Keyword

1 Introduction

Suspension bridges have developed rapidly due to their advantages of simple structure, clear stress distribution, and reduced material consumption with larger spans. Suspension bridges fully utilize the advantages of high-strength steel wire tension on the main cable and suspension rods, and their span capacity is the largest among various bridge types, reaching over a kilometer [1].

At present, only some research has been conducted abroad on the construction sequence of stiffening beams and bridge decks for tower single span steel truss suspension bridges. There is little research on the impact of the lifting sequence of stiffened beam segments and the different paving methods of the bridge deck on the stiffened beam [2, 3].

The long span suspension bridge built in China has limited design and construction experience as well as computational research on the unique asymmetric single tower and single span steel truss suspension bridge type. This article mainly studies the construction sequence of this special bridge type stiffening beam and bridge deck [4]. By comparing and analyzing the calculation of different construction schemes, the vertical displacement of the main cable, the stress parameters of the steel truss stiffening beam members, suspension rods, and cable towers are controlled, and a reasonable and convenient construction sequence of the stiffening beam and bridge deck is obtained, providing reference for the construction of similar projects.

2 Engineering Background and Research on the Hoisting Sequence of Steel Truss Stiffening Beam Segments

A special bridge adopts a single tower and single span suspension bridge structure with a main span of 256 m, with a bridge deck width of 13.4 m. The main cable saddle body is cast entirely of cast steel, the cable tower is a portal tower with a reinforced concrete structure, and its crossbeam is a prestressed reinforced concrete structure with a rectangular cross-section. The bearing platforms of the two tower columns are designed as a whole. The longitudinal floating system is adopted between the tower beams at the main tower.

The overall layout of the bridge is shown in the following Fig. 1:

Fig. 1.
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Overall Layout (Unit: m)

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Design speed: 40 km/h; Design load: Highway - Class II.

By using finite element software to establish a finite element model and analyzing the initial equilibrium state of the suspension bridge, the linear shape of the main cable and empty cable states, as well as the stress free length of the main cable and suspension rod, were determined.

These were used as important parameters for construction control. Simulation calculations were conducted on the super large bridge, and the optimal construction sequence for the installation of stiffening beams and bridge decks of a single tower and single span steel truss suspension bridge built in mountainous areas was analyzed [5].Compare and analyze the on-site measured data with the theoretical data calculated by the model, and summarize simple and effective construction methods.

The lifting of steel truss stiffening beams is very important for the construction monitoring of large-span suspension bridges, and the primary problem to be solved is the sequence of lifting steel truss stiffening beam segments [2, 6]. Therefore, when determining the lifting sequence of steel truss stiffened beam segments, the main consideration is its impact on the changes in the main cable shape. This article mainly verifies whether the lifting sequence is reasonable from the following two aspects: ① The absolute vertical deformation of the main cable at each step during the lifting of the stiffening beam segment is relatively minimum. ② During the lifting process, the vertical deformation of the main cable is small.

When determining the lifting sequence of stiffened beam segments, all segments should be freely suspended in a certain order [7]. Option 1: Starting from the support platform of the cable tower, lift the section of the stiffening beam and close it at the beam end. Starting from lifting section B1, to lifting section B26.Option 2: Lift the stiffened beam segment from the beam end until the B6 segment of the stiffened beam is closed.

The displacement changes of the nodes in the main span of the main cable during the lifting stage of each section in Scheme 1 are shown in Fig. 2. (Up is positive, down is negative).

Fig. 2.
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Displacement diagram of nodes in the main span of the main cable during the lifting process of the stiffened beam in Scheme 1

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The displacement changes of the nodes in the main span of the main cable during the lifting stage of each section in Scheme 2 are shown in Fig. 3. (Up is positive, down is negative).

Fig. 3.
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Displacement diagram of the main cable mid span node during the lifting process of the stiffened beam in Scheme 2

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Comparing the displacement change curves of the nodes in the main span of the main cable during the lifting process of the stiffened beam segment between two schemes, Scheme 1 shows that the change in elevation of the nodes in the main cable span during the lifting process of the stiffened beam segment is greater than Scheme 2. According to the principle that the absolute vertical deformation of the main cable in each step of lifting the stiffening beam segment is relatively minimum, and the vertical deformation of the main cable upwards is small during the lifting process, the specific calculation results of the two schemes are compared in Table 1.

Table 1. Comprehensive comparison of displacement changes of main cable nodes in the middle span during the installation of stiffened beam segments
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From the above table, it can be seen that the maximum absolute vertical displacement and the maximum vertical upward deformation of the main cable at the mid span node of each step of lifting the stiffening beam section in Scheme 1 are greater than those in Scheme 2, namely 0.3550 m > 0.1923 m, 1.3530 m > 0.3533 m. So in Scheme 2, the deformation of the main cable is relatively gentle during the lifting of each section of the stiffening beam. Finally, Option 2 was chosen as the sequence for lifting the steel truss stiffening beam segments.

3 Research on the Sequence of Bridge Deck Pavement

According to calculation and analysis, different bridge deck paving sequences have a greater impact on the stress of stiffening beam members and suspension rods, while the impact on the change of the main cable shape is relatively small. Therefore, this article focuses on the rationality of the stress distribution of the stiffening beam members and suspension rods during the entire construction control process when determining the paving sequence of the bridge deck [8]. The specific division of the bridge deck corresponds to the segment division of the steel truss stiffening beam. Combining the simplicity and convenience of construction, after screening, three paving sequence schemes for the bridge deck were obtained [9].

After selecting three options, calculate the stress changes of the stiffening beam members and suspension rods for each option. The stress changes of the stiffening beam members and suspension rods in the PD1 scheme are shown in Fig. 4, Fig. 5, and Table 2.

Fig. 4.
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Maximum Stress Envelope Diagram of Stiffening Beam Member in Scheme 1

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Fig. 5.
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Maximum stress diagram of suspension rod in Scheme 1

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Table 2. Maximum stress values of main components in Scheme 1
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According to the calculation, the maximum tensile stress of the stiffened beam member is 288.0 MPa > 200.0 MPa, the maximum compressive stress is 282.2 MPa > 200.0 MPa, and the maximum tensile stress of the suspension rod is 319.5 MPa < 501.0 MPa.

The stress changes of the stiffening beam members and suspension rods in Scheme 2 are shown in Fig. 6, Fig. 7, and Table 3.

Fig. 6.
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Maximum Stress Envelope Diagram of Stiffening Beam Member in Scheme 2

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Fig. 7.
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Maximum stress diagram of suspension rod in Scheme 2

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Table 3. Maximum stress values of main components in Scheme 2
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According to the calculation, the maximum tensile stress of the stiffened beam member is 278.0 MPa > 200.0 MPa, the maximum compressive stress is 274.1 MPa > 200.0 MPa, and the maximum tensile stress of the suspension rod is 318.0 MPa < 501.0 MPa.

The stress changes of the stiffening beam members and suspension rods in Scheme 3 are shown in Fig. 8, Fig. 9, and Table 4.

Fig. 8.
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Maximum Stress Envelope Diagram of Stiffening Beam Member in Scheme 3

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Fig. 9.
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Maximum stress diagram of suspension rod in Scheme 3

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Table 4. Maximum stress values of main components in Scheme 3
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Comparing the stress changes of stiffening beam members and suspension rods in the process of bridge deck pavement with three schemes, it can be seen that different bridge deck pavement sequences have a significant impact on the stress of suspension rods and stiffening beams; In Scheme 1, Scheme 2, and Scheme 3, the stress of the stiffening beam member is relatively high and there is an exceeding limit phenomenon, and the stress of the suspension rod meets the requirements of the specification; However, the relationship between the maximum stress of the stiffened beam members in three different paving schemes is: Scheme 3 < Scheme 2 < Scheme 1, which is 207.0 MPa < 278.1 MPa < 282.2 MPa, and the stress of the stiffened beam members significantly increases. The stress situation of the stiffened beam in Scheme 3 is the most reasonable, so Scheme 3 is determined as the final paving sequence for the bridge deck. During the process of bridge deck pavement, the stress of the stiffening beam members exceeds the limit. Part of the factors is that the paving sequence of the bridge deck is different, and another important reason is that the connection method between the stiffening beam segments adopts rigid connection. Therefore, after determining the sequence of symmetrical paving of the bridge deck from the middle of the stiffened beam span to both ends, the impact of different connection methods between segments during the lifting process of the stiffened beam on the internal force of the structure should be analyzed.

4 Verification of Measured Data

During bridge construction, three test sections are arranged on the steel truss stiffening beam, namely the 1/4, 1/2, and 3/4 cross sections of the steel truss stiffening beam. Four test points are arranged on each test section, and specific sensors are installed on the upper and lower chords of the test section [10]. According to the measured data of the steel truss stiffened beam during construction monitoring, the maximum tensile stress of the chord in the mid span test section is about 118.2 MPa, and the maximum compressive stress is about 124.6 MPa, which is smaller than the theoretical calculation value; During the entire construction monitoring process, the measured stress of the 1/4 and 3/4 span test sections of the steel truss stiffening beam is within 70 MPa, so the construction process is safe and reliable. The comparison between the measured stress and theoretical stress of the 1/2 mid span section of the steel truss stiffening beam during construction is shown in Table 5.

Table 5. 1/2 span mid section steel truss beam chord measured stress (MPa)
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From the above table, it can be seen that after the completion of bridge deck pavement, the measured stress value of the chord of the steel truss stiffening beam is less than the theoretical calculation value, which meets the requirements of the specifications and ensures structural safety during the construction process. During the construction process, the measured maximum tensile stress of the suspension rod was 303.21 MPa, which is less than the theoretical calculation value of 310.2 MPa and even less than its allowable stress value of 501.0 MPa. Therefore, the stress of each component in each construction stage meets the requirements of the specifications.

5 Conclusion

This article studies the reasonable construction sequence of the stiffening beam and bridge deck of a single tower and single span steel truss suspension bridge through simulation calculation of a finite element model. When lifting the stiffening beam, the lifting sequence from the beam end to the closure section of the cable tower is determined based on the principle of the relative minimum vertical deformation absolute value of the main cable and the minimum vertical upward deformation of the main cable during each section lifting. When laying the bridge deck, based on the principle of the most reasonable stress situation of the steel truss stiffening beam members and suspension rods, a plan was determined to symmetrically pave from the middle of the steel truss stiffening beam span to both ends. By comparing the measured data and theoretical calculation values during the construction monitoring process, the maximum tensile stress of the upper chord is 153.0 MPa, and the maximum compressive stress of the lower chord is 159.6 MPa, both of which are less than the allowable stress of 200.0 MPa; The measured maximum tensile stress of the suspension rod is 303.21 MPa, which is less than its allowable stress value of 501.0 MPa. Verified that the determined lifting sequence plan for the stiffening beam and bridge deck is reasonable.