One of our current projects is a pre-engineered metal building with a 12-foot eave height. The rigid frames are 30 feet on center. The exterior walls are 8-inch concrete masonry, to be supported at the top with a structural steel member spanning 30 feet between frames. Horizontal deflection of the top wall support and the rigid frame must be considered together. What are your minimum deflection recommendations? Can the concrete masonry tolerate greater than L/600 deflections?
A simple, efficient way to control deflection along the top edge of the wall is to design and detail the roof structure to act as a diaphragm, rather than to design a beam spanning 30 feet. If the top and bottom of the wall are continuously supported, then the masonry wall will span 12 feet vertically. If you cannot use the roof structure to support the top edge of the wall, install a very stiff steel beam or a beam combined with horizontal reinforcement in the top several feet of the wall. In either case, don't use rule-of-thumb deflection limitations to design the beam. Instead, design the steel beam stiff enough to prevent stresses in the wall from exceeding levels allowed by code. The percentage of loads handled by the masonry should be based on relative stiffness. The size of the steel beam may be reduced greatly by using reinforced masonry in conjunction with the beam.