The grouted mortise-tenon joint, invented as the connection between the large prefabricated components, is the key to the prefabricated underground structures, and the double-tenon joint is most widely used in the prefabricated metro station structure. This paper conducts characteristic analysis of bending stiffness with a 1 : 1 prototype test in key working direction of different joint types for grouted double mortise-tenon joint. The results show that the double-tenon joint is characteristic of variable stiffness under different loads. Change laws of double-tenon joint bending stiffness without and with auxiliary pretightening device in tension side and compression side are also discussed. The correlations for calculating double-tenon joint bending stiffness with various axial loads and bending moments are derived at last, which offers the theory foreshadowing of similar joints.
With the rapid development of rail transit construction in China, social-environmental awareness increases continuously. Moreover, long and tight construction period, large resource consumption, decrease of young laborers of civil engineering causing shortage of skilled labor, and not guaranteed structure quality bring great challenges to traditional construction technology of metro station. The above situations are particularly prominent in the northeast of China like Changchun city, located in the northeast region, which is so cold that 4∼5 months’ winter break is needed for metro construction, which causes huge deadline pressure and is hard to guarantee construction quality under low temperature [
To address these problems, a new prefabricated technology has been researched, developed, and implemented by Yang et al. team for constructing metro stations [
The six cut-and-cover stations are all supported by an anchor-pile system. All those horseshoe-shaped two-storey stations are 20.5 m wide and 17.45 m high. The full prefabricated station structure is assembled by seven 2 m width prefabricated components (see Figure
Prefabricated metro station display and arrangement of grouted double-tenon joint.
In the past, scholars have carried out a more in-depth study on the mechanical behavior of shield tunnel segment joints [
In this study, the bending properties of three kinds of grouted double-tenon joints under various axial loads were tested, including (1) the joint without auxiliary pretightening device; (2) the joint with auxiliary pretightening device in tension side; (3) the joint with auxiliary pretightening device in compression side. The test conditions and loading mode are shown in Table
Test cases and layout.
Cases | Axial load: 0 kN, 500 kN, 1000 kN, and 1250 kN | Without auxiliary pretightening device | ||
With auxiliary pretightening device | ||||
Test content | The amount of opening and compression of the joint (so as to obtain the joint rotation angle); the whole process record of crack development details of the joint | |||
Loading layout | ||||
In the test, the edge measurement method is used to indirectly measure the joint rotation angle (as shown in Figure
Measurement method for rotation-displacement of grouted double-tenon joint.
The main structural components of prefabricated metro station are in an eccentric compression state. For the joint interface, compression effects of axial force, the connection force of joint (such as the force of steel bar, the bond force of interface, etc.), and the embedment of long tenon are a pair of equilibria corresponding to the bending action of the joint. The bending moment formed by the axial force, the joint connection force, and the embedment effect of the long tenon in bending deformation are used to resist the bending moment of the joint. It is an important factor to keep the joint stable, which we call the “resistance” of the joint.
Calculation model of resistance bending moments
Calculation model of resistance
The expression of “resistance” is The resistance moment (resistance caused by the active force) formed by the axial force of the components changes with the axial force, and the resistance moment where The resistance force When When Due to the low tensile strength of concrete, the resistance of The mortise and tenon of the joint embedded together can also provide a certain resistance under the action of bending moment. In calculation, the moment
The joint resistance, especially the resistance moment, is an important index to analyze the mechanical behavior of the joints.
According to the joint rotation angle
Comparison of
It can be found that
Through in-depth analysis of
After clarifying the physical meaning of the above joint behavior, we can divide the Linear stage: in this stage, the peak load moment does not exceed the resistance moment of the joint. The joint is not damaged at this stage, and the bending moment is basically linear with the rotation angle. Particularly in the initial stage of loading, there is almost no rotation for the joint surface, and the rotation angle comes from the rotation of the beam. Nonlinear stage (joint softening stage): the main reason of joint softening at this stage is that the integrity of joint becomes worse after cracks appear in the joint, and the other reason is that the concrete has entered a significant plastic stage. Joint instability stage: at the end of the softening stage, the cracks of the joint are completely penetrated, transient failure and instability occur in the joint, and the bearing capacity is completely lost. The joint rotation angle develops rapidly. At this time, the
Figure
Comparison of
Figure
Comparison of
Figure
Comparison of
There are significant differences in the form of
Figure
Comparison of
The bending stiffness
In the past, a fixed value of joint bending stiffness is usually used for structural analysis according to experience in the structural design of shield tunnel. According to the
In this paper, the
Using the In the whole loading range, with the change of the load atmosphere, the stiffness of the joint is also in a state of constant change, and the variable stiffness characteristics of the joint are obvious. The specific change trend is as follows: under a certain axial force, the stiffness of the joint decreases with the increase of bending moment. Under the same bending moment, the stiffness of the joint increases with the increase of axial force. The joint stiffness of load-bearing zone and design utilization zone under different axial loads is present in Table
Bending stiffness curve of double-tenon joints without auxiliary pretightening device under different axial loads.
Design ranges and stiffness values of double-tenon joints without auxiliary pretightening device under different axial loads.
Axial load (kN) | Design bearing range (kN·m) | Design using bending stiffness (kN·m/rad) |
---|---|---|
500 | 0∼325 | 1.96 |
1000 | 0∼450 | 1.94 |
1250 | 0∼562.5 | 1.52 |
From the
The The characteristics of variable stiffness of the joint are basically consistent with those of the joint without auxiliary pretightening device when the auxiliary pretightening device is located in tension side. However, due to the reinforcement effect of the pretightening steel bar, the joint stiffness is in a constant change state with the change of the load atmosphere, and the variable stiffness characteristics of the joint are obvious. The specific change trend is as follows: under a certain axial force, the stiffness of the joint decreases with the increase of bending moment. Under the same bending moment, the stiffness of the joint increases with the increase of axial force. The joint stiffness of load-bearing zone and design utilization zone under different axial loads is present in Table
Bending stiffness curve of double-tenon joints with auxiliary pretightening device in tension side under different axial loads.
Design ranges and stiffness values of double-tenon joints with auxiliary pretightening device in tension side under different axial loads.
Axial load (kN) | Design bearing range (kN·m) | Design using bending stiffness (kN·m/rad) |
---|---|---|
500 | 0∼380 | 3.90 |
1000 | 0∼490 | 3.02 |
1250 | 0∼637.5 | 4.06 |
Change law of joint bending stiffness under different axial load and bending moment.
It can be seen that the joint reflects the characteristics of variable stiffness, and the bending stiffness increases with the increase of the axial force of the structure and decreases with the increase of the bending moment. In the beginning, the joint stiffness is large, with the bending moment loading exceeding the resistance moment, the joint crack appears, the joint stiffness decreases, and in the later stage of bearing, the permanent crack forms and enters the instability stage and the bending stiffness decreases rapidly.
Comparison of joint bending stiffness among different double-tenon joints (1000 kN).
The joint without auxiliary pretightening device enters the instability stage at the earliest stage (560 kN·m), followed by the joint with auxiliary pretightening device in compression side (680 kN·m), and the joint with auxiliary pretightening device in tension side enters the instability stage at the latest (717 kN·m). At this moment, the bending stiffness is reduced to the order of 4 of 10 and then rapidly decays.
From the view of resistance moment, when the load of the joint without auxiliary pretightening device exceeds the resistance moment of 450 kN·m, the bending stiffness of the joint without auxiliary pretightening device is 1.94
Through the analysis of
Relation curve of parameter (A) and (B) and axial force (double-tenon joint without auxiliary pretightening device).
Empirical formula of bending stiffness for grouted double mortise-tenon joint.
Joint types | Empirical formulas |
---|---|
Without auxiliary pretightening device | |
Auxiliary pretightening device in tension side |
This paper analyzes the loading test data of 1 : 1 prototype joint under the combined action of axial force and bending moment for various types of grouted double-tenon joints and obtains the basic law of bending stiffness characteristics of grouted double-tenon joint based on the The test study reveals that the double-tenon joints have variable stiffness characteristics. The joint also shows different joint stiffness properties under different load atmosphere. The stiffness changes with the change of axial force and bending moment. For the same type of joint, with the increase of axial force, the bearing capacity and bending stiffness of the joint increase. Under the action of constant axial force, the stiffness is closely related to the loading moment. The On the whole, the setting of auxiliary pretightening device in tension side is helpful to improve the bending bearing capacity of the joint. However, the auxiliary pretightening device in compression side forms a negative moment in compression side, which reduces the bending stiffness in the early stage of loading. The auxiliary pretightening device in tension side provides additional resistance moment, delays the deterioration process of the joint, and improves the ductility of the joint. The empirical formulas for calculating the bending stiffness of different types of double-tenon joints with different axial loads and bending moments are obtained, which is helpful for the design and calculation of similar joints in the future.
The data used to support the findings of this study are available from the corresponding author upon request.
The authors declare that they have no conflicts of interest.
Xiuren Yang supervised the project and developed the concept and methodologies with Meiqun Huang. Fang Lin performed the experimental and theoretical studies.
This research was funded by the National Key S&T Special Projects, under Grant no. 2017YFB1201104.