A double-layer steel truss beam shearing lag analysis test method

Abstract

The application discloses a kind of double-layer steel truss shearing lag analysis test methods, test device includes four-column type loading hydraulic press, top plate distribution beam, jack, bottom plate distribution beam and rigid spring;Top plate distribution beam is horizontally laid and is fixedly connected with the lower end of loading hydraulic press piston rod in middle part, and top plate distribution beam is pressed on the top plate of double-layer steel truss when using, and top plate distribution beam is used to exert pressure load for double-layer steel truss top plate;Bottom plate distribution beam is horizontally laid in the bottom of top plate distribution beam, and rigid spring includes multiple and is connected between the left and right ends of top plate distribution beam and bottom plate distribution beam;When using, bottom plate distribution beam is arranged between the top plate and bottom plate of double-layer steel truss and presses down double-layer steel truss bottom plate, and bottom plate distribution beam is used to exert pressure load for double-layer steel truss bottom plate;Jack includes multiple and is arranged between top plate distribution beam and bottom plate distribution beam.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of civil engineering mechanics test, and particularly relates to a double-layer steel truss shear lag analysis test method. BACKGROUND

[0002] The double-layer steel truss bridge is widely used in the fields of urban traffic, expressway and railway due to its obvious advantages. The double-layer steel truss bridge fully utilizes the vertical space of the bridge, improves the traffic capacity and passing efficiency, has excellent mechanical properties and stability, and can bear larger load and deformation. The overall structure is affected by the randomness and asymmetry of the upper and lower loads, and the steel truss will be accompanied by bending, shear lag, torsion and distortion effect, and the spatial stress state is particularly prominent. The model test is the most convenient means for researching the overall spatial effect of the structure, and the current loading device for the double-layer steel truss bridge only acts on the upper bridge deck (top plate), and cannot realize the simultaneous loading of the upper and lower bridge decks of the double-layer steel truss bridge. Figure 3 Simultaneous loading diagram of upper and lower bridge decks of double-layer steel truss). SUMMARY

[0003] In order to solve the problem that the current double-layer steel truss top and bottom plate bridge decks cannot be simultaneously loaded, the application provides a double-layer steel truss shear lag analysis test method, which provides technical support for the actual load test research of the structure.

[0004] Therefore, the application adopts the following technical scheme:

[0005] A double-layer steel truss shear lag analysis test method, comprising a test device for loading the top and bottom plates of the double-layer steel truss, the test device comprising a four-column loading hydraulic machine, a top plate distribution beam, a jack, a bottom plate distribution beam and a rigid spring;

[0006] The top plate distribution beam is horizontally arranged and the middle part thereof is fixedly connected with the lower end of the piston rod of the loading hydraulic machine, and in use, the top plate distribution beam is pressed on the top plate of the double-layer steel truss, and the top plate distribution beam is used for applying pressure load to the top plate of the double-layer steel truss;

[0007] The bottom plate distribution beam is horizontally arranged below the top plate distribution beam, the rigid spring comprises a plurality of springs and is connected between the left and right ends of the top plate distribution beam and the bottom plate distribution beam; in use, the bottom plate distribution beam is arranged between the top and bottom plates of the double-layer steel truss and presses the bottom plate of the double-layer steel truss, and the bottom plate distribution beam is used for applying pressure load to the bottom plate of the double-layer steel truss;

[0008] The jack comprises a plurality of jacks and is arranged between the top plate distribution beam and the bottom plate distribution beam;

[0009] The top plate distribution beam bottom surface is provided with two spaced upper loading blocks, and the bottom plate distribution beam bottom surface is provided with two spaced lower loading blocks, the distance between the two upper loading blocks is adjustable, and the distance between the two lower loading blocks is adjustable;

[0010] The test method comprises the following steps:

[0011] Step 1: design loading conditions

[0012] ① Load condition 1

[0013] Symmetrically adjust the positions of the upper loading blocks and the lower loading blocks, the upper loading blocks are to the intersection of the top plate of the double-layer steel truss girder and the web, the lower loading blocks are to the intersection of the bottom plate of the double-layer steel truss girder and the web, and are fixed by bolts; the hydraulic machine loading load is set to 2 P , the maximum pressure of the jack is P / 2, the hydraulic machine is started, and the jacks on the left and right sides are controlled to be tested synchronously, and the loading of the top plate and the lower layer of the double-layer steel truss girder is conducted through mechanics conduction, as shown in load condition 10;

[0014] ② Load condition 2

[0015] Symmetrically adjust the positions of the upper loading blocks and the lower loading blocks, the upper loading blocks are to the 1 / 4 and 3 / 4 positions of the top plate of the double-layer steel truss girder, the lower loading blocks are to the 1 / 4 and 3 / 4 positions of the bottom plate of the double-layer steel truss girder, and are fixed by bolts; the hydraulic machine loading load is set to 2 P , the maximum pressure of the jack is P / 2, the hydraulic machine is started, and the jacks on the left and right sides are controlled to be tested synchronously, and the loading of the top plate and the lower layer of the double-layer steel truss girder is conducted through mechanics conduction, as shown in load condition 10;

[0016] ③ Load condition 3

[0017] Symmetrically adjust the positions of the upper loading blocks and the lower loading blocks, the upper loading blocks are to the center of the top plate of the double-layer steel truss girder, the lower loading blocks are to the center of the bottom plate of the double-layer steel truss girder, and are fixed by bolts; the hydraulic machine loading load is set to 2 P , the maximum pressure of the jack is P / 2, the hydraulic machine is started, and the jacks on the left and right sides are controlled to be tested synchronously, and the loading of the top plate and the lower layer of the double-layer steel truss girder is conducted through mechanics conduction, as shown in load condition 10;

[0018] ④ Load condition 4

[0019] Symmetrically adjust the positions of the upper loading blocks and the lower loading blocks, the upper loading blocks are to the 1 / 4 and 3 / 4 positions of the top plate of the double-layer steel truss girder, the lower loading blocks are to the center of the bottom plate of the double-layer steel truss girder, and are fixed by bolts; the hydraulic machine loading load is set to 2 P , the maximum pressure of the jack is P / 2, start the hydraulic machine, and control the jacks on the left and right sides to test synchronously, and the loading of the double-layer steel truss roof and the lower layer is shown in the working condition 10 through mechanical transmission;

[0020] 5. Load working condition 5

[0021] Symmetrically adjust the positions of the upper loading block and the lower loading block, the upper loading block is to the center of the double-layer steel truss roof, the lower loading block is to 1 / 4 and 3 / 4 of the double-layer steel truss bottom plate, and is fixed by bolts; set the load of the hydraulic machine to 2 P , the maximum pressure of the jack is P / 2, start the hydraulic machine, and control the jacks on the left and right sides to test synchronously, and the loading of the double-layer steel truss roof and the lower layer is shown in the working condition 10 through mechanical transmission;

[0022] 6. Load working condition 6

[0023] Symmetrically adjust the position of the upper loading block, the upper loading block is to the intersection of the double-layer steel truss roof and the web, and is fixed by bolts; set the load of the hydraulic machine to 2 P , the maximum pressure of the jack is P / 2, start the hydraulic machine, and control the jacks on the left and right sides to test synchronously, and the loading of the double-layer steel truss roof and the lower layer is shown in the working condition 10 through mechanical transmission;

[0024] Step 2: Paste strain gauges

[0025] Paste longitudinal strain gauges on the double-layer steel truss roof and the double-layer steel truss bottom plate in the middle section, and the strain gauge is symmetrically pasted in the middle, as shown in Figure 11 ;

[0026] Step 3: Calculate the shear lag longitudinal stress

[0027] When calculating the shear lag longitudinal stress of the double-layer steel truss roof and the double-layer steel truss bottom plate, the longitudinal stress is obtained according to the longitudinal strain of the measuring point obtained by the test:

[0028]

[0029] Wherein is the shear lag longitudinal stress of the measuring point; E s is the elastic modulus of the steel material; is the longitudinal strain of the measuring point.

[0030] Further, the roof distribution beam and the bottom plate distribution beam are I-beam structures.

[0031] Further, the top surface and the bottom surface of the jack are respectively lined with pressure plates.

[0032] The beneficial effects of the present application are that:

[0033] 1. The hydraulic machine vertical load is applied to the double-layer steel truss bottom plate by setting the jack between the top plate distribution beam and the bottom plate distribution beam. Two rigid infinite distribution beams (the loading position of which can be adjusted) are placed on the top plate and the bottom plate of the double-layer steel truss, the hydraulic machine applies load to the top plate distribution beam, the jack is started and the reading is controlled, and the hydraulic machine vertical load can be transmitted to the bottom plate of the double-layer steel truss.

[0034] 2. The top plate distribution beam and the bottom plate distribution beam are connected by rigid springs, and the use of rigid spring connection bottom plate distribution beam reduces the self-weight influence of the bottom plate distribution beam during loading, and the structure is easy to disassemble, which provides convenience for the assembly of the device.

[0035] 3. Six different loading conditions are designed, and the loading size and loading position are controlled, and the position and load size of the loading can be adjusted according to the experimental requirements or the structure of the material, so that the double-layer steel truss shear lag longitudinal stress under any symmetrical load can be obtained. BRIEF DESCRIPTION OF DRAWINGS

[0036] Figure 1 It is the structure front view of the test device of the application;

[0037] Figure 2 It is the structure side view of the test device of the application;

[0038] Figure 3 It is a top and bottom plate loading schematic diagram of a double-layer steel truss;

[0039] Figure 4 It is a whole loading stress schematic diagram of a double-layer steel truss;

[0040] Figure 5 It is a top plate distribution beam stress schematic diagram;

[0041] Figure 6 It is a bottom plate distribution beam stress schematic diagram;

[0042] Figure 7 It is a double-layer steel truss stress schematic diagram;

[0043] Figure 8 It is a jack working stress schematic diagram;

[0044] Figure 9 It is the flow chart of the test method of the application;

[0045] Figure 10 It is the load condition diagram of the application;

[0046] Figure 11 It is a strain gauge layout schematic diagram;

[0047] Figure 12is a schematic diagram of a cross section of a double-layer steel truss girder according to an embodiment of the present application;

[0048] Figure 13 is a finite element model and deformation diagram of six working conditions according to an embodiment of the present application;

[0049] Figure 14 is a cross section shear stress and longitudinal normal stress curve diagram of a midspan upper wing plate according to an embodiment of the present application;

[0050] Figure 15 is a cross section shear stress and longitudinal normal stress curve diagram of a midspan lower wing plate according to an embodiment of the present application;

[0051] In the figure: 1-filter, 2-liquid-filled oil tank, 3-main oil cylinder, 4-first round nut, 5-cross beam, 6-flange plate, 7-piston rod, 8-pressure sensor, 9-first pressing plate, 10-second round nut, 11-stand column, 12-top plate distribution beam, 13-upper loading block, 14-second pressing plate, 15-piston top plate, 16-piston top rod, 17-oil cylinder, 18-rigid spring, 19-lower loading block, 20-bottom plate distribution beam, 21-oil pipe, 22-shunt collector valve, 23-pressure gauge, 24-motor, 25-pressure control valve, 26-oil pump. DETAILED DESCRIPTION

[0052] The present application will be further described below in conjunction with the accompanying drawings and specific embodiments, as shown in the drawings and specific embodiments, the present application comprises the following components: Figure 1 and 2 The present application comprises the following components:

[0053] Filter 1: can effectively remove impurities and pollutants in the oil, reduce the wear of particles on the valve body, pump, hydraulic cylinder and other key components, reduce the failure rate due to impurities blocking the valve.

[0054] Liquid-filled oil tank 2: store hydraulic oil, cool oil, precipitate impurities and provide bubble separation to reduce cavitation in the hydraulic system, ensure stable operation of the system.

[0055] Main oil cylinder 3: provides a power source for driving the hydraulic machine through the action of hydraulic oil, the piston rod 7 of the main oil cylinder 3 produces linear motion under the action of pressure oil to provide corresponding pressure source.

[0056] First round nut 4: the first round nut bears part of the gravity and other forces of the slider during movement, and transmits these forces to the stand column of the hydraulic machine.

[0057] Cross beam 5: the pressure generated by the hydraulic cylinder is transmitted to the cross beam 5 through the slider, and the cross beam 5 uniformly distributes the pressure to the four stand columns, ensuring the stability and uniform stress of the entire hydraulic machine structure.

[0058] Flange plate 6: During the operation of the hydraulic machine, the flange plate 6 can transmit the torque generated by the hydraulic cylinder to the slider or other working parts, ensuring the effective transmission and distribution of force, and the flange plate 6 is made of high-strength material, which can withstand the high pressure generated during the operation of the hydraulic machine, ensuring the stability and safety of the connected parts.

[0059] Piston rod 7: used to convert the pressure provided by the hydraulic oil into mechanical energy and provide guiding transmission to the lower pressure plate. During the operation of the hydraulic machine, not only the force needs to be transmitted, but also the weight of the slider and other parts needs to be supported to maintain the stability of the hydraulic machine.

[0060] Pressure sensor 8: used to monitor the pressure value in the hydraulic system, ensure that the hydraulic machine works within the set pressure range, prevent system overload, protect the key parts of the hydraulic machine from damage and record the pressure data during the operation of the hydraulic machine.

[0061] First pressure plate 9: used to connect the top plate distribution beam 12 and evenly distribute the pressure of the hydraulic machine to the top plate distribution beam 12.

[0062] First round nut 4: the first round nut 4 bears part of the gravity and other forces of the slider during movement, and transmits these forces to the column of the hydraulic machine.

[0063] Column 11: provides a solid support frame for the cross beam and workbench of the hydraulic machine, ensuring the stability of the entire machine. During the operation of the hydraulic machine, the column 11 bears the huge pressure generated by the hydraulic cylinder.

[0064] Top plate distribution beam 12: evenly distributes the pressure generated by the hydraulic machine to the top plate of the double-layer steel truss, and uses I-beam for distribution, so that the upper loading block can change the loading position when the steel truss is loaded horizontally.

[0065] Upper loading block 13: evenly applies the pressure on the top plate distribution beam 12 to the top plate of the double-layer steel truss, and the loading position can be changed by adjustment.

[0066] Second pressure plate 14: adjusts the distance between the split hydraulic machine and the top plate, and evenly applies the pressure on the top plate, which is used to adjust the pressure on the top plate.

[0067] Piston top plate 15: increases the contact area of the jack and the distribution beam, so that the pressure generated by the jack can be evenly applied to the top plate distribution beam.

[0068] Piston top rod 16: converts the pressure provided in the oil cylinder into mechanical energy and moves upward, playing a role in force transmission and movement during the operation of the jack.

[0069] Oil cylinder 17: The oil cylinder is the part of the jack that contains the hydraulic oil and generates the thrust. The piston moves within the oil cylinder, thereby causing the extension or retraction of the ram.

[0070] Rigid spring 18: Fix the bottom plate distribution beam 20, adjust the bottom plate distribution beam 20, reduce the self-weight influence of the bottom plate distribution beam 20.

[0071] Lower load block 19: Apply pressure on the bottom plate distribution beam 20 evenly to the bottom plate of the double-layer steel truss, and change the loading position by adjusting.

[0072] Bottom plate distribution beam 20: Distribute the pressure generated by the hydraulic machine evenly to the bottom plate of the double-layer steel truss, and use I-beams for distribution, so that the load block can change the loading position when loading the steel truss transversely.

[0073] Oil pipe 21: Used to connect the oil pump and oil cylinder of the jack, to ensure that the hydraulic oil can flow under the pressure generated by the oil pump, thereby transmitting the pressure to the piston in the oil cylinder.

[0074] Flow distribution and collection valve 22: Divides a single oil flow into multiple branches, controls the flow of each branch, and ensures that each cylinder or actuator obtains appropriate operating pressure and speed.

[0075] Pressure gauge 23: Used to detect the pressure generated by the jack to prevent excessive pressure from damaging the components.

[0076] Electric motor 24: Provides energy source for the oil pump of the jack.

[0077] Pressure control valve 25: Adjusts the maximum pressure generated by the jack.

[0078] Oil pump 26: The oil pump is the power source of the hydraulic system, which converts mechanical energy into hydraulic energy to provide pressure and flow for the entire hydraulic system, so that the hydraulic oil can circulate in the system and transmit energy.

[0079] The design principle of the invention is as follows:

[0080] (1) Based on the existing extension and compression type four-column hydraulic machine, add the bottom plate distribution beam.

[0081] Use the existing four-column loading hydraulic machine, first fix the top plate distribution beam 12 on the pressure plate of the loading machine, then pass the bottom plate distribution beam 20 through the web and press it on the corresponding position of the bottom plate of the double-layer steel truss. To reduce the influence of the self-weight of the bottom plate distribution beam 20 on loading, rigid springs 18 are added between the top plate distribution beam 12 and the bottom plate distribution beam 20.

[0082] (2) Use separate jacks to apply vertical load to the top and bottom plate distribution beams.

[0083] Two jacks are placed on the top and bottom plate distribution beams respectively, and the load of the double-layer steel truss bottom plate is applied by changing the readings of the two symmetrical jacks.

[0084] (3) Consider the pressure control of the split jack

[0085] The two jacks use the same pump source, and a hydraulic pump with synchronization control function is used. Before the test, the pressure control valve is adjusted to make the jack can provide the required pressure.

[0086] The stress principle of the present application is as follows:

[0087] As Figures 3 to 8 shown in the loading stress analysis diagram of the double-layer steel truss, the top plate distribution beam 12 is fixed on the loading device, and the bottom plate distribution beam 20 is connected with the top plate through the rigid spring 18. The specific stress process is as follows:

[0088] (1) The hydraulic machine and the jack start working at the same time. When the top plate distribution beam 12 is downward, the downward force of the top plate distribution beam 12 will be reduced due to the existence of the jack, and the reduced force will be distributed to the double-layer steel truss bottom plate.

[0089] (2) The jack parameters are set so that the pressure applied is just the size of the pressure required by the double-layer steel truss top plate. Assuming that the overall force required to be loaded is P, the jack parameters are adjusted so that the maximum pressure is P / 6. During the overall device working process, the left and right jacks will generate a pressure of P / 6. At this time, the jack is supported by the bottom plate distribution beam 20, and a mutual force of P / 6 will be generated between the jack and the double-layer steel truss top plate. Similarly, at this time, the force generated on the top plate distribution beam 12 due to the action of the jack is 2P / 3. At this time, after analyzing the stress of the top plate distribution beam 12, it is found that the force acting on the double-layer steel truss top plate is 2P / 3.

[0090] (3) As Figure 8 shown, the stress of the jack is analyzed. The bottom plate distribution beam 20 will generate a support force of P / 6. According to Newton's third law, it is found that the force acting on the double-layer steel truss bottom plate by the bottom plate distribution beam is P / 3. Thus, the distribution of the force loaded on the double-layer steel truss top plate and the double-layer steel truss bottom plate can be realized.

[0091] (4) As Figure 5 shown, the stress of the top plate distribution beam is analyzed. The upward force is: the upward pressure of the left jack P / 6, the upward pressure of the right jack P / 6, and the upward force of the double-layer steel truss top plate on the top plate distribution beam 12. The total upward force is P. In the vertical direction, the external force is zero, and it is in a balanced state, which conforms to the stress law.

[0092] (5) As shown in Figure 6 , the force analysis of the bottom plate distribution beam 20 is that the left-side jack downward pressure is P / 6, and the right-side jack downward pressure is P / 6. The reaction force direction of the double-layer steel truss bottom plate to the bottom plate distribution beam 20 is upward, and the vertical direction external force sum is zero, which conforms to the force law.

[0093] (6) As shown in Figure 7 , the force analysis of the steel truss is that all downward pressure sums are P, which is equal to the size of the expected loading force, and conforms to the loading purpose. Specific embodiments:

[0095] To reveal the distribution law of the shear lag effect of the double-layer steel truss, a 72m simply-supported double-layer steel truss is taken as an example, the cross-sectional size and web size are as shown in Figure 12 , the steel type is Q345qD, the concentrated load in the span is 2 P =1200kN, the elastic modulus is Es =2.06×10 5 MPa, the Poisson's ratio is µ =0.3; the Abaqus software is used to analyze 6 load cases as shown in Figure 13 , and the steel truss deformation diagram is as shown in Figure 13 . It can be seen that the double-layer steel truss cross-section deflection in the span has certain differences under different load cases, compared with the concrete beam, the steel truss is more significantly affected by the local effect, and the deformation and stress concentration effect is particularly obvious. The cross-section shear lag longitudinal stress in the span is as shown in Figure 14 and 15 , the longitudinal stress peak values of the top plate and the cantilever plate have great differences, which change with the change of the concentrated load position, and the stress peak values corresponding to each load case are not equal, which is completely different from the law that the cross-section longitudinal stress peak values of the concrete box girder bridge under the load transverse displacement are equal. Except for the working condition 6, the longitudinal stress of the bottom plate under other load cases is greatly affected by the local bending deformation, and all show compressive stress, and the stress distribution pattern is approximately a horizontal line, which is mainly due to the large local flexibility of the orthotropic steel bridge deck and the prominent deformation caused by the local flexibility.

Claims

1. A method for analyzing shear lag of a double-layer steel truss beam, characterized by, The test device comprises a double-layer steel truss top plate and a bottom plate, and comprises a four-column loading hydraulic machine, a top plate distribution beam, a jack, a bottom plate distribution beam and a rigid spring. The top plate distribution beam is horizontally arranged and fixedly connected with the lower end of the piston rod of the loading hydraulic machine at the middle part, and is pressed on the top plate of the double-layer steel truss during use. The bottom plate distribution beam is horizontally arranged below the top plate distribution beam, and the rigid spring comprises a plurality of springs and is connected between the left and right ends of the top plate distribution beam and the bottom plate distribution beam. During use, the bottom plate distribution beam is arranged between the top plate and the bottom plate of the double-layer steel truss and presses the bottom plate of the double-layer steel truss, and the bottom plate distribution beam is used to apply a pressure load to the bottom plate of the double-layer steel truss. The jacks are arranged between the top plate distribution beam and the bottom plate distribution beam. Two upper loading blocks are arranged on the bottom surface of the top plate distribution beam, and two lower loading blocks are arranged on the bottom surface of the bottom plate distribution beam. The distance between the two upper loading blocks is adjustable, and the distance between the two lower loading blocks is adjustable. The test method comprises the following steps: Symmetrically adjust the position of the upper loading block and the lower loading block, the upper loading block is to the intersection of the double-layer steel truss roof and web, the lower loading block is to the intersection of the double-layer steel truss bottom plate and web, and is fixed by bolts; the hydraulic machine loading load is 2 P , the maximum pressure of the jack is P / 2, the hydraulic machine is started, and the jacks on the left and right sides are controlled to be synchronous for testing; Step 1: design the loading condition Symmetrical adjustment of the position of the upper and lower loading blocks, the upper loading block to the top plate of the double-layer steel truss 1 / 4 and 3 / 4, the lower loading block to the bottom plate of the double-layer steel truss 1 / 4 and 3 / 4, and fixed with bolts; set the hydraulic machine load to 2 P , the maximum pressure of the jack is P / 2, start the hydraulic machine, and control the jacks on the left and right sides to test synchronously; ① Load condition 1 Symmetrically adjust the position of the upper loading block and the lower loading block, the upper loading block is to the center of the top plate of the double-layer steel truss, the lower loading block is to the center of the bottom plate of the double-layer steel truss, and is fixed by bolts; set the loading load of the hydraulic machine to 2 P , the maximum pressure of the jack is P / 2, start the hydraulic machine, and control the jacks on the left and right sides to be synchronous to test; ② Load condition 2 Symmetrical adjustment of the position of the upper and lower loading blocks, the upper loading block to the top plate of the double-layer steel truss 1 / 4 and 3 / 4, the lower loading block to the center of the bottom plate of the double-layer steel truss, and fixed with bolts; set the hydraulic machine load to 2 P , the maximum pressure of the jack is P / 2, start the hydraulic machine, and control the jacks on the left and right sides to test synchronously; ③ Load condition 3 Symmetrically adjust the position of the upper and lower loading blocks, the upper loading block is to the center of the top plate of the double-layer steel truss, the lower loading block is to 1 / 4 and 3 / 4 of the bottom plate of the double-layer steel truss, and is fixed by bolts; set the loading load of the hydraulic machine to 2 P , the maximum pressure of the jack is P / 2, start the hydraulic machine, and control the jacks on the left and right sides to be synchronous to test; ④ Load condition 4 Symmetrical adjustment of the position of the loading block, loading block to the double-layer steel truss roof and truss intersection, and fixed with bolts; set the hydraulic machine load of 2 P , the maximum pressure of the jack is P / 2, start the hydraulic machine, and control the left and right jacks to test synchronously; ⑤ Load condition 5 ⑥ Load condition 6 Step 2: paste strain gauges Paste longitudinal strain gauges on the top plate and the bottom plate of the double-layer steel truss at the mid-span section, and the strain gauges are symmetrically arranged at the center. wherein is the measured point shear lag longitudinal stress; E s is the steel modulus of elasticity; is the measured point longitudinal strain.

2. The method of claim 1, wherein the method is characterized by: Step 3: calculate the shear lag longitudinal stress 3. The method of claim 1, wherein the method is characterized by: When calculating the shear lag longitudinal stress of the top plate and the bottom plate of the double-layer steel truss, the longitudinal strain of the measuring points obtained by the test is used to obtain the longitudinal stress as follows:

4. The method of claim 1, wherein the method is characterized by: The top plate distribution beam and the bottom plate distribution beam are I-beam structures. The top surface and the bottom surface of the jack are respectively provided with pressure plates. In step 2, the strain gauges are arranged at the center of the top plate and the bottom plate of the double-layer steel truss, at the intersection with the web, and at other positions according to the spacing; the longitudinal strain of the corresponding measuring points is obtained by a strain acquisition instrument, and data is collected.

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