A Numerical Test Study On Concrete Creep & Shrinkage And Temperature Effects For Large Span PC Box Girder Bridges

Large span prestressed concrete box girder bridge features great mechanical ability, high structural stiffness, and strong spanning ability, and has thus been widely applied in modern bridge engineering. But, due to improper design, construction, use and maintenance, most prestressed concrete box girder bridges have problems of cracking and bigger deflection; with the increase of bridge span, the problem becomes more and more serious. Currently the issue has been general concerned in bridge engineering area. Theoretical researches and engineering experiments show that the concrete creep and shrinkage as well as the temperature effects are the important causes of the cracking and bigger deflections. Although those effects have been considered in bridge design and construction process, the problems still arise at last. That means the current computation models still exist defects. That is, to make an in-depth study on those computation models and the corresponding structural effects has significance for systematically solving the concrete bridge’s cracking and bigger deflection problem.In this dissertation, to make a further study on the concrete bridge’s cracking and bigger deflection problem, nine bridges are carefully organized as the engineering background. And numerical tests of those bridges are performed to study the influences of the concrete creep and shrinkage models, the uniform temperature, the gradient temperature models, and the span and horizontal alignment of the bridges on the structure internal force and deflection in stages from construction to 50 years after fold.The main works in this paper include:(1). Unified the symbolic parameters of the six common concrete creep and shrinkage models including CEB-FIP MC78、CEB-FIP MC90、ACI209R-92、AASHTO LRFD-2007、RILEM B3 and GL2000. A quantitative analysis are performed on the influence of the concrete shrinkage strain and the creep coefficient to the factors including concrete shrinkage age, loading age, calculation time, environmental relative humidity, member theoretical thickness and concrete strength.(2). According to the grey system theory, a correlation analysis on the main factors impact on creep and shrinkage are carried out, and the rank of various influence factors has been found out.(3). Based on the research of concrete shrinkage and creep rules, a uniform expression form for creep and shrinkage calculation models is proposed. The corresponding equivalent models for the six common models and a new calculation model for prediction of shrinkage and creep are established respectively. All the new established models adopt the form of product of coefficients, in which the factors are independent of each other. The difference among the models is only varying coefficient value. As to the influence coefficient factors, except that the time point takes a logarithmic form expression, all the other factors adopt linear function or quadratic function expression based on the accuracy requirement.(4). Based on the chosen nine engineering projects, a computer program is developed, and a quantitative and comparative studies of the concrete shrinkage and creep effects is performed for PC box girder bridges. The considered factors include the calculation models for concrete shrinkage and creep, the span of bridge, and the horizontal alignment of bridge. The considered stages include the largest cantilever construction stage, the completion stage, and the normal use stage, all in 18 time points. Based on the obtained conclusions, some suggestions are forwarded for the choosing of shrinkage and creep model and determining the construction camber in engineering.(5). Based on the research of gradient temperature field, a unified expression in the form of index is put forwarded. The corresponding equivalent models for the six common code gradient temperature models are deduced respectively. Based on the chosen nine engineering projects, a comprehensive quantitative and comparative analysis on temperature effects in the normal use stage is made for PC box girder bridges. The system’s temperature changes, the temperature gradient differences, the span of bridge, and the horizontal alignment of bridge, etc. are considered in the analysis. Furthermore, a study on the stress distribution and deformation characteristics as well as their influence on concrete cracking of the box girder section is performed through refined finite element analysis. To reduce the negative influence of the temperature effect on bridge cracking and bigger deflection, based on the analysis conclusions, some suggestions are given on the bridge’s cracking-proof design and the fold time selection in bridge construction.