A simple numerical modelling technique is proposed for estimating the shear-lag behaviour of framed-tube systems with multiple internal tubes. The system is analysed using an orthotropic box beam analogy approach in which each tube is individually modelled by a box beam that accounts for the flexural and shear deformations, as well as the shear-lag effects. The method idealises the tube(s)-in-tube structure as a system of equivalent multiple tubes, each composed of four equivalent orthotropic plate capable of carrying loads and shear forces. The numerical analysis so developed is based on the minimum potential energy principle in conjunction with the variational approach. The shear-lag phenomenon of such structures is studied taking into account the additional bending stresses in the tubes. Structural parameters governing the shear-lag behaviour in tube(s)-in-tube structures are also investigated through a series of numerical examples.