A framed-tube system with multiple internal tubes is analyzed using an orthotropic box beam analogy approach in which each tube is individually modeled by a box beam that accounts for the flexural and shear deformations as well as the shear-lag effects. The method idealizes the tubes-in-tube structure as a system of equivalent multiple tubes, each composed of four equivalent orthotropic plate panels capable of carrying axial loads and shear forces. By simplifying the assumptions in relation to the patterns of strain distributions in external and internal tubes, the structural analysis is reduced to the mere solution of a single second-order linear differential equation. The proposed method, which is intended to be used as a tool for preliminary design purposes, can be applied for the analysis of framed-tube structures with single and multiple internal tubes as well as those without internal tubes. The simplicity and accuracy of the proposed method is demonstrated through the analysis of three framed-tube structures (of different heights) without internal tubes. A 3D frame analysis program and two existing approximate methods are also included in the comparative study. Furthermore, three other framedtube structures with one, two, and three internal tubes are analyzed to verify the applicability and reliability of the proposed method.
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