The earthquake effects on two structures supporting steam generation plant at the New Zealand Forest Service Waipa State Mill were examined using an inelastic dynamic analysis computer program. The geometry and structure of the plant and support systems indicated that the equivalent static method would not be appropriate for analysis. One of the support structures incorporated axial energy dissipating devices and performance of these was confirmed by testing of a full-scale prototype. The second structure was characterised by an unsymmetrical distribution of stiffness. An equivalent two-dimensional representation of the structure plan was analysed to assess the effect of torsion on the inelastic response to seismic loading.