K-series and LH-series joists are most commonly used with masonry. The span and depth of these open-web joists determine their bearing requirements, as established by the Steel Joist Institute. It is sometimes necessary, however, for a steel joist to bear on masonry less than the required dimensions. In such circumstances, the structural engineer should give special consideration to the design of the masonry and steel joist connection. When a roof joist spans its maximum length and carries its maximum allowable load, the ends of the joist will rotate. This inward movement (joist shortening) is the result of deflection. One way to eliminate this problem is to specify joists stiff enough to resist maximum deflection. You also can minimize deflection by increasing the number of joists, thereby reducing the space between them. If joists are erected in hot weather and subjected to heavy cooling or refrigeration upon completion, they will contract. Joists erected in cold weather and later subjected to heating will expand. To accommodate this thermal movement, you can specify that the steel fabricator modify the camber (upward bowing) of the top chord of the joist. A special slippage connection to accommodate joist lengthening may be required if the walls are braced laterally by steel joists. In addition, do not design joists to be in direct contact with the outer face shell of concrete masonry units. Provide a clearance of 3/4 inch. If the two materials are in contact, any structural movement of the joist can stress the concrete masonry, causing cracks. The clearance is especially critical in the design of single-wythe walls, where any cracks that develop would be on the exterior and exposed.