Bridge Expansion Joint for Composite Steel-Concrete Bridges

Composite steel-concrete bridges have a deck consisting of a concrete slab connected to steel girders by shear connectors. The thermal movement of the composite deck is influenced by both the steel and concrete components, creating a complex movement pattern that must be considered in joint design. Thermal movement of composite bridges is intermediate between that of pure steel bridges and pure concrete bridges. The coefficient of thermal expansion of steel (12 x 10^-6 per degree C) is similar to that of concrete (10-12 x 10^-6 per degree C), so the differential thermal movement between the steel and concrete components is small. The overall thermal movement of the composite deck is calculated using the weighted average of the steel and concrete coefficients. Temperature gradient effects are more significant in composite bridges than in concrete bridges due to the different thermal properties of steel and concrete. The steel girders heat up and cool down faster than the concrete deck, creating a temperature gradient through the depth of the composite section. This gradient causes the deck to camber upward in the morning as the steel heats up faster than the concrete, and to camber downward in the evening as the steel cools faster. Shrinkage of the concrete deck in composite bridges creates a compressive force in the deck and a tensile force in the steel girders. This shrinkage force causes the composite bridge to shorten over time, reducing the joint gap. The shrinkage movement must be included in the joint gap calculation to ensure that the joint does not close completely in hot weather. Creep of the concrete deck under the sustained compressive stress from the composite action also contributes to long-term shortening. The creep movement is typically 1.5-2.5 times the elastic shortening, and it occurs over a period of 10-20 years. The joint gap must be set to accommodate the full creep movement over the design life. Fatigue design of expansion joints on composite bridges must account for the higher frequency of stress cycles compared to concrete bridges. The steel girders in a composite bridge are more flexible than a concrete bridge of the same span, resulting in larger deflections under traffic loading. These deflections create higher stress cycles in the joint components, requiring more careful fatigue design.