The Research On Thermal Gradient Model Of The Concrete Box Structure

Crack severely impacts the concrete bridge structure's safety and durability, thermal stress is one of main factors causing concrete cracking on box structures. Reasonable thermal gradient is the prerequisite for determination of structural thermal stress. Researches about box pier and fish-bellied box girder is limited, so the profound study on thermal effect of bridge structure is necessary. According to the engineering project, the thermal effect of bridge box structure was researched in this paper based on the in-situ test results and parametric studies. The main investigations are summarized as follows:(1)According to continuous observation data under solar radiation on a concrete box pier, the temperature field and its variation laws were discussed. The effect of wall azimuth was researched based on the parametric studies, and the temperature stress was calculated under different temperature gradient models, the temperature gradient model was also determined for box pier in different directions. The results show that: the maximal temperature difference and its appears moment are both different for box pier in different directions; the relationship between wall azimuth and maximal temperature difference changes in a sinusoidal fashion.(2)The effect of cooling rate, cooling amplitude and wall thickness on negative thermal gradient of concrete box pier was researched based on the in-situ test results and parametric studies, and the corresponding negative thermal gradient and thermal stress for box pier with different thickness were presented. The results show that: the thermal gradient in each wall of the concrete box pier under cold current is about the same; the negative thermal gradient is relative to the wall thickness of box piers; the initial thermal distribution has little effect on the increment of structural response under the same cooling amplitude; the horizontal tensile stress is up to 3.6mpa for the concrete box pier with wall thickness of 0.6m under cooling amplitude of 20?.(3)The temperature gradient model under solar radiation was proposed based on the calculation results of transient temperature field. The temperature effect was analyzed for a three spans prestressed continuous box grider bridge, and the calculation result was compared with the result by current specifications. The results show that: the effect of transverse temperature deviation can be ignored on fish-bellied box girder; the temperature gradient model stipulated by current specifications has a greater difference with physical truth, it is suggested that exponential function should be adopted for temperature gradient model in the design.