What are the similarities and differences between bridge seismic analysis and building seismic analysis?

The specifications for the seismic design of bridges in the registration examination mainly include the "Code for Seismic Design of Urban Bridges" CJJ 166-2011 (hereinafter referred to as "City Bridge Resistance Regulations") and the "Code for Seismic Design of Highway Bridges" JTGT2231-01-2020 (hereinafter referred to as "Public Bridge Resistance Regulations"). Railway engineering seismic design specificationsGB 50111-2006 (2009 Edition). Purely from the perspective of review and preparation, you only need to master the "City Bridge Anti-Regulation" and "Public Bridge Anti-Regulation". There are questions about bridge earthquake resistance every year, and the test questions in recent years are as follows:

2024 2nd Circuit, 39, 40

2023 1 Way, 40

2022 2nd Circuit, 35, 40

2021 2nd Circuit, 39, 40

2020 1 Way, 37

2019 1 Way, 33

It can be seen that bridge seismic design is a compulsory test every year, and the test questions account for 1-2 points every year!! Therefore, it is necessary to pay enough attention to and review carefully!

The concept and analysis method of bridge seismic design are the same as those of building structures, but the design service life of bridge structures, the importance of structures, and the characteristics of structural stress are different, and bridge codes are specially used as different regulations.

1. The following is a brief analysis of several concepts:

1. Magnitude: Magnitude indicates the amount of energy released by the earthquake itself, and there is only one magnitude for an earthquake.

2. Earthquake intensity: Simply put, it is the impact and damage of an earthquake on the surface of the area, buildings and structures. The seismic intensity caused by an earthquake to different regions is different. Generally speaking, the intensity of earthquakes is highest in the epicenter area and decreases with the increase of the distance between the epicenters. For example, if a strong earthquake occurs in the United States, the magnitude is very large, but it has no impact on our country, then the intensity of the earthquake to our country will be low.

3.The intensity of seismic fortification：Compared with the seismic intensity, the word fortification is added, and according to the impact of earthquakes in history, the corresponding seismic fortification standards are determined by statistical knowledge, so that the structure can resist the seismic intensity and achieve the purpose of three-level fortification, that is, small earthquakes are not damaged, medium earthquakes can be repaired, and large earthquakes cannot be knocked down. We know that earthquakes are a very destructive and random natural disaster. Then there is a probability of its occurrence, and the probability method is used to predict the maximum intensity that may occur in a certain area in a certain period of time, and it is divided into: frequent earthquakes, design earthquakes, and rare earthquakes.

4. Seismic resistance: It is the attribute of the building structure, the more important, the more susceptible to earthquake attacks, and the more it needs to be protected, the higher the seismic resistance level, the higher the requirements for seismic resistance.

5. Design grouping: Why design grouping? Earthquakes of different sizes (magnitude or epicenter intensity) may cause the same intensity to a certain area, but the destructive effects of structures with different dynamic characteristics are different. Generally speaking, earthquakes with large magnitude and farther epicenter distance damage long-term flexible structures are more severe than earthquakes of the same intensity but with smaller magnitude and closer epicenter distance. In order to distinguish the destructive effects of earthquakes of different magnitudes and epicenter distances on structures with different dynamic characteristics under the same intensity, under the same seismic fortification intensity, the earthquake impact of an area should be divided into different design earthquake groups according to the difference in magnitude and epicenter distance, which is divided into three groups.

6. Site category: The site soil is the medium of seismic wave propagation, which has a great impact on the seismic effect, and has amplification and filtering effects. For this reason, the bridge seismic code is the same as the "Building Seismic Design Code", which divides the site into four categories: I, II, III, and IV, and the basis for classification is determined by two factors: site soil type and cover thickness. (Foundation and foundation are often tested).

2. The main earthquake damage of bridges

1) Superstructure failure: falling beams (caused by tilting and misalignment of piers), collision (too small spacing between adjacent beam joints)

The earthquake knocked down the beams

2) Bridge pier and foundation failure: pier bending and shear failure, due to the excessive shear force and bending moment transmitted by the upper part of the foundation, causing pile foundation damage.

The Kobe section of the Hanshin Expressway was bent and the bridge collapsed

Wuxi Bridge seismic pier shear failure

3) Bearing failure: bearing toppling, falling off, displacement, bolt pull-out and shearing, and structural damage

Seismic bearing damage

3. The biggest differences between bridge seismic design and building structure are:

1. The design base period and service life of bridge structures are generally 100 years, while the building structure is 50 years, and the difference is reflected in the structural importance coefficient.

In order to unify the seismic fortification intensity of bridge seismic design, the seismic intensity with a probability of exceeding about 10% within 50 years, also known as the basic intensity or fortification intensity, that is, the medium earthquake, is not taken, and the seismic intensity with a probability of exceeding about 10% within 100 years is not taken, with the purpose of unifying with the current national standards. It is mainly reflected by the structure of different fortification categories and different importance coefficients.

For example, the fortification categories of highway bridges are divided into: A, B, C, D

The importance coefficient of bridge seismic resistance

Municipal bridge design categories are divided into: A, B, C, D

The importance coefficient of bridge seismic resistance

2. The seismic idea of two-level fortification (rare, rare) and two-stage design (strength design and ductility design) is mainly adopted, and is adopted according to the characteristics of the bridgeDuctile seismic designand the principle of ability protection, through the elastoplastic deformation of ductile members, extend the structural cycle and reduce the seismic reaction of the structure. Achieve the purpose of protecting the protection of components (bearings, foundations, pier columns) to achieve the purpose of shear resistance.

1) There are two main types of bridge seismic methods:

(1) Ductile seismic resistance is to extend the structural cycle and reduce the seismic response of the structure through the selection of suitable elastoplastic deformation and energy-consuming parts.

②Vibration isolation design, is to control the deformation and energy consumption of the structure by setting up seismic isolation bearings, dampers or limiting devices at the upper and lower connections of the bridge to protect the superstructure, piers and foundation from damage and maintain within the elastic range.

Under normal circumstances, for high piers, the stiffness of the pier columns is small, and the top and bottom of the piers are easy to form plastic hinges, which can be used for ductile seismic resistance, as shown in Figure 3. For bridges with short pier height and large pier stiffness, brittle shear failure is prone to brittle shear failure and it is difficult to form plastic hinges, so the seismic isolation design is adopted.

2) Bridge seismic fortification objectives and design methods

Bridge seismic design methods can be divided into the following 3 categories:

Category 1, Seismic analysis and seismic calculation under the action of E1 earthquake and E2 earthquake action shall be carried out, and the requirements of the seismic system of the bridge structure in Section 3.4 of this chapter shall be met, as well as the requirements of related structures and seismic measures.

Category 2, seismic analysis and seismic calculation under the action of E1 earthquake should be carried out, and the requirements of relevant structures and seismic measures should be met.

3 categories, which should meet the requirements of relevant structures and seismic measures, and can not be subjected to seismic analysis and seismic calculation.

The seismic design methods of Class B, C and D bridges are divided into three categories: A, B and C according to the basic seismic intensity of the bridge site and the seismic fortification of the bridge structure, and shall comply with the following provisions:

Class A: Seismic analysis and seismic calculation under the action of E1 and E2 earthquakes should be carried out, and the requirements of the bridge seismic system and related structures and seismic measures should be met in Section 3.4 of this chapter.

Class B: Seismic analysis and seismic calculation under the action of E1 earthquake should be carried out, and the requirements of relevant structures and seismic measures should be met; Erzhou Elementary School does not refer to the attached mountain

Class C: It should meet the requirements of relevant structures and seismic measures, and there is no need for seismic analysis and seismic calculation.

3) The main process of bridge seismic analysis and calculation

Take the "Highway Bridge Seismic Design Code" as an example topic to explain the seismic calculation

4) Analytical modeling principles and seismic analysis methods (there is a high possibility of future questions)

E1 Under the action of earthquakes, the structure is in the elastic working range and can be adoptedReaction spectroscopy methodFor regular bridges, since their dynamic response is mainly controlled by the first-order mode, the simplified single-mode response spectrum method can be used to calculate.

Under the action of E2 earthquake, although the bridge structure is allowed to enter the plastic working range, the elastic seismic displacement response of the structure can be modified by using the isodisplacement principle and the isoenergy principle in structural dynamics to represent the nonlinear seismic displacement response of the structure.

However, for complex structures such as multi-link long-span continuous beam bridges, only the nonlinear time history method can correctly predict the nonlinear seismic response of the structure.

5) The seismic effect of highway bridges should be considered according to the following principles.

1 Under normal circumstances, highway bridges can only consider horizontal seismic action, and straight bridges can consider the seismic action of parallel bridge direction X and horizontal bridge direction Y, respectively.

2 When one of the following conditions is met, both horizontal and vertical earthquakes should be considered.

1) Class A bridges.

2) Bridges in areas with seismic fortification intensity of IX degrees.

3) Bridges with seismic fortification intensity in VIII. area and significant seismic effect caused by vertical seismic action.

When analyzing the seismic response of straight bridges, the ground motion inputs along the forward and horizontal directions of the bridge can be considered. When analyzing the seismic response of curved bridges, the multi-directional ground motion input can be considered along the direction of the connecting line (secant) of the piers at both ends of the first link and perpendicular to the horizontal direction of the connection line to determine the most unfavorable seismic response.