چكيده به لاتين
There are many masonry arch bridges in Iran’s railway network, as well as all over the world, which are still in operation despite their long lifetime. Given the long life that has passed since the design and construction of these structures, design and construction methods have indeed made significant progress during this period. Besides, increasing axial loads and train speeds in all railway networks have been considered necessary in recent years. On the other hand, the components of railway tracks, including bridges, must tolerate the increased axial load and speed of trains. Strengthening is one of the common ways to achieve this purpose in masonry arch bridges. Several factors affect the performance of the strengthening process, the most prominent of which are methods of strengthening, bridge materials, behavioral models in bridge materials and reinforcements, adhesion between reinforcements, and arched bridges. New lines are usually designed and developed based on higher axle loads (25 tons) than the axle loads of older lines (18.5, 20 and 22.5 tons). However, these newly designed lines are operated simultaneously with other aged components and infrastructures of the network. If less operating axle load is selected, part of the ideal line performance will not be used, and the use of larger axial load may cause damage and failures to old structures and then to the entire network. Hence, according to the evaluation of the structure's load-bearing capacity, strengthening should be conducted if needed. One of the approaches for increasing the bridge's efficiency is superstructure improvement, and another approach is to strengthen the bridge structure. One of the most widely used methods in strengthening bridge structures is the use of composite materials, the most common of which in civil structures is Fiber-Reinforced Polymer (FRP). In a part of this dissertation, the effect of infrastructure rehabilitation on the response of bridge structures is investigated. It can be seen that the root mean squares of the vertical acceleration have decreased about 1.75 times in the case of assessing ballast operation and about 2.3 times in the case of ballast mat execution. On the other hand, in the construction of masonry arch bridges and during their lifetime, many uncertainties play an essential role in estimating these bridges' behavior. It seems that achieving a specific safety level of these structures is not simply possible because of different reasons, such as various components and inherent uncertainties. One of the most important uncertainties in evaluating masonry arch bridges is the uncertainty of the impact factor. In this thesis, by using long-term health monitoring data, several methods for estimating the probabilistic distribution of the impact factor are presented. The study of the uncertainty of this variable shows that the impact factor equivalent to 95% of the output results is 1.5 times greater than what is proposed in the regulations. Considering the complex performance of masonry arch bridges and to evaluate them accurately, a more reasonable model than conventional models is desirable. In this study, a separate block-mortar model based on the plane-strain finite element is introduced and developed. Compared to the results of field experiments, the output of the model indicates higher accuracy and better identification of the position of peaks than conventional methods. Also, the capability of modeling existing cracks in the masonry arch can be highlighted as the primary advantage of this model. The results reveal that cracks reduce the tensile stress due to live load by about 19% and increases the compressive stress up to three times. Analysis and study of strengthening in the structure of the masonry arch bridge is another study done in this dissertation. First, for simple arcs and conventional models, the strengthening effect is investigated, and about 12% reduction in stresses is observed. However, by using more accurate modeling and considering the more realistic performance of blocks and mortars in masonry arch bridges, as well as the application of cracks in different parts of the arch, a reduction of up to 35% in stresses can be observed. Then, by considering various uncertainties in the problem variables such as the bridge structure, reinforcement and adhesion system, train axle load, and impact factor, the problem is transferred from deterministic to stochastic state. Ultimately, the reliability index and failure probability for the strengthened bridge are calculated. The results of these studies in the cracked bridge show a reduction of 78% in failure probability.
