Steel plate girder bridge web gap beach grain distortion fatigue test loading device

Abstract

A steel plate girder bridge web gap beach grain distortion fatigue test loading device is characterized in that a hydraulic actuator is connected with a loading disc, the loading disc is connected with the lower portion of a loading cross beam, two sets of connecting pieces are arranged on the upper portion of the loading cross beam in a bilateral symmetry mode, each set of connecting pieces is connected with the lower portion of a test piece, and the upper portion of the test piece is connected with a fatigue testing machine chuck; according to the device, cyclic load is applied to the web vertical stiffening rib through the loading cross beam and the connecting assembly by the servo hydraulic actuator, so that the device can accurately simulate the actual acting force of a cross brace in a steel plate girder bridge on the web stiffening rib, and a multi-parameter test of fatigue details of a steel bridge is realized; in addition, visual observation and quantitative analysis of microcosmic fatigue crack initiation and expansion behaviors can be achieved with the help of a beach grain method, obtained data are reliable, and an important basis can be provided for web gap anti-fatigue design and research of the fatigue crack expansion process.

Description

Technical Field

[0001] The invention belongs to the technical field of bridge engineering tests, and in particular relates to a loading device for a steel plate girder bridge web gap beach grain distortion fatigue test. Background Art

[0002] Due to the combined effects of numerous factors, including manufacturing quality, service environment, and long-term heavy traffic loads, fatigue issues in steel bridges are prominent. The hidden nature of fatigue issues and the sudden nature of failure pose a serious threat to public life and property safety. Web gap fatigue details are a typical sensitive detail of distortional fatigue in steel plate girder bridges. In the design of steel plate girder bridges, to ensure structural integrity, the two main beams are typically connected by vertical stiffeners welded to the webs of the beams via cross braces. To prevent fatigue failure in the welds between the vertical stiffeners and the tension flanges, the vertical stiffeners in this area are typically disconnected, creating a gap between the ends of the vertical stiffeners and the flanges. This gap is called the web gap. Under vehicle loads, multi-girder steel plate girder bridges experience frequent deflection differences between adjacent beams. Furthermore, the cross braces between the two beams transfer lateral forces to the vertical stiffeners and, in turn, to the webs. Under the constraints of the bridge deck and girder flanges, the web gap experiences out-of-plane bending deformation, resulting in large fatigue stress amplitudes at the weld toe between the web and vertical stiffener, which in turn promotes the initiation and growth of fatigue cracks. Because traditional fatigue testing cannot directly observe the initiation and growth of microscopic fatigue cracks, the beach pattern method is used to indirectly reveal the microscopic fatigue crack initiation and growth patterns.

[0003] The beach grain method uses a well-designed fatigue loading sequence to create visible fatigue arcs on the fatigue cross-section. Because the resulting fatigue arcs resemble beach grains, it is called the beach grain method. The formation of beach grains can be explained by the fact that when the load amplitude of cyclic loading changes, the stress magnitude and stress state at the fatigue crack front change, causing changes in the crack growth rate and direction, resulting in dark plastic deformation marks on the fracture surface.

[0004] The fatigue behavior and stress distribution in the web gap details are complex. To explore the fatigue damage mechanism and verify the effectiveness of repair and reinforcement methods, a systematic study of this critical structural detail through fatigue testing is required to improve the durability and safety of bridge structures.

[0005] Fatigue testing of the gaps in the webs of steel plate girder bridges is a key means of studying their damage mechanisms and verifying reinforcement methods, and can provide a scientific basis for repairing fatigue cracks in real bridges. This test helps to reveal structural weaknesses and guide the optimization of reinforcement schemes in engineering practice. The loading devices and methods of traditional fatigue tests of the gaps in the webs of steel plate girder bridges have obvious limitations, which are mainly reflected in the following aspects: (1) Existing tests usually use large-sized steel beam segments as test specimens, which are not only complex to make and time-consuming to install, but also expensive. This test method is difficult to achieve multi-parameter variable research, which limits the systematic exploration of fatigue damage mechanisms. In addition, large-scale test specimens have high requirements for the test site, and large-tonnage loading equipment is required to simulate the stress state of the real bridge, which further increases the test cost and makes it difficult to carry out fatigue tests on a large scale. (2) The current research on the fatigue performance of the gap details of the vertical stiffener webs of steel bridges is still insufficient, especially the lack of test data on multiple parameters (such as gap size, load amplitude, etc.). Most existing loading devices cannot accurately simulate the actual distortion and stress characteristics of this detail, resulting in deviations between the test results and the actual bridge working conditions. (3) There are still obvious deficiencies in the research on the initiation and propagation mechanism of micro-fatigue cracks in the web gap of steel plate beam bridges. In particular, the existing test device has significant limitations when using the beach ripple method to observe the micro-fatigue crack propagation process, making it difficult to effectively track and visualize the initiation and propagation process of micro-fatigue cracks in the web gap area. Therefore, in order to overcome the above problems, it is urgent to develop a new fatigue test device. The device should have the following characteristics: (1) adopt a miniaturized and modular specimen design to reduce the manufacturing and installation costs; (2) optimize the loading method to accurately simulate the distortion behavior of the web gap; (3) support multi-parameter variable efficient testing, and provide a reliable basis for fatigue mechanism research and reinforcement scheme optimization. (4) The beach ripple method can be used to achieve visual observation and quantitative analysis of the micro-fatigue crack propagation behavior. The development of such a device will significantly improve the test efficiency and promote the in-depth development of fatigue performance research of steel bridges. Summary of the Invention

[0006] The technical problem to be solved by this invention is to overcome the shortcomings of the aforementioned prior art and provide a loading device for testing beach grain distortion fatigue in web gaps of steel plate girder bridges. This device can realistically simulate the forces exerted by cross braces on vertical stiffeners under vehicle loads, ensuring that the stress and deformation states of the web gaps closely align with actual bridge operating conditions. Furthermore, the device can utilize beach grain technology to visualize the initiation and propagation behavior of microscopic fatigue cracks. Compared with traditional methods, this test device offers advantages such as high test accuracy and reliability, while significantly reducing the requirements for test sites and loading equipment. Its modular design allows for easy operation and quick assembly and disassembly, effectively reducing testing costs and providing an efficient and reliable testing method for in-depth research on web gap distortion fatigue performance using the beach grain method.

[0007] The technical solution adopted to solve the above technical problems is: a loading device for the beach grain distortion fatigue test of the web gap of a steel plate beam bridge, the hydraulic actuator is connected to the loading disc, the loading disc is connected to the lower part of the loading beam, and two groups of connecting parts are symmetrically arranged on the upper part of the loading beam, each group of connecting parts is connected to the lower part of the specimen, and the upper part of the specimen is connected to the chuck of the fatigue testing machine.

[0008] The connecting piece of the present invention comprises two channel steels, which are symmetrically arranged front and back on the upper part of the loading beam, with a filling plate arranged in the middle and lower parts and a test piece arranged in the middle and upper parts of the two channel steels.

[0009] The channel steel of the present invention is: a U-shaped channel steel body is processed with a first threaded hole on the upper edge and a fourth threaded hole on the lower edge; a channel steel body bottom plate is processed with a second threaded hole on the upper portion and a third threaded hole on the lower portion.

[0010] The test specimen of the present invention is: webs are horizontally symmetrically arranged on both sides of the wing plate, two groups of stiffening ribs are vertically arranged in the middle of the web, the two groups of stiffening ribs are symmetrically arranged about the wing plate, the stiffening ribs are divided into two parts and are symmetrically arranged about the upper and lower parts of the web, stiffening rib threaded holes are processed on the lower part of the stiffening ribs, the stiffening rib threaded holes correspond to the second threaded holes, and web threaded holes are processed on the web, the web threaded holes correspond to the first threaded holes.

[0011] The thickness of the stiffening rib of the present invention is consistent with the thickness of the filling plate.

[0012] The reinforcing rib of the present invention is perpendicular to the wing plate, and the distance between the end of the reinforcing rib close to the wing plate and the wing plate is 40 to 80 mm.

[0013] The cross-section of the loading beam body of the present invention is rectangular. A beam threaded hole is processed on the upper portion of the loading beam body. The beam threaded hole corresponds to the fourth threaded hole.

[0014] The loading disc of the present invention has a mounting hole processed at the center of the disc body, the hydraulic actuator is installed in the mounting hole, and fixing holes are symmetrically arranged on the upper part of the disc body, which correspond to the threaded holes on the lower part of the loading beam.

[0015] Because the present invention uses friction-type high-strength bolts to fix the specimen, and applies cyclic loads to the vertical stiffeners of the web through a servo-hydraulic actuator via a loading beam and a connecting assembly, the device can accurately simulate the actual force of the cross brace on the web stiffener in a steel plate beam bridge, realize multi-parameter testing of fatigue details of the steel bridge, and can use the beach grain method to achieve visual observation and quantitative analysis of the microscopic fatigue crack initiation and propagation behavior. The obtained data is reliable and can provide an important basis for the fatigue resistance design of the web gap and the study of the fatigue crack propagation process. At the same time, the modular design of the connecting assembly and specimen of the device has the following advantages: 1. Compact structure, which greatly reduces the specimen size and saves test space; 2. Simplified structure, which reduces the loading tonnage requirement; 3. The specimen can be quickly replaced, which is convenient for multi-parameter testing; 4. Strong applicability. The loading device can perform not only vertical fatigue loading but also transverse fatigue loading and can be applied to various types of fatigue testing machines. This economical and efficient testing scheme provides a new technical means for the fatigue performance research of steel bridges and has broad engineering application prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 It is a structural schematic diagram of the present invention.

[0017] Figure 2 yes Figure 1 Schematic diagram of the structure of the loading disk 2.

[0018] Figure 3 yes Figure 1 Schematic diagram of the structure of the loading beam 3.

[0019] Figure 4 yes Figure 1 Schematic diagram of the structure of the connected components.

[0020] Figure 5 yes Figure 1 Schematic diagram of the structure of the middle channel steel 4.

[0021] Figure 6 yes Figure 1 Schematic diagram of the structure of specimen 7.

[0022] In the figure: 1. Servo hydraulic actuator; 2. Loading disc; 3. Loading beam; 4. Channel steel; 5. Filler plate; 6. Chuck; 7. Test piece; 2-1. Mounting hole; 2-2. Fixing hole; 2-3. Disc body; 3-1. Loading beam body; 3-2. Beam threaded hole; 4-1. First threaded hole; 4-2. Second threaded hole; 4-3. Channel steel body; 4-4. Third threaded hole; 4-5. Fourth threaded hole; 7-1. Stiffener; 7-2. Flange; 7-3. Web; 7-4. Stiffener threaded hole; 7-5. Web threaded hole. DETAILED DESCRIPTION

[0023] The present invention will be further described in detail below with reference to the accompanying drawings and examples, but the present invention is not limited to these examples.

[0024] Example 1 Figures 1 to 6 In the present invention, a loading device for a steel plate girder bridge web gap beach grain distortion fatigue test is described. A hydraulic actuator 1 is connected to a loading disc 2. In this embodiment, the loading disc 2 comprises a disc body 2-3 with an outer diameter of 146 mm, an inner diameter of 28 mm, and a thickness of 35 mm. A mounting hole 2-1 is machined at the center of the disc body 2-3. The hydraulic actuator 1 is mounted within the mounting hole 2-1. Fixing holes 2-2 are symmetrically arranged on the upper portion of the disc body 2-3. The fixing holes 2-2 have a diameter of 10 mm and are 64 mm from the center of the fixing holes 2-2 to the center of the mounting hole 2-1. The fixing holes 2-2 correspond to threaded holes on the lower portion of the loading beam 3. High-strength bolts are passed through the fixing holes 2-2 and the threaded holes to secure the loading disc 2 to the bottom of the loading beam 3. The loading beam 3 comprises a loading beam body 3-1, which has a rectangular cross-section with a length of 585 mm, a width of 200 mm, and a height of 85 mm. In this embodiment, the loading beam 3 is designed as a solid rectangular structure, which reduces processing errors and makes the upper surface of the loading beam 3 smoother. In addition, the solid rectangular structure can ensure that the loading beam has sufficient rigidity, making the deformation of the specimen 7 closer to the actual project. Two groups of connecting parts are symmetrically arranged on the upper part of the loading beam 3, and each group of connecting parts consists of two channel steels 4 and a filling plate 5. The two channel steels 4 are symmetrically arranged front and back on the loading beam body 3-1. The filling plate 5 is located in the lower middle part of the two channel steels 4, and the test piece 7 is located in the upper middle part of the two channel steels 4. Specifically, the channel steel 4 is composed of a U-shaped channel steel body 4-3. The upper edge of the channel steel body 4-3 is processed with two first threaded holes 4-1, and the lower edge is processed with four fourth threaded holes 4-5. The upper part of the bottom plate of the channel steel body 4-3 is processed with four second threaded holes 4-2, and the lower part is processed with two third threaded holes 4-4. The bottom plate of the channel steel body 4-3 is 220 mm long, 180 mm wide, and 15 mm thick. The upper and lower edges are 180 mm long, 82 mm wide, and 15 mm thick. The filler plates 5 are fastened between the symmetrically arranged channel steels 4 by means of friction-type high-strength bolts installed in the third threaded holes 4-4. The filler plates 5 are rectangular plates with a length of 180 mm, a width of 110 mm, and a thickness of 6 mm. The filler plates 5 are used to control the spacing between the two relative channel steels 4, making the force transmission path more reasonable and ensuring that the friction-type high-strength bolts can provide reliable preload. The thickness of the filler plates 5 is consistent with the thickness of the stiffening ribs 7-1. Sixteen beam threaded holes 3-2 are machined on the upper part of the loading beam body 3-1. The beam threaded holes 3-2 correspond to the fourth threaded holes 4-5. The loading beam 3 is fixedly connected to the lower edge of the channel steel 4 by self-drilling and self-tapping screws provided in the beam threaded holes 3-2 and the fourth threaded holes 4-5.

[0025] The specimen 7 of this embodiment is composed of stiffening ribs 7-1, flanges 7-2, and webs 7-3. Webs 7-3 are horizontally symmetrically welded on both sides of the flanges 7-2. Two groups of stiffening ribs 7-1 are vertically welded in the middle of the webs 7-3. The two groups of stiffening ribs 7-1 are symmetrically arranged about the flanges 7-2. The stiffening ribs 7-1 are divided into two parts and are welded symmetrically about the webs 7-3 up and down. The stiffening ribs 7-1 are perpendicular to the flanges 7-2, and the distance between the end of the stiffening rib 7-1 close to the flange 7-2 and the flange 7-2 is 60 mm. The lower portion of the stiffener 7-1 is machined with four threaded holes 7-4, corresponding to the second threaded holes 4-2. Friction-type high-strength bolts installed in these holes secure the lower stiffener 7-1 to the two channel steels 4. Two web threaded holes 7-5 are machined in the web 7-3, corresponding to the first threaded holes 4-1. Friction-type high-strength bolts installed in these holes secure the web 7-5 to the upper edge of the channel steel 4. The upper end of the flange 7-2 is connected to the chuck 6 of the fatigue testing machine. The friction-type high-strength bolts are all M-20 friction-type high-strength bolts. The flange 7-2 of specimen 7 is 408 mm long, 200 mm wide and 25 mm thick. The webs 7-3 on both sides are of the same size, 280 mm long, 200 mm wide and 8 mm thick. The stiffening ribs 7-1 on both sides are of the same size, 220 mm long and 6 mm thick, and the width of the upper and lower parts is 100 mm.

[0026] Example 2

[0027] In the above-mentioned Example 1, the cross-sectional shape of the loading beam body 3-1 of this embodiment is rectangular, with a length of 585 mm, a width of 200 mm, and a height of 85 mm. The bottom plate of the channel steel body 4-3 is 220 mm long, 180 mm wide, and 15 mm thick, while the upper and lower edges are 180 mm long, 82 mm wide, and 15 mm thick. The filler plate 5 is a rectangular plate with a length of 180 mm, a width of 110 mm, and a thickness of 6 mm. The flange 7-2 of the specimen 7 is 408 mm long, 200 mm wide, and 25 mm thick. The webs 7-3 on both sides are of the same size, with a length of 280 mm, a width of 200 mm, and a thickness of 8 mm. The stiffening ribs 7-1 on both sides are of the same size, with a length of 240 mm, a thickness of 6 mm, and a width of 100 mm for both the upper and lower parts.

[0028] The distance between the end of the stiffening rib 7-1 close to the wing plate 7-2 and the wing plate 7-2 is 40 mm, and the remaining components and their connection relationships are exactly the same as those in Example 1.

[0029] Example 3

[0030] In the above-mentioned Example 1, the cross-sectional shape of the loading beam body 3-1 of this embodiment is rectangular, with a length of 585 mm, a width of 200 mm, and a height of 85 mm. The bottom plate of the channel steel body 4-3 is 220 mm long, 180 mm wide, and 15 mm thick, while the upper and lower edges are 180 mm long, 82 mm wide, and 15 mm thick. The filler plate 5 is a rectangular plate with a length of 180 mm, a width of 110 mm, and a thickness of 6 mm. The flange 7-2 of the specimen 7 is 408 mm long, 200 mm wide, and 25 mm thick. The webs 7-3 on both sides are of the same size, with a length of 280 mm, a width of 200 mm, and a thickness of 8 mm. The stiffening ribs 7-1 on both sides are of the same size, with a length of 200 mm, a thickness of 6 mm, and a width of 100 mm on both sides.

[0031] The distance between the end of the stiffening rib 7-1 close to the wing plate 7-2 and the wing plate 7-2 is 80 mm, and the remaining components and their connection relationships are exactly the same as those in Example 1.

[0032] Example 4

[0033] In the above-mentioned Example 1, the cross-sectional shape of the loading beam body 3-1 of this embodiment is rectangular, with a length of 585 mm, a width of 200 mm, and a height of 85 mm. The bottom plate of the channel steel body 4-3 is 220 mm long, 120 mm wide, and 15 mm thick, while the upper and lower edges are 120 mm long, 82 mm wide, and 15 mm thick. The filler plate 5 is a rectangular plate with a length of 120 mm, a width of 110 mm, and a thickness of 6 mm. The flange 7-2 of the specimen 7 is 408 mm long, 200 mm wide, and 25 mm thick. The webs 7-3 on both sides are of the same size, with a length of 280 mm, a width of 200 mm, and a thickness of 8 mm. The stiffening ribs 7-1 on both sides are of the same size, with a length of 220 mm, a thickness of 6 mm, and a width of 100 mm on both sides.

[0034] The distance between the end of the stiffening rib 7-1 close to the wing plate 7-2 and the wing plate 7-2 is 60 mm, and the connection relationship of the other components is exactly the same as that of Example 1.

[0035] In the above four embodiments, the distances between the end of the stiffening rib 7-1 close to the flange 7-2 and the flange 7-2 are different, that is, the web gap sizes are different. The test results show that the distortion fatigue strength of the web gap details decreases significantly with the decrease of the web gap size, and the fatigue life of the details is reduced.

[0036] The working process of the present invention is as follows:

[0037] During installation, the center of specimen 7's flange 7-2, the center of the fatigue testing machine's chuck 6, the center of the loading beam 3, the center of the loading disc 2, and the center of the servo-hydraulic actuator 1 were aligned. Before the test began, a static load test was performed to determine the loading sequence. After the external hydraulic pump supplied oil to the fatigue testing machine's chuck 6, the chuck 6 began operating, clamping the flange 7-2 of specimen 7. During operation, the servo-hydraulic actuator 1 transferred the load to the loading beam 3 via the loading disc 2, and then applied two concentrated cyclic loads to specimen 7 through the connecting assembly. During the test, strain gauges were attached to fatigue measurement points in the web gap between the vertical stiffener 7-1 and the flange 7-2 and web 7-3 of specimen 7 to monitor stress changes in real time. Stress data was transmitted via wires to a dynamic tester, where stress fluctuations were recorded every 30 minutes. The initiation and growth of fatigue cracks in this area were also observed and recorded. After each 100,000 and 50,000 dynamic load cycles, a static test must be performed. Compare with the results of the previous static test to see if there are any major changes. This monitoring method can accurately capture the evolution of fatigue damage in key areas. As the number of cycles continues to increase, fatigue damage at the web gap continues to accumulate, and when the fatigue cracks reach a certain level, the test ends. Analyze the fatigue fracture morphology, observe the characteristics of the upper beach lines on the fatigue section, and accurately measure the spacing between adjacent fatigue arcs. After completing a specimen 7, loosen the fatigue testing machine chuck 6, remove the friction-type high-strength bolts connecting the channel steel 4 to the specimen 7, and then replace the specimen 7 to conduct the next parameter specimen 7 test.

[0038] The loading method of the present invention:

[0039] In order to leave visible fatigue arcs, namely "beach lines", on the fatigue fracture, the beach mark fatigue load is a variable amplitude load sequence in which the reference load and the mark load are applied alternately. Before the fatigue test begins, the loaded specimen 7 is statically loaded, and the load sequence for fatigue loading is determined based on the stress results analysis of the web gap area. Usually, the maximum loads of the reference load and the mark load are the same, and their minimum loads can be determined based on stress ratios of 0.1 and 0.5, respectively. The number of cycles of the reference load is greater than or equal to the number of cycles of the mark load. During loading, after the external hydraulic oil pump supplies oil to the chuck 6, the chuck 6 starts working and can clamp the wing plate of the loaded specimen 7. When the servo hydraulic actuator 1 is working, the load is transferred to the loading beam 3 through the loading disc 2, and then the cyclic load is provided to the loaded specimen 7 through the connecting assembly.

Claims

1. A loading device for a steel plate girder bridge web gap beach grain distortion fatigue test, characterized by: The hydraulic actuator (1) is connected to the loading disc (2), the loading disc (2) is connected to the lower part of the loading beam (3), the upper part of the loading beam (3) is symmetrically provided with two groups of connecting parts, each group of connecting parts is connected to the lower part of the test piece (7), and the upper part of the test piece (7) is connected to the chuck (6) of the fatigue testing machine.

2. The loading device for the steel plate girder bridge web gap beach grain distortion fatigue test according to claim 1, characterized in that: The connecting piece comprises two channel steels (4), which are symmetrically arranged front and back on the upper part of the loading beam (3), and a filling plate (5) is arranged in the lower part of the two channel steels (4) and a test piece (7) is arranged in the upper part.

3. The loading device for the steel plate girder bridge web gap beach grain distortion fatigue test according to claim 2 is characterized in that The channel steel (4) is as follows: a U-shaped channel steel body (4-3) is processed with a first threaded hole (4-1) on its upper edge and a fourth threaded hole (4-5) on its lower edge; a second threaded hole (4-2) is processed on the upper portion of the bottom plate of the channel steel body (4-3) and a third threaded hole (4-4) is processed on the lower portion.

4. The loading device for the steel plate girder bridge web gap beach grain distortion fatigue test according to claim 3 is characterized in that The test piece (7) is as follows: webs (7-3) are horizontally symmetrically arranged on both sides of the wing plate (7-2); two groups of stiffening ribs (7-1) are vertically arranged in the middle of the web plate (7-3); the two groups of stiffening ribs (7-1) are symmetrically arranged about the wing plate (7-2); the stiffening ribs (7-1) are divided into two parts and are symmetrically arranged about the web plate (7-3) in the upper and lower parts; a stiffening rib threaded hole (7-4) is processed on the lower part of the stiffening rib (7-1); the stiffening rib threaded hole (7-4) corresponds to the second threaded hole (4-2); a web threaded hole (7-5) is processed on the web plate (7-3); the web threaded hole (7-5) corresponds to the first threaded hole (4-1).

5. The loading device for the steel plate girder bridge web gap beach grain distortion fatigue test according to claim 4, characterized in that: The thickness of the reinforcing rib (7-1) is consistent with the thickness of the filling plate (5).

6. The loading device for the steel plate girder bridge web gap beach grain distortion fatigue test according to claim 4, characterized in that: The reinforcing rib (7-1) is perpendicular to the wing plate (7-2), and the distance between the end of the reinforcing rib (7-1) close to the wing plate (7-2) and the wing plate (7-2) is 40 to 80 mm.

7. The loading device for the steel plate girder bridge web gap beach grain distortion fatigue test according to claim 2, characterized in that: The cross-section of the loading beam body (3-1) is rectangular. A beam threaded hole (3-2) is processed on the upper portion of the loading beam body (3-1). The beam threaded hole (3-2) corresponds to the fourth threaded hole (4-5).

8. The loading device for the steel plate girder bridge web gap beach grain distortion fatigue test according to claim 1 is characterized in that The loading disc (2) is as follows: a mounting hole (2-1) is machined at the center of the disc body (2-3); the hydraulic actuator (1) is mounted in the mounting hole (2-1); and fixing holes (2-2) are symmetrically arranged on the upper part of the disc body (2-3); the fixing holes (2-2) correspond to the threaded holes on the lower part of the loading beam (3).

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