The purpose of this research is to develop an active-control mechanism and its control law, to reduce the deformation of the relief bridge and weight of construction materials and improve the effective span of the bridge. The counter-force (control force) mechanism, provided by Neutral Equilibrium Mechanism, NEM, is applied to exert a counter-force at the selected position of the bridge and maintain the deflection at this selected position to be zero. This proposed NEM is used as a counter-force mechanism, which is an automatic control device consisting of a pair of pressed steel strands, rotary cantilever arm mechanisms, a displacement sensor and a controller to form as a virtual pier. In this study, these proposed counter-force mechanisms are installed at the positions of 1/3 and 2/3 span of the bridge: the effective span is only 1/3 of the original span, the maximum bending moment and maximum deflection is only 1/9 and 1/27 of the original bridge, respectively. The experimental results display that the ideal behavior of this NEM can be fully achieved in the dynamic moving load test. The deflection and the bending moment of the bridge at the position of installing these NEMs are near zero. The goals of reducing deformation and improving the carrying capacity of the bridge are realized.
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