Masonry Magazine May 1975 Page. 18
Reinforced portions of partially reinforced masonry walls are considered to be those portions of masonry which have an effective width "b" not greater than six times the wall thickness in running bond, nor more than three times the wall thickness in stack bond. For example, in an 8 inch partially reinforced concrete masonry wall laid in running bond and having reinforcement spaced 6 ft. on centers, the reinforced portions would be assumed to have effective widths of 48 inches (6 t) and the nonreinforced portions would be assumed to have effective widths of 24 inches (72-48). The design assumptions and provisions which are used for the reinforced portions or elements of partially reinforced concrete masonry walls are identical to those used in designing reinforced concrete masonry.
Design Tables
As mentioned previously, partially reinforced concrete masonry walls can be used advantageously in panel and curtain wall applications where the wall spans exceed those permitted for nonreinforced concrete masonry. Accordingly, Tables 1 and 2 are presented as design aids and give maximum vertical and horizontal spans, respectively, permitted for hollow nonreinforced walls of various thicknesses and subjected to different magnitudes of wind load. Tables 1 and 2 are also useful in determining the maximum span (either vertical or horizontal) of nonreinforced portions of partially reinforced masonry walls between reinforced portions or elements such as horizontal bond beams or vertical grouted and reinforced cores.
Table 3 presents the minimum area of steel reinforcement which is required for "reinforced" concrete masonry walls. If the area of steel is less than that shown in Table 3 or the spacing of the principal reinforcement exceeds 6 t or 48 inches, the wall is considered to be "partially reinforced." Areas and perimeters of reinforcing steel for different sizes of joint reinforcement and bar sizes at spacings varying from 8 inches to 96 inches are given in Table 4.
Design Example
Given: Exterior hollow concrete masonry non-load-bearing panel walls; Continuous 20-ft. horizontal spans between lateral supports; Wind load, w 15 psf; f'm = 2,000 psi
Required: (a) Nonreinforced wall thickness. (b) Partially reinforced wall thickness and area of reinforcing steel.
Solution: (a) Based on Table 2, a 12-in. nonreinforced wall is adequate for a simple span of 20.7 ft.
(b) Allowable stresses:
fm = 0.30 f'm (1.33) = 798 psi
f = 20,000 (1.33) = 26,600 psi
V = 1.1/fm (1.33) = 65 psi
U = 80 (1.33) = 106 psi (plain bars)
U 160 (1.33) = 213 psi (deformed bars)
Maximum moment M = 0.10 wL2 = (0.10) (15) (20)2 = 600 ft.-lb. 7,200 in.-lb.
Maximum shear V = 0.60 wL = (0.60) (15) (20) = 180 lb.
Assume 6-in. wall thickness; and Determine approximate area of steel required:
M / fjd = 7,200 / (26,600)(7/8)(4.87) = 0.063 sq.in./ft.
(Note: Depth, d, is based on use of joint reinforcement; d 5.625"-0.625" (cover) -0.125" (1/2 bar diameter) = 4.87")
From Table 4, A, = 0.147 sq.in./ft. for 2-1/4" wires spaced 8" o.c. Since only one wire is on tension side, A = 0.0735 sq.in./ft.
Σο = 1.177 in./ft.
P = A / bd = .0735 / (12)(4.87) = .00126
n= Em / E = 2 x 10 psi / 29 x 106 = 2 x 106 / 14.5
pn (.00126)(14.5) = 0.01827
kpn2pn + (pn)2 =-.01827+2(.01827) + (.01827)2 = 0.174
j1k/310.174/3 0.942
Check masonry stress (fm):
fm=2M / jkbd2 = 2(7,200) / (.942)(.174) (284.6) = 309 psi 798 psi, OK
Check steel stress (f,):
M / Ajd = 7,200 / (.0735)(.942)(4.87) = 21,353 psi 26,600 psi, OK
Check shear (v):
v = 180 / bd = 12(4.87) = 3 psi 65 psi, OK
Check bond (u):
V / Zojd = 180 / (1.177)(.942)(4.87) = 33.3 psi 106 psi, OK
Therefore, use 6-in. wall with joint reinforcement consisting of 2-1/4" wires spaced 8" о.с.