Keywords

1 Introduction

The main girder of a hybrid girder cable-stayed bridge consists of a steel girder and a concrete girder, which exist independently in the structural hierarchy and are combined into a single unit using connectors to jointly bear and transmit loads. The main girder consists of both steel and concrete structures along the longitudinal direction. Hybrid girders are often engineered in the form of main-span steel girders and side-span concrete girders [1]. In this system, the use of concrete forms for the side spans not only reduces the internal forces and deformations of the main span steel girders but also enhances the main span spanning and reduces the negative reaction forces at the end supports of the side spans [2]. It also allows for relatively shorter side span lengths, making the choice of bridge site more flexible.

To ensure structural safety and smooth load transmission, a large number of load-bearing components have been incorporated in the steel-concrete composite segments of the hybrid beam, resulting in a complex structural configuration, significant cross-sectional variations, and pronounced deviation of shear flow. It is difficult to carry out the theoretical study of the structural force transfer mechanism, so the domestic and international research on the force transfer mechanism of the steel-concrete composite segments is mainly carried out through finite element simulation [3] and model test [4 ~ 5].

Zou et al. [6] proposed a simplified mechanical model for the steel-concrete composite segments of hybrid beams based on the partial combination theory. They obtained the simplified calculation methods for the interface slip distribution, maximum slip, effective utilization rate of the connector group, and the proportion of load borne by the connectors.

Zhou et al. [7] investigated the force transfer performance of the steel-concrete composite segments through model tests and finite element simulations, and their test and finite element simulation results showed that the shear nails in steel-hybrid sections had an obvious group nailing effect.

This paper carries out a study on the force transmission path of the structural steel-mixed section of the Dianbu River Bridge in Anhui Province. Duanbu River Bridge is a (84 + 152)m single tower single cable-stayed steel-mixed girder cable-stayed bridge, which is characterized by the following features compared with conventional cable-stayed bridges: The main tower has a special shape, a shaped one-tower cable-stayed bridge; The bridge is a tower-girder-pier consolidation system; it is a steel-mixed girder cable-stayed bridge. Due to the special structure, the force transmission mechanism of the steel-hybrid section of this type of bridge needs to be studied independently.

2 Project Overview

Fig. 1.
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Arrangement of the main bridge of the Dianbu River Special Bridge

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Dianbu River Bridge adopts (84 + 152)m single tower single cable-stayed steel-mixed girder cable-stayed bridge. The main girder of side span adopts prestressed concrete box girder, the main girder of middle span adopts steel box girder, the bridge tower adopts reinforced concrete crescent tower, and the tower-pier-beam cementation system. The length of steel-mixed girder section is 6m, including 2m of steel-mixed section and 3.5m of stiffness change section, and the main part of the tower is in the shape of a moon, and the lower tower columns are set up with circular hollowing. The bridge layout is shown in Fig. 1. The units in the figures are centimeters.

3 Modelling

ABAQUS 2020 is used to establish the finite element model of the whole bridge, and the base model will be the concrete girder end and main tower section with a solid finite element model, and the steel box girder section with a shell unit model. Among them, the concrete solid finite element model, the tower-beam bonding section with 16mm thickness of covered steel plate was simulated by the C3D8R unit. Steel girders, stiffening ribs, and diaphragms are simulated using S4R units, while steel bars and diagonal cables are simulated using T3D2 units. Reinforcing steel units are built into the concrete units, and four types of steel box girders exist in the baseline model, all of which are bound and combined; Transverse bulkheads, stiffening ribs, and steel box girders are connected by binding bond; steel-hybrid bonding section will be steel plate and concrete block binding link, steel plate and concrete connecting plate is built into the concrete and bound to the steel plate.

The ABAQUS full bridge finite element model is shown in Fig. 2.

Fig. 2.
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Finite element model of Dianbu River Bridge and steel-hybrid section

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ABAQUS was used to create a refined model of the whole bridge. For the steel-concrete composite segments, a combination of blueprints and on-site photographs were employed to ensure model fidelity with the actual construction. The design of the steel-concrete composite segments of the Dianbu River Bridge is illustrated in Fig. 3, while the detailed finite element modelling is shown in Fig. 2. To enhance the visual representation, a perspective view is presented in Fig. 4.

It is observed from Fig. 2 that both the head plate, stiffening ribs, diaphragm plate, rear pressure plate, and the steel plate of the combined section. All are represented in the model and the dimensions of each structure are consistent with the actual project.

Fig. 3.
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Combined steel and concrete section of the Dianbu River Bridge

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Fig. 4.
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Perspective view of finite element model of steel-hybrid combined section

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4 Assumption of Force Transmission Path

The steel-concrete composite segments of the hybrid beam consist of the steel transition segment, the composite segment, and the concrete transition segment. Among these, the composite segment serves as the primary load-bearing component. In a macroscopic sense, the load transfer pathway within the steel-concrete composite segments of the hybrid beam can be generalized as follows: internal forces within the steel structural segment are transmitted through the steel transition segment to the composite segment and further transferred to the concrete transition segment.

The structural form classification of the steel-hybrid combined section can be divided into two categories according to the presence or absence of the lattice chamber, on this basis and then based on the form of the pressure plate and then divided in detail [8–10], its simple classification is shown in Fig. 5.

Fig. 5.
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Classification of Steel-Concrete Composite Segments

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It can be observed from Fig. 3 that the structure has a lattice chamber and no front bearing plate, so it is a steel-hybrid section with a lattice chamber and rear bearing plate type.

It is considered that the specific force transfer paths of the steel transition section in the steel-hybrid bonded section of the hybrid beam that transfers the internal forces to the concrete transition section through the bonded section are the five shown in Fig. 6.

Fig. 6.
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Common force transfer paths in steel-cement sections with compartmentalized rear bearing plate type

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5 Finite Element Result Analysis

The red and orange areas in Fig. 7 are subjected to tensile stresses, and tensile phenomena are observed in the middle of the transverse bridge direction as well as on both sides, while the rest of the area is subjected to compressive stresses. Meanwhile, the region of maximum compressive stress is located in the region of the bond section concrete near the concrete girder section, opposite to the tensile zone, indicating that the structure is stressed in both directions in the direction of the bridge. At the same time, the structure has a large volume share and is connected to a concrete box girder at one end. One end is connected to a steel-concrete section, which is a mandatory path for the longitudinal force flow.

Fig. 7.
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Stress Contour Plot of Concrete and Steel Plate in Composite Steel-Concrete Section

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The steel plate of the bond section is divided into two pieces, one covering the uppermost part of the concrete of the bond section and the other covering the lowermost part of the concrete of the bond section. In Fig. 7, the pressurized portion of the structure is marked in colour. The whole steel plate is subjected to compressive stress in the majority of the area, and the middle part of both plates is subjected to large compressive stress. It can reach 150 ~ 300 Mpa, of which the peak compressive stress is distributed in the middle of the steel plate in the lower combined section, reaching 319.4 Mpa. Slightly lower than the yield strength of the material. The structure as a whole is under high stress and is therefore considered to be a more important part of the force transfer path during the service phase.

Fig. 8.
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Stress cloud of reinforcement rib and steel lattice chamber plate

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It is observed that the full interface of the stiffening rib is under compression, for the upper stiffening rib, the middle part is subjected to higher compressive stresses, while for the lower stiffening rib, it is subjected to higher compressive stresses on both sides.

Fig. 9.
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Stress cloud of steel plate of end socket and steel plate at the end of the bonding section

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Figure 8 illustrates the stress cloud of reinforcement rib and steel lattice chamber plate. The steel latticework panels are interlocked with concrete to form several steel lattice chambers. Among them, the stress of the steel grating plate is less than 50 Mpa, which is much lower than the material strength, and it is considered that the structure has a role in the force transfer process, but it is not the most dominant force transfer path. At the same time, the peak stress of the steel plate of the head only reaches 16 Mpa, and it is considered that the structure does not play the role of force transmission, but the construction requirements, and plays the role of blocking. There are two steel plates at the end of the combined section, which are close to the head steel plate and are located at the upper and lower part of the head steel plate. Figure 9 illustrates the stress distribution in the steel plate of the end socket and the steel plate at the end of the bonding section. The structure has a peak stress of 73 MPa, but it is limited to a smaller area which is less than 5% for the structure, and isolating this area, the majority of the area has a stress of less than 30 MPa. It is considered that the structure, like the head plate, does not act as a force transfer but is more of a construction requirement.

Fig. 10.
figure 10

Stress cloud of the rear pressure plate

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Figure 10 illustrates the stress cloud of the rear bearing plate. The stress distribution of the rear bearing plate is complex, but the value is low, and the whole is kept in the role of 20 Mpa, and the peak stress is 30 Mpa, which is located in the lower side rear bearing plate. Considering the low stresses, it is not considered to act as a major force transfer path. Considering that its relative position to the end concrete is similar to that of the head plate, it is considered to act more as a blocking similar to that of the head plate.

6 Conclusion

This paper utilizes a single-tower, solid-pier, single-cable-plane, irregular single-tower cable-stayed bridge as the subject. A comprehensive finite element model of the entire bridge is established for numerical simulation to analyze the load transmission pathways in the steel-concrete composite segments of such bridges. The conclusions are as follows:

  1. 1.

    The steel plates in the combined section are subjected to high compressive stresses and play an important role in the force transfer process. Stiffening ribs and steel lattice plates are also involved in force transfer, but not the most important pathï¼› the head steel plate, combined section end steel plate, and back pressure plate have low stress, mainly play the role of blocking and structure, not the main force transmitting parts.

  2. 2.

    Through finite element analysis, the primary load transmission pathways in the steel-concrete composite segments of the Dianbu River Bridge have been identified as follows: steel transition segments, composite segment steel plates, composite segment concrete, and concrete transition segments, conforming to bidirectional load transfer principles.

  3. 3.

    The construction method of the steel-concrete composite segments of the Dianbu River Bridge is reasonable and can safely and reliably transmit internal structural forces, meeting the design requirements.

In this paper, only the force transfer mechanism of the steel-hybrid section under static force is considered, and its effect caused by seismic loading is not taken into account, which will be further explored in the subsequent research.