1.Importance and advantage of isolation design in bridge
1.1.Importance of isolation design in bridge
Isolation design works mainly through the installation of seismic isolation device that forms the isolation layer which works in a relatively elastic state to greatly reduce the possibility of the bridge collapse so as to ensure the safety and stability of bridge. The main purpose of the bridge isolation design is to balance the damping effect caused by the usage of the bridge. At the same time, the lateral support force can meet the requirements of overall stability of the bridge structure. The introduction of isolation technology to the bridge structure design can improve the overall stability of the bridge structure. Especially in some geological disasters, the isolation design can effectively reduce the destructive force on the bridge.
1.2.Advantage of isolation design in bridge
For one thing, the introduction of seismic isolation design to the bridge structure can prevent the bridge structure from collapsing, making the bridge balance forces which can not result in the collapse of bridge structure owing to forcing a single point. For another thing, the isolation design can adjust the lateral stiffness of the bridge structure to reduce the impact of the bridge by the seismic force in earthquake and other geological disasters, which reflects the advantages of isolation design in improving the reliability, safety and overall stability of bridge structure.In addition, the upper design of the isolation design can reduce or even eliminate the performance that the lower part of the structure extends beyond the elastic range. The partial structure is buried in the structures which are difficult to be inspected and repaired, such as piers, thereby reducing the inelastic deformation caused by these structures. Therefore, it is very meaningful, advantageous and essential for the quality and safety of the bridge and the reliability of the bridge structure to introduce the isolation design to the bridge structure.
2.Isolation design and principles of bridge
2.1.Isolation design of bridge
As an emerging design concept, isolation design is an important part of the bridge structure design. The design of bridge isolation is an important measure to enhance the seismic performance. The quality of the isolation design directly affects the safety and reliability of the bridge.
According to the related experience of the introduction of isolation design in the bridge structure design in China in recent years, the isolation design should follow the following principles:
Through field survey, the constructor needs to conduct a comprehensive survey to conduct the bridge construction site and then analyzes whether the bridge can be used in isolation design, whether it is appropriate to employ isolation design, whether it can absorb the energy it sends in event of earthquake.
Especially the bridge design in the earthquake region, the design structure should be consider more. For example, the bridge structure design must be relatively simple to avoid the piers suddenly changing in the bridge structure, and should reduce the introduction of the design with holes. Furthermore, the constructor tries to ensure the symmetry of the bridge structure in the overall design.
Firstly, the designers and architects need to ensure the rationality of introducing the isolation design into the bridge structure design and avoid the blindness and following-up of the bridge design.
Secondly, the designers shall give full consideration to the introduction of the other technologies to assist the usage of the other design methods, while introducing the isolation design to the bridge structure,
Then, The designers also shall fully consider earthquakes and other geological disasters at the time of designing the isolation devises to reduce unnecessary vibration to the greatest extent and to ensure the stability of the foundation of the bridge, for ensure the rationality of the isolation design.
Finally, the designers shall strictly follow the guidance of mechanics and other related knowledge and selectively use isolation devices to ensure the scientific of isolation design.
2.2.Principles of bridge design
The principles of the bridge design mainly Increase the flexible pillars of the bridge structure, weaken the coupling of the various structures of the bridge and thus enhance the reaction speed of the bridge reaction, so that the bridge design which is the basic principle of isolation design greatly reduces the destruction and other geological disasters caused by the earthquake. However, the isolation design is not completely for resisting the earthquake, but for protecting the bridge structure from the earthquake's destructive, but the isolation design is also intended to consume a large amount of energy as fast as possible carried by seismic waves, which is fully taking the possible structural deformation of the bridge into account. Therefore, the isolation design of each member shall have strict requirements to ensure that each member has good plasticity. The bracket of bridge isolation design is shown in Figure 1.
Figure 1 The bracket of bridge isolation design
3.Main content of isolation design
The design and placement of isolation device is the key to isolation design.
The elastic response spectrum methods and time-history analysis methods are two important design methods of isolation design for now. There are different norms in different countries and regions for these two kinds of design methods, and these differences are mainly reflected in the differences in the formulas of structural design methods. The two formulas are mainly to calculate the equivalent stiffness and the equivalent damping of isolation devices. However, In the current design of bridge isolation, the elastic response spectrum method is widely used. Because of its convenient operation, simple calculation and high standardization, the elastic response spectrum method is widely used in the design of modern bridge isolation.
Bridge isolation design is an important measure to enhance seismic isolation performance, and the quality of seismic isolation design directly affects the safety and reliability of the bridge. The designers needs to consider the maximum amplitude of the isolation device and select the appropriate design method after that. For those bridge isolation devices with complicated or irregular structures in some areas, the designers shall consider the vibration period of bridge structure and adopt the time-history analysis method. During the construct, it is necessary to strictly monitor the elastic properties of the isolation devices, In particular, the designers should pay attention to the range of elastic changes of isolation devices to ensure that the isolation device has good flexibility, and can greatly improve the safety and reliability of the bridge structure.
Bridge isolation device can not be completely separated from the earthquake, so the designers introducing isolation design into the bridge structure shall fully take the introduction of other technologies into account to help the use of other design methods.
Although only as a accessory structure, the accessory structure in the bridge design also plays an extremely important role and is an indispensable part of the isolation design. For example, these accessory structures are like screw cap in some key structure of the machine, if a screw is lack of screw cap, the stability of the machine will be greatly reduced. These accessory structures include anti-drop-beam devices, displacement limiting devices, expansion joints and so on. Based on the past experience of bridge design, some accessory structures in the bridge isolation design shall be of good quality. The fine design of accessory structures such as bridge isolation devices also needs continuous optimization and improvement.
In summary, the importance of isolation design in the design of bridge structures is self-evident. Only by introducing seismic isolation technology, the bridge deformation and carrying capacity can be greatly improved to effectively deal with geological issues such as earthquakes and reduce the losses caused by natural disasters caused.
Therefore, bridge architects and designers whose country’s bridge isolation is not yet mature enough or in start-up stage can learn more from the mature and successful design experiences in other countries and advanced technologies related to bridge isolation combing with own country’s bridge and own design features to constantly summarize and explore.