Abstract: A case study was carried out on the seismic behavior of a complex reinforced concrete(RC) plant structure with large slab openings and an auxiliary facility with large mass ratio. The absence of effective slab constraints for the structural members can be observed in structures with large slab openings, leading to evident transverse vibration. As a result, the structural members fail under the combination of bi-directional moments and other internal forces. A fiber beam-column element with distributed plasticity approach should be used to elaborately capture this significant phenomenon, and much attention should be paid to the transverse flexural capacity of such structural members. The simple formula suggested by Tsai and Lin can be applied to optimize the parameters of the large mass auxiliary facility as a non-conventional tuned mass damper(TMD) to evidently reduce the dynamic response of the main structure. For the auxiliary facility covering a relatively large area with multiple supports, the acceleration amplification effect due to the torsion of auxiliary facility should be considered, and the evident decreasing of the floor response spectrum due to large mass ratios should not be neglected.