Bearing Performance Assessment Method Of Continuous Box Girder With P.C. Variable Section After Cracking

In the dissertation, it was reviewed the domestic and foreign diseases investigation and research of box girders'cracks. Details of the crack influence to the mechanical behavior of box girder was introduced. Destructive model test of continuous box girder model with P.C. variable section was loaded using repeated loading method. Combined with nonlinear finite element analysis to study the stiffness degeneration and shear lags of box girder at different crack level.The three dimensional finite element equations for thin shell structures were deduced. The total strain crack model was used to simulate cracks. The combination element equation to analyze prestressed steel was proposed. Incremental iterative method was used to solve nonlinear equations. The parameters of numerical model was verified with observation of test. Based on the above works, The cracks'influence to beams'mechanical property was researched and cracks was at mid-span,1/4 span and supporting position respectively.Strain distribution of box girder under different load levels was analyzed. Stress redistribution property of box girder was researched after cracking. The shear lag coefficient variation of top and bottom flange of box girder was analyzed. Based on Levenberg-Marquardt optimization algorithm, a programe using MATLAB language was developed. A continuous box girder model with P.C. variable section finite element model was bulided using using ANSYS APDL language. The above analysis module were integraed, the beam bending stiffness of damage assessment method was achieved basedon optimization theory. At the same time, another method was used to assess stiffness damage assessment of box girders. The beam stiffness damage index directly builded based on the load-deflection curve information. The compute results of two methods were compared and analyzed to prove the later method's validity.The main innovative research results of this paper are summarized as follows:1) Continuous box girder model with P.C. variable section carrying capacity model test was researched.The same type test was first in China.2) The basic equation of combined prestressed steel finite element model were deduced. The combined model was applied at non-linear finite element calculation.This method overcomed shortcomings of the excessive number of discrete finite element model, improved the accuracy of nonlinear finite element method effectively and reducd computation time.3) Shear lags of box girder with P.C. variable section was researched after cracking. 4) The proposed box girder crack density index that was used for quantitative description of web cracking.Correlation model of box girder box girder stiffness damage factor and crack density parameter was proposed. Based on analysis results of 21 finite element model, mathematical model of box girder stiffness damage factor considering the wide span ratio, span ratio and the load factor is presented by statistical regression calculation.