In this research, fatigue tests on fullsize specimens are conducted for a steel crossbeam joint before and after reinforcement. Combined with a threedimensional (3D) numerical simulation, the 3D stress parameters, and their redistribution rules study anew with a web crack and new crack initiation locations and fatigue weakness details are predicted. The research results include the following: 1) The empirical formula parameter m of the Zaxis stress for the new crack tip is approximately 0.05. 2) The fatigue performance of the web’s new crack tips is significantly improved by bolting reinforced steel plates, the stress range is reduced by 60%98.78%, and the original crack stops growing in size. The health monitoring system can choose the predicted weak details as valid monitoring points so that the fatigue damage can be intelligently perceived after the reinforcement of steel bridges.
En este estudio, la prueba de fatiga de una muestra a gran escala antes y después del refuerzo se llevó a cabo en la junta de viga transversal de un puente de acero. Combinado con la simulación numérica 3D, se estudiaron los parámetros de tensión 3D y las leyes de redistribución de las redes agrietadas, y se predijeron nuevas ubicaciones de grietas y detalles débiles por fatiga. Los resultados muestran que el refuerzo de la placa de acero atornillada puede mejorar la resistencia a la fatiga de la viga, y la amplitud de tensión de la punta de grieta de la placa de banda se puede reducir en más del 60%. Los puntos de monitoreo y medición se pueden establecer en los detalles débiles de la predicción, de modo que el daño por fatiga en el funcionamiento del puente de acero en la etapa posterior del refuerzo se encuentre en un estado de percepción inteligente.
Fatigue cracking is common in many old steel bridges built in China in the last century. The longest crack detected at the beam joints of steel truss bridges in this research program is 230 mm (
Many scholars have predicted the positions of fatigue crack formation in steel truss bridges, Ju Xiaochen took the Ibeam web of a steel bridge as his research object and predicted the fatigue crack propagation behavior with the displacement extrapolation method combined with the maximum energy release rate method (
Wang Teng proposed the probability prediction method using a Monte Carlo integral (
In summary, many scholars have performed a large amount of research on the crack growth of steel bridges, and the applicability has been partially verified. However, there have been few research studies on the fatigue performance of old steel bridges after reinforcement, particularly because it is difficult to identify and monitor the fatigue cracks hidden behind reinforced plates. In this research, based on the fatigue test of the fulllength specimens of steel truss bridge transverse longitudinal beam joints, combined with finite element software, a threedimensional simulation model of beam joints with 230 mm cracks through the web before and after reinforcement is established. The stress field parameter changes at the crack tip and the stress redistribution law after reinforcement are studied, and the effectiveness of bolted steel plate reinforcement is evaluated. The prediction of the new crack nucleation location and the fatigue weakness details is of great significance for the intelligent sensing and monitoring of fatigue damage in the later operation of steel bridges.
Bolted steel plate reinforcement has the following advantages compared with welding, rivets, pasting steel plates, etc.: higher bearing capacity; stress concentration is improved, and the fatigue resistance of the connector is improved; construction speed is fast, installation is easy, and replacement is flexible, the steel bridge was reinforced with bolted steel plates in 2017. According to the maximum crack detected by the real bridge in the subject study, five fullsize test specimens of a steel cross beam joint are designed in this experiment ^{[}
Each specimen has strain gauges affixed at key points according to the results of the numerical simulation, as shown in
As shown in
The crack tip is meshed with singular finite elements, as shown in
A contact surface is set between the bolt and the connected member for simulation. Both ends of the beam model are boundary constraints, and only the longitudinal beam bearing area at the upper center of the beam is simulated under pressure, which makes the model conform to the actual working conditions and constraints of the test.
In the fatigue test carried out on beam specimen T1, the maximum fatigue load is the sinusoidal constant amplitude load of 80 kN880 kN, and the frequency is 2.5 Hz.
The data for the loading test are compared with the stress data of the corresponding measuring points calculated with the numerical simulation of the finite element model. The data for typical measuring points 1, 7, and 9 are selected for comparison, as shown in
Measuring point 1  contrast %  Measuring point 7  contrast %  Measuring point 9  contrast %  

Load (t)  The experimental data (MPa)  Simulated data (MPa)  The experimental data (MPa)  Simulated data (MPa)  The experimental data (MPa)  Simulated data (MPa)  
10  13.79  11.05  19.85  8.70  8.24  5.23  5.92  5.99  1.11 
20  22.94  22.08  3.73  17.00  17.11  0.65  12.63  12.70  0.55 
30  31.84  33.12  4.00  31.66  30.04  5.11  18.25  18.52  1.49 
40  41.65  44.00  5.64  48.24  44.17  8.44  23.42  23.18  1.05 
By comparing the data in
The maximum amplitude of cyclic load is 80,000kg applying to the center of the beam (y, gravity direction), combined with the results of the test and the numerical simulation analysis, the threedimensional stress states in the areas with large amplitudes of the fatigue stress, near the web crack, the lower flange plate, and the stiffening rib weld (as shown in
1) Web with a crack
In the web crack area, the stress is concentrated on the crack tip.
The threedimensional stress data at the crack tip are shown in
Node  Sx  Sy  Sz  m=Sz/ (Sx+Sy)  

Unreinforced  300731  201.68  198.18  18.53  0.05 
300733  201.89  198.25  18.48  0.05  
Reinforced  1500798  20.56  1.29  1.04  0.05 
1500801  12.08  0.36  0.56  0.05 
2) Bottom flange plate
The distribution of the main tensile stress S1 of the lower flange plate is shown in
3) Weld of stiffened rib plate
The stress diagram of the threedimensional stress at the welding seam of the stiffened rib along the height direction (Y direction) is shown in
The fatigue failure of the specimen is shown in
It can be seen that the failure crack location in the fatigue test is in good agreement with the crack initiation location predicted by the numerical simulation.
The maximum amplitude of cyclic load is 40,000kg applying to the center of the beam (y, gravity direction). The vertical stress amplitude of the web crack tip, the stiffener plate weld and the bottom flange plate before and after reinforcement are compared. It can be seen from
Stress  Before reinforcement (MPa)  After reinforcement (MPa)  The stress amplitude is reduced (%)  

Web crack tip 

131.28  25.38  80.67 

256.97  3.13  98.78  

61.29  2.12  96.54  
Stiffened rib weld 

96.58  34.22  64.57 

15.96  1.44  90.99  

91.41  44.50  51.32  
Bottom flange plate 

19.87  5.02  74.73 

0.11  0.04  60.00  

0.12  0.03  71.90 
In this research, the fullsize test of a steel bridge beam joint is carried out, and the stress variation of five test beams is analyzed, processed, and compared with the theoretical simulation data, which verifies the availability of the model. Based on the analysis of the threedimensional stress amplitude field of the measured points, test data, and simulated data, the possible fatigue detail position of the cracks after reinforcement is predicted, and the stress estimation parameters and the distribution law of the fatigue stress amplitude are given for the web thickness direction of the steel beams.
The results for the fatigue test and the numerical simulation of the steel beam joints before and after reinforcement show that the following:
The structure after reinforcement is complex, and once damage occurs, multiple cracks may appear at multiple weak points. The traditional single crack growth mode cannot simulate the overall damage state of the real structure. There are several weak details in the final failure of the specimen (
The results of the 3D finite element numerical model presented in this study are in good agreement with the measured data and can be used in research on fracture mechanical properties and crack propagation rules.
The empirical formula parameter
The reinforcement improves the local stress distribution of the beam joints, reduces the fatigue stress range in the reinforcement area obviously, and slows down the growth of the cracks. However, for the fatigue load with a large stress amplitude, the beam will create new cracks at new weak positions, and the damage will continue to accumulate and develop.