Masonry Magazine January 1966 Page. 40
bond beams
Bond beams serve both as structural elements and as a means of crack control. They are constructed with special shape masonry units which are filled with concrete or grout and reinforced with embedded steel. Their value in crack control is due to the increased strength and stiffness they give a masonry wall. Since they are capable of structural function as well as crack control, bond beams will be found serving: (1) as lintel beams over doors and windows, (2) below the sill in walls with openings, (3) at the top of walls and at floor level to distribute vertical loads, and (4) as horizontal stiffeners incorporated into masonry to transfer flexural stresses to columns and pilasters when unusually high lateral loading is encountered. As a means of crack control, the bond beam's area of influence normally is presumed to extend 24 inches above and below its location in the wall. In walls without openings they are spaced four feet apart and may be any length up to 60 feet maximum. Table 1.
Reinforcement for bond beams must satisfy structural requirements but should not be less than two No. 4 steel bars. The beams are always discontinuous at expansion joints, and joints should be designed to transfer lateral forces along the wall. Beams may be discontinuous at control joints; practice varies depending upon structural requirements. Dummy joints are formed when a bond beam is continuous at a control joint.
joint reinforcement
Horizontal joint reinforcement serves much the same purpose in crack control as bond beams-it increases (tensile) resistance to cracking. Due to closer spacing in the wall, joint steel may be more effective in crack control than bond beams, and it appears to be more economical in most cases.
Reinforcement is embedded in horizontal joints at intervals of 8, 16, and 24 inches depending upon panel length (L), height (H) and the number and type of wall openings. Table I gives L/H ratios (panels length to height ratios) recommended for masonry walls constructed of Type "I" Q BLOCK (moisture controlled) and containing different amounts of joint reinforcement. L/H values are approximate and provide an adequate margin of safety against cracking when employed in the design of walls without openings. The suggested ratios presume the use of Moisture Controlled, Type "I" Q BLOCK concrete masonry units. A panel is a wall element in one plane which lies between; (1) wall ends, (2) control joints, or (3) a control joint and wall end.
control joints
Control joints are employed in crack control to reduce restraint by accommodating movement of the masonry wall, or movement of structural elements adjacent to the wall. They are vertical separations built into the wall at locations where cracking is likely due to excessive horizontal stress. Their spacing along the wall length will depend upon: (1) expected movement (s) of wall or other elements, (2) resistance of wall to horizontal tensile stress, and (3) the extent and location in the wall of windows, doors, recesses, chases, and other causes of stress concentration. Control joints may be used alone for crack control or they may be used in conjunction with either or both bond beams and joint reinforcement.
Common methods of constructing control joints are illustrated in Fig 2. The joints permit free longitudinal movement, but they should have sufficient shear and flexural strength to resist lateral loads. They also must be