Structural behavior of Cold-formed Steel (CFS) face-to-face connected built-up closed cross-section columns is investigated in the present study. The CFS built-up columns are designed as long (L > 20r) and locally slender (λl > 1) to verify the influence of intermediate longitudinal connection spacing and check the appropriateness of the current AISI's Direct Strength Method (DSM) design. A total of 31 axial compression tests were carried out with fixed–fixed end conditions, the design parameters such as local slenderness (λl), global slenderness (λc), intermediate longitudinal fastener spacing (a), and length of the column (L) are varied. The failure modes of the column are summarized and the reason for them is elucidated. The influence of intermediate longitudinal connection spacing was observed in the failure modes and ultimate loading capacity. The test results including ultimate load and failure modes were compared with the current direct strength method design predictions. As a preliminary work towards improving the current AISI design approach, a modified local slenderness (λlm) expression is suggested to consider the influence of intermediate longitudinal fastener spacing in the design strength of cold-formed steel built-up closed cross-section columns. © 2021 Elsevier Ltd