Method for quickly determining optimal measurement frequency of existing bridge pile foundation

Abstract

The invention relates to the technical field of existing bridge pile foundation detection, in particular to a method for rapidly determining the optimal detection frequency of existing bridge pile foundations. The method comprises the following steps: selecting an excitation point on the existing pile foundation; obtaining the pile diameter of the existing pile foundation, the height of the upper structure and the distance between the excitation point and the upper structure; and based on the empirical relation among the pile diameter, the height of the upper structure, the distance between the excitation point and the upper structure, the excitation frequency and the three-dimensional effect coefficient, the optimal excitation frequency is determined according to the pile diameter, the height of the upper structure, the distance between the excitation point and the upper structure and the set three-dimensional effect coefficient. The problem that the detection result is inaccurate due to improper detection frequency when a conventional detection hammer is adopted for knocking according to experience in the prior art can be solved.

Description

Technical Field

[0001] The invention relates to the technical field of existing bridge pile foundation detection, and in particular to a method for quickly determining an optimal detection frequency of an existing bridge pile foundation detection. Background Art

[0002] The inspection of existing bridge pile foundations is crucial to the operation and maintenance of bridges. Currently, a large number of bridges have entered the maintenance period from the construction period. The performance of the foundations of the serving bridges, especially the pile foundations, whether they have defects, and whether the pile length is consistent with the design requirements directly affect the direct use of the bridges. In particular, some bridges are very old, and the construction units and operating units have changed. Many relevant materials are missing and need to be supplemented and re-inspected. The existing bridge pile foundation integrity inspection technology is urgently needed in actual projects.

[0003] The non-destructive testing of existing bridge pile foundations is currently a world-class technical problem. There are currently two main solutions: ① Improve the acquisition and signal analysis methods based on the original pile foundation testing technology, such as the pile side excitation method based on the low-strain reflection wave method, the dual-velocity method, and the detection technology based on the side wall R wave velocity; ② Detection technology based on new principles and new models, such as engineering CT technology such as sound waves or electromagnetic waves, side hole transmission method, load test method, borehole radar method, bridge pile testing technology (PST method), etc. Although domestic and foreign scholars have carried out relevant technical research, due to the principle limitations of various detection technologies themselves and the influence of complex working conditions, there are few successful cases in actual application. At present, there has been no systematic and in-depth research in China, and the detection of in-service pile foundations has become the development trend of the entire detection industry.

[0004] At present, the excitation method is more mainstream. The optimal excitation frequency for each diameter of pile is different. Usually, the inspection personnel ignore the optimal frequency and use conventional inspection hammers based on experience, which lacks scientificity. The selection of excitation frequency, especially the optimal excitation frequency for piles of different diameters, is directly related to the results of pile foundation defects and integrity inspection. Improper inspection frequency will lead to inaccurate inspection results. Summary of the invention

[0005] In view of the defects existing in the prior art, the purpose of the present invention is to provide a method for quickly determining the optimal testing frequency of existing bridge pile foundation inspections, which can solve the problem in the prior art that the use of conventional testing hammers based on experience will lead to inaccurate inspection results due to improper testing frequency.

[0006] In order to achieve the above purpose, the technical solution adopted by the present invention is:

[0007] The present invention provides a method for quickly determining the optimal frequency of pile foundation testing of an existing bridge, comprising the following steps:

[0008] Select the excitation point on the existing pile foundation;

[0009] Obtain the pile diameter, superstructure height, and distance between the excitation point and the superstructure of the existing pile foundation;

[0010] Based on the empirical relationship between pile diameter, superstructure height, distance between the excitation point and the superstructure, excitation frequency and three-dimensional effect coefficient, the optimal excitation frequency is determined according to the pile diameter, superstructure height, distance between the excitation point and the superstructure and the set three-dimensional effect coefficient.

[0011] In some optional schemes, based on numerical models with different parameters, the empirical relationship is obtained by fitting the three-dimensional effect coefficients after excitation at different excitation frequencies.

[0012] In some optional schemes, the numerical model with different parameters is used to fit the three-dimensional effect coefficient after excitation with different excitation frequencies to obtain an empirical relationship, including the following steps:

[0013] Establish numerical models for different pile diameters, superstructure heights, and distances between the excitation point and the superstructure;

[0014] For each numerical model, different excitation frequencies are used to excite the numerical model to obtain the corresponding three-dimensional effect coefficient;

[0015] The empirical relationship is obtained by fitting the three-dimensional effect coefficients corresponding to all pile diameters, superstructure heights, distances between the excitation point and the superstructure, and the excitation frequency.

[0016] In some optional schemes, for each numerical model, a first sensor receiving point and a second sensor receiving point are set below the excitation point, and a first sensor and a second sensor are set at the first sensor receiving point and the second sensor receiving point respectively, and the excitation point, the first sensor receiving point and the second sensor receiving point are located on the same vertical line.

[0017] In some optional solutions, an excitation window is opened at a set distance below the upper structure in the numerical model, and the lower surface of the excitation window is made flat.

[0018] In some optional schemes, the numerical model is excited at an excitation point, and the first sensor and the second sensor are used to receive the wave signal propagated after the excitation point is excited, and the three-dimensional effect coefficient is obtained based on the signals received by the first sensor and the second sensor.

[0019] In some optional solutions, the three-dimensional effect coefficient obtained according to the wave signals received by the first sensor and the second sensor includes:

[0020] collecting speed time information based on the wave signals received by the first sensor and the second sensor;

[0021] Based on the collected speed-time information, draw the speed-time curves of the two receiving points;

[0022] According to the velocity time history curve, the interference signal amplitude and the pile bottom signal amplitude are obtained;

[0023] The three-dimensional effect coefficient is determined based on the amplitude of the interference signal and the amplitude of the pile bottom signal.

[0024] In some alternative schemes, according to the formula Determine the three-dimensional effect coefficient σ, where V a is the interference signal amplitude, V b is the signal amplitude at the pile bottom.

[0025] In some optional schemes, the empirical relationship is:

[0026] σ=0.439X1+0.134X2-0.085X3+0.0004X4-0.251, where X1 is the pile diameter, X2 is the height of the superstructure, X3 is the distance between the excitation point and the superstructure, and X4 is the excitation frequency.

[0027] In some optional schemes, the three-dimensional effect coefficient is set to 0.8.

[0028] Compared with the prior art, the advantages of the present invention are: the technical method only needs to determine the pile diameter, the height of the superstructure, the position of the excitation point, and based on the empirical relationship between the pile diameter, the height of the superstructure, the distance between the excitation point and the superstructure, the excitation frequency and the three-dimensional effect coefficient, the three-dimensional effect coefficient can be set within a reasonable range to determine the optimal range of the excitation frequency. Conventional methods cannot determine the reasonable range of different pile diameters, and can only use different materials to strike the excitation hammer, and rely on the data rules and experience collected by the sensor to determine whether the frequency is appropriate. Obviously, the traditional method is relatively inefficient and has low accuracy. It is necessary to adopt this type of new method to determine the relevant parameters before detection, so as to facilitate rapid detection and increase the accuracy and precision of detection. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the drawings required for use in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work.

[0030] Figure 1 Flow chart of a method for quickly determining the optimal frequency of pile foundation testing of an existing bridge in an embodiment of the present invention;

[0031] Figure 2 is a schematic diagram of a numerical model in an embodiment of the present invention;

[0032] Figure 3 is a schematic diagram of an excitation window in a numerical model in an embodiment of the present invention;

[0033] Figure 4 A schematic diagram of the definition of a three-dimensional effect coefficient in an embodiment of the present invention;

[0034] Figure 5 Schematic diagram of the optimal value line of the three-dimensional effect coefficient in an embodiment of the present invention;

[0035] Figure 6 It is the velocity time history curve of hammering with different frequencies in the test of the embodiment of the present invention. DETAILED DESCRIPTION

[0036] In order to make the purpose, technical solution and advantages of the embodiments of the present application clearer, the technical solution in the embodiments of the present application will be clearly and completely described below in conjunction with the drawings in the embodiments of the present application. Obviously, the described embodiments are part of the embodiments of the present application, not all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of this application.

[0037] The embodiments of the present invention are further described in detail below with reference to the accompanying drawings.

[0038] like Figure 1 As shown, on the one hand, the present invention provides a method for quickly determining the optimal frequency of pile foundation testing of an existing bridge, characterized in that it includes the following steps:

[0039] S0: According to the numerical models with different parameters, the three-dimensional effect coefficients after excitation with different excitation frequencies are fitted to obtain the empirical relationship, including the following steps:

[0040] S01: Establish a numerical model with different pile diameters, superstructure heights and distances between the excitation point and the superstructure.

[0041] In this example, the numerical model uses three different diameters: 1m, 1.5m, 2m, and 2.5m; nine superstructure heights: 1m, 1.2m, 1.3m, 1.4m, 1.5m, 1.7m, 2m, 2.5m, and 3m; and nine distances from the excitation points to the superstructure: 0.5m, 0.6m, 0.7m, 0.8m, 0.9m, 1m, 1.2m, 1.5m, and 2m.

[0042] Preferably, for each numerical model, a first sensor receiving point and a second sensor receiving point are set below the excitation point, and a first sensor and a second sensor are set at the first sensor receiving point and the second sensor receiving point, respectively, and the excitation point, the first sensor receiving point and the second sensor receiving point are located on the same vertical line. Such a design can facilitate the setting of sensors at the first sensor receiving point and the second sensor receiving point to collect the excitation wave generated by the excitation point.

[0043] In addition, an excitation window is opened at a set distance below the upper structure in the numerical model, and the lower surface of the excitation window is made flat.

[0044] like Figure 3 As shown, in this example, an excitation window with a flat lower surface is used, and the excitation point is set on the lower surface of the excitation window. Such a design can ensure that the excitation wave is transmitted vertically downward.

[0045] like Figure 2 As shown, the excitation window is 2m away from the pile top, the first sensor receiving point is 3m away from the pile top, the second sensor receiving point is 4m away from the pile top, and the excitation window and the first and second sensor receiving points are kept in a vertical line. The superstructure height is 2m, and the excitation point is 2m away from the superstructure.

[0046] S02: For each numerical model, different excitation frequencies are used to excite the numerical model to obtain corresponding three-dimensional effect coefficients.

[0047] Now, we will take the case of 750Hz excitation frequency, 2m pile diameter, 2m pile height of superstructure, and 2m distance between the excitation point and the superstructure as an example. Velocity sensors are arranged at the first sensor receiving point and the second sensor receiving point to receive the signal of the wave propagated after the excitation point is excited.

[0048] In this example, the numerical model is excited at the excitation point, and the first sensor and the second sensor are used to receive the wave signal propagated after the excitation point is excited. The three-dimensional effect coefficient is obtained according to the signals received by the first sensor and the second sensor. Specifically, it includes:

[0049] Speed ​​time information is collected based on the wave signals received by the first sensor and the second sensor.

[0050] Based on the collected speed-time information, draw the speed-time curves of the two receiving points. The speed-time curve signals received by the two sensors are as follows: Figure 4 shown.

[0051] According to the velocity time history curve, the interference signal amplitude and the pile bottom signal amplitude are obtained. The first interference wave amplitude with a larger amplitude after the excitation wave is defined as the interference signal amplitude V a , the pile bottom signal amplitude is defined as Vb , similarly, see Figure 4 .

[0052] The three-dimensional effect coefficient is determined based on the interference signal amplitude and the pile bottom signal amplitude.

[0053] In this example, according to the formula Determine the three-dimensional effect coefficient σ, where V a is the interference signal amplitude, V b is the signal amplitude at the pile bottom.

[0054] The three-dimensional effect coefficient is about 0.8 when the pile diameter is 1m and the excitation frequency is 500Hz. Therefore, it is determined that the more reasonable value of the three-dimensional effect at a higher excitation frequency is σ=0.8.

[0055] For all numerical models respectively: the diameters are 1m, 1.5m, 2m, and 2.5m; the nine superstructure heights are 1m, 1.2m, 1.3m, 1.4m, 1.5m, 1.7m, 2m, 2.5m, and 3m; the distances between the excitation point and the superstructure are 0.5m, 0.6m, 0.7m, 0.8m, 0.9m, 1m, 1.2m, 1.5m, and 2m, and three excitation frequencies of 500Hz, 750Hz, and 1000Hz are simulated, and the corresponding three-dimensional effect coefficient can be obtained according to the above data acquisition and processing method.

[0056] S03: The empirical relationship is obtained by fitting the three-dimensional effect coefficients corresponding to all pile diameters, superstructure heights, distances between the excitation point and the superstructure, and the excitation frequency.

[0057] The three-dimensional effect coefficients corresponding to all pile diameters, superstructure heights, distances from the excitation point to the superstructure and excitation frequencies were fitted, and the empirical relationship obtained was σ=0.439X1+0.134X2-0.085X3+0.0004X4-0.251, among which X1 is the pile diameter, X2 is the superstructure height, X3 is the distance from the excitation point to the superstructure, and X4 is the excitation frequency.

[0058] like Figure 5 As shown in the figure, the part where the ratio of the interference signal to the pile bottom signal is greater than 1, that is, the area where the three-dimensional effect coefficient is greater than 1 is called the interference area; when the interference signal is smaller than the amplitude of the pile bottom signal, the pile bottom can be identified and distinguished, and this area is defined as the identifiable area.

[0059] According to the existing data and the pile bottom identification, the excitation frequency needs to be in a higher range. When the three-dimensional effect coefficient is greater than 1, the interference signal amplitude is greater than the pile bottom signal amplitude, and the upper limit of the three-dimensional effect coefficient is 1; considering the influence of other factors, the three-dimensional effect coefficient is set to 0.8.

[0060] S1: Select the excitation point on the existing pile foundation.

[0061] When selecting the excitation point on the existing pile foundation, it is selected according to the actual environment on site, and it is ensured that the excitation point is consistent with the situation of numerical simulation and set on a plane to ensure that the excitation wave is transmitted vertically downward.

[0062] S2: Obtain the pile diameter, superstructure height, and distance between the excitation point and the superstructure of the existing pile foundation.

[0063] In this scheme, the three-dimensional effect coefficients corresponding to all pile diameters, superstructure heights, distances from the excitation point to the superstructure and excitation frequencies were fitted, and the empirical relationship between the three-dimensional effect coefficients corresponding to the pile diameters, superstructure heights, distances from the excitation point to the superstructure and excitation frequencies was obtained. The three-dimensional effect coefficient was set to 0.8, and the pile diameters, superstructure heights, and distances from the excitation point to the superstructure were obtained on site. Then the only unknown variable left was the excitation frequency.

[0064] S3: Based on the empirical relationship between pile diameter, superstructure height, distance between the excitation point and the superstructure, excitation frequency and three-dimensional effect coefficient, the optimal excitation frequency is determined according to the pile diameter, superstructure height, distance between the excitation point and the superstructure and the set three-dimensional effect coefficient.

[0065] The empirical relationship is: σ=0.439X1+0.134X2-0.085X3+0.0004X4-0.251. The pile diameter, superstructure height, and distance between the excitation point and the superstructure have been obtained. By substituting them into the empirical relationship formula, the optimal excitation frequency can be obtained.

[0066] In order to verify the accuracy of the above scheme, the following verification tests were carried out:

[0067] At a highway bridge in Qingyuan City, Guangdong Province, the case implementation steps are as follows:

[0068] 1. A pile with a radius of 1m and a superstructure height of 2m was selected on site. The optimal frequency determination calculation method was adopted. The three-dimensional effect coefficient was taken as 0.8 according to the numerical simulation empirical formula. The optimal excitation frequency calculated according to the empirical relationship was 1128Hz. It was determined that the excitation effect of this pile at a frequency of about 1000Hz was better.

[0069] 2. Install sensors at intervals of 1m on the same vertical line on the same side of the pile foundation, with sensor 1 on the top and sensor 2 on the bottom. Open an opening platform 10cm long and 10cm wide 0.5m above sensor 1, which is 5cm into the pile body, and grind the platform flat with an angle grinder. Also grind the place where the sensor is installed flat. After the sensors are installed, ensure that they are in good contact with the pile body.

[0070] 3. Use an exciting hammer in the exciting window to ensure that the hammer head is perpendicular to the exciting window, the exciting wave propagates downward, and sensors 1 and 2 collect the exciting signal.

[0071] 4. First, a nylon hammer of 1000Hz, which is close to the optimal frequency according to theoretical calculation, is used to hammer the excitation window, and the sensor obtains the speed time curve. Since the highway is not closed during the collection process, driving has an impact on the excitation signal. Try to perform the excitation hammer when the vehicle is driving and the bridge vibration is small.

[0072] 5. Use a conventional dual-speed steel hammer (frequency of about 2000 Hz) and a rubber hammer (frequency of about 500 Hz) to repeat the above process and collect the velocity time history curve.

[0073] 6. According to the field test results, the curve is as follows: Figure 6 As shown in the figure. The first curve is the velocity time-history curve collected by the sensor after the rubber hammer (frequency of about 500Hz) hits. It can be seen that the waveforms of some sensors 1 and 2 in the range of 0-6m are quite different. This type of waveform is not very effective for extracting the upgoing wave. The second curve is the velocity time-history curve collected by the sensor after the nylon hammer (frequency of about 1000Hz) hits. It can be seen that the signal rules of sensors 1 and 2 are basically the same, and the signal strength near the bottom of the pile is still relatively high. This type of curve is used to extract the upgoing wave to judge the pile length signal. The third curve is the velocity time-history curve collected by the sensor after the common steel hammer (frequency of about 2000Hz) hits. It can be seen that the waveforms of the curves of sensors 1 and 2 are very different, and the curves are very flat after the wave reaches the bottom of the pile, close to the axis with a value of 0, indicating that the excitation signal has been attenuated when it reaches the bottom of the pile, and the sensor cannot collect effective signals. Finally, the nylon hammer with a frequency of 1000Hz close to the optimal frequency calculation method has the best effect, indicating that this calculation method has both practical value on site.

[0074] In summary, the velocity time history curves obtained from the tests of piles of different types and superstructures are used to define the three-dimensional effect coefficient based on the velocity time history curves. The optimal frequency calculation formula is fitted according to the three-dimensional effect coefficient, different pile diameters, different superstructure heights, different distances between the excitation point and the superstructure, and different excitation frequencies. According to the frequency fitting empirical relationship formula, the optimal frequency for the detection of existing bridge pile foundations can be determined after determining the pile diameter, superstructure height, excitation point position, and three-dimensional effect coefficient.

[0075] Compared with the prior art, the advantages of the present invention are: in this technical solution, it is only necessary to determine the pile diameter, the height of the upper structure, and the position of the excitation point, and to obtain the set three-dimensional effect coefficient within a reasonable range according to the defined three-dimensional effect coefficient, and to bring it into the empirical relationship formula to determine the optimal range of the excitation frequency. Conventional methods cannot determine the reasonable range of different pile diameters, and can only use different materials to strike the excitation hammer, and rely on the data rules and experience collected by the sensor to determine whether the frequency is appropriate. Obviously, the traditional method is relatively inefficient and has low accuracy. It is necessary to adopt this type of new method to determine the relevant parameters before detection, so as to facilitate rapid detection and increase the accuracy and precision of detection.

[0076] In the description of the present application, it should be noted that the terms "upper", "lower", etc. indicate orientations or positional relationships based on the orientations or positional relationships shown in the accompanying drawings, and are only for the convenience of describing the present application and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as a limitation on the present application. Unless otherwise clearly specified and limited, the terms "installed", "connected", and "connected" should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection, or it can be an indirect connection through an intermediate medium, or it can be a connection between the two elements. For those of ordinary skill in the art, the specific meanings of the above terms in the present application can be understood according to the specific circumstances.

[0077] It should be noted that, in this application, relational terms such as "first" and "second" are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Moreover, the terms "include", "comprise" or any other variants thereof are intended to cover non-exclusive inclusion, so that a process, method, article or device including a series of elements includes not only those elements, but also other elements not explicitly listed, or also includes elements inherent to such process, method, article or device. In the absence of further restrictions, the elements defined by the sentence "comprise a ..." do not exclude the presence of other identical elements in the process, method, article or device including the elements.

[0078] The above description is only a specific implementation of the present application, so that those skilled in the art can understand or implement the present application. Various modifications to these embodiments will be apparent to those skilled in the art, and the general principles defined herein can be implemented in other embodiments without departing from the spirit or scope of the present application. Therefore, the present application will not be limited to the embodiments shown herein, but will conform to the widest range consistent with the principles and novel features applied for herein.

Claims

1. A method for quickly determining the optimal frequency of pile foundation testing of an existing bridge, characterized in that: The following steps are involved: Select the excitation point on the existing pile foundation; Obtain the pile diameter, superstructure height, and distance between the excitation point and the superstructure of the existing pile foundation; Based on the empirical relationship between pile diameter, superstructure height, distance between the excitation point and the superstructure, excitation frequency and three-dimensional effect coefficient, the optimal excitation frequency is determined according to the pile diameter, superstructure height, distance between the excitation point and the superstructure and the set three-dimensional effect coefficient.

2. The method for rapidly determining the optimal frequency of pile foundation testing of an existing bridge as claimed in claim 1, characterized in that: According to the numerical models with different parameters, the empirical relationship is obtained by fitting the three-dimensional effect coefficient after excitation at different excitation frequencies.

3. The method for quickly determining the optimal frequency of pile foundation testing of an existing bridge as claimed in claim 2, characterized in that: The numerical model with different parameters is used to fit the three-dimensional effect coefficient after excitation with different excitation frequencies to obtain an empirical relationship, which includes the following steps: Establish numerical models for different pile diameters, superstructure heights, and distances between the excitation point and the superstructure; For each numerical model, different excitation frequencies are used to excite the numerical model to obtain the corresponding three-dimensional effect coefficient; The empirical relationship is obtained by fitting the three-dimensional effect coefficients corresponding to all pile diameters, superstructure heights, distances between the excitation point and the superstructure, and the excitation frequency.

4. The method for quickly determining the optimal frequency of pile foundation testing of an existing bridge as claimed in claim 3, characterized in that: For each numerical model, a first sensor receiving point and a second sensor receiving point are set below the excitation point, and a first sensor and a second sensor are set at the first sensor receiving point and the second sensor receiving point respectively, and the excitation point, the first sensor receiving point and the second sensor receiving point are located on the same vertical line.

5. The method for quickly determining the optimal frequency of pile foundation testing of an existing bridge as claimed in claim 3, characterized in that: In the numerical model, an excitation window is opened at a set distance below the upper structure, and the lower surface of the excitation window is made flat.

6. The method for quickly determining the optimal frequency of pile foundation testing of an existing bridge as claimed in claim 4, characterized in that: The numerical model is excited at an excitation point, and a first sensor and a second sensor are used to receive wave signals propagated from the excitation point. The three-dimensional effect coefficient is obtained according to the signals received by the first sensor and the second sensor.

7. The method for quickly determining the optimal frequency of pile foundation testing of an existing bridge as claimed in claim 6, characterized in that: The three-dimensional effect coefficient obtained according to the wave signals received by the first sensor and the second sensor includes: collecting speed time information based on the wave signals received by the first sensor and the second sensor; Based on the collected speed-time information, draw the speed-time curves of the two receiving points; According to the velocity time history curve, the interference signal amplitude and the pile bottom signal amplitude are obtained; The three-dimensional effect coefficient is determined based on the amplitude of the interference signal and the amplitude of the pile bottom signal.

8. The method for quickly determining the optimal frequency of pile foundation testing of an existing bridge as claimed in claim 7, characterized in that: According to the formula Determine the three-dimensional effect coefficient σ, where V a is the interference signal amplitude, V b is the signal amplitude at the pile bottom.

9. The method for quickly determining the optimal frequency of pile foundation testing of an existing bridge as claimed in claim 1, characterized in that: The empirical relationship is: σ=0.439X1+0.134X2-0.085X3+0.0004X4-0.251, where X1 is the pile diameter, X2 is the height of the superstructure, X3 is the distance between the excitation point and the superstructure, and X4 is the excitation frequency.

10. The method for quickly determining the optimal frequency of pile foundation testing of an existing bridge as claimed in claim 1, characterized in that: The three-dimensional effect coefficient is set to 0.8.

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