Passive Control Of Highway Structures — Two Shake Tables Experimental Studies of the Effectiveness of Damping–Augmentation Devices in Cable-Stayed Bridges (Volumes 1 & 2)

Rapid progress has been made over the past twenty years in the design techniques for cable-stayed bridges; this progress is largely due to the use of electronic computers, the development of box girders with orthotropic plate decks, and the manufacturing of high strength wires that can be used for cables. This progress has also let to increased competition among the bridge engineers in Japan, Europe and the United States. Cable-stayed bridges are now entering a new era, reaching to medium and long span lengths with a range of 1300 ft (400m) to 3000 ft (1000m) for the center span.

Cable-stayed bridges are increasing in numbers and popularity. This, in addition to the increase in the span lengths of these flexible structures raise many concerns about their behavior under environmental dynamic loads such as wind, earthquake and service loads such as vehicular traffic-loads. From the analysis of various observational data, including ambient forced vibration test of cable-stayed bridges, it is known that these bridges have very small mechanical or structural damping (0.3% -2%). Moreover these bridges occasionally experience extreme loads, especially during a strong earthquake or in a high wind environment. For such circumstances, the response should be controlled within certain limits for serviceability (human comfort) and for safety (risk of damage of failure).

For typical span highway bridges. Modem seismic bridge codes and provisions have now been developed to the point where the basic earthquake-resistant requirements to be imposed on a "standard" bridge are specified adequately, and intelligent consideration of these requirements will lead to the design of a safe and economical structure. For new cable-stayed bridges, however, the provisions of the highway bridge seismic codes may not be applicable, and accordingly, there is an urgent need to develop general seismic design guidelines tailored especially for these bridges and based on research, experimental studies and full-scale observational data. Furthermore, due to the large displacements and member forces induced by strong ground shaking in this type of structure, energy absorption devices and special bearings should be provided at the supporting points to dissipate seismic energy, thus assuring the serviceability of the bridge.

The response of a cable-stayed bridge to applied loads is highly dependent on the manner in which the bridge deck is connected to the towers. If the deck is swinging freely at the towers, the induced seismic forces will be kept to minimum values, but the bridge may be very flexible under service loading conditions (i.e. dead loads and live loads). On the other hand, a rigid connection between the deck and the towers will result in reduced movements under service loading conditions but will attract much higher seismic forces during an earthquake. Therefore, it is extremely important to provide special bearings or devices at the deck-tower connections to absorb the' large seismic energy and reduce the response amplitudes. Good examples of these devices, which make it possible to control the natural period of vibration, are rubber-lead block bearings elastic links, spring shoes and elastomeric bearings. These devices should be dimensioned so that they provide adequate stiffness high enough to produce acceptable performance under day-to-day service conditions, yet soft enough to prevent high seismic inertial forces from being transmitted to the towers from the deck. These devices should also constitute a multi-defense line; that is, they should be composed of different, tough structural subsystems which are interconnected by very tough structural elements.(structural fuses) whose inelastic behavior would permit the whole bridge to fmd its way of the critical range of dynamic response.

Long span prestressed concrete cable-stayed bridges are often designed as a floating structure in which the girder is not supported by bearing but suspended by only cables. The aim of the use of this type of structure is to reduce the inertial force of the girder by extending the natural period of the structure in the longitudinal direction. This structure, however, has some drawbacks too, such as larger horizontal displacements of the girder and larger bending moment of the tower than in bridges where the girder and the main pier are rigidly connected.

The goal of this study is to solve these problems and improve the earthquake resistance of floating-type prestressed concrete cable-stayed bridges by introducing passive vibration control systems.