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Boundary Elements of Special Structural Walls per ACI 318-19 with ideCAD

How does ideCAD control boundary elements of special structural walls according to ACI 318-19?

  • Boundary Elements of Special Structural Wall requirements are met automatically.

  • Dimention of boundary elements are determined automatically. In addition, the user-defined boundary element length can also be defined.


As = area of nonprestressed longitudinal tension reinforcement, in2
Ag = gross area of concrete section, in2
Ach = cross-sectional area of a member measured to the outside edges of transverse reinforcement, in.2
Acv = gross area of concrete section bounded by web thickness and length of section in the direction of shear force considered in the case of walls, and gross area of concrete section in the case of diaphrams. Gross area is total area of defined section minus area of any openings, in.2
Av = area of shear reinforcement within spacing s, in.2
b = width of compression face of member, in.
c = distance from extreme compression fiber to neutral axis, in.
fc' = specified compressive strength of concrete, psi
√fc = square root of specified compressive strength of concrete, psi
fy = specified yield strength for nonprestressed reinforcement, psi
fyt = specified yield strength of transverse reinforcement, psi
h = overall thickness, height, or depth of member, in.
hu = laterally unsupported height at extreme compression fiber of wall or wall pier, in. equivalent to lu for compression members
hw = height of entire wall from base to top, or clear height of wall segment or wall pier considered, in.
hwcs = height of entire structural wall above the critical section for flexural and axial loads, in.
hx =maximum center-to-center spacing of longitudinal bars laterally supported by corners of crossties or hoop legs around the perimeter of a column or wall boundary element, in.
ln = length of clear span measured face-to-face of supports, in.
lu = unsupported length of column or wall, in.
lw = length of entire wall, or length of wall segment or wall pier considered in direction of shear force, in.
Mu = factored moment at section, in.-lb
Ve = design shear force for load combinations including earthquake effects, lb
Vu = factored shear force at section, lb
ρs = ratio of volume of spiral reinforcement to total volume of core confined by the spiral, measured out-to-out of spirals
δc = wall displacement capacity at top of wall, in.
δu = design displacement, in.

According to ACI, wall or wall piers with hwcs/lw ≥ 2.0 that is designed to have a single critical section for flexure and axial loads and are effectively continuous from the base of the structure to the top of the wall should satisfy two conditions given below;

  1. Compression zones should be reinforced with special boundary elements that satisfy ACI Eq. ( c means the largest neutral axis depth calculated for the factored axial force and nominal moment strength consistent with the direction of the design displacement δu.
    The minimum δu/hwcs ratio is 0.005

  2. If special boundary elements are required by Condition 1, then (a) and either (b) or (c) should be satisfied. (a), (b) and (c) are given below.

    1. Transverse reinforcement of special boundary elements should extend vertically below and above the critical section at least the maximum value of lw and Mu/4Vu.

    2. b ≥ (0.025*l*cw)0.5

    3. δc /hwcs ≥ 1.5δu /hwcs and the maximum δc /hwcs ratio is 0.015.

According to ACI; if special boundary elements are required by ACI or, the conditions given below should be satisfied;

  1. The boundary element shall extend horizontally from the extreme compression fiber a distance at least greater of c – 0.1lw and c/2.

  2. The minimum width of the flexural compression zone, b, over the horizontal distance calculated by condition 1, including flange if present, should be hu /16.

  3. For walls or wall piers with hw/lw ≥ 2.0 that are designed to have a single critical section for flexure and axial loads and with c/lw ≥ 3/8, the minimum width of the flexural compression zone b over the length calculated in condition 1 should be 12 in.

  4. In flanged sections, the boundary element should include the effective compression width and extend at least 12 in. into the web.

  5. The boundary element transverse reinforcement should be satisfied column transverse reinforcement requirements ACI and ACI Instead of "one-fourth of the minimum column length" in ACI, “one-third of the least dimension of the boundary element” should be used.

  6. Maximum transverse reinforcement spacing hx between laterally supported longitudinal bars around the perimeter of the boundary element is the lesser of 14 in. and two-thirds of the boundary element thickness. A seismic hook of a crosstie or corner of a hoop should provide lateral support. The maximum length of a hoop leg is two times the boundary element thickness, and adjacent hoops shall overlap at least the lesser of 6 in. and two-thirds the boundary element thickness.

  7. The amount of transverse reinforcement should be in accordance with ACI Table;

  8. Concrete within the thickness of the floor system at the special boundary element location should have a specified compressive strength at least 0.7fc'. (fc' is the concrete compressive strength of the wall).

  9. For a distance above and below the critical section, web vertical reinforcement should have lateral support provided by the corner of a hoop or by a crosstie with seismic hooks at each end. Maximum transverse reinforcement spacing is 12 in. and the diameter should satisfy ACI

  10. If the critical section occurs at the wall base, transverse reinforcement of the boundary element at the wall base should extend into the support at least ld in accordance with ACI 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 should extend at least 12 in. into the footing, mat, or pile cap unless a greater extension is required by ACI

  11. Horizontal reinforcement in the wall web should extend to within 6 in. of the end of the wall. Reinforcement should 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.

The transverse reinforcement area of boundary elements should be in accordance with ACI Table;

Boundary elements transverse Reinforcement Area

Applicable expressions

Ash for rectengular hoop

Maximum value of (a) and (b)

ρs for spiral or cicular hoop

Maximum value of (c) and (d)

According to ACI; if special boundary elements are not reqired by ACI or, conditions given below should be satisfied;

  1. Except where Vu in the plane of the wall is less than λ√fc‘Acv, 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 should be enclosed in U-stirrups having the same size and spacing as, and spliced to, the horizontal reinforcement.

  2. If the maximum longitudinal reinforcement ratio is greater than 400/fy , boundary transverse reinforcement should satisfy ACI (except over the distance calculated in accorance with ACI (condition 1). The vertical transverse reinforcement spacing at the wall boundary should be in accordance with ACI Table

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