^{1}

^{2}

^{2}

^{2}

^{1}

^{2}

To investigate the load transfer mechanism of the steel-concrete hybrid pylon joint with cells and bearing plates, a theoretical model based on the continuous elastic interlayer method was established. Both the slip effect at the steel-concrete interface and the local compression effect of the bearing plate were considered in the proposed theoretical model. A segment model test with a 1 : 3 scale was carried out to obtain the strain distribution of the hybrid joint and the relative slip between steel and concrete components. Finite element analysis was implemented on the tested segment model, and the structural performance of the tested hybrid joint was compared with the FEA results. The test and analysis results show that the stress of steel and concrete components is at a lower level, and the relative slip between steel and concrete components is extremely limited. The bearing plates and shear connectors are the two load-transferring components and could transfer 40% and 60% of the vertical force into the lower concrete pylon, respectively. The vertical force of shear connectors is at a much lower magnitude within 0.6 times the length of the hybrid joint from the bearing plate and will increase gradually within 0.6 to 1.0 times the length of the hybrid joint. The FEA results are in good agreement with the model test results, and the maximum shear force difference between the theoretical analysis results and the FEA results is less than 10%, proving that the proposed theoretical model can reasonably predict the shear force distribution at the steel-concrete interface of the hybrid joint. In addition, the stiffness of shear connectors has limited effect on the shear force distribution at the steel-concrete interface.

Steel and concrete have been the two most practical and prevailing building materials in the construction of bridge infrastructures for many decades. Generally, there are four types of bridge components according to the arrangement of these two construction materials, including steel components, concrete components, steel-concrete composite components, and steel-concrete hybrid components. The composite component is an effective connection of the steel member and the concrete member in the cross-sectional level, while the hybrid component is a reasonable combination of the steel member and the concrete member along the longitudinal direction of the component [

In recent years, the newly built cable-stayed bridges in China normally need to provide a much wider bridge deck to meet the growing traffic volume. The increasing operating traffic load and the much larger structural gravity of the bridge deck will be transferred in the bridge pylon through the cable-girder anchorage system, the cables, and the cable-pylon anchorage system. The huge cable force in these cable-stayed bridges requires an advanced cable-pylon anchorage system to ensure the load transfer reliability. If the concrete pylon scheme is selected, the steel-concrete composite cable-pylon anchorage structure could be adopted for the accelerated construction, while the configuration of the composite cable-pylon anchorage structure will be complicated. The steel-concrete hybrid pylon could be an alternative for the construction of the cable-stayed bridges, since the lower part of the pylon could be constructed as a concrete structure and the upper part of the pylon could be fabricated as a steel structure. Besides, the upper steel pylon is also beneficial to the cable-pylon anchorage system and the accelerated bridge construction [

Many studies have been conducted to investigate the performance of steel-concrete hybrid structures especially for hybrid girders. Kim and Nguyen [

In this paper, a theoretical model for exploring the load transfer mechanism in the steel-concrete connecting part of the hybrid pylon is introduced firstly. Then, a scaled model of the connecting part, taken the hybrid pylon of Jishui Gan River Second Bridge as the prototype structure, was fabricated and tested. The load-sharing ratio by the bearing plate and the shear connector was measured, and the load transfer mechanism in the connecting part of the hybrid pylon was analyzed.

Figure

Jishui Gan River Second Bridge.

Figure

Schematic view of the hybrid joint structure.

The steel cells in the hybrid pylon joint are the basic component for transferring the vertical axial force from the upper steel pylon to the lower concrete pylon. As shown in Figure

The strain of the pylon steel and concrete parts accords with the assumption of the plane section

Small axial deformation is assumed and bending and shear deformation is ignored

Both PBL and headed stud connectors are equivalent to the continuous spring layer

The adhesive friction between the steel pylon wall and the concrete structure in the steel cells is neglected

The structure detail of the steel cell in the hybrid joint.

Figure _{s}(_{c}(

Relative slip and load distribution of the microbody. (a) Relative slip model. (b) Microbody.

In the microbody of the hybrid joint as shown in Figure _{s}(_{c}(_{τ}(

It is assumed that both the steel plate and the concrete wall need to satisfy the Hooke law under the axial compression. The relationship between the axial force and the relative slip for these two elements in the microbody could be expressed as_{s} and _{c} represent the elastic modulus of the steel plate and the concrete wall, respectively, and _{s} and _{c} stand for the cross-sectional area of the steel plate and the concrete wall, respectively.

For the shear connector at the steel-concrete interface, it is assumed that the shear force transferred by the shear connector including PBLs and headed studs is proportional to the relative slip at the steel-concrete interface; that is, all the shear connectors are in the linearly elastic state in the analysis. The constitutive model of the shear connector is shown in the following equation, which presents the relationship between the relative slip _{s} stands for the equivalent shear stiffness of the shear connector at the steel-concrete interface, which could be estimated based on the following equation:_{ss} and _{sp} refer to the number of headed studs and perfobond plate connectors in the cross section, respectively; _{ss} and _{sp} mean the shear stiffness of headed studs and perfobond plate connectors, respectively; and

To obtain the displacement and the shear force distribution at the steel-concrete interface, the relative slip at the steel-concrete interface is taken as the elementary unknown. Firstly, the second-order derivation is conducted on equations (

The second-order derivation of _{s}(_{c}(

Then, equations (_{s}(_{c}(

Afterwards, the shear force density _{τ}(_{s}_{s}+1/_{c}_{c})_{s}).

Equation (_{1} and _{2} are two undetermined coefficients, which can be determined by the boundary condition of the hybrid joint.

Figure _{s} resisted by the steel plate and _{c} transferred to the top surface of the concrete structure in the steel cell. At the end of the steel plate in the connecting part of the hybrid pylon, i.e.,

Theoretical analysis model for the joint. (a) Simplified model. (b) Force boundary condition. (c) Displacement condition. (d) Local compressive stress.

It needs to be noticed that, at the interface between the bearing plate and the concrete structure, the concrete below the bearing plate is compressed unevenly, and the compressive stress of the concrete near the steel wall plate will be much larger. For simplifying the mechanical model of the concrete below the bearing plate, it is assumed that the concrete is only partially compressed at the edge of the steel wall plate. A vertical displacement still occurs in this part concrete under the axial load _{c}. Therefore, the supporting effect on the bearing plate by the local concrete near the steel wall plate needs to be considered. In this paper, this supporting effect is regarded as an elastic spring, and the axial stiffness _{n} of the elastic spring can be predicted as shown in the following equation:_{s} is the thickness of the bearing plate and _{z} is the area of the partially confined concrete, which could be obtained through diffusing by 45° from the corner of the upper steel wall and the bearing plate to the top steel-concrete interface as shown in Figure

According to the above analysis, the displacement boundary conditions at

The displacement boundary conditions at

And then, the following equations could be obtained:

By substituting equation (_{1} and _{2} could be obtained:

The total shear force of connectors at _{i} is the integral of the shear force of the continuous spring layer in the range of (_{i} − _{i} +

The shear force _{ss}(_{i}) for headed studs and the shear force _{sp}(_{i}) for PBLs at _{i} could be obtained according to their shear stiffness, as follows:

Figure

Details of the test specimen (unit: mm).

Mechanical properties of the steel component.

Specimen | Plate thickness |
Yield strength _{y} (MPa) |
Tensile strength _{t} (MPa) |
Modulus of elasticity _{c} (GPa) |
---|---|---|---|---|

Longitudinal steel web | 13 | 347 | 466 | 213 |

Outer wall plate | 10 | 350 | 471 | 211 |

Stiffened steel plate | 8 | 377 | 512 | 196 |

Steel bar | — | 480 | 548 | 199 |

Mechanical properties of the concrete component.

Compressive strength _{cu} (MPa) |
Tensile strength _{t} (MPa) |
Modulus of elasticity _{c} (GPa) |
---|---|---|

67.8 | 3.2 | 41.5 |

The loading test setup is shown in Figure

Loading test setup.

The measuring point layout for the test specimen is shown in Figure

Layout of measuring positions.

The embedded strain gauges were set in the steel lattice cells to measure the compressive strain of the concrete. The strain gauge numbers were F1, F2, F3, F4, and F5 in sequence as shown in Figure

Figure

Load-slip curve of the test specimen.

Figure

Stress distribution of the steel structure.

Figure

Stress distribution of the concrete structure.

Figure

Distribution of axial force along the vertical direction shared by the steel and concrete structures.

The finite element model of the joint part was established using the finite element software ANSYS. Steel plates were modelled using the shell element SHELL63, and concrete components were modelled using the solid element SOLID65. The contact pressure at the steel-concrete interface was simulated by the contact element, while the adhesive friction between the contact surfaces was ignored. The shear connectors at the steel-concrete interface were modelled by a linear spring element, and the shear stiffness of the perfobond plate connector _{ps} is shown by the following equation according to the Chinese specification JTG/T D64-1 [_{s} is the diameter of the reinforcement, _{c} is the modulus of elasticity of the concrete, and _{ck} is the characteristic compressive cylinder strength of concrete.

The shear stiffness of the headed stud connector _{ss} is shown by the following equation according to the study by Lin et al. [_{s} is the diameter of the headed stud and _{s} is the modulus of elasticity of the steel.

Figure

Comparison between FEA and model test results.

Substituting the calculated parameters into equation (

Contrast of shear force distribution.

The theoretical analysis method proposed in this paper is adopted to implement a parameter analysis based on the hybrid joint structure of the Jishui Gan River Second Bridge; meanwhile, the load transfer mechanism of the hybrid joint is discussed to provide a useful reference for the similar hybrid joint design.

Figure

Load-slip curve with various shear stiffness.

Figure

Effect of hybrid joint length on shear force distribution.

Figure _{s}_{s} and _{c}_{c} represent the axial tensile/compressive stiffness of the steel plate and the concrete component, respectively. It can be seen from Figure

The relationship between the shear force and the axial stiffness of the hybrid joint.

Figure

Effect of the connector spacing on the shear force distribution.

Based on the elastic continuous layer method, a theoretical prediction method for the hybrid joint with the cells and rear bearing plates is given, which could be employed to predict the shear force distribution of the hybrid joint. The slip effect at the steel-concrete interface and the local bearing effect on the internal concrete by the steel bearing plates are both considered in the proposed theoretical model.

A scaled test model for the hybrid joint of the Jishui Gan River Second Bridge was conducted; the stress distribution of the steel and concrete components and the relative slip at the steel-concrete interface were measured and compared with the finite element analysis and the theoretical analysis results. The theoretical analysis results have a good agreement with the model test results, proving the accuracy of the proposed theoretical analysis method.

The proposed theoretical analysis method was employed to explore the load transfer mechanism; it is concluded that, with the increase of the distance from the top bearing plate to the bottom of the hybrid joint, the relative slip will decrease firstly and then increase till the bottom of the hybrid joint. The maximum shear force of the connectors at the steel-concrete interface will have an increasing tendency with the increase of the axial tensile/compressive stiffness of the steel plate. With the increase of the connector spacing, the sharing proportion of the initial axial force for the steel structure decreases, and the sharing proportion of initial axial force for the concrete column component increases.

The test results of the steel-concrete hybrid joint in this paper are available from the corresponding author upon request.

The authors declare that they have no conflicts of interest.

This work was financially supported by the National Natural Science Foundation (Grant no. 51578406) of People’s Republic of China.