Building Design and Construction Handbook - Merritt - Ventech!

9.102 SECTION NINE
The allowable average compressive stress ƒ c for a wall is obtained by dividing
P u in Eq. (9.82) by A g.
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 7 1 ⁄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
A � 0.0025A (9.83)
h wv
where Awv � gross area of the vertical cross-section of wall.
Area of vertical reinforcement should be at least
Av � 0.0015A wh
(9.84)
where A wh � 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
0.0020A wv and 0.0012A wh, 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 1 1 ⁄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.