The seismic behaviour of regular multistorey structures supported laterally by a number of reinforced concrete cantilever walls with markedly different lengths is addressed. It is shown that the shear force distribution among the walls is strongly affected by the different yield displacement levels of the walls. The share of the shear force carried by the shorter walls is shown to be larger than their share in the bending moment. The dynamic amplification of the shear forces due to multi-mode effects and the implications for capacity design are noted. Numerical examples illustrate the problem.