Masonry Magazine January 1969 Page.24
Transfor
Fig. 4 Effect of Geometric and Elastic Asymmetry
on Sess Distribution Wall Section
CA
1000
2000
3000
4000
Compressive Strength of Mortar
Fig. 5. General Effect of Mortar Strength on Compressive Strength of Hollow Block Prians Aally Loaded
marily on ultimate masonry strength,
f'm:
fm=20m [1-(40)]
Effect of Eccentric Loading
Maximum fiber stress in walls tested under eccentric loading is somewhat greater than the ultimate unit stress obtained from axial load tests; the relationship is approximately 1.5 times the average ultimate stress. Thus, new criteria provides for maximum allowable stress in flexural compression of non-reinforced concrete masonry of 0.30f'm.
Effect of Mortar Strength
Recent research which included a study of the influence of mortar strength on the ratio of prism to block strength indicated that mortar strength has very little influence on the axial load strength of concrete masonry prisms, Fig. 5, at least for types M, S, and N mortars made with both portland cement-lime and masonry cement. Earlier research on concrete masonry walls showed similar results except for the weaker type O and K mortars, where a more significant influence on wall strength was noted.
Effect of Slenderness
The strength of bearing walls having end conditions fixed at the bottom and supported at the top is not drastically affected by wall slenderness up to and beyond the limits permitted by NCMA design criteria of 20 for non-reinforced and 30 for reinforced walls. While the slenderness reduction factor, [1-(40t)], mentioned previously, implies that a wall with an unsupported height 40 times its thickness can carry no load, research reveals that at this slenderness the wall is able to carry a high percentage of the ultimate load on a wall with lower slenderness. Buckling is not a factor until well beyond an h/t value of 40.
Effect of Mortar Bedded Area
The observed increase in the strength of hollow walls with full mortar bedding over that of face shell bedding is considerably less than the increase in net bearing area at the joint. Test data that includes these factors indicates that full mortar bedding tends to increase the wall strength 10 to 20 percent over face shell mortar bedding. The theoretical stress distribution shown in Fig. 3a may explain why the increase is not as great as the percent difference in mortar bedded area.
NATIONAL CONCRETE MASONRY ASSOCIATION
P.O. Box 9185, Rosslyn Station Arlington, Virginia 22209