Study Of Crack Growth Probability Of Mass Bridge Tower Under The Thermal Fatigue Load

With the poor heat conduction performance and the influence of external environment factors, the mass concrete bridge tower, exposing to the natural environment, is in nonlinear temperature distribution and leading to tension and compression cycle temperature fatigue stress. Furthermore, the conventional way by establishing expansion joints to dissipate temperature stress doesn’t work because of the bridge tower structure requirement. At the same time, the crack is an inevitable problem of concrete, and the stress concentration phenomenon existing in the crack tip makes the concrete fatigue performance greatly degrade and the crack extension possibility is very large. At present, the research of pre-crack concrete fatigue performance is very few because the structure design concept is that the concrete tensile strength is zero. Based on the research and analysis of pre-crack concrete fatigue characteristic test in combination with observation and calculation of the temperature stress of the cable-stayed bridge tower, the reliability analysis of crack extension is done. With the national natural science fund project (50908184), the following researches are carried out:(1) A tension and compression fatigue test of pre-crack concrete is designed and carried out based on the in-depth understanding of existing concrete fatigue test methods. The fatigue test results are analyzed earnestly and the S-N, P-N, P-S-N fatigue curves are plotted. Furthermore, the maximum likelihood method of fatigue performance curves is used to verify the reliability of test results. The pre-crack concrete fatigue stress (S) and fatigue life (N) corresponding formula is established by deforming the index form S-N expression to more intuitive linear, quadratic, cubic polynomials on the fatigue stress (S) and fatigue life logarithm form (lgN).(2) By analyzing the a-Ncurve of the deformation and fatigue loading cycle times, the crack propagation process of pre-crack concrete is divided into four stages and it is concluded that the aggregate quality (strength and shape) plays a very important role in pre-crack concrete fatigue life.(3) The equivalent hidden crack model is put forward to solve the problem that it is difficult to accurately grasp the true propagation process of the cracks at different locations because of the discreteness of concrete material itself. Meanwhile the simplified formula that can ignore the initial crack depth and width is obtained and the analysis results are more applicable in the actual engineering projects in which the initial crack depth and width is variable.(4) By using the secant method and the least square method, the relationship between the fatigue crack propagation rate and the stress intensity factor variation is established and the important fatigue parameters in Pairs formula, C and m, at different survival rates are given. The constant life fatigue diagram of C50pre-crack concrete is drawn by the means of the Goodman linear and Gerber quadratic parabolic curve.(5) To felicitously simplify the system and build a feasible model, the six fundamental assumptions, which can be used to report the influencing factors and inherent laws on the temperature field and stress of mass concrete bridge tower, are supposed. By comparing the third boundary condition and the first boundary condition, the appropriate boundary conditions on the bridge tower temperature stress analysis is chosen.(6) According to the observation of the external natural environment for four months, the temperature and stress field distribution analysis of the selected bridge tower segment is calculated by using ABAQUS finite element analysis software. The temperature fatigue load spectrum is made based on the temperature and stress field distribution results.(7) To get mean and amplitude cycle number distribution and mean-amplitude joint probability density diagram, the process flow of the temperature fatigue load spectrum, performed by the rain flow method, is edited by using MATLAB software. It can be proved that both the mean and amplitude cycle number distribution of temperature fatigue loading obey the normal distribution and the load cycle characteristics are in two-parameter lognormal distribution.(8) The crack propagation probability density formula is derived by using Lin-Yang crack propagation model and introducing random variable process. The possibility of cracks extending to the damaging level is analyzed with infinite life safety reliability model and the cumulative probability of crack extension over time is founded on the safety reliability model of crack extension randomness.