Masonry Magazine July 1981 Page. 15
Stiffness Compatibility of Veneer and Flexible Back-up Systems
Generally metal stud back-up systems are more flexible than the masonry veneer which they support. To prevent excessive deflection and cracking the metal stud back-up must be designed with sufficient stiffness to control tensile stress in the veneer. The stiffness criteria for the back-up is determined by what the veneer can tolerate. Cracking of the veneer on either face due to wind pressure or suction should not be permitted. Cracks destroy the appearance and resistance to water penetration of the veneer and in some extreme cases may become safety hazards if units dislodge and fall from the facade. Cracks in the veneer are controlled by limiting the tensile stress to those permitted in Table 4. The tensile stress of the veneer may be controlled by incorporating sufficient stiffness into the back-up system. A rigid back-up system will carry virtually all of the lateral load and limit the deflection of the veneer to a minimum. Where flexible back-up systems are used, a significant portion of the lateral load is shared by the flexural resistance of the veneer. If the share of lateral load distributed to the veneer is greater than its flexural strength, cracks in the veneer may open. The second example illustrates how to design the veneer and back-up to prevent excessive flexural stress in the veneer. First, the maximum resisting moment of the veneer is selected from Table 3. The maximum load resisted by the veneer may be calculated from the following equation:
Wv-
MR X 8
h²
(psf)
where Wy maximum load resisted by the veneer (psf)
Maximum load resisted by the veneer may be substituted into the load distribution equation which is then solved for the moment of inertia of the back-up.
W-W
Evlv
Evly + EBIB
18-[WW-Evlv][]
inch/Ft of wall
The above equation gives the minimum moment of inertia for the back-up per foot of wall to control excessive tension and cracking in the veneer.
TABLE 3
Flexural Resistance of Concrete Masonry(1)
Resisting Moment, Mg F15 (1.33) (ft./12 in.)
| Nominal wythe thickness inch | Hollow Units Mor S Mortar Ft-Lbs | N Mortar Ft-Lbs | Solid or Grouted Units Mor S Mortar Ft-Lbs | N Mortar Ft-Lbs |
| ----------- | ----------- | ----------- | ----------- | ----------- |
| 4 | 53.5 | 372 | 1137 | 78.4 |
| 6 | 118.0 | 82.1 | 273.6 | 189.4 |
| 8 | 206.5 | 143.6 | 502.7 | 348.0 |
| 10 | 300.3 | 208.9 | 801.0 | 554.5 |
| 12 | 407.6 | 283.6 | 1.168.4 | 808.9 |
(1) Based on inspected workmanship. Allowable stresses of Table 2. Stresses increased by one-third for wind. Reference NCMA TR 758 "Specification for Design and Construction of Load-bearing Concrete Masonry."
Vertical dead load stress may be added to the allowable tensile stress, F, in calculating Mg.
TABLE 4
ALLOWABLE TENSILE BOND STRESS
Normal to Bed Joints, Fr(1)
| Type of Unit | Mortar Type Mor S | N |
| ----------- | ----------- | ----------- |
| Hollow Units | 23 psi | 16 psi |
| Solid or Grouted units | 39 psi | 27 psi |
(1) Based on inspected workmanship. NCMA TR 758 "Specification for Design and Construction of Load-bearing Concrete Masonry."
Example Two:
Determine the required moment of inertia of metal stud back-up to support a 4" solid concrete masonry veneer using Type N mortar. The design wind load is 20 psf and the wall height between supports is 8 feet.
The maximum resisting moment of the veneer is 78.4 ft.-lbs. selected from Table 3. The veneer will resist a maximum distributed wind load of
Wv
MR X 8
h²
Wv-
78.4 x 8
82
9.8 psf
The equation for wind load distributed to the veneer can be solved by substituting in the known parameters and solving for the moment of inertia of the back-up.