Study On The Key Factors Of Temperature Effect Of Concrete Box-girder Bridges

It is known that temperature stresses and strains are comparable to that induced by thedead load or vehicle load upon concrete box-girder bridges.As one of the most importantfactors that lead to the occurrence of cracks in concrete bridges during construction oroperation period,temperature effect may deteriorate the safety,durability and adaptabilityof the bridge structures.Tracing from different originations the temperature effect ofconcrete box-girder bridge is classified into three categories:climatic temperature effect,artificial temperature effect and material temperature effect (the third one here representsthe effect caused by the hydration heat released by the cement-based concrete materials intheir hardening phase).Taking the general finite element modeling package ANSYS asplatform,a wide variety of thermo-mechanical problems of concrete box-girder bridge areanalyzed by simulating the structure in 3 dimensions.The main works are as follows:(1)A new timing system named Radiation Calendar is established.In RadiationCalendar the ranges of solar altitude angle and azimuth angle are redefined,which makesthe angles calculated from the improved formulas single-valued varying to RC hour angle.These improved formulas may be used conveniently when the 3D structural temperatureeffect caused by the solar radiation is to be quantified.(2)A parametric finite element modeling of a box-girder segment with any axialdirection is established.The external surface of the segment model is divided into differentareas according to their different characteristics reflected in the heat exchange betweenthese areas and thermal surroundings.By solving the equations that describes the heatequilibrium between these surface areas and thermal surroundings the valves of equivalentradiation air temperature and their variation to RC hour angle can be obtained.Taking useof the ANSYS auxiliary processor (AUX12)that supports the application of Hemi-cubemethod,the recognition of the elements exposed to and shaded from the sunlight is realized.So the thermal conditions that vary not only to time but also to spatial position can be lively simulated.Following these methods,parametric analysis of the transient temperature fieldof box-girder segments located in different geographical positions and axial directions arecarried out.In a comparison the simulation results agree well with the observed values froma practical engineering.(3)The definition of linearity of vertical gradient temperature forms is proposed.Through comparing the results calculated from the solid finite element model,theprevailing simplified method through which the thermal stresses are calculated in usualdesign work is proved to be lack of safety from the overall.The reasons for the differencebetween these two series of results are expounded.(4)Sudden drop of air temperature causes stresses in the body of box-girder bridge.To investigate the characteristics this kind of temperature effect,the parametric analysisabout the influence of characteristic length of cross section,convective heat transfercoefficient and severe degree of temperature drop on the magnitude of temperature stressesare performed.High temperature asphalt pavement is another factor that changes thetemperature field of box-girder in short time.Making use of the elements'"death and birth"function,the course of asphalt pavers'marching along the girder is simulated,throughwhich the temperature distribution and the stresses caused by asphalt paving is analyzed.(5)According to the relaxation coefficient method finite element scheme,takingtime-varying and age-varying properties of concrete into account,a program written inAPDL is proposed to calculate the temperature stresses in massive concrete structuresduring early ages.Through this section of APDL a calculation of temperature field andtemperature stress distribution of an original segment of a long span continuous box-girderbridge is implemented.