Experiments, Analysis, and Design of Built-Up Cold-Formed Steel Columns
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The buckling and collapse behavior of built-up, cold-formed steel (CFS) columns are investigated in this thesis through a series of experimental and numerical analyses for the development of suitable design approaches. Built-up columns are composed of multiple CFS sections, which are fastened together with screws, welds, or bolts. They are used widely in CFS-framed buildings as higher capacity columns, shear wall chord studs, truss members, headers, and jambs. This thesis is focused on: (1) a review of current design approaches for built-up sections and motivation for further investigation and revision of their design guidelines; (2) three phases of column testing to study composite action, effects of sheathing, and effects of fastener layouts on column buckling behavior and strength; (3) numerical modeling of the elastic buckling and collapse behavior of the tested columns; and (4) rational design approaches based on tested observations and state-of-the art numerical modeling capabilities. The tested columns ranged in height from 3 ft (0.91 m) to 6 ft (1.83 m) and were composed of two, back-to-back CFS lipped channel sections connected by screws through their webs. Results from testing and numerical modeling indicate that column strength is highly sensitive to buckling mode interactions, fastener layouts, column end rigidity, the presence of sheathing, and initial geometric imperfections. New design approaches which aim to properly address these effects on buckling and strength are proposed. These approaches include analytical and numerical methods to estimate elastic buckling loads, which are used as parameters along with squash loads to empirically estimate column strength using the Direct Strength Method (DSM). Good agreement is achieved when numerical approaches are used in design.