Reliability Analysis And Ultimate Bearing-Load Capacity Assessment Of New Composite Truss Bridge

Since the 21th century, with the rapid development of steel truss bridge construction in china, many new composite steel truss bridges have been constructed and opened to traffic. Their common characteristics are novel in Structure, dramatically special in mechanic properties and complex in construction technology. Meanwhile, they were located in the traffic arteries and were the lifeline of national economy. So their reliability and the ultimate bearing-load capacity are quite important in service period. But rarely studies about large-span steel truss bridges'reliability could be found at the present time, and there also exited disputes about the assessment ways of elastic-plastic ultimate capacity of the bridge structure. Therefore, based on the Dongjiang Bridge, a steel truss bridge with rigid cable, this paper unfolded some research in following ways:1) Characters and principles of several commonly used reliability methods are introduced, and a composite method is presented on the basis of their own advanced. Firstly, the Response Surface Method (RSM) to fit the true curved surface of structure's ultimate state is used to get the approximate equation in ultimate state. Secondly, random sampling on the equation by Monte-Carlo (MC) is proceeded to get the probability distributions and numerical characteristic about equation in ultimate state. Finally utilize the method of First Order Second Moment (FORM) or the Moment Method (MM)to calculate reliability index and failure probability. And found that the response surface method with cross terms have a better accuracy and efficiency, which is suitable to be used in the reliability analysis of large special structures;2) Considered the variability of the structural materials, geometric dimensions and loads, a static stochastic finite element model was established to analyze whole construction processes of Dongjiang Bridge, and the Response Surface Method (RSM) was used to study the mean value, standard deviation and probability distribution of structural response. And parameter sensitivity analysis was adopted to research the influence degrees of each parameter on structural response. Then the equations in ultimate state of trusses'stress and mid-span's deflection were established to get the reliability index and failure probability. Then make an assessment on the reliability of the bridge.3) Firstly to consider the randomicity of installation errors and simulate it as to be random interval. By the composite methods, researched the influence on reliability of Dongjiang Bridge and studied the sensitive degree about structural response to installation errors. In consequence, the installation errors have a great affection on structural reliability.Many domestic research about the uncertainty of loads and resistances was concentrated on the statistical analysis of geometric parameters, material properties and all kinds of loads, while rarely research about installation errors in the construction process. So this paper proposed that according to the complexity of construction technology, the corresponding target should raise reliability by 1,0.5 and 0 on basement of the original codes to ensure the structural reliability.4) For large bridge structure, they have strong non-linear character when reaching their ultimate state, then the linear elastic analysis method is not applicable any more. To analyze the probability distributions and numerical characteristic of the structure's ultimate load factors and deflection, then the Stochastic finite element model taking the double non-linear into account are established. Finally the reliability of the structural as a system in construct's non-linear stage is researched.5) Respecting no unite assessment standard on the elastic-plastic ultimate bearing-load capacity of bridge structure at present, a safety factors assessment method based on reliability theory was proposed: According to the single factor design criteria, the reliability index is converted to safety factor which can easily accepted by engineers and provides reference for evaluating the elastic-plastic ultimate bearing-load capacity of bridge structure. And based on this way, the elastic-plastic ultimate bearing-load capacity of Dongjiang Bridge in largest cantilever stage and finished stage were assessed.