This chapter shall apply to the design of nonprestressed and prestressed walls including (a) through (c):
(b) Precast in-plant
(c) Precast on-site including tilt-up
Design of special structural walls shall be in accordance with Chapter 18.
Design of plain concrete walls shall be in accordance with Chapter 14.
Design of cantilever retaining walls shall be in accordance with 22.2 through 22.4, with minimum horizontal reinforcement in accordance with 11.6.
Design properties for concrete shall be selected to be in accordance with Chapter 19.
Design properties for steel reinforcement shall be selected to be in accordance with Chapter 20.
Materials, design, and detailing requirements for embedments in concrete shall be in accordance with 20.7.
For precast walls, connections shall be designed in accordance with 16.2.
Connections of walls to foundations shall satisfy 16.3.
Unless otherwise demonstrated by an analysis, the horizontal length of wall considered as effective for resisting each concentrated load shall not exceed the lesser of the center-to-center distance between loads, and the bearing width plus four times the wall thickness. Effective horizontal length for bearing shall not extend beyond vertical wall joints unless design provides for transfer of forces across the joints.
Walls shall be anchored to intersecting elementssuch as floors or roofs; or to columns, pilasters, buttresses, of intersecting walls and footings with reinforcement at least equivalent to No. 4 bars at 12 inches (305 mm) on center for each layer of reinforcement.
Minimum wall thicknesses shall be in accordance with Table 188.8.131.52. Thinner walls are permitted if adequate strength and stability can be demonstrated by structural analysis.
Table 184.108.40.206—Minimum wall thickness h
|Wall type||Minimum thickness h|
|Bearing||Greater of:||4 in.||(a)|
|1/25 the lesser of unsupported length and unsupported height||(b)|
|Nonbearing||Greater of:||4 in.||(c)|
|1/30 the lesser of unsupported length and unsupported height||(d)|
|Exterior basement and foundation||7.5 in.||(e)|
Required strength shall be calculated in accordance with the factored load combinations in Chapter 5.
Required strength shall be calculated in accordance with the analysis procedures in Chapter 6.
Walls shall be designed for the maximum factored moment Mu that can accompany the factored axial force for each applicable load combination. The factored axial force Pu at given eccentricity shall not exceed ϕPn,max, where Pn,max shall be as given in 220.127.116.11 and strength reduction factor ϕ shall be that for compression-controlled sections in 21.2.2. The maximum factored moment Mu shall be magnified for slenderness effects in accordance with 6.6.4, 6.7, or 6.8.
For each applicable factored load combination, design strength at all sections shall satisfy ϕSn ≥ U, including (a) through (c). Interaction between axial load and moment shall be considered.
(a) ϕPn ≥ Pu
(b) ϕMn ≥ Mu
(c) ϕVn ≥ Vu
If the resultant of all factored loads is located within the middle third of the thickness of a solid wall with a rectangular cross section, Pn shall be permitted to be calculated by:
Effective length factor k for use with Eq. (18.104.22.168) shall be in accordance with Table 22.214.171.124.
Table 126.96.36.199—Effective length factor k for walls
|Walls braced top and bottom against lateral translation and:|
|(a) Restrained against rotation at one or both ends (top, bottom, or both)||0.8|
|(b) Unrestrained against rotation at both ends||1.0|
|Walls not braced against lateral translation||2.0|
Pn from Eq. (188.8.131.52) shall be reduced by ϕ for compression-controlled sections in 21.2.2.
Wall reinforcement shall be at least that required by 11.6.
Vn shall be calculated in accordance with 184.108.40.206 through 220.127.116.11. Alternatively, for walls with hw ≤ 2ℓw, it shall be permitted to design for in-plane shear in accordance with the strut-and-tie method of Chapter 23. In all cases, reinforcement shall satisfy the limits of 11.6, 11.7.2, and 11.7.3.
For in-plane shear design, h is thickness of wall and d shall be taken equal to 0.8ℓw. A larger value of d, equal to the distance from extreme compression fiber to center of force of all reinforcement in tension, shall be permitted if the center of tension is calculated by a strain compatibility analysis.
Vn at any horizontal section shall not exceed .
Vn shall be calculated by:
|Vn = Vc+ Vs||(18.104.22.168)|
It shall be permitted to calculate Vc in accordance with Table 22.214.171.124, where Nu is positive for compression and negative for tension, and the quantity Nu/Ag is expressed in psi.
Table 126.96.36.199—Vc: nonprestressed and prestressed walls
|Calculation option||Axial force||Vc|
|Detailed||Tension or compression||Lesser of:||(d)|
Equation shall not apply if (Mu/Vu— ℓw/2) is negative.
Vs shall be provided by transverse shear reinforcement and shall be calculated by:
If in-plane Vu ≤ 0.5ϕVc, minimum ρℓ and minimum ρt shall be in accordance with Table 11.6.1. These limits need not be satisfied if adequate strength and stability can be demonstrated by structural analysis.
Table 11.6.1—Minimum reinforcement for walls with in-plane Vu ≤ 0.5ϕVc
|Wall type||Type of nonprestressed reinforcement||Bar/wire size||fy, psi||Minimum longitudinal, ρℓ||Minimum transverse, ρt|
|Cast-in-place||Deformed bars||≤ No. 5||≥ 60,000||0.0012||0.0020|
|> No. 5||Any||0.0015||0.0025|
|Welded-wire reinforcement||≤ W31 or D31||Any||0.0012||0.0020|
|Precast||Deformed bars or welded-wire reinforcement||Any||Any||0.0010||0.0010|
Prestressed walls with an average effective compressive stress of at least 225 psi need not meet the requirement for minimum longitudinal reinforcement ρℓ.
In one-way precast, prestressed walls not wider than 12 ft and not mechanically connected to cause restraint in the transverse direction, the minimum reinforcement requirement in the direction normal to the flexural reinforcement need not be satisfied.
Concrete cover for reinforcement shall be in accordance with 20.6.1.
Development lengths of deformed and prestressed reinforcement shall be in accordance with 25.4.
Splice lengths of deformed reinforcement shall be in accordance with 25.5.
Spacing s of longitudinal bars in cast-in-place walls shall not exceed the lesser of 3h and 18 in. If shear reinforcement is required for in-plane strength, spacing of longitudinal reinforcement shall not exceed ℓw/3.
Spacing s of longitudinal bars in precast walls shall not exceed the lesser of (a) and (b):
If shear reinforcement is required for in-plane strength, s shall not exceed the smallest of 3h, 18 in., and ℓw/3.
For walls with h greater than 10 in., except basement walls and cantilever retaining walls, distributed reinforcement for each direction shall be placed in two layers parallel with wall faces in accordance with (a) and (b):
(a) One layer consisting of at least one-half and not exceeding two-thirds of total reinforcement required for each direction shall be placed at least 2 in., but not exceeding h/3, from the exterior surface.
(b) The other layer consisting of the balance of required reinforcement in that direction, shall be placed at least 3/4 in., but not greater than h/3, from the interior surface.
Flexural tension reinforcement shall be well distributed and placed as close as practicable to the tension face.
Spacing s of transverse reinforcement in cast-in-place walls shall not exceed the lesser of 3h and 18 in. If shear reinforcement is required for in-plane strength, s shall not exceed ℓw/5.
If longitudinal reinforcement is required for compression and if Ast exceeds 0.01Ag, longitudinal reinforcement shall be laterally supported by transverse ties.
In addition to the minimum reinforcement required by 11.6, at least two No. 5 bars in walls having two layers of reinforcement in both directions and one No. 5 bar in walls having a single layer of reinforcement in both directions shall be provided around window, door, and similarly sized openings. Such bars shall be anchored to develop fy in tension at the corners of the openings.
Perimeters of precast walls shall be reinforced continuously with a minimum of one No. 5 bar extending the full height and width of the wall panel. Where wall panels do not connect to columns or other wall panels to develop at least 75 percent of the horizontal wall steel as noted below, vertical perimeter bars shall be retained by hooked wall bars.
A continuous tie or bond beam shall be provided at the roof line either as a part of the roof structure or part of the wall panels as described in the next paragraph below. This tie may be designed as the edge member of the roof diaphragm but, in any case, shall not be less than equivalent to two No. 6 bars continuous. A continuous tie equivalent to two No. 5 bars minimum shall also be provided either in the footing or with an enlarged section of the floor slab.
Wall panels of shear wall buildings shall be connected to columns or to each other in such a manner as to develop at least 75 percent of the horizontal wall steel. No more than half of this continuous horizontal reinforcing shall be concentrated in bond or tie beams at the top and bottom of the walls and at points of intermediate lateral support. If possible, cast-in-place joints with reinforcing bars extending from the panels into the joint a sufficient distance to meet the splice requirements of ACI 318, Section 25.5.2, for Class A shall be used. The reinforcing bars or welded tie details shall not be spaced over eight times the wall thickness vertically nor fewer than four used in the wall panel height. Where wall panels are designed for their respective overturning forces, the panel connections need not comply with the requirements of this paragraph.
Exception: Nonbearing, nonshear panels such as nonstructural architectural cladding panels or column covers are not required to meet the provisions of this section.
It shall be permitted to analyze out-of-plane slenderness effects in accordance with this section for walls satisfying (a) through (e):
(a) Cross section is constant over the height of the wall
(b) Wall is tension-controlled for out-of-plane moment effect
(c) ϕMn is at least Mcr, where Mcr is calculated using fr as provided in 19.2.3
(d) Pu at the midheight section does not exceed 0.06f'cAg
(e) Calculated out-of-plane deflection due to service loads, Δs, including PΔ effects, does not exceed ℓc/150
Concentrated gravity loads applied to the wall above any section shall be assumed to be distributed over a width equal to the bearing width, plus a width on each side that increases at a slope of 2 vertical to 1 horizontal, but not extending beyond (a) or (b):
(b) The edges of the wall panel
Mu at midheight of wall due to combined flexure and axial loads shall include the effects of wall deflection in accordance with (a) or (b):
(a) By iterative calculation using
|Mu = Mua+ PuΔu||(188.8.131.52a)|
where Mua is the maximum factored moment at midheight of wall due to lateral and eccentric vertical loads, not including PΔ effects.
Δu shall be calculated by:
where Icr shall be calculated by:
and the value of Es/Ec shall be at least 6.
(b) By direct calculation using:
Out-of-plane deflection due to service loads, Δs, shall be calculated in accordance with Table 184.108.40.206, where Ma is calculated by 220.127.116.11.
Table 18.104.22.168—Calculation of Δs
Δcr and Δn shall be calculated by (a) and (b):
Horizontal reinforcing of concrete foundation walls for wood-frame or light-steel buildings shall consist of the equivalent of not less than one No. 5 bar located at the top and bottom of the wall. Where such walls exceed 3 feet (914 mm) in height, intermediate horizontal reinforcing shall be provided at spacing not to exceed 2 feet (610 mm) on center. Minimum vertical reinforcing shall consist of No. 3 bars at 24 inches (610 mm) on center.
Where concrete foundation walls or curbs extend above the floor line and support wood-frame or light-steel exterior, bearing or shear walls, they shall be doweled to the foundation wall below with a minimum of No. 3 bars at 24 inches (610 mm) on center. Where the height of the wall above the floor line exceeds 18 inches (457 mm), the wall above and below the floor line shall meet the requirements of ACI 318, Section 11.6 and 11.7.