Masonry Magazine July 1981 Page. 13
NCMA-TEK
An Information series from National Concrete Masonry Association
Structural Backup
Systems for Concrete
Masonry Veneers
114A
National Concrete Masonry Association 34 Concrete Units
Introduction
Concrete masonry veneers are typically composed of "architectural" units such as concrete facing brick, split block, fluted or scored block, slump block of combinations such as the split-fluted, or split-ribbed unit. Generally masonry veneers have a thickness of 4 inches and are not designed as structural members. Veneers do not carry the vertical loads of the structure; and the lateral loads, such as wind, are transferred to the back-up system. It has been the practice with traditional masonry design to neglect the structural properties of veneers because their only function was to provide an architectural facade and to prevent water from penetrating the exterior walls. The designer could comply with the required spacing and strength of the ties and assume that no further consideration need be given to the structural adequacy of the veneer. Today, flexible systems such as light gauge metal studs are being proposed for masonry veneer back-up in multi-story buildings. To insure this system is compatible, the designer must consider the stiffness properties of the masonry veneer and the flexible back-up. Unfortunately the design literature for flexible back-up systems currently available does not properly consider stiffness compatibility. Deflection criteria of the stud back-up vs. span length is available from metal stud manufacturers; however, the design literature has not established a rational criteria for limiting the deflection of flexible back-up systems or masonry veneers. The purpose of this TEK is to provide a design methodology by which the stiffness properties of the veneer and back-up can be proportioned to achieve compatibility.
Design
The lateral wind pressure and suction exerted on the exterior walls of a building are resisted by the masonry facing (veneer) and its structural back-up. Lateral loads are shared between the veneer and back-up when an air space separates the veneer wythe from the back-up and where both wythes are tied together with metal ties or joint reinforcing. The lateral load will be distributed to both the facing wythe and the back-up in proportion to the stiffness of each element. The parameters which determine stiffness are the modulus of elasticity, E; moment of inertia, I; and height of span for a uniformly loaded wall. The height of span is similar for both the veneer and back-up; therefore, the relative stiffness is proportional to the product of E x I.
Stiffness - E x I
where: E = Modulus of elasticity (psi)
I = Moment of inertia (inch)
TABLE 1
Strength Properties of Concrete Masonry
Type M or S Mortar
Type N Mortar
Net Area Compressive Modulus of Compressive Modulus of
Compressive strength of
Strength masonry
fm
Im
Elasticity
strength of Elasticity
E
masonry
E
of Units
(1000 fm)
(1000 (m)
(psi)
(psi)
(psi)
(pl)
(psi)
1,500
1,150
1,150,000
875
875,000
2,000
1,350
1.350.000
985
985,000
2,500
1,550 1,550,000
1,100
1,100,000
3,000
1,700 1,700,000 1,150
1,150,000
3,500
1,850 1,850,000 1,200
1,200,000
4,000
2,000 2,000,000 1,250
1,250,000
4,500
2,100 2,100,000 1,275
1,275,000
5,000
2,200
2,200,000 1,300
1,300,000
5,500
2.300 2,300,000 1,325
1,325,000
6,000
2,400
2,400,000 1,350
1,350.000
TEK 114A © 1981 National Concrete Masonry Association