Master's Thesis
Civil Engineering – Earthquake Engineering
1392
Abstract:
Concrete beam and slab bridges with neoprene supports are among the most common highway bridges in the country. In these bridges, due to the need to provide free movement of the deck, due to temperature changes and not using a shear key along the length of the bridge, the deck falling from the foundations or the deck hitting the abutment, during the longitudinal vibration of the deck, is one of the important cases of bridge failure in past earthquakes. One of the effective ways to prevent the deck from falling or hitting the bag is to use a motion restraint along the length of the bridge. The main goal of this research is to investigate the effect of using longitudinal fasteners on the seismic performance of conventional concrete beam and slab bridges. For this purpose, a three-span bridge with equal spans and a three-span bridge with unequal spans equipped with longitudinal ties at the base and in the abutments have been modeled and the seismic performance of the bridges has been investigated using nonlinear time history dynamic analysis. The obtained results show that the use of longitudinal restraints significantly reduces the relative displacement of the deck and also reduces the severity of the impact of the deck on the bag. In addition, the seismic performance of the bridge foundations is also significantly improved.
Key words
Bridge, earthquake, vital artery, damper, binding, nonlinear analysis
Chapter 1
Introduction
Today, roads and transportation infrastructures and Transfer plays a major role in the fate of societies, so that the quantitative and qualitative development of communication ways is one of the most important indicators for evaluating the degree of development of societies. Bridges are also important and integral components of roads. The experience of past earthquakes has shown that bridges are one of the most vulnerable road components. In our country, where the regulations for the seismic design of bridges have a very short history, and due to the seismicity of many parts of our country, the seismic vulnerability of a large number of existing bridges is probable. Multi-span girder and slab bridges with neoprene supports are among the most common highway bridges in the country. In these bridges, expansion joints are usually installed in the middle foundations and abutments in order to provide longitudinal movement due to temperature changes. The seismic performance of this type of bridge is completely different along the length and width of the bridge. In the transverse direction, the inertial force of the bridge deck is transferred to the substructure elements (base, pile and foundation) through supports and shear blocks. The mass of the bridge deck is relatively high and therefore the inertial force caused by its transverse vibration can cause serious damage to the elements under the structure. In the longitudinal direction, due to the lack of shear key and the relatively low resistance of the neoprene supports, the possibility of the deck falling from the foundations or the deck hitting the pile is high, and this type of failure has been observed abundantly in past earthquakes. Along the length of the bridge, the problem of the deck falling from the foundation or the deck hitting the bag is serious, and it is necessary to take measures along the length of the bridge. One of the effective ways to prevent this type of failure is to use a motion limiter along the length of the bridge. In this research, the effect of using different types of restraints, including restraining cables, locking restraints, and viscous dampers, on the seismic performance of beam and concrete slab bridges is studied. The different aspects of the current research compared to similar foreign researches are:
The bridges studied in most foreign studies are simple span bridges with in-span hinges, while the common bridges of our country are implemented as simple supports on foundations. The judgment of binding systems has been taken into consideration, while in the present research, the formation and development of plastic joints in foundations has been investigated.
In the researches that have studied steel cables as a longitudinal binding system, the main basis has been the elastic behavior of steel cables. While in this research, the plastic behavior of the cables has also been examined.
Types of longitudinal restraints
Of the usual joint restraints, in bridges, one can use cable restraints, high strength rods, Locking fasteners, viscous dampers, flowing dampers, FRP fasteners, and SMA fasteners are mentioned. During an earthquake, adjacent frames may vibrate out of phase and cause two types of displacement issues. The first type is the problems caused by the frames hitting each other in the joints. In general, this local damage will not cause the bridge to collapse and is therefore not a major concern. The second type happens when the expansion joint is opened and allows very large movements of the frames and the lack of seating of the support. Restraints are used to fasten the frames to each other and limit relative displacements from one frame to another and provide a suitable path for force transmission across the joint. The main goal is to prevent the openings from falling from their supports. Some factors such as periodicity of the structure, flexibility, joint resistance, bending frame, diaphragm, tensile capacity of the superstructure and to some extent the geometry of the superstructure are effective in choosing the type of containment. (Images can be seen in the main file) The history of seismic design of bridges and the need to check vulnerability. Seismicity of Existing Bridges
In America, until 1940 AD (1319 A.D.), the bridge design code was used without mentioning the need to consider the force caused by an earthquake. Since 1940, the consideration of earthquake force in the design of bridges has been required first in California and subsequently since 1949 in the 5th edition of Ashto throughout the United States (without providing guidelines for determining this force). In 1961, the method of determining the force caused by an earthquake according to the bridge construction, between 4% of the weight of the bridge for structures located on wide foundations, and 6% of the weight of the bridge for structures located on piles, was included in the design regulations of American bridges, which remained unchanged until 1975. After the 1971 San Fernando earthquake, first in 1973 in the state of California and from 1975 by Ashto for the whole of America, new regulations for the seismic design of bridges including the connection of the location with the active fault, the seismic response of the local soil and the dynamic characteristics of the bridge were presented. In 1992, after the numerous earthquakes of Nortrich 1984 and Mexico 1985, a fundamental change in Ashto's seismic regulations was introduced and published as a separate section. On the other hand, in Japan, extensive damage to bridges in various earthquakes, including the 1995 Kobe earthquake, along with extensive research conducted after these earthquakes, led to fundamental changes in the seismic design code of bridges in Japan in 1996. It was compiled by the Building and Housing Research Center in 1372 and was approved by the government board in 1375 for compliance by consultants. In 2014, more complete regulations were issued by the Ministry of Roads. And Transportation was compiled and published, which is mandatory for the design of bridges outside the city. In this way, the bridges that were built before 1375 can be considered unsafe due to the lack of codified regulations for bridge construction in Iran. Although many of these bridges have been used by experienced consulting engineers, the design and existing knowledge of the time have been used in their design, but the re-examination of the design and calculations of these bridges is inevitable due to the progress achieved in the knowledge of earthquake engineering in recent years. Whether or not this review is necessary for all bridges depends on several factors. One of these factors is the importance of these bridges in providing aid to different regions through them. These bridges may be found to be acceptable or unsafe after investigation.