Masonry Magazine July 1981 Page. 16
Wv = W = Evlv
Evlv + EBIB
Ev = 1,350,000 psi (selected from Table 1 assuming 2000 psi unit strength)
lv = 47.6 in⁴ (selected from Table 2)
Ев = 29,000,000 psi (modulus of elasticity for metal back-up)
Ів = Unknown
9.8 - 20 = (1350000 x 47.6)
(1350000 x 47.6 + 29,000,000 IB)
18= 2.31 in/Ft of wall
Composite Wall Design
Masonry walls may be designed with a solidly filled collar joint bonding the veneer wythe to the back-up. Composite action will insure both wythes act together in resisting lateral loads. The resisting moment may be calculated for the section modulus of the wall and its allowable tensile stress.
MR= S Ft (1.33) (ft./12 in.)
where: S = Section modulus of wall (Table 5)
Ft = Allowable tensile stress (Table 4)
1.33 = One-third increase for wind
TABLE 5
SECTION PROPERTIES OF DOUBLE WYTHE WALLS 3/8 INCH COLLAR
JOINT BETWEEN SOLID 4 INCH VENEER AND BACK-UP FILLED WITH
MORTAR
| Back-up Block inch | Section Modulus inch3 | Resisting Moment(1) (Fi-lbs/Ft) Mor S Mortar | N Mortar |
|---|---|---|---|
| Hollow Unit | | | |
| 4 | 63.8 | 162.6 | 113.1 |
| 6 | 93.8 | 239.1 | 166.3 |
| 8 | 134.8 | 343.6 | 239.0 |
| 10 | 173.8 | 443.0 | 308.2 |
| 12 | 218.8 | 557.8 | 388.0 |
| Solid Unit | | | |
| 4 | 116.3 | 502.7 | 348.0 |
| 6 | 185.3 | 801.0 | 554.5 |
| 8 | 270.3 | 1,168.4 | 808.9 |
| 10 | 371.3 | 1.604.9 | 1,111.1 |
| 12 | 488.3 | 2,110.7 | 1,461.2 |
(1) Based on inspected workmanship and one-third increase for wind load.
NCMA TR 758 "Specification for Design and Construction of Load-bearing Concrete Masonry."
Adequate design requires that the resisting moment, MR equal or exceed the applied moment due to wind.
Example 3:
Design a 4 inch veneer and block back-up wall with a filled collar joint. The wind load is 40 psf and the wall height between supports is 8 feet.
M= Wh²
8
M= 40 x 8²
8 = 320 ft.-lbs./ft.
Select an 8 inch hollow block back-up with Type S mortar from Table 5.
Resisting moment MR = 343.6 ft.-lbs./ft.
NATIONAL CONCRETE MASONRY ASSOCIATION
P.O. Box 781, Herndon, Virginia 22070