Railway Expansion Joint Thermal Gap Calculation for Long Railway Bridges

Long railway bridges require careful thermal gap calculation to ensure that the expansion joint can accommodate the full thermal movement of the bridge deck. Incorrect gap calculation can lead to joint locking in hot weather or excessive gap opening in cold weather. Temperature range determination is the first step in thermal gap calculation. The design temperature range is the difference between the maximum and minimum temperatures that the bridge deck will experience during its design life. For railway bridges, the deck temperature is measured at the rail level, which may be higher than the air temperature due to solar radiation on the rail. Coefficient of thermal expansion varies with the deck material. Steel decks have a coefficient of 12 x 10^-6 per degree C, while concrete decks have a coefficient of 10-12 x 10^-6 per degree C. Composite steel-concrete decks have an intermediate coefficient that depends on the proportions of steel and concrete. Thermal movement calculation multiplies the bridge length by the coefficient of thermal expansion and the temperature range. For a 500-meter steel railway bridge with a temperature range of 70 degrees C, the thermal movement is 500 x 12 x 10^-6 x 70 = 420 mm. This large movement requires a modular expansion joint system. Creep and shrinkage movements must be added to the thermal movement for concrete and composite bridges. Creep and shrinkage cause the bridge deck to shorten over time, reducing the joint gap. The joint must be installed with a larger gap to accommodate the expected creep and shrinkage movement over the design life. Gap setting at installation must account for the installation temperature relative to the mean temperature. If the joint is installed at a temperature above the mean, the gap must be set smaller than the mean gap to allow for thermal expansion. If the installation temperature is below the mean, the gap must be set larger than the mean gap to allow for thermal contraction.