چكيده به لاتين
Abstract
Bridges are fundamental components in railways. In fact, the bridges provide a complete service path by providing the project line level along the railways. The complete and without defect performance of bridges ensure the correct performance of the route. In our country, There are a lot of masonry bridges in railway path, most of them are ancient, in order to their well performance, accurate examination is required. According to different design rules, some cracking is acceptable in masonry structures, but if the depth and dimensions of cracking are high, it can lead to a change in the behavior of the bridge. Considering the recent demand for increasing axial load and passing speed, it is necessary to control the axial load bearing capacity and seismic load bearing capacity of these bridges with accurate methods and assumptions.
There are several methods for modeling a cracked structure, among which one can consider the characteristics of materials with the assumption of cracking, open crack modeling with elastic and plastic specification, also using viscous element in the crack region and in the finite element environment. In the cohesive region model, it is assumed that the stresses are at the cross section of an open crack with a low width. In this model, the cracks are described using crack openings against stress relationship, where the adhesion stresses at the crack level vary between the tensile strength of the materials at the tip of the cracks and the zero at the crack opening. In this method, it is assumed that the crack is formed when the stress is reached to the concrete tensile strength at the tip of the crack.
In this thesis, it has been tried to use the numerical modeling, to obtain axial bearing capacity and seismic bearing capacity of the bridge at 23 km of Tehran-Qom railway as a field test case with the assumption that the bridge is cracked. In this regard, the validation of the numerical model with the aid of three models: with crack in bridge pier, the cohesive element in bridge pier and the cohesive element in the pier and the quadrant of the arch span is verified, and in each step the response curves were updated with the field test results by analyzing the sensitivity of the Vertical displacement parameter and Mode frequency. The result of this research has shown that the maximum load capacity of the bridge 23 km for the bridge model with the departure of the cracks in bridge pier is 1120 tons, for a model with a cohesive element in bridge pier, equals to 1,165 tons, and the ultimate capacity of the bridge model with cohesive element in the pier and in The quarter of arch span is 1134 tons. Also, to evaluate the seismic capacity with regard to the results of the arc in all three modes, the existence of a cohesive element in the middle of the span, a cohesive element in the middle of the span and two cohesive elements at the critical point of a quarter of the arch span, and in the case that cracks are in the middle of the arch span under Longitudinal earthquake is responsive.
Key words: masonry bridges, axial load increment, cracking, cohesive region model, finite element environment
