural walls not designed to the provisions of 7.7.6.2 shall have special boundary
elements at boundaries and edges around openings of structural walls where the maximum extreme fiber compressive stress, corresponding to factored forces including E, load effects for earthquake effect, exceeds 0.2fc’. The special boundary element shall be permitted to be discontinued where the calculated compressive stress is less than 0.15 fc’. Stresses shall be calculated for the factored forces using a linearly elastic model and gross section properties. For walls with flanges, an effective flange width as defined in 7.7.5.2 shall be used.
7.7.6.4 Where special boundary elements are required by 7.7.6.2 or 7.7.6.3, (a) through (e) shall
be satisfied:
(a) The boundary element shall extend horizontally from the extreme compression fiber a distance not less than the larger 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) In flanged sections, the boundary element shall include the effective flange width in compression and shall extend at least 300 mm (12 in.) into the web;
(c) Special boundary element transverse reinforcement shall satisfy the requirements of 7.3.4.1 through 7.4.4.3, except Eq. (7.4) need not be satisfied;
(d) Special boundary element transverse reinforcement at the wall base shall extend into the support at least the development length of the largest longitudinal reinforcement in the special boundary element unless the special boundary element terminates on a footing or mat, where special boundary element transverse reinforcement shall extend at least 300 mm (12 in.) into the footing or mat;
(e) Horizontal reinforcement in the wall web shall be anchored to develop fy within the confined core of the boundary element;
7.7.6.5 Where special boundary elements are not required by 7.7.6.2 or 7.7.6.3, (a) and (b) shall
be satisfied:
(a) If the longitudinal reinforcement ratio at the wall boundary is greater than 2.8/fy (400/fy), boundary transverse reinforcement shall satisfy 7.4.4.1(c), 7.4.4.3, and 7.7.6.4(a). The maximum longitudinal spacing of transverse reinforcement in the boundary shall not exceed 200 mm (8 in); (b) Except when Vu in the plane of the wall is less than0.083Acv fc′ (Acv
f
c'
) , 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.7.7.7 Coupling Beams
7.7.7.1 Coupling beams with aspect ratio, (ln /h) > 4, shall satisfy the requirements of 7.3. The provisions of 7.3.1.3 and 7.3.1.4 need not be satisfied if it can be shown by analysis that the beam has adequate lateral stability.
7.7.7.3 Coupling beams with aspect ratio, (ln /h) < 2, and with Vu exceeding 0.33Acw fc′
(4Acw
f
c'
) 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 capacity of the structure, or the egress from the structure, or the integrity of nonstructural components and their connections to the structure.7.7.7.4 Coupling beams reinforced with two intersecting groups of diagonally placed bars
symmetrical about the midspan shall satisfy (a) through (f):
(a) Each group of diagonally placed bars shall consist of a minimum of four bars assembled in a core having sides measured to the outside of transverse reinforcement no smaller than bw / 2 perpendicular to the plane of the beam and bw / 5 in the plane of the beam and perpendicular to the diagonal bars;
(b) Vn shall be determined by
VVVVnnnn =2222 A A A Avdvdvdvd ffffyyyy sinsinαsinsinααα ≤0.830.830.830.83 ffffcccc''''AAAAcwcwcwcw (VVVVnnnn=2222 A A A Avdvdvdvd ffffyyyy sinsinsinαsinααα ≤10 ffffcccc''''AAAAcvcvcvcv) (7.10)
where α is the angle between the diagonally placed bars and the longitudinal axis of the coupling beam.
(c) Each group of diagonally placed bars shall be enclosed in transverse reinforcement satisfying 7.4.4.1 through 7.4.4.3. For the purpose of computing Ag for use in Eq. (7.3) and Eq. (7.5), the minimum concrete cover as required in 7.7 of ACI 318-2005 shall be assumed on all four sides of each group of diagonally placed reinforcing bars;
(d) The diagonally placed bars shall be developed for tension in the wall;
(e) The diagonally placed bars shall be considered to contribute to Mn of the coupling beam; (f) Reinforcement parallel and transverse to the longitudinal axis shall be provided and, as a
minimum, shall conform to ACI318-2005, Secs 11.8.4 and 11.8.6.
7.7.8 Construction Joints
All construction joints in structural walls shall conform to 6.4 of ACI 318-2005 and contact surfaces shall be roughened as in 11.7.9 of ACI 318-2005.
7.7.9 Discontinuous Walls
Columns supporting discontinuous structural walls shall be reinforced in accordance with 7.4.4.5.