Deflection gap study for cold‐formed steel curtain wall systems

Abstract

Cold‐formed steel has become a preferred building material for wall framing in many different types of structures. One of its main uses has been as non‐structural members in curtain wall assemblies of structural steel framed buildings. In an exterior wall application, the main purpose of the curtain wall is to transfer out of plane loads to the steel frame while not supporting any superimposed gravity loads. Therefore, when the curtain wall is in the plane of the structural steel frame, the vertical deflection of the spandrel beam directly above the wall must be known to provide the appropriate deflection gap between the beam and the curtain wall so that gravity loads are not transferred to the wall. Common practice is to size the gap for the deflection from 100% of the live load. In some cases, the deflection gap may be significant, and since this gap must also be provided in the exterior cladding of the wall, it creates a design issue for the architect. This report presents the results of an investigation into the feasibility of reducing the size of the deflection gap when the wall is located directly under the spandrel beam. In this study, analytical models were developed for common design situations of curtain walls constructed of cold‐formed steel studs in structural steel framed buildings. This study investigates two common stud heights combined with different floor live loads. Taking into account that wall studs have some available axial compressive strength, a procedure was developed to determine an appropriate reduction for the gap. Using an iterative process a relationship is made between the axial compressive strength of the stud and the amount of axial load the stud can support to establish a factor which gives the percentage the live load gap for 100% live load can be safely reduced by.