Accommodating Building Deflections

The ASCE-SEI Committee on Cold-Formed Steel is charged to "disseminate and interpret information on the behavior and design of structural steel members, cold-formed to shape from flat materials..." The committee consists of both practitioners and educators. Over the last several years, the focus of the committee's ongoing discussions has been on deflections in cold-formed steel systems. The opinion of the committee is that a variety of issues related to deflections in secondary cold-formed steel systems are poorly understood by key parties involved in building construction. This lack of understanding negatively impacts building performance, as well as initial cost and operating cost. Further, current conditions indicate these issues are growing, rather than lessening, importance. Therefore, the committee set out to provide a brief article of interest to the Engineer of Record (EOR), who may subcontract out secondary cold-formed steel (CFS) systems. The article concentrates on problems, ramifications and hopefully some solutions and guidance when dealing with how CFS systems should be designed to properly accommodate building deflections: both vertical deflections and lateral drifts. The phrase, secondary cold-formed steel system, is used here to describe cold-formed structural members that support exterior finishes, but are not considered part of the primary structural frame. These cold-formed members may support a variety of exterior finishes: metal panels, exterior insulation and finish systems (EIFS), concrete masonry units (CMU), brick etc. The committee's focus is on accommodating the primary building movements, i.e., in-plane lateral and vertical deflections.