Masonry Magazine July 1976 Page. 14
The panels may be constructed in place to span horizontally to isolated footings or vertically prefabricated at the job site and placed in position.
Design: The walls are designed and reinforced in accordance with the Code for reinforced solid or hollow concrete concrete masonry construction. They may be grouted by the low or high lift method as specified in Section 2413. Allowable working stresses shall be as specified in Section 2404 or as determined by prism testing under the same Code Section. The walls are supported by foundation pads located at the ends, each having a bearing length not exceeding one-tenth of the distance between supporting vertical edge members.
Concentrated loads at supporting reaction points are considered by assuming an effective column having a width equal to the length of bearing plus twice the panel thickness that resists the entire panel reaction. The allowable compressive stress for this effective column under working stress design shall be:
Fa= 0.20 fm [1-()]
If the panel height to length ratio exceeds four-fifths, the effects of deep beam stress distribution must be considered in the design.
Design
Special design requirements for deep reinforced concrete flexural members are contained in Sections 10.7 and 11.9 of ACI 318-71 (3). According to Section 10.7, "flexural members with overall depth-span ratios greater than 2/5 for continuous spans, or 4/5 for simple spans, shall be designed as deep beams taking account of nonlinear distribution of stress and lateral buckling." Section 11.9 contains special shear design requirements for deep beams having a clear span to beam depth ratio less than 5 and loaded at the top or compression face.
Additional design information which may be helpful in the design of concrete masonry wall beams is contained in references 4 and 5, as well as in the previously mentioned ICBO Research Recommendation. In general, other design requirements (including allowable stresses) and construction requirements for concrete masonry wall beams should comply with provisions contained in the "Specification for the Design and Construction of Load-bearing Concrete Masonry", NCMA-1970.
Two examples illustrating the design of concrete masonry wall beams are presented in the following paragraphs. Design aids and provisions used in these examples have been taken from the previously cited references.
EXAMPLE 1.
Design a single-span deep beam for which the following data are given.
fm 1500 psi
Length of support, c 2 ft.
Length of span,1 = 24 ft.
Height of beam, h= 24 ft.
Width of beam, b 7.625 in.
Uniform load, w 2500 lb./ft.
208.3 lb./in.
(a) Check compressive stress at support.
Allowable comp, stress,
Fa 0.20 fim [1-4]
-(0.2)(1500) [1-(48)(702]
Fa 153 psi
(2500)(12)
Actual comp, stress=
(7.625) 12+2(7.625)|
144 psi; OK
(b) Compute flexural stresses and required reinforcement.
For single-span beams, e
24
2(24)
2
Flexural stresses
(Coefficient from Table 1)()
From Table 1, Coefficient
Coefficient
Flexural stress (top) = 0.75
+0.75 (top of beam)
1.2 (bottom of beam)
208.3
() -0.75 (7.625)
20essionl
Flexural stress (bottom)-1.2()-1.2 (7.625)
-32.8 psi (tension)
Allowable comp, stress 0.33 fm (0.33)(1500) - 500 psi
From Figure 2, find coefficient for T, resultant of tensile stresses, and multiply by w21 for single-span beams.
T
0,095 w21
(0.095)(2500)(2)(24)
11,400 lb.
T
11,400
0.57 sq.in.
Ass 20,000
Select 1-No. 7 bar (As 0.60 sq.in.; 2 2.75 in.)
Assume steel location near bottom of beam, d=2883-285 in.
Assume critical section for shear as specified in ACI 318-71, Section 11.9.3, is located 0.15 In from face of support
(c) Compute shear stresses and required reinforcement.
(where In clear span between supports)
0.15 10 0.15 (24-2) = 3.3
At critical section,
v=[(3.3+1)] (2500) = 19,250 lb.
V
bd
19,250
(7.625)(285)
= 8.9 psi
Allowable shear, v. 1.1 fim 42.6 psi
(without shear reinforcement)
Therefore, no special shear reinforcement is required.
Minimum wall steel 0.002 (7.625)(12)
0.183 sq.in./ft.
(*Total in both directions; no more than 2/3 in one direction)
In horiz. direction, use No. 5 bars
32" (0.115 sq.in./ft.)
In vertical direction ue No 5 bars 48" 077 in