Dissertation for Master's Degree in Structural Engineering
Abstract
Today, due to the increasing space limitation and economic savings, it is inevitable to use diagonal bridges to avoid lengthening the route and increase road safety. The diagonal issue of the bridge will cause a different and sometimes complex behavior in the bridge under the effect of the forces acting on it, and according to the discussion of the occurrence of earthquakes in recent decades, which opened a new chapter in the design of structures, the discussion of the seismic performance of diagonal bridges has added to their behavioral complexity. The ability and benefits of seismic isolators as a passive control tool for earthquake forces that allow designers to reduce or balance earthquake forces on bridge piers and piers have been well proven. In this research, the simultaneous effect of bridge diagonality and the use of lead core seismic isolator has been evaluated in the seismic response of bridges, and the results show that the use of seismic isolator in diagonal bridges can be very useful and reduces the increasing effect of the seismic response of diagonal bridges. In other words, the effect of the isolator in reducing the seismic responses of diagonal bridges is much higher than that of non-diagonal bridges. Also, the results show that considering the vertical component in diagonal bridges without isolators, which have significant spans, can increase the seismic response of such bridges, and the use of isolators has been able to reduce this increase to a large extent. Keywords: diagonal bridge, seismic isolation, earthquake, horizontal, vertical. In today's world, communication channels play an important role in the economic, social and political structure of countries. In order to bring passengers to their destination faster, any road must pass through natural obstacles such as mountains, rivers, and valleys, and bridges are needed to pass through most of the mentioned areas. This doubles the importance of bridges. Bridges play a key, strategic and decisive role in a country's communication network, especially in economic, military and political affairs. For this reason, bridges incur a lot of costs to the economy of a country. Therefore, maintenance and technical inspections and management of the bridge must be done with a complete management system in order to avoid the waste of national and exorbitant costs. Today, due to the increasing space limitation and economic savings, it is inevitable to use cheap bridges [1] to prevent the length of the route and increase the safety of the road. The diagonal issue of the bridge will cause a different and sometimes complex behavior in the bridge under the effect of the forces acting on it, and according to the discussion of the occurrence of earthquakes in recent decades, which opened a new chapter in the design of structures, the discussion of the seismic performance of diagonal bridges has increased their behavioral complexity. On the other hand, the development of communications and the need for optimal use of urban spaces, as well as geometric limitations when crossing natural obstacles, have caused diagonal bridges to form an important part of the urban and intercity transportation system. In fact, diagonal bridges have been more vulnerable to seismic loads than straight bridges in past earthquakes.
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1-2- The purpose of this thesis
Diagonal bridges are intended to reduce the effects of lateral forces. Also, in this research, the rules of Ashto [2] regarding the design of seismic isolators have been studied. It seems that diagonal bridges are subject to additional reactions compared to normal bridges, so it is expected that additional reactions of diagonal bridges can be significantly reduced by using a lead core seismic isolator.
Since in reality, structures face bidirectional earthquake excitation during an earthquake, therefore, in this research, diagonal bridges without isolators and isolated are subjected to bidirectional earthquake excitation. Also, in order to influence the vertical component of the earthquake, diagonal bridges have been subjected to three-way excitation (taking into account the vertical component) and the seismic response of diagonal bridges in two and three-way excitation modes have been evaluated and compared.
-3- Defining the problem and stating the main research objectives
The ability and benefits of seismic isolators as a tool for passive control of earthquake forces that allow designers to reduce or balance the earthquake forces on the foundations and bridge decks have been well proven. Considering the types of damage caused to bridges in past earthquakes, which are mainly due to the incorrect estimation of earthquake forces, using separators can be effectively used in the improvement of existing bridges or the design of new bridges located in seismic areas. In this research, the simultaneous effect of the diagonal of the bridge and the use of the lead core seismic isolator has been evaluated in the seismic response of the bridges. For this purpose, the studied bridges have been modeled in three dimensions with the help of SAP2000V14.2.2 software. In order to evaluate the seismic response of diagonal bridges and the impact of the lead core seismic isolator, the bridges with torque angles of 0 to 50 degrees (with 10 degrees growth) have been modeled once without isolators and again with seismic isolators. Also, in order to observe the impact of the vertical component of the earthquake on the seismic response of irregular and diagonal bridges, the models have been subjected to two-way stimulation (without considering the vertical component) and once again three-directional stimulation (taking into account the effect of the vertical component of the earthquake). The things that are investigated are:
Investigation of base shear in diagonal bridges without isolator (fixed support) and comparing it with bridges isolated with lead core rubber isolator, in the case of two-way earthquake excitation (without considering the effect of the vertical component)
Investigation of the maximum shear force and axial force of piles in diagonal bridges without isolator (fixed support) and comparing it with isolated bridges with isolation device lead core rubber, in bidirectional earthquake excitation mode (without considering the effect of the vertical component)
Investigating the maximum axial force of the columns and the maximum column foot twist in diagonal bridges without isolators (fixed support) and comparing it with isolated bridges with lead core rubber isolators, in the bidirectional earthquake excitation mode (without considering the effect of the vertical component)
Investigating the maximum deck corner displacement in diagonal bridges without Isolator (fixed support) and its comparison with bridges isolated with a lead core rubber isolator, in two-way earthquake excitation mode (without considering the effect of the vertical component)
1-4- Structure of the thesis
All the content of this thesis is in five chapters, which are as follows.
Chapter one: Introduction and overview of the necessity of using foundation isolation in diagonal bridges
Chapter two: Definition of diagonal bridges (beehives) and studies conducted regarding diagonal bridges, the performance of bridges in recent earthquakes and their improvement methods, and historical studies on bridges with Isolators and a brief description of the types of isolators and their application and the difference between the application of seismic isolation in bridges and buildings
Chapter three: How to model, load and analyze the bridges examined in this thesis
Chapter four: Examining the results of the analysis of diagonal bridges with fixed support and comparing the results with diagonal bridges isolated with lead core seismic isolators in two modes of two-way and three-way excitation, as well as checking the protection factor of the bridge foundations Diagonal in three excitation modes, one, two, and three directions
Chapter five: Conclusions and suggestions
Appendix A: Rules of Ashto regulations regarding the design of seismic isolation bridges
Appendix B: Design rules of isolated bridges according to Eurocode regulations [3]
Chapter Two
2-1- Introduction
Bridge As relatively simple structural systems, they have a special place in the minds of structural designers because their structural form is a simple expression of their functional needs. For this reason, the structural solutions that are established for them must be confirmed both in terms of bridge performance and aesthetics. Despite the structural simplicity of bridges, especially those made of reinforced concrete or prestressed concrete, they have not been able to perform at the level of designers' expectations under various earthquakes. The recent earthquakes in California, Japan, and Central America showed that many bridges designed according to earthquake regulations suffered severe damage or even overturned, while the intensity of these earthquakes was lower than the values ??reported in the design regulations [1].