Modular Expansion Joint for Cable-Stayed Bridges: Movement Characteristics

Cable-stayed bridges have unique movement characteristics that affect the design of expansion joints at the deck ends. The cable forces change under live load and temperature variations, causing complex movements at the joint location. Thermal movement of cable-stayed bridges is similar to other bridge types for the deck, but the cable forces also change with temperature. When the temperature increases, the cables lengthen and the deck moves longitudinally. The thermal movement at the joint is the sum of the deck thermal movement and the movement caused by the change in cable forces. Live load movement at the ends of a cable-stayed bridge is caused by the deflection of the deck under traffic loading. When traffic loads the main span, the deck deflects downward and the cable forces increase. This increase in cable forces causes the deck to move longitudinally at the joint location. The live load movement depends on the stiffness of the cable system and the magnitude of the traffic load. Wind-induced movement at the ends of a cable-stayed bridge is caused by the lateral deflection of the deck under wind loading. The lateral movement at the joint location depends on the wind speed, the deck width, and the stiffness of the cable system. For wide-span cable-stayed bridges, the lateral movement can be significant. Creep and shrinkage of concrete cable-stayed bridges cause long-term shortening of the deck, reducing the joint gap over time. The creep and shrinkage movements must be included in the joint gap calculation to ensure that the joint does not close completely over the design life. Seismic movement at the ends of a cable-stayed bridge is complex due to the interaction between the deck, the cables, and the towers. The seismic response analysis must consider the full dynamic behavior of the cable system to correctly predict the joint displacements. The joint must be designed for the maximum seismic displacement from the response analysis.