Masonry Magazine January 1969 Page.22
A
B
At web section
At core section
(a) Axial compression at A-A & B-B
عاه
Compression
Tension
At web section
At web section
At core section
(b) Eccentric compression
At core section
(c) Pure bending
Fig. 2 Unit Stress Diagrams for Hollow Block Walls Laid with Full Mortar Bedding.
Stress diagrams for single wythe, hollow block wal walls laid with full mortar bedding are shown in Fig. 2 and for similar walls laid with face-shell mortar bedding in Fig. 3. With full mortar bedding (webs as well as face shells aligned and fully bedded in mortar) the fiber stresses in the web section assume the same pattern as shown for solid unit construction when the load is axial compression but differ somewhat with respect to eccentric compression loading. The stress diagrams for the core section are quite different from those for either the web sections or for solid unit walls.
Fig. 3 is of particular interest as it relates to the stress diagrams for concrete block walls as commonly laid, with face-shell mortar bedding. Since webs are not in contact they do not transmit stress directly one to the other and at the mortar joints the full vertical load is carried by the face shells. At intermediate points between joints some stress is transferred from the face shells to the webs with the stress paths and distribution being somewhat as shown in sketch (a) for walls under axial compression.
Referring to sketch (b) of this figure, it will be noted that vertical loading applied eccentrically at from the centerline does not result in the extreme maximum and minimum stresses as in the case of a solid section. If the load line lies anywhere between the inside edges of the opposite face shells, the eccentricity will not be sufficient to cause tensile stress as it does theoretically in solid sections with eccentricities exceeding
Fiber stresses due to axial or eccentric vertical loading are calculated from the usual formulas:
Axial loading: 1-
P
Eccentric loading: f+POC
However, for some purposes the approximate method shown in Fig. 2c will give sufficiently accurate values of the eccentric load stresses and does not involve the moment of inertia, I.
For design purposes and in reporting results of research on strength tests of non-reinforced masonry, the area traditionally is taken as the gross cross section of hollow walls and the moment of inertia I is also based on gross cross section. Since true specific values are obtained only when calculations are based on the actual section which may be considered as resisting stress, new design criteria uses the net cross section for area and moment of inertia of hollow walls. The difference between the two premises accounts for the marked difference between the stress diagrams for Figs. 2 and 3. As previously suggested vertical
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