Question: In masonry walls built in one-third bond rather than conventional running bond, there are places in the wall where the overlap between units is less than 1/3 the length of the unit. As the overlap is reduced, I think the walls would function more like stack bond masonry rather than running bond. However, stack bond masonry walls are supposed to be reinforced at the bed joints to prevent vertical cracking.
At what point should these walls be reinforced?
Answer:The Building Code Requirements for Masonry Structures (ACI 530-99/ASCE 5-99/TMS 402-99, Reported by The Masonry Standards Joint Committee 1999) defines running bond as "The placement of masonry units such as that head joints in successive courses are horizontally offset at least one-quarter of the unit length."
In most cases I have seen one-third bond used with 12-inch "jumbo" masonry units, and 9-inch non-modular masonry units. With 12-inch units the overlap needs to be greater than 2-7/8 inches to be considered running bond. For 9-inch non-modular units the overlap must be greater than 2 1/4 inches. When walls are constructed with horizontal offsets less than these figures, bed joint reinforcement spaced at a maximum of 18 inches on center vertically is required.