Reinforced concrete bearing walls may be designed as eccentrically loaded columns or by an empirical method given in the ACI 318 Building Code. The empirical method may be used when the resultant of the applied load falls within the middle third of the wall thickness. This method gives the capacity of the walls as
The allowable average compressive stress ƒc for a wall is obtained by dividing Pu in Eq. (9.82) by Ag. Length. The effective length of wall for concentrated loads may be taken as the center-to-center distance between loads, but not more than the width of bearing plus 4 times the wall thickness.
Thickness. The minimum thickness of bearing walls for which Eq. (9.82) is applicable
is one-twenty-fifth of the least distance between supports at the sides or top, but not less than 4 in. Exterior basement walls and foundation walls should be at least 71⁄2 in thick. Minimum thickness and reinforcement requirements may be waived, however, if justified by structural analysis.
Reinforcement. The area of horizontal steel reinforcement should be at least
where Awh gross area of the horizontal cross-section of wall. For Grade 60 bars, No. 5 or smaller, or for welded-wire fabric, these steel areas may be reduced to 0.0020Awv and 0.0012Awh, respectively.
Walls 10 in or less thick may be reinforced with only one rectangular grid of rebars. Thicker walls require two grids. The grid nearest the exterior wall surface should contain between one-half and two-thirds the total steel area required for the wall. It should have a concrete cover of at least 2 in but not more than one-third the wall thickness. A grid near the interior wall surface should have a concrete cover of at least 3⁄4 in but not more than one-third the wall thickness. Minimum size of bars, if used, is No. 3. Maximum bar spacing is 18 in. (These requirements do not apply to basement walls, however. If such walls are cast against and permanently exposed to earth, minimum cover is 3 in. Otherwise, the cover should be at least 2 in for bar sizes No. 6 and larger, and 11⁄2 in for No. 5 bars or 5⁄8-in wire and smaller.)
At least two No. 5 bars should be placed around all window and door openings.
The bars should extend at least 24 in beyond the corners of openings.
Design for Eccentric Loads. Bearing walls with bending moments sufficient to cause tensile stress must be designed as columns for combined flexure and axial load, including slenderness effects if applicable. Minimum reinforcement areas and maximum bar spacings are the same as for walls designed by the empirical method.
Lateral ties, as for columns, are required for compression reinforcement and where the vertical bar area exceeds 0.01 times the gross horizontal concrete area of the wall. (For column capacity, see Art. 9.82.)
Under the preceding provisions, a thin, wall-like (rectangular) column with a steel ratio less than 0.01 will have a greater carrying capacity if the bars are detailed as for walls. The reasons for this are: The effective depth is increased by omission of ties outside the vertical bars and by the smaller cover (as small as 3⁄4 in) permitted for vertical bars in walls. Furthermore, if the moment is low (eccentricity less than one-sixth the wall thickness), so that the wall capacity is determined by Eq. (9.82), the capacity will be larger than that computed for a column, except where the column is part of a frame braced against sidesway.
If slenderness effects need to be considered, slender walls must comply with the slenderness requirements for columns (Art. 9.86). For slender precast concrete wall panels, where the panels are restrained at the top, an alternative design procedure can be used. The alternative approach was introduced into Chapter 14 of the ACI 318-99 Building Code. Complying with the provisions in the alternative procedure is deemed to satisfy the Code’s slenderness requirements for columns.