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. With large masonry units, head joints are more difficult to fill than bed joints and therefore may not be filled completely. For these reasons, there is generally much lower bond across head joints than across bed joints. Another reason for stronger bond across bed joints relates to the forming of wire-cut clay masonry units. In these units, the bed joint surface is wire-cut. Wire-cut surfaces often have a rough surface texture, which will provide greater mechanical keying and therefore better bond. Head joints are often smooth extruded faces, which can have lower bond. The Masonry Standards Joint Committee's ACI 530/ASCE 5/TMS 402 Building Code Requirements for Masonry Structures requires that bed-joint reinforcement be placed in stack bond masonry to provide a minimum area of horizontal reinforcement of 0.0003 times the vertical cross-sectional area of the wall. The reinforcing shall be spaced not more than 48 inches on center. This prescribed horizontal reinforcement is intended to replace the flexural resistance parallel to the bed joints, which is present in running bond masonry but normally absent in stack bond masonry.