Comparison of Different Specifications and Field Measurement on Vertical Temperature Gradient of Steel Box Girder

Steel bridges are built in a complex natural environment, and thus their temperature varies with solar radiation and air temperature.Such temperature variation includes not only a uniform temperature rise or temperature fall but also non-uniform distribution in the transverse direction of a bridge in the vertical direction of the main girder.Temperature variation and non-uniform distribution inside the bridge may result in thermal expansion and contraction in the structure, which leads to structural displacement or deformation.When this displacement or deformation is constrained, large secondary internal force and secondary thermal stresses will be generated in the structure, and under some unfavorable conditions, the secondary thermal stress developed in some parts of the structure may be greater than the stress produced by vehicles or other live loads.If such temperature loads are ignored or not properly predicted in the design stage, it may result in bridge damage or even collapse.
To investigate the magnitude and distribution characteristics of the vertical temperature gradient of a closed steel box girder under solar radiation, specifications in four major codes on the vertical temperature gradient of a main girder or steel box girder in China and other countries are compared.The comparison reveals that only Eurocode 1 specifies the vertical temperature gradient pattern of a steel box girder with a steel bridge deck.Then, temperature measurement is carried out inside the steel box girder of the Sanchaji Bridge across the Xiangjiang River in Changsha with two temperature measuring sections set up on the same diaphragm in the transverse direction of the bridge.Under strong solar radiation and high ambient temperature in summer, the temperature at the measurement points on different sections was measured several times to obtain the temperature of the top surface of the deck overlay, the deck, interior air, and bottom plate of the steel box, as well as the ambient temperature in 24 h.The change laws of vertical temperature with time at different measurement points on the diaphragm are analyzed.
It was found that under the effect of hot weather and strong solar radiation, the temperature variations at the top surface of the deck overlay and the deck, interior air, and bottom plate of the steel box girder shared the same trends with air temperature.Under solar radiation, the temperature rose rapidly on the deck overlay and reached its maximum at around 14:00, while the maximum temperature of the deck was recorded at around 16:00.Great temperature differences between the deck and bottom plate occurred from 14:00 to 18:00, with the maximum value of 16.8℃ presented at around 14:30.It was also found that the positive vertical temperature gradient showed nonlinear distribution, and the girder top had a larger positive temperature difference but a significantly smaller negative temperature difference.On the basis of the maximum temperature difference obtained between the deck and the bottom plate, a suggested pattern of vertical temperature gradient was fitted using a four-broken-line, where the five reference points are 0, 100 mm, 300 mm, 650 mm, and the girder depth h away from the deck, with their corresponding temperature gradient values of ΔT₁=17℃, ΔT₂=13℃, ΔT₃=8℃, ΔT₄=4.5℃, and 0℃, respectively.Studies showed that the four-broken-line mode specified in the Eurocode 1 is applicable to the vertical temperature gradient of steel box girders of this steel bridge.