Masonry Magazine January 1969 Page.21
An Information series from National Concrete Masonry Association
NCMA TEK 15
©1969 National Concrete Masonry Association
Compressive Strength of Concrete Masonry
For many years, structural design of non-reinforced masonry walls has been based on criteria developed from experience, assumptions, and some research involving the behavior of walls subject to various types of loading. The simple, conservative rules that relate minimum wall thickness to building height, maximum spacing of elements which provide lateral support, and allowable stresses in axial compression were developed to provide for safe construction even though all of the variables which affect the structural performance of the wall were not considered in the design.
The need to provide for more economical use of concrete masonry has prompted a reexamination of existing research data (some which dates back many years), and has generated the development of new data. This analysis and research resulted in the establishment of new criteria which takes into account more variables, and thus provides better and more efficient design of concrete masonry assemblages.
While there are numerous factors that can influence the compressive strength of a concrete masonry wall it has become apparent from results of structural testing that compressive strength of the units is the important variable for a given type of loading and wall size. Other variables that influence wall load-carrying capacity, include eccentricity of vertical load, slenderness of wall, mortar bedding (full or face-shell), workmanship, and mortar strength. For a given masonry unit strength, the type of vertical loading and slenderness are more significant to the strength of a wall than mortar strength. The type of loading largely determines the general shape of the stress distribution diagram for the wall section. For solid walls, Fig. 1, axial loading results in a rectangular stress diagram, the fiber stresses being uniform over the entire cross section and equal to P/A. If the vertical load is applied eccentrically or off-center by a distance of one-sixth the wall thickness (1/6) the unit stress varies from a maximum of 2P at the wall face nearest the load line, to zero stress at the opposite face, Fig. 1b. Eccentricity greater than 1/6 would produce tensile stresses at the opposite face and a stress diagram which would show zero stress at some point between the wall faces. Eccentricity less than 1/6 would result in some compression at the opposite face and a stress diagram of trapezoid shape. In Figs. la and Ib the average compressive stress is the same in each case, assuming the same vertical load, but as noted the maximum fiber stress for the eccentrically loaded wall is twice that for the axially loaded wall. A logical deduction is that a given wall will support a greater axial than eccentric load. This is borne out by tests which indicate that bending stress due to eccentric location of vertical load or other causes, reduces the ultimate vertical load capacity of masonry below its axial load strength.
Distribution of stress to different parts of the wall section is also influenced by the geometrical design of the section; it is not the same for hollow unit as for solid unit walls or for unsymmetrical sections (such as composite walls) as for sections symmetrical about their centerline.
October, 1909
Concrete Onits