Masonry Magazine June 1973 Page. 31
NCMA-TEK
An Information series from National Concrete Masonry Association
Design of Composite
Masonry Walls
48
Concrete Units
Introduction
Most building codes and masonry standards define a composite masonry wall as "a multiple-wythe wall in which at least one of the wythes is dissimiliar to the other wythe or wythes with respect to type or grade of masonry unit or mortar." Typically, composite masonry walls are used as exterior walls, and in many such applications, this composite wall construction consists of a facing wythe such as concrete brick or split block and a backing wythe composed of hollow or solid concrete block.
A great deal of information has been published concerning properties and design of walls built with either hollow or solid units, and this discussion relates to engineered design where two types of units are put together to form a composite wall.
Design
General. Vertical loading applied at the centerline of a wall is true axial loading only when the wall section is geometrically and elastically symmetrical about the centerline. In order to obtain uniform fiber stress in all parts of the wall section, the line of load application must coincide with the center of resistance of the section. Referring to Fig. 1(a). the wall section is both geometrically and elastically symmetrical. Therefore, centerline loading produces the same intensity of stress in both face shells and is true axial
1973 National Concrete Masonry Association
loading. In Fig 1(b), the solid and hollow units are assumed to be of the same material, therefore this composite wall section is symmetrical as regards the stiffness of the materials but it is obviously geometrically unsymmetrical. The centroidal axis, which in this case is also the center of resistence, is located to the left of the centerline and the load actually is eccentric instead of axial as might be assumed. Fig. 1(c) illustrates the transformed area of the wall section shown in Fig. 1(b) assuming that the modulus of elasticity of the brick wythe is greater than that of the block wythe. The transformation utilizes the well known transformed area principle used in reinforced concrete design. The center of resistance of the actual section becomes the geometric centroidal axis of the transformed area and this axis, it will be noted, has moved farther to the left, compared to the section in Fig. 1(b), thus increasing eccentricity of the centerline load. Making the usual assumptions that sections that are plane before bending remain plane after bending and that stress is the product of strain times the elastic modulus, the fiber stresses will vary somewhat as shown in Fig. 1(c). The brick wythe being solid and stiffer may carry the larger portion of the total load although the load line is substantially eccentric toward the
FIGURE 1.
Effect of Geometric and Elastic Asymmetry on Stress Distribution in Wall Section.
Centroidal axis
at
(a) Symmetrical
section
0
C.A.
bE (brick)
E (block)
C.A.
(c) Transformed section (b)
(b) Non symmetrical section
Stress diagrams
with vertical loading
G.A.C.A.
2