With the rapid increase of bridge spans, research on controlling earthquake-induced vibration of long-span bridges has been a problem of great concern. The concept of varying the normal force in a frictional interface is investigated to enhance the energy dissipation from a vibrating structure and improve the seismic performance of bridge structures. A semi-active optimal control algorithm is formulated to determine the controllable clamping force of a variable friction device; this algorithm uses measurements of the absolute acceleration and device relative displacements for determining the control action to ensure that the algorithm would be implementable on a physical structure. The friction device UHYDE-fbr is designed and manufactured such that the normal force in the friction interface can be influenced with air pressure chamber, hence the normal force and friction damping can be controlled. The friction device is a controllable energy dissipation device that cannot add mechanical energy to the structural system; the proposed control strategy is fail-safe in that bounded-input, bounded-output stability of the controlled structure is guaranteed.

The spatial variability of the input ground motion at the pporting foundations plays a key role in the structural response of flexible long span bridges such as cable-stayed and suspension bridges, herefore the spatial variation effects should be included in the analysis and design of effective vibration control systems for such orizontally extended structures. The control of long-span bridges represents a hallenging and unique problem, with many complexities in modelling, control design and implementation, the control system should be esigned not only to mitigate the dynamic component of the structural response but also to counteract the effects of the seudo-static component of the response. The feasibility and efficiency of seismic control systems for the vibration control of cable-stayed bridge under multiple support excitations are nvestigated. The effect of the spatial variability of the ground motion in the analysis of seismically controlled long span bridges s considered based on the decomposition of the total structural response into a dynamic component and a pseudo-static component. he assumption of uniform earthquake motion along the entire bridge could be unrealistic for long span bridges since the differences in round motion among different supports due to travelling seismic waves may esult in quantitative and qualitative differences in seismic response as compared with those produced by niform motion at all supports. Comparison of the response due to non-uniform input with the response due to uniform input emonstrates the importance of accounting for spatial variability of excitations; different wave propagation velocities are undertaken n the dynamic analysis of the controlled cable-stayed bridg