In the field of civil engineering,magnetorheological fluid (MRF) damper-based semi-active control systems have received considerable attention for use in protecting structures from natural hazards such as strong earthquakes and high winds.In this paper,the MRF damper-based semi-active control system is applied to a long-span spatially extended structure and its feasibility is discussed.Meanwhile,a trust-region method based instantaneous optimal semi-active control algorithm (TIOC) is proposed to improve the performance of the semi-active control system in a multiple damper situation.The proposed TIOC describes the control process as a bounded constraint optimization problem,in which an optimal semi-active control force vector is solved by the trust-region method in every control step to minimize the structural responses.A numerical example of a railway station roof structure installed with MRF-04K dampers is presented.First,a modified Bouc-Wen model is utilized to describe the behavior of the selected MRF-04K damper.Then,two semi-active control systems,including the well-known clipped-optimal controller and the proposed TIOC controller,are considered.Based on the characteristics of the long-span spatially extended structure,the performance of the control system is evaluated under uniform earthquake excitation and travelling-wave excitation with different apparent velocities.The simulation results indicate that the MR fluid damper-based semi-active control systems have the potential to mitigate the responses of full-scale long-span spatially extended structures under earthquake hazards.The superiority of the proposed TIOC controller is demonstrated by comparing its control effectiveness with the clipped-optimal controller for several different cases. In the field of civil engineering, magnetorheological fluid (MRF) damper-based semi-active control systems have received attractive attention for use in protecting structures from natural hazards such as strong earthquakes and high winds.In this paper, the MRF damper-based semi -active control system is applied to a long-span spatially extended structure and its feasibility is discussed. Meanwhile, a trust-region method based instantaneous optimal semi-active control algorithm (TIOC) is proposed to improve the performance of the semi-active control system in a multiple damper situation. proposed TIOC describes the control process as a bounded constraint optimization problem, in which an optimal semi-active control force vector is solved by the trust-region method in every control step to minimize the structural responses. A numerical example of a railway station roof structure installed with MRF-04K dampers is presented. First, a modified Bouc-Wen model is utilized to describe the behavior of the selected MRF-04K damper. Chen, two semi-active control systems, including the well-known clipped-optimal controller and the proposed TIOC controller, are considered.Based on the characteristics of the long-span spatially extended structure, the performance of the control system is evaluated under uniform earthquake excitation and traveling-wave excitation with different apparent velocities. The simulation results indicate that the fluid dampening-based semi-active control systems have the potential to mitigate the responses of full-scale long-span spatially extended structures under earthquake hazards. The superiority of the proposed TIOC controller is demonstrated by comparing its control effectiveness with the clipped-optimal controller for several different cases.