Longitudinal damage diagnosis of beam bridge based on intersection of strain curves

Abstract

This invention relates to the field of beam bridge quality inspection technology, specifically disclosing a longitudinal damage diagnosis method for beam bridges based on the intersection of strain curves, including: testing the cross-sectional strain influence lines at different spans of the longitudinal beam N of the beam bridge under test; calculating the non-damage strain influence lines at different spans of the cross-section of the longitudinal beam N; and [further details about the method]. L / n Strain influence lines at the cross section and L / m Intersection of strain influence lines at the cross section N L / n ‑ L / m Intersection of strain influence lines at the test section N L / n ‑ L / m The longitudinal damage of beam bridges is identified by the change in the position of the intersection point of the strain influence line with the corresponding undamaged strain influence line. In the longitudinal damage diagnosis of beam bridges based on the intersection point of strain curves of the present invention, longitudinal damage of beam bridges is identified by the intersection point index of strain influence lines of strain sections at different spans of the beam bridge. It can quickly identify damage in the middle beam and the side beams, locate the damage position, and the identification method is simple, effective and highly accurate.

Description

Technical Field

[0001] This invention belongs to the field of beam bridge quality inspection and monitoring technology, and specifically relates to longitudinal damage diagnosis of beam bridges based on the intersection of strain curves. Background Technology

[0002] Transportation plays a vital role in economic development, cultural exchange, and national defense, and bridges are an important component of transportation infrastructure. In bridge engineering, beam bridges offer numerous advantages, including convenient construction, short construction time, simple structure, and low cost, leading to their widespread application and large proportion in my country's bridge engineering. During their service life, with the increasing duration of service, due to the combined long-term effects of various natural environmental factors, some bridges gradually suffer damage and aging, resulting in reduced strength, decreased load-bearing capacity, and reduced reliability. In severe cases, this can even endanger pedestrian and vehicular safety. Damage to precast bridges mainly manifests as damage to transverse diaphragms and longitudinal beams. Longitudinal damage to the beams, represented by concrete cracking, breakage, and carbonation, is a significant factor jeopardizing bridge safety. To ensure the full utilization of bridges in service, damage diagnosis is essential, enabling targeted reinforcement and repair to ensure their safety, suitability, and durability.

[0003] Currently, bridge structural inspection and damage assessment mainly include diagnostic methods based on visual inspection, bridge damage assessment methods based on dynamic information, and bridge damage assessment methods based on static information. Visual inspection-based methods primarily rely on the engineering experience and subjective judgment of inspectors to statistically analyze the bridge's visual defects. The analysis results are then combined with highway bridge technical condition assessment standards to evaluate the bridge's technical condition level. However, this method suffers from drawbacks such as cumbersome process, inconvenient operation, strong subjectivity, and low reliability of diagnostic results. The basic principle of bridge damage assessment methods based on dynamic information is that damage to the bridge structure causes changes in its dynamic characteristics. By deploying acceleration sensors on the bridge to measure its vibration signal data, and then processing the data to derive the bridge's dynamic characteristics, the bridge damage can be diagnosed by comparing the measured values ​​with the theoretical values ​​of the bridge in its undamaged state. This type of method has high requirements for the number of measuring points, sensor measurement accuracy, and modal parameter identification methods. Furthermore, information acquisition during implementation is susceptible to environmental interference, is time-consuming, and has poor economic efficiency. The basic principle of bridge damage evaluation methods based on static information is that bridge damage causes changes in the stiffness of the bridge structure, which in turn manifests as changes in its static information such as strain and deflection. This method diagnoses bridge damage by comparing measured static information values ​​with theoretical values ​​of the bridge in its undamaged state. This method has a relatively rich variety of algorithms and a simple data processing method, and is widely used in bridge inspection and damage diagnosis. In this method, appropriate static indicators are selected as diagnostic indicators, and changes in indicator values ​​reveal the location and extent of damage, thus improving the accuracy of bridge damage diagnosis. Chinese Patent Application No. 202211606901.6 discloses a rapid diagnostic method for longitudinal damage in beam bridges, which uses strain gauges for comparison and can comprehensively diagnose longitudinal damage in beam bridges. However, it requires fitting the strain curve and calculating related parameters such as strain gauges. The strain gauge calculation contains a significant amount of random error, and the area under the strain curve used in the interval can interfere with the identification of the loss location and damage amount. This invention provides a longitudinal damage diagnosis method for beam bridges based on the intersection of strain curves, which can easily and effectively identify the location and degree of longitudinal damage in beam bridges. Summary of the Invention

[0004] The purpose of this invention is to provide a longitudinal damage diagnosis method for beam bridges based on the intersection of strain curves, which can easily and effectively identify the location and degree of longitudinal damage in beam bridges by using the location information of the intersection of strain curves.

[0005] To achieve the above objectives, the present invention provides a longitudinal damage diagnosis method for beam bridges based on the intersection of strain curves, comprising the following steps:

[0006] (1) Test the cross-sectional strain influence lines of the longitudinal beam N of the beam bridge under test at different spans;

[0007] (2) Calculate the cross-sectional strain influence lines (theoretical strain influence lines) of the longitudinal beam N at different spans;

[0008] (3) Based on the strain influence lines from steps (1) and (2), obtain the intersection point N of the strain influence line at section L / n on longitudinal beam N and the strain influence line at section L / m. L / n-L / m The intersection point N of the strain influence lines of the comparative test section L / n-L / m Longitudinal damage in beam bridges can be identified by the change in the position of the intersection point with the corresponding undamaged strain influence line.

[0009] Preferably, in the above-mentioned longitudinal damage diagnosis of beam bridge based on the intersection of strain curves, in step (1), the different spans of the longitudinal beam N are the spans L / 4, L / 2 and 3L / 4 of the middle beam C, the side beam A and the side beam E.

[0010] Preferably, in the above-mentioned longitudinal damage diagnosis of beam bridges based on the intersection of strain curves, when the intersection of strain influence lines C... L / 4-L / 2 and C L / 4-3L / 4 Compared to the intersection of the theoretical strain influence lines under the undamaged state, the ordinate value decreases, indicating damage to the middle beam; when the intersection of the strain influence lines is C... L / 4-L / 2 and C L / 4-3L / 4 The vertical coordinate value increases compared to the intersection of the strain influence lines in the undamaged state, indicating damage to the side beam.

[0011] Preferably, in the above-mentioned longitudinal damage diagnosis of beam bridges based on the intersection of strain curves, when the middle beam and the side beam are damaged, the intersection of the strain influence lines of the damaged longitudinal beam section moves longitudinally towards the damage location compared to the intersection in the undamaged state, indicating the damage location.

[0012] Preferably, in the above-mentioned longitudinal damage diagnosis of beam bridge based on the intersection of strain curves, in step (2), the cross-sectional strain influence lines of the longitudinal beam N at different spans are calculated by the finite element method or by a simplified algorithm.

[0013] Preferably, in the above-mentioned longitudinal damage diagnosis of beam bridges based on the intersection of strain curves, the simplified algorithm calculates that when the path of the moving load is fixed on longitudinal beam i, the analytical expression of the strain influence line at a distance l from the beam end on longitudinal beam j is:

[0014]

[0015]

[0016] In the formula, ε(x) is the strain influence line of longitudinal beam j, L is the total span of the bridge, EI is the stiffness of the measuring point section, l is the distance of the measuring point section from the bridge end, and y sη is the distance from the lower edge of the section where the measuring point is located to the neutral axis; x is the distance between the location of the moving load F and the bridge end; η is the distance from the point where the moving load F is applied to the bridge end. ij Let be the transverse distribution coefficient of longitudinal beam j.

[0017] Preferably, in the above-mentioned longitudinal damage diagnosis of beam bridges based on the intersection of strain curves, the transverse distribution coefficient of the longitudinal beam is calculated as follows:

[0018]

[0019]

[0020] In the formula, n is the number of longitudinal beams, and K1 and K0 are influence coefficients.

[0021] Preferably, in the above-mentioned longitudinal damage diagnosis of beam bridge based on the intersection of strain curves, step (1) specifically involves: arranging multiple strain test points at different spans of the longitudinal beam N on the beam bridge to be tested, applying a moving load to the beam bridge by a test loading vehicle traveling at a uniform low speed along the bridge deck from the bridge end to the bridge tail, collecting the cross-sectional strain data of the strain test points, and obtaining the strain influence line of the test section.

[0022] Compared with existing technologies, the present invention has the following advantages:

[0023] 1. In the longitudinal damage diagnosis of beam bridges based on the intersection of strain curves of the present invention, longitudinal damage of beam bridges is identified by the intersection index of strain influence lines of strain sections at different spans of the beam bridge. It can quickly identify damage in the middle beam and the side beams, locate the damage location, and the identification method is simple, effective and highly accurate.

[0024] 2. In the longitudinal damage diagnosis of beam bridges based on the intersection of strain curves, compared with existing identification technologies, the invention uses the cross relationship of strain curves at different measuring points for damage localization and characterization. The intersection is two-dimensional information, and the data reflects rich information and physical meaning, which can reduce interference and improve the accuracy of identification. Attached Figure Description

[0025] Figure 1 This is a schematic diagram of the T-beam bridge structure in Application Example 1 of the present invention.

[0026] Figure 2 This is a schematic diagram of the damaged section of the longitudinal beam in Application Example 1 of the present invention.

[0027] Figure 3 The coordinates of the intersection points of the strain influence lines in damage conditions n1 to n4 in Example 1 of this invention are: (a) Intersection point A L / 4-L / 2 Location; (b) Intersection C L / 4-L / 2 Location; (c) Intersection A L / 4-L3 / 4 Location; (d) Intersection C L / 4-L3 / 4 Location.

[0028] Figure 4 The coordinates of the intersection points of the strain influence lines in damage conditions o1 to o4 in Example 1 of this invention are: (a) Intersection point A L / 4-L / 2 Location; (b) Intersection C L / 4-L / 2 Location; (c) Intersection A L / 4-L3 / 4 Location; (d) Intersection C L / 4-L3 / 4 Location.

[0029] Figure 5 The coordinates of the intersection points of the strain influence lines in damage conditions p1 to p4 in Example 1 of this invention are: (a) Intersection point A L / 4-L / 2 Location; (b) Intersection C L / 4-L / 2 Location; (c) Intersection A L / 4-L3 / 4 Location; (d) Intersection C L / 4-L3 / 4 Location.

[0030] Figure 6 Here are schematic diagrams of the model in application example 2 of the present invention (unit: mm): (a) elevation view; (b) cross-sectional view.

[0031] Figure 7 This is a physical image of the T-beam model in application example 2 of the present invention.

[0032] Figure 8 This is a schematic diagram of the longitudinal beam elevation crack in a bridge component in Application Example 2 of the present invention.

[0033] Figure 9 This is a schematic diagram of the strain gauge bonding position in application example 2 of the present invention.

[0034] Figure 10 This is a diagram of the positive load application path in application example 2 of the present invention.

[0035] Figure 11 This is a finite element model elevation view of the T-beam component in Application Example 2 of the present invention.

[0036] Figure 12 The location of the intersection of the strain influence lines of the positive load path section in application example 2 of this invention: (a) Intersection point C L / 4-L / 2 Location; (b) Intersection C L / 4-3L / 4 Location. Detailed Implementation

[0037] The specific embodiments of the present invention will be described in detail below, but it should be understood that the scope of protection of the present invention is not limited to the specific embodiments.

[0038] Example 1

[0039] A longitudinal damage diagnosis method for beam bridges based on the intersection of strain curves includes the following steps:

[0040] (1) Strain measuring points are arranged at the spans L / 4, L / 2 and L3 / 4 of the middle beam C and the side beam A. The test loading vehicle is loaded with normal load and moves at low speed along the bridge deck from the end of the bridge to the end of the bridge to apply a moving load to the beam bridge. The cross-sectional strain data of the strain measuring points are collected to obtain the strain influence line of the test section.

[0041] (2) Calculate the damage-free strain influence lines of the cross sections at spans L / 4, L / 2, and L3 / 4 of the middle beam C and the side beam A using the finite element method or a simplified algorithm (GM method (quasi-orthotropic plate method)). Specifically, when the path of the moving load is fixed on the i-th longitudinal beam, the analytical expression for the strain influence line of the cross section at a distance l from the beam end on the j-th longitudinal beam is:

[0042]

[0043]

[0044] In the formula, ε(x) is the strain influence line of longitudinal beam j, L is the total span of the bridge, EI is the stiffness of the measuring point section, l is the distance of the measuring point section from the bridge end, and y s η is the distance from the lower edge of the section where the measuring point is located to the neutral axis; x is the distance between the location of the moving load F and the bridge end; η is the distance from the point where the moving load F is applied to the bridge end. ij The transverse distribution coefficient of longitudinal beam j;

[0045] The transverse distribution coefficient of the longitudinal beam is calculated as follows:

[0046]

[0047]

[0048] In the formula, n is the number of longitudinal beams, and K1 and K0 are influence coefficients. The influence coefficients K1 and K0 can be found by interpolation in the calculation table of analogous orthotropic plates.

[0049] (3) Intersect the strain influence line at section L / n on longitudinal beam N with the strain influence line at section L / m at point N. L / n-L / m The form numbering, when the strain influence line intersection point C L / 4-L / 2 and C L / 4-3L / 4 Compared to the intersection of the strain influence lines in the undamaged state, the ordinate value decreases, indicating damage to the middle beam; when the intersection of the strain influence lines is C... L / 4-L / 2 and C L / 4-3L / 4 Compared to the intersection of the strain influence lines in the undamaged state, the ordinate value increases, indicating damage to the edge beam. When the middle and edge beams are damaged, the intersection of the strain influence lines of the damaged longitudinal beam section moves towards the damage location compared to the intersection in the undamaged state. The magnitude of the movement increases with the increase of damage. The location and extent of damage are determined based on the direction and magnitude of the movement of the strain influence line intersection. For example, when the middle beam is damaged, C... L / 4-L / 2 Move to the lower right, C L / 4-3L / 4Moving to the lower left will cause the damaged area to be closer to the mid-span.

[0050] Application Example 1

[0051] Taking a 40m span, 5-section T-beam bridge as an example, the main parameters of the T-beam bridge are as follows: span is 40m, bridge width is 12.25m, T-beam height is 2.5m, and the material parameters of the T-beam are: elastic modulus 35GPa. The structural schematic diagram is shown below. Figure 1 As shown. The longitudinal damage of the bridge was simulated by reducing the elastic modulus of the material; the damaged sections are shown in [the diagram / image / reference]. Figure 2 The damage conditions are shown in Table 1.

[0052] Table 1 Damage Condition Table

[0053]

[0054]

[0055] When the longitudinal beams of a T-beam bridge lose stiffness, the distribution coefficient η of the damaged area... ij This will change; in this region, the damaged longitudinal beam, due to its reduced stiffness, will bear a smaller load based on the load stiffness distribution principle, while the other undamaged beams will bear a larger load. Assume a simply supported beam bridge has a fixed-size single-point moving load F. s Moving along longitudinal beam i, the strain influence line at the section l from the beam end on longitudinal beam j shows that, after the bridge longitudinal beam loses a certain amount of stiffness, the following exists:

[0056]

[0057]

[0058] Here, the parameter ψ is less than 1 for undamaged longitudinal beams and greater than 1 for damaged longitudinal beams. Its specific calculation formula can be expressed according to the formula for calculating the ordinate of the lateral influence line of the rigid beam method load:

[0059]

[0060]

[0061] In the formula, e is the rotational eccentricity of the load from the cross-section, and α i Let η' be the distance from beam i to the center of rotation of the cross section. ij This is the lateral load distribution coefficient after the bridge stiffness is changed.

[0062] The strain influence line after damage is calculated using the above formula, and the intersection point is obtained under different damage conditions.

[0063] When the stiffness of region n of the beam decreases, the finite element method can also be used to calculate the intersection points of strain influence lines in damage conditions n1 to n4. Figure 3 .Depend on Figure 3 It can be seen that when the central beam is damaged, the intersection point C L / 4-L / 2 Location, intersection C L / 4-3L / 4 The position of the intersection point C is lower than that of the undamaged state. L / 4-L / 2 Location, intersection C L / 4-3L / 4 The positions all point to the lower left, indicating that the damage is located near the L / 4 span of the middle beam, and the range of movement increases as the damage increases.

[0064] The locations of the intersection points of the strain influence lines in damage conditions o1 to o4 are shown in the figure. Figure 4 .Depend on Figure 4 It can be seen that when the central beam is damaged, the intersection point C L / 4-L / 2 Location, intersection C L / 4-3L / 4 The position of the intersection point C is lower than that of the undamaged state. L / 4-L / 2 Move the position to the lower right, intersection point C L / 4-3L / 4 The position shifts to the lower left, indicating that the damage is located near the middle beam span L / 2, and the magnitude of the shift increases as the damage increases.

[0065] The locations of the intersection points of the strain influence lines in damage conditions p1 to p4 are shown in the figure. Figure 5 .Depend on Figure 5 It can be seen that when the edge beam is damaged, the intersection point C L / 4-L / 2 Location, intersection C L / 4-3L / 4 The ordinate of the intersection point is higher than that in the undamaged state, indicating that the intersection point A of the side beams (damaged longitudinal beams) is higher. L / 4-L / 2 Move the position to the upper right, intersection point A L / 4-3L / 4 The position shifts to the lower left, indicating that the damage is located near the span L / 2 of the side beam, and the magnitude of the shift increases as the damage increases.

[0066] Application Example 2

[0067] The prototype bridge used in this experiment was a 40-meter T-beam bridge template issued by the Ministry of Transport in 2009. A scaled-down test model was fabricated at a 20:1 scale, using PMMA acrylic material by a third party. The model consists of 5 longitudinal beams with a span of 2000mm, a bridge width of 600mm, a height of 120mm, a T-beam flange thickness of 18mm, a diaphragm thickness of 10mm, a web thickness of 18mm, and diaphragm spacing of 340mm×2+330mm×4. Elevation and cross-sectional views of the model are shown below. Figure 6 The scaled-down test model of the T-beam bridge can be seen in the physical specimen. Figure 7 .

[0068] A distributed dynamic signal acquisition and analysis system was adopted, which includes dynamic signal acquisition units, network cables, connecting instruments, a computer and a switch, and signal input lines. Two dynamic signal acquisition units were used, each with eight signal test channels. The acquisition units were connected to the switch via network cables, and the switch was connected to the computer via network cables. The dynamic strain range was 0–50000 με, and the resolution frequency was 0.1 με, fully meeting the experimental accuracy requirements. Before data acquisition, the strain values ​​at the acquisition points were zeroed through the system's self-check, and the strain acquisition frequency was set. This experiment used a range of 0–1000 με and employed a half-bridge method to connect the specimen and temperature compensation plate.

[0069] The vehicle model is a dual-axle, four-wheel counterweight model car, with iron blocks used to balance the weight to 20.38 kg. These iron blocks are symmetrically arranged on the vehicle chassis to ensure equal weight on all four wheels. The vehicle has a wheelbase of 200 mm and a track width of 80 mm. The vehicle is traction-based using a variable frequency motor, and the speed can be controlled by adjusting the motor speed via a frequency regulator.

[0070] 2. Experimental Procedure

[0071] This model test consisted of two parts, arranged in sequence: a test without longitudinal beam damage and a test with damage to the central beam. The damage test was conducted based on the model from the previous test. Because the elastic modulus of acrylic glass is quite sensitive to changes in temperature, the indoor temperature was adjusted to 20°C using air conditioning before each test, and maintained for 3 hours before the test began.

[0072] The specific damage conditions of the T-beam are as follows:

[0073] To fully represent the influence of cracks in bridge components on the strain line, this experiment simulated longitudinal damage to the bridge by cutting cracks in the longitudinal beams. Due to the small model scale and the high sensitivity of the testing instruments, the quasi-static load test was susceptible to errors caused by environmental factors. To minimize the impact of these errors and highlight the mechanical performance characteristics of the damaged bridge structure, larger cracks were introduced in the predetermined damage area. The damage condition in the middle beam was located 5cm to 25cm to the left of the mid-span point, with a crack height of 18mm and 10 cracks. The cracks were observed... Figure 8 Assuming that the area with cracks completely loses its bending resistance, the bending stiffness loss in the cracked area of ​​the model is set to 50%. Finite element simulation verification shows that after damage occurs, the area of ​​the strain influence line envelope recorded at measuring points near the damaged area changes by about 10%, highlighting the effect of damage on the influence line characteristics.

[0074] Strain gauges were placed at spans L / 4, L / 2, and 3L / 4 of the middle beam C and the side beam A. The strain gauges were uniformly attached to the bottom edge of the T-beam. Temperature compensation blocks, made of the same material and thickness as the bottom edge of the T-beam, were attached next to the strain sections. The locations of the strain gauges are shown in the attached diagram. Figure 9 .

[0075] Applying moving loads: Using positive load quasi-static load tests (see...) Figure 10 At the start of the test, the test vehicle must be placed at the front end of the acceleration ramp and its travel track strictly corrected. Then, the frequency regulator is set to 20 r / min, and the traction motor is started to make the vehicle model travel at a constant speed. After the rear axle of the test model vehicle enters the deceleration ramp, the motor is turned off, and the recording of dynamic strain values ​​is stopped. After waiting 10 minutes, the vehicle is reset to begin the next test. The test under the same working condition is repeated 15 times to ensure that relatively ideal data can be obtained.

[0076] (4) Analysis of the intersection point of strain influence lines:

[0077] The locations of the intersection points of the strain influence lines at measuring point C on the beam during the normal load test were statistically analyzed, and the results are shown in Table 2. Table 2 shows that the intersection point C of the strain influence lines on the beam in load condition 2 is... L / 4-L / 2 and C L / 4-3L / 4 The decrease in the ordinate value of the corresponding intersection point compared to the point without longitudinal beam damage indicates damage to the middle beam, which is consistent with the facts.

[0078] Table 2 Information on the location of the intersection of the influence lines

[0079]

[0080]

[0081] The simulation data in Table 2 were obtained using a finite element model of acrylic glass built with Midas Civil. The model facade is shown below. Figure 11 As shown. According to the data provided by the manufacturer, at room temperature of 20℃, the model material has an elastic modulus E = 2.93 GPa, a Poisson's ratio γ = 0.37, and a unit weight ρ = 11.9 kN / m³. 3 The dynamic load on the model vehicle was simulated by establishing multiple static load cases along the dynamic load path. Data was collected every 0.01 seconds the vehicle moved along the path, and a total of 225 static load cases were set. The model consisted of 3803 elements and 1408 nodes.

[0082] Compare the data in Table 2, where the intersection point C is... L / 4-L / 2 and C L / 4-3L / 4 The positions in operating conditions 1 and 2 are as follows: Figure 12 As shown in the figure, the intersection point C L / 4-L / 2 Due to damage to the central beam, it moved to the lower right, intersection point CL / 4-3L / 4 Moving to the lower left, the coordinate changes of the two intersection points are basically consistent in both the simulation and experimental results. Measured intersection point C L / 4-L / 2 The point shifted 0.014 to the right due to the damage, close to the simulated value of 0.02; the measured intersection point C... L / 4-3L / 4 The beam shifted 0.055 units to the left due to the damage, which is close to the simulated value of 0.06. All intersection points moved in the direction of the damage location, indicating that the damage location was the middle beam near the mid-span, which is consistent with the facts.

[0083] In summary, the direction of movement of the intersection point of the strain influence lines indicates the location of damage. This movement occurs through the intersection point C of the strain influence lines. L / 4-L / 2 and C L / 4-3L / 4 Compared to the intersection of the strain influence lines in the undamaged state, the ordinate value decreases, indicating damage to the middle beam; when the intersection of the strain influence lines C... L / 4-L / 2 and C L / 4-3L / 4 Compared to the intersection of the strain influence lines in the undamaged state, the ordinate value increases, indicating damage to the side beam. When the middle and side beams are damaged, the intersection of the strain influence lines of the damaged longitudinal beam section moves towards the damage location compared to the intersection in the undamaged state. The magnitude of the movement increases with the increase of damage. The location and magnitude of damage can be determined based on the direction and magnitude of the movement of the intersection of the strain influence lines, which has been verified in the case studies.

[0084] The foregoing description of specific exemplary embodiments of the invention is for illustrative and explanatory purposes. These descriptions are not intended to limit the invention to the precise forms disclosed, and it will be apparent that many changes and variations can be made in accordance with the foregoing teachings. The exemplary embodiments were chosen and described in order to explain the specific principles of the invention and its practical application, thereby enabling those skilled in the art to implement and utilize various different exemplary embodiments of the invention, as well as various different choices and variations. The scope of the invention is intended to be defined by the claims and their equivalents.

Claims

1. A method for longitudinal damage diagnosis of beam bridges based on the intersection of strain curves, characterized in that, Includes the following steps: (1) Test the cross-sectional strain influence lines at different spans of the longitudinal beam N of the beam bridge to be tested; the different spans of the longitudinal beam N are the middle beam C, the side beam A, and the side beam E at spans L / 4, L / 2, and 3L / 4; (2) Calculate the strain influence lines of the cross section without damage at different spans of the longitudinal beam N; (3) Based on the strain influence lines from steps (1) and (2), the longitudinal beam N is obtained. L / n Strain influence lines at the cross section and L / m Intersection of strain influence lines at the cross section N L / n - L / m Intersection of strain influence lines at the test section N L / n - L / m The longitudinal damage of the beam bridge is identified by the change in the position of the intersection point with the corresponding undamaged strain influence line; specifically, when the intersection point C of the strain influence line changes... L / 4-L / 2 and C L / 4-3L / 4 Compared to the intersection of the strain influence lines in the undamaged state, the ordinate value decreases, indicating damage to the middle beam; when the intersection of the strain influence lines is C... L / 4-L / 2 and C L / 4-3L / 4 The vertical coordinate value increases compared to the intersection of the strain influence lines in the undamaged state, indicating damage to the side beam.

2. The longitudinal damage diagnosis of beam bridges based on the intersection of strain curves according to claim 1, characterized in that, When the middle beam and the side beam are damaged, the intersection of the strain influence lines of the damaged longitudinal beam section moves towards the damage location compared to the intersection in the undamaged state, indicating the damage location.

3. The longitudinal damage diagnosis of beam bridges based on the intersection of strain curves according to claim 1, characterized in that, In step (2), the cross-sectional strain influence lines at different spans of the longitudinal beam N are calculated by the finite element method or by a simplified algorithm.

4. The longitudinal damage diagnosis of beam bridges based on the intersection of strain curves according to claim 3, characterized in that, Calculated using the simplified algorithm, when the path of the moving load is fixed on longitudinal beam i, the distance from the beam end on longitudinal beam j is... l The analytical expression for the strain influence line at the cross section is: ； In the formula, for j Strain influence line of longitudinal beam No. 1 L The total span of the bridge EI For the cross-sectional stiffness of the measuring point, l The distance from the measuring point section to the bridge end. y s This is the distance from the lower edge of the section where the measuring point is located to the neutral axis; x For moving loads F The distance between the point of action and the end of the bridge. for j Lateral distribution coefficient of longitudinal beam No.

5. The longitudinal damage diagnosis of beam bridges based on the intersection of strain curves according to claim 4, characterized in that, The transverse distribution coefficient of the longitudinal beam is calculated as follows: In the formula n This refers to the number of longitudinal beams. K 1 and K 0 This is the influence coefficient.

6. The longitudinal damage diagnosis of beam bridges based on the intersection of strain curves according to claim 1, characterized in that, The specific steps (1) are as follows: Multiple strain test points are arranged at different spans of the longitudinal beam N on the beam bridge to be tested. A test loading vehicle is used to apply a moving load to the beam bridge by moving at a constant low speed along the bridge deck from the end of the bridge to the end of the bridge. The cross-sectional strain data of the strain test points are collected to obtain the strain influence line of the test section.

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