Joint Reinforcement in Stack Bond Masonry Lincoln NE

Why do codes require the reinforcing of stack bond masonry with horizontal joint reinforcement in Lincoln? I understand that stack bond masonry does not have overlapping units as in a running bond pattern.

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Joint Reinforcement in Stack Bond Masonry

Provided By:

Source: MASONRY CONSTRUCTION MAGAZINE
Publication date: October 1, 1998

Why do codes require the reinforcing of stack bond masonry with horizontal joint reinforcement? I understand that stack bond masonry does not have overlapping units as in a running bond pattern. This overlapping, however, does not occur across bed joints in any masonry walls, yet the codes do not require vertical reinforcement.
The purpose of horizontal joint reinforcement in stack bond masonry is to limit vertical cracking. The overlapping units in running bond masonry will provide more strength across vertical planes because they eliminate the continuous vertical joints that occur in stack bond walls. There is very little mortar-to-brick bond strength at head joints. Running bond patterns do not depend on the head joints for strength in a horizontal direction but rather on the strength of the units themselves.The reason head joints typically have lower bond strengths than bed joints is related to the orientation of the mortar joint when the walls are constructed. In running bond masonry, the horizontal mortar joint is easy to place with full joints. Also, the bed joints are under compression when the wall is laid up due to the masonry's weight. Head joints are formed by buttering the end of the units with mortar and shoving them into the adjacent units. Unlike bed joints, they are not under compression.

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