December 30, 2025
In recent years, significant achievements have been made in bridge construction in China, with various new types of bridges emerging one after another. Among them, tied arch bridges occupy an important position in the field of large-span bridges due to their unique structural advantages - the mechanical properties of beam bending resistance and arch ring compression. With the construction and operation of a large number of tied arch bridges, daily inspection and maintenance of bridges have gradually become key links to ensure their safe operation.
Part of the mid span tied arch bridges adopt steel hanging hole design in the mid span area, which significantly improves the bridge's crossing capacity. However, the lightweight nature of steel box girders also poses challenges during construction: when the cable tension is too high, the mid span main beam may be lifted as a whole, resulting in a detachment of the contact surface between the bracket at the cow leg position and the beam body. When vehicles pass through, the vibration of the beam body is abnormal, especially the vibration difference at the end of the beam is more prominent.
In response to this phenomenon, this article systematically analyzes the causes of the disease and proposes corresponding maintenance plans through static pressure tests on the supports, tension tests on the main bridge suspension rods, and vertical modal tests on the main bridge. The research results can provide reference for the inspection and maintenance work of similar bridges.
Project Overview
The main bridge of a certain bridge is a three span flying swallow shaped special-shaped mid supported tied arch bridge, with a span combination of 51.0m+158.0m+51.0m. The mid span also uses a steel box girder hanging hole with a span of 120m, which is supported above the concrete beam in a simply supported form. There are 16 pairs of 32 suspension rods under the main arch ribs of the main bridge, with a specification of LZM7-61 type. The standard strength of the cable is fpk=1670MPa, the nominal cross-sectional area of the steel wire bundle is 23.48cm2, and the breaking load is 3920kN. The bridge was built in 2009 and the design load level is City Class A. In 2015, maintenance personnel of the bridge discovered abnormal vibration in the steel hanging holes when vehicles passed by during daily inspections, and severe damage to the expansion joints of the steel frame at the cow leg position. The maximum vertical displacement difference between the steel box beam and the concrete beam at the expansion joint reached 50mm. Due to limitations in conditions, it was not possible to directly observe and judge the support situation at the cow leg position on site. Therefore, through the static pressure test of the support, the tension test of the main bridge suspension rod, and the vertical modal test of the main bridge, the causes of the disease are analyzed, and maintenance measures are proposed, providing reference for the detection and maintenance of similar diseases in the later stage.
Static pressure test of cow leg position support
According to the design drawings of the bridge, there are three spherical longitudinal supports on one side of the cow leg position, totaling six for the entire bridge. The support models are all LYQZ8000/1600DX (F). In this experiment, a displacement meter will be installed at the bottom of the steel box beam and concrete beam directly below the bracket at the cow leg position. The displacement difference between the two will be measured to determine whether there is a detachment phenomenon in the bracket (see Figure 1 and Figure 2). The principle of testing is: if the support is intact, the displacement difference at the same position should be zero or close to zero; If there is a void phenomenon in the support, due to the fact that the vehicle load is mainly applied above the steel hanging hole during the test loading, the downward displacement of the steel box beam under the load will be much greater than that of the concrete beam, and there is a significant displacement difference between the two.
Figure 1 Plan of Main Bridge Support System (Unit: mm)
The analysis of the experimental results is as follows:
The experiment used 6 three axle load trucks each weighing 350kN (a total of 2100kN) to conduct static pressure on the bearings, while considering the correction of the deformation of the spherical bearings under load. The specific experimental results are shown in Tables 1 and 2.
According to Tables 1 and 2, considering the deformation of the supports, the displacement differences between the supports at the west end of the cow leg position from north to south are 8.62mm, 1.19mm, and -2.80mm, respectively. The displacement differences between the supports at the east end of the cow leg position from north to south are 2.81mm, 0.59mm, and 1.69mm, respectively. Except for the negative displacement difference of the support seat on the south side of the west end, the displacement differences of the other supports are all greater than 0, with a maximum value of 8.62mm, indicating that the above support positions are all in a state of detachment.
Figure 2 Schematic diagram of the installation position of displacement sensor (unit: mm)
Table 1: Statistical Summary of Support Displacement at the Western End of the Steel Hanging Hole Bull Leg Position
Table 2: Statistical Summary of Support Displacement at the Eastern End of the Steel Hanging Hole's Bull Leg Position
The displacement difference between the supports below the concrete beam and steel box beam at measuring points 1-6 is negative. This is because under the action of load, the concrete beam will only produce downward displacement, while the displacement of the three supports below the steel box beam may produce upward or downward displacement due to the different sizes of deformation. In this experiment, the downward displacement generated by the north support under the load was much greater than the other two measuring points on the same section, resulting in a seesaw effect with the middle support as the fulcrum, with the north support and the south side forming a seesaw effect, with one side upward and the other side downward.
The test results show that there is varying degrees of detachment in the support positions of the main bridge's cow legs, with the western end support experiencing more severe detachment, which is consistent with the detection result that the vibration deformation of the steel box girder at the western end is greater than that at the eastern end during on-site inspection.
Main bridge suspension rod tension
Testing principle: If the supports on both sides of the hanging hole cow leg position have become empty, and the supports are not bearing the load, then the self weight of the lower steel hanging hole and the load above it are all borne by the suspension rod. That is, the measured cable force value of the suspension rod should be basically equal to the sum of the self weight of the steel box girder, bridge deck pavement, railings and other facilities. And compare the measurement results with the design cable force to determine whether there is over tension phenomenon in the cable force.
Due to the absence of pressure sensors embedded at the tensioning end of the suspension rods during the construction process of the bridge, it is not possible to directly read the tension values of each suspension rod. This cable force test adopts vibration measurement method, which measures the vibration frequency of the suspension rod and converts it into cable force according to a certain formula (see Table 3). In order to ensure the accuracy of the measurement, the entire bridge traffic will be temporarily closed during the cable force test, and vehicles will be prohibited from passing to prevent interference from external loads. At the same time, each suspension rod is tied with two vibration pickups at the same measuring point position, and the average value of the two measurements is calculated to reduce measurement errors.
Table 3 Summary of measured frequencies and cable forces for each suspension rod
Due to the missing completion drawings of the bridge, some key data cannot be accessed. According to the general description of the bridge and some design drawings, the total dead load weight of the steel hanging hole box girder and its upper components is approximately 39500kN (estimated value). When designing, a single support for the cow leg position is designed with a vertical bearing capacity of 1500kN. The support reaction force of six supports is approximately 9000kN, and the total design cable force of the suspension rod is approximately 30500kN (estimated value).
According to Table 3, the measured cable force value of the suspension rod is 38900kN, accounting for 98.4% of the theoretical calculated total dead load of 38900/39500. The two data are almost identical, indicating that the force of the main bridge's rigid hanging hole box girder dead load is almost entirely borne by the suspension rod, and the support is in an unstressed state. In addition, there is a certain degree of deviation between the measured and estimated cable forces of the bridge, and there may be over tension in the cable forces.
The results of the constant load cable force test indicate that there may be over tension in the cable force of the bridge, and the bearings have not played their due role. The rigid hanging hole box girder has formed a floating system to a certain extent, resulting in abnormal vibration of the bridge when vehicles pass through.
Vertical modal testing of the main bridge
Modal testing mainly measures the vibration modes, natural frequencies, and damping ratios of bridge structures under random environmental vibrations, in order to reflect the damage situation of the bridge structure and the overall stress performance and changes in the stress system of the structure.
The modal parameter testing points for the main bridge structure are mainly arranged at the suspension rod position for the steel box girder, and at the quarter point of the side span, arch seat position, and cow leg position for the prestressed concrete box girder. The measurement results are shown in Table 4 and Figure 3.
Table 4: First order vertical frequency and damping ratio of bridge modal vibration test
Comparison of measured and theoretical vibration mode diagrams in Figure 3
According to the test results, the actual vibration pattern of the bridge deviates significantly from the theoretical vibration pattern of the intact structure, and the measured frequency value is 0.815Hz, which is much lower than the theoretical calculated value of 1.62Hz. This indicates that the structural stress system of the bridge has undergone significant changes compared to the original design state, and the actual overall stiffness of the main bridge is worse than the theoretical value. According to the measured vibration pattern, the rigid hanging hole mainly vibrates vertically up and down as a whole, which is consistent with the phenomenon of significant abnormal vibration of the bridge when vehicles pass through.
Conclusion
According to the static pressure test, cable force test analysis, and vertical vibration mode analysis of the cow leg support at the hanging hole position mentioned above, it can be concluded that there is varying degrees of detachment in the support at the hanging hole position of the main bridge. The hanging hole of the main bridge has formed a floating system on the stress system, which cannot bear the force normally, resulting in abnormal vibration of the steel box girder hanging hole when passing vehicles, especially when heavy vehicles cross the bridge.
The reason for the detachment of the main bridge bearings should be due to improper control of cable tension during the bridge construction process, which may result in over tension of the cables, causing the hanging holes to be pulled up as a whole. The management and maintenance unit subsequently arranged a professional technical team to repeatedly demonstrate the plan and adjust the cable tension of the bridge. The vibration situation of the steel box girder hanging hole was significantly improved.
From the above research, it can be seen that the construction and testing experience of bridges with suspension rods and steel box girder hanging holes provide important warnings for similar bridges. In the early stage of construction, it is necessary to strictly monitor the tension of cables and the displacement of supports to avoid over tensioning; During the handover acceptance, it is advisable to add a static pressure test condition for bearing displacement to ensure that the bearing is under normal stress. During operation, regular inspections should be strengthened, and an early disease warning mechanism should be established. Static pressure testing, cable force testing, and modal analysis methods should be comprehensively used to timely take targeted measures such as cable force adjustment for bridges with problems, in order to restore the structural stress system and ensure long-term safe operation of the bridge.
Westcoast New Area, Qingdao City ,Shandong Province ,China
