Talking about the seismic design of highway bridges

Our country is in the world's two major seismic zones———Pacific Rim seismic zoneand the Eurasian seismic zone, it is a country prone to strong earthquakes, Wenchuan,An earthquake occurred in YushuIt shows that strong earthquakes will cause long-term socio-political and economic problems, and bring emotional trauma that is difficult to comfort. In earthquake relief, the highway transportation network is to rescue people's lives and property, resume production, rebuild homes, and alleviate as soon as possibleSecondary disastersTherefore, highway bridges are an important part of life system engineering, and the ability of highway bridges to resist earthquake damage is one of the key issues in bridge design. This paper discusses some theoretical and practical discussions on the seismic design of highway bridges, including the analysis of seismic damage and its causes of bridge structures and the key points of bridge seismic absorption design, and introduces the seismic fortification measures of highway bridges.

1. Analysis of bridge structure earthquake damage and its causes

In order to establish correct seismic design methods and take effective seismic measures, it is essential to investigate and analyze the earthquake damage of highway bridges and their causes. From the statistics of earthquake cases in various countries around the world, the main earthquake damage phenomena of highway bridges are as follows:

1. The seismic displacement of the beam bridge causes damage caused by the magnetic collision of the falling beam or the beam body at the upper movable node due to the insufficient width of the cover beam, while the arch structure is mainly manifested in the damage of the arch building and the abdominal arch, the broken cracks caused by the arch ring at the vault and arch foot, and even the deformation of the entire uplift.

2. The liquefaction of the foundation soil caused by the earthquake increases the ground displacement and intensifiesStructural reactions, which greatly increases the possibility of falling beams.

3. Insufficient consideration of the seismic requirements of the bearing will cause excessive displacement and deformation of the bearing, resulting in structural damage to the bearing itself, which will have an adverse impact on other parts of the structure.

4. BridgessubstructureThe lower part cracks, deforms and fails during earthquakes caused by insufficient resistance, which adversely affects the whole bridge.

5. During the earthquake, the bridge on the soft foundation will be slipped on the river bank, resulting in a shortening of the length of the whole bridge.

2. Key points of bridge shock absorption design

For the selection of bridge types in seismic areas, the following principles should be followed: reduce the weight of the structure as much as possible and reduce its center of gravity to reduce the seismic effect of the structureInternal forceThe length and height of the structure should be coordinated to reduce the harmful effects caused by vibration of different properties of each part, appropriately reduce the stiffness of the structure, and use ductile materials to improve its deformation ability, so as to reduce the seismic effect, strengthen the adjustment and treatment of the foundation, so as to reduce the deformation of the foundation and prevent the failure of the foundation.

2.1 The stiffness symmetry of the structure is conducive to earthquake resistance, and bridges with unequal spans are prone to earthquake damage. In particular, the height difference of the pier body in a bridge is too large, and there will be a large seismic horizontal force on the shorter piers, and the span is different. It also generates large seismic forces on the piers of long-span bridge holes. In terms of design, try to avoid using this bridge type in high-intensity areas, if it is unavoidable. It is advisable to set up energy dissipation measures on unfavorable piers to reduce the integrated stiffness of the pier roof, such as seismic bearings.

2.2 Pair of bridgesEarthquake resistanceAfter analysis and research, a certain type of structure cannot be proposed to be built in the seismic area, and the original structure should be analyzed and studiedStructural seismic performanceOn the basis of the structure, it adapts to the structure of earthquakes. Secondly, the seismic design of the structure is not passively used as an earthquakeStructural strengthFrom the design point of view, it is necessary to improve the anti-vibration ability of the structure, and systematically consider the behavioral ability design of the structure.

2.3 Research based on our country's national conditionsStructural controlTo strengthen seismic measures, it is necessary to adopt the idea of "overcoming rigidity with softness" to consider the old traditional fortification view of "overcoming rigidity with rigidity" of seismic fortification of the structure of the earthquake area.Liquefaction of sandIn-depth research should also be made on how to resist seismic fortifications of the bridge structure.

2.4 In view of the lessons of the current collapse and destruction of a large number of viaducts, it is necessary to carry out research on the ductility of anti-seismic bearings and various types of piers, and use the concept of constrained concretePrestressed concrete, and it can improve its ductility. Not only the ductility of reinforced concrete, concrete structure, and mixed structure needs to be studied.

3. Seismic fortification measures for highway bridges

Reasonable structural form and successful seismic design, that is, reasonable conceptual design, can greatly reduce or even avoid the occurrence of earthquake damage.One is conceptual design, the other is constructionDetail design。 It should be noted that these two things actually have little to do with the specific seismic calculation, and the calculation is only an auxiliary means, only to verify the rationality of concepts and details. Therefore, what designers need is a deep understanding of the basic concepts and principles of bridge seismic design. From a structural point of view, it is necessary to know which structures are conducive to earthquake resistance and which are not suitable for earthquake resistance, including the treatment of bridge type, superstructure, substructure, pier, foundation, etc. The construction details include some basic measuresSeismic measures, such as the selection of supports, the setting of stops, etc., but also include the structural measures of component details, such as the stirrup configuration of piers, and the reinforcement structure of nodes. Bridge seismic researchers at home and abroad have been studying the reasonable structural measures of bridges, which can improve the overall ductility and hysteresisEnergy dissipation capacity。 When determining the general direction and main control points of the route, try to avoid themBasic intensityhigher areas and areas with greater risk of earthquake damage; In the route design, it is necessary to make reasonable use of the terrain, correctly grasp the standards, and try to adopt the design scheme of shallow excavation and low filling to reduce the impact on natureEquilibrium conditionsof destruction. For the selection of bridge type in the seismic area, it is advisable to follow the following principles: reduce the weight of the structure as much as possible and reduce its center of gravity to reduce the structureSeismic actionand internal force, improve stability; Strive to coincide with the center of stiffness of the structure to reduce the attachment caused by torsion in earthquakesSeismic force； The length and height of the structure should be coordinated to reduce the harmful effects caused by vibrations of different properties of each part. appropriately reduce the structural stiffness, use ductile materials to improve its deformation ability, so as to reduce the seismic effect; Strengthen the adjustment and treatment of the foundation to reduce foundation deformation and prevent foundation failure.

4. Conclusion

Although earthquakes cannot be effectively predicted, as long as we recognize the damage law of earthquakes to structures through research, we can formulate relevant seismic fortification measures and control the construction quality through certain seismic fortification principles, so as to minimize the impact of earthquake damage.

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