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Building a Better Understanding of Build-Up Cold-Formed Steel Member Buckling Behavior and Capacity

David C. Fratamico Johns Hopkins University Baltimore, MD

Built-up cold-formed steel members are integral parts of shear walls and are frequently included in frames as king and corner studs. Current predictions for buckling capacity in AISI S100-12 Sec-tion D1.2 employ the modified flexural slenderness ratio, which reduces the buckling capacity of columns in part due to a loss of shear rigidity in the overall member’s interconnections (fasteners). There exist provisions for calculating fastener spacing and layout, whether screws or welds are used to connect two sections together. However, a detailed understanding of the effective section rigidities of these composite, thin-walled members does not yet exist.

Interestingly, connecting two standard CFS channel sections together does of-fer a boost in flexural capacity. Elastic flexural buckling loads of back-to-back channel sections frequently used in de-sign, for example, are theoretically more than twice the buckling load of the individual channel sections. A recent approach to understanding the behavior of built-up columns at Johns Hopkins University employs the concept of composite action, in which this boost in flexural capacity can begin to be quantified.

A parametric study using elastic buck-ling analysis was conducted on a representative population of built-up structural columns in ABAQUS (using discrete fasteners) and Finite Strip Method-based software CUFSM (using smeared constraint interconnections). Member cross-sections, fastener spacing, and fastener grouping at the column ends were varied. Prevailing buckling modes are shown in Figure 1. Buckling loads from the study are compared to code-based equation predictions and show considerable composite action (illustrated by the signature curve in Figure 2), which can increase a column’s flexural buckling load by up to 85% from its non-composite lower bound, for trials in both CUFSM and ABAQUS. Future work includes more accurate modeling of fastener stiffness and experimental studies.

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