Method for calculating shear bearing capacity of steel pipe and external concrete under freeze-thaw fatigue

Abstract

The invention relates to the technical field of steel pipe concrete structure design and safety evaluation, in particular to a method for calculating the shear bearing capacity of a steel pipe and external concrete under freeze-thaw fatigue. Establishing a three-section constitutive model according to the load slip curve of the stud-free test piece; according to the load slip curve of the stud-free test piece under the action of N freeze-thaw cycles and n fatigue loads and the three-section constitutive model, a shear capacity formula of the stud-free test piece is established; according to the load slip curve of the test piece with the stud in the target combined test piece under the action of N freeze-thaw cycles and n fatigue loads, obtaining a shear capacity formula of the stud of the test piece with the stud; and establishing a shear bearing capacity calculation formula of the steel pipe and the external concrete under the freeze-thaw fatigue. The method has the positive effects of realizing safety evaluation and structural durability prediction of the composite structure with the concrete-filled steel tube wrapped by the external concrete under the action of freeze-thaw cycle and fatigue load.

Description

Technical Field

[0001] The invention relates to the technical field of steel tube concrete structure design and safety assessment, and in particular to a method for calculating the shear bearing capacity of a steel tube and external concrete under freeze-thaw fatigue. Background Art

[0002] In modern building structures, the composite structure of steel tube concrete wrapped with external concrete is an efficient structural form. Its core advantage lies in the synergistic effect of steel, studs and concrete. Due to its high bearing capacity, good plasticity and ductility, and superior seismic performance, it is widely used in high-rise buildings, subways, bridges and other projects. Shear performance is the key to determining the overall effectiveness of the structure and directly affects durability. However, the current calculation method for the shear bearing capacity of steel tubes and external concrete under freeze-thaw environments and fatigue loads is still unclear. The reasons include the following aspects: On the one hand, the internal steel tube concrete structure is a smooth-surfaced steel component. The shear performance of the composite structure formed after being wrapped by external concrete is different from that of ordinary reinforced concrete structures with and without studs. Therefore, the traditional steel-concrete shear performance calculation formula cannot be used to effectively analyze and calculate the shear capacity of the composite structure of steel tubes and external concrete. On the other hand, composite structures with concrete-encased steel tubes (CFSTs) face dual challenges in cold regions: freeze-thaw cycles damage the external concrete's microstructure (volume expansion and contraction due to phase changes in moisture), weakening the bond between the steel tubes and the external concrete and the shear resistance of the studs. Furthermore, long-term fatigue loads (such as vehicle and wind loads) exacerbate the accumulation of slip damage, further compromising the durability of the structure. Furthermore, the lack of ability to assess the shear performance of composite structures with concrete-encased steel tubes (CFSTs) hinders the prediction of structural durability. Summary of the Invention

[0003] The purpose of the present invention is to provide a method for calculating the shear bearing capacity of steel pipes and external concrete under freeze-thaw fatigue, which solves the technical problem of calculating the shear bearing capacity of the composite structure of steel pipe concrete wrapped with external concrete under freeze-thaw cycles and fatigue loads, so as to achieve the purpose of realizing safety assessment and structural durability prediction of the composite structure of steel pipe concrete wrapped with external concrete under freeze-thaw cycles and fatigue loads.

[0004] The method for calculating the shear bearing capacity of a steel pipe and external concrete under freeze-thaw fatigue provided by the present invention is characterized by comprising the following steps: Step 1: Determine the number of target composite specimens after N freeze-thaw cycles and n fatigue loads based on the number of freeze-thaw cycles and fatigue loads to which the target composite specimens are subjected, wherein the target composite specimens are a composite structure of an outer concrete structure and a steel tube concrete-filled structure with or without studs, including specimens with and without studs. Step 2: Conduct material property tests on the target combination specimens with and without studs; Step 3, performing N freeze-thaw cycles and n fatigue load tests on the specimens without studs and the specimens with studs in the target combination specimens; Step 4: Perform push-out tests on the target combination specimens with and without studs under freeze-thaw cycles and fatigue loads, and obtain load-slip curves of the specimens with and without studs under N freeze-thaw cycles and n fatigue loads; Step 5: Establish a three-segment constitutive model based on the load-slip curve of the specimen without studs; Step 6: Based on the load-slip curve and three-segment constitutive model of the specimen without studs under N freeze-thaw cycles and n fatigue loads, establish the shear capacity formula of the specimen without studs; Step 7: Obtain a shear capacity formula for the studs of the target composite specimen based on the load-slip curve of the stud specimen under N freeze-thaw cycles and n fatigue loads; Step 8: Based on the shear capacity formula of the specimen without studs and the shear capacity formula of the studs in the specimen with studs, establish a calculation formula for the shear bearing capacity of the steel pipe and the external concrete under freeze-thaw fatigue.

[0005] Furthermore, in step 1, the concrete selected is ordinary concrete; a hot-rolled, weld-free ordinary carbon structural cylindrical steel pipe with a smooth surface is selected as a component for the built-in steel tube concrete; the studs are ordinary studs, and the required number of target combination specimens is determined based on the number of freeze-thaw cycles and the number of fatigue load actions, wherein the target combination specimens include specimens with studs and specimens without studs on the outer surface of the steel pipe.

[0006] Furthermore, in step 2, a material property test is performed on the materials constituting the target composite test piece to determine the material properties of the target composite test piece.

[0007] Furthermore, in step 3, fatigue loading tests are performed on the target combination specimen n times.

[0008] Furthermore, in step 4, a push-out test is performed on the target composite specimen under freeze-thaw cycles and fatigue loads, and the load-slip curve of the target composite specimen under N freeze-thaw cycles and n fatigue loads is obtained through a pressure control computer.

[0009] Furthermore, for the specimens without studs, the load obtained in the push-out test is converted into the target bond strength of the composite specimen according to the ratio of the load obtained in the push-out test to the cross-sectional perimeter and embedded length of the internal steel tube concrete. The calculation method is as follows:

[0010] Draw the load-slip curve of the specimen without studs under N freeze-thaw cycles and n fatigue loads. Where, τ is the bond strength; P is the load size; A is the total surface area of ​​the steel pipe anchor; C a is the cross-sectional perimeter of the internal steel tube concrete; L is the embedded length of the internal steel tube concrete.

[0011] Furthermore, for the specimen with studs, the load-slip curve of the specimen with studs under N freeze-thaw cycles and n fatigue loads is obtained through a pressure-controlled computer.

[0012] Furthermore, in step 5, the load-slip curve of the specimen without studs is analyzed, and the bond-slip characteristic values ​​are obtained by the pressure control computer, which are recorded as the peak bond strength τ u , residual bond strength τ r Peak slip S u and residual slip S r , a three-segment constitutive model is obtained; the three-segment constitutive model is described segmentally according to the bond-slip characteristic value, and the model functions of the rising segment, the descending segment and the residual segment are obtained; The rising segment is a power function, the falling segment is a linear function, and the residual segment is a constant about τ. The three-segment constitutive model is expressed as follows:

[0013] Where, τ u is the peak bond strength, α, β, γ are the rising segment parameters affected by the number of freeze-thaw cycles and fatigue loads, K is the descending segment parameter affected by the number of freeze-thaw cycles and fatigue loads, δ is the residual segment parameter affected by the number of freeze-thaw cycles and fatigue loads, S is the slip amount corresponding to the bond strength, S u 、S r are the slip amount and residual slip amount corresponding to the peak bond strength, respectively.

[0014] Furthermore, in step 5, the origin software is used to fit the ascending segment, descending segment, and residual segment of the three-segment constitutive model of the specimen without studs, and the ascending segment parameters, descending segment parameters, and residual segment parameters are substituted into the three-segment constitutive model to establish the three-segment constitutive model affected by freeze-thaw cycles and fatigue load times as follows: .

[0015] Furthermore, in step 6, the shear capacity of the specimen without studs is calculated by obtaining the relationship between the peak bond strength of the specimen without studs and N freeze-thaw cycles and n fatigue load times in the target composite specimen. The calculation formula for the shear capacity of the specimen without studs is as follows:

[0016] Among them, P u (N,n) is the shear capacity of the specimen without studs, τ0 is the initial bond strength of the specimen without studs in the target combination specimen under 0 freeze-thaw cycles and 0 fatigue loads, α1 is the influence parameter of N freeze-thaw cycles on the specimen without studs in the target combination specimen, that is, the freeze-thaw damage parameter, N is the number of freeze-thaw cycles applied to the target combination specimen, β1 is the influence parameter of n fatigue loads on the specimen without studs in the target combination specimen, that is, the fatigue gain parameter, n is the number of fatigue loads applied to the target combination specimen, and A is the total surface area of ​​the steel pipe anchorage.

[0017] Furthermore, in step 6, the freeze-thaw damage parameter α1 and fatigue gain parameter β1 of the specimen without studs are quantified by the origin software to calculate the shear capacity of the specimen without studs. The calculation formula of the shear capacity of the specimen without studs is: .

[0018] Furthermore, in step 7, the shear capacity of the studs in the target combination specimen is obtained by the relationship between the peak load in the load-slip curve of the stud specimen and the number of freeze-thaw cycles (N) and fatigue load actions (n). The calculation formula is as follows:

[0019] Among them, P s (N,n) is the shear capacity of the studs in the specimen with studs, P s0 is the shear bearing capacity of the stud specimen in the target combination specimen under 0 freeze-thaw cycles and 0 fatigue loads, γ1 is the influence parameter of N freeze-thaw cycles on the stud specimen in the target combination specimen, that is, the freeze-thaw contribution factor, and δ1 is the influence parameter of n fatigue loads on the stud specimen in the target combination, that is, the fatigue attenuation factor.

[0020] Furthermore, in step 7, the freeze-thaw contribution factor γ1 and fatigue attenuation factor δ1 of the specimen with studs were quantified using the origin software, and the calculation formula for the shear capacity of the studs under freeze-thaw cycles and fatigue loads was obtained, which is expressed as: .

[0021] Furthermore, in step 8, the shear capacity calculation formula of the steel pipe and the external concrete under freeze-thaw fatigue is established by using the shear capacity calculation formula of the specimen without studs and the shear capacity calculation formula of the studs in the specimen with studs, which is expressed as:

[0022] Where P(N,n) is the shear bearing capacity of the steel pipe and the external concrete under freeze-thaw fatigue; the freeze-thaw damage parameter α1 and the fatigue gain parameter β in the shear capacity formula of the specimen without studs are 1， The freeze-thaw contribution factor γ1 and fatigue attenuation factor δ1 of the shear bearing capacity formula of the stud in the stud specimen are obtained as follows: .

[0023] The method provided by the present invention for calculating the shear bearing capacity of a steel tube and external concrete under freeze-thaw fatigue conditions allows the shear bearing capacity of the target composite specimen to be determined by subjecting it to N freeze-thaw cycles and n fatigue loads, eliminating the need for repeated testing. This reduces the number of experimental steps and saves costs. The present invention has the positive effect of enabling safety assessment and structural durability prediction of composite structures of steel tubes and concrete encased in external concrete under freeze-thaw cycles and fatigue loads. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 It is a working principle diagram of the present invention; Figure 2 Dimensional parameter diagram of the cross section and longitudinal section of the target composite test piece in the embodiment of the present invention; Figure 3 Design drawing of the stud in the embodiment of the present invention; Figure 4 Load-slip curves of the nail-free specimen under 0 freeze-thaw cycles and 0, 9,000, and 15,000 fatigue loads in an embodiment of the present invention; Figure 5 Load-slip curves of the nail-free specimens under 90 freeze-thaw cycles and 0, 9,000, and 15,000 fatigue loads according to the embodiment of the present invention; Figure 6 Load-slip curves of the nail-free specimens under 150 freeze-thaw cycles and 0, 9,000, and 15,000 fatigue loads in accordance with the present invention; Figure 7 Load-slip curves of the studded specimen under conditions of 0 freeze-thaw cycles and 0, 9,000, and 15,000 fatigue loads in an embodiment of the present invention; Figure 8 Load-slip curves of the studded specimens subjected to 90 freeze-thaw cycles and 90, 9,000, and 15,000 fatigue loads according to an embodiment of the present invention; Figure 9 Load-slip curves of the studded specimens under 150 freeze-thaw cycles and 1500, 9000, and 15000 fatigue loads in an embodiment of the present invention; Figure 10 Schematic diagram of a three-segment constitutive model in an embodiment of the present invention; Figure 11 Graphs showing the three-segment constitutive model verification under conditions of 0 freeze-thaw cycles and 0, 9,000, and 15,000 fatigue loads in an embodiment of the present invention; Figure 12 Verification diagram of the three-segment constitutive model under 90 freeze-thaw cycles and 0, 9000, and 15000 fatigue loads in the embodiment of the present invention; Figure 13 Verification diagram of the three-segment constitutive model under the conditions of 150 freeze-thaw cycles and 0, 9000, and 15000 fatigue loads in the embodiment of the present invention; Figure 14 This is a diagram illustrating the calculation formula for the shear capacity of a specimen without studs according to an embodiment of the present invention; Figure 15 This is a verification diagram of the calculation formula for the shear bearing capacity of the steel pipe and external concrete under freeze-thaw fatigue in an embodiment of the present invention. DETAILED DESCRIPTION

[0025] The following further describes the method for calculating the shear bearing capacity of a steel pipe and external concrete under freeze-thaw fatigue provided by the present invention through a specific embodiment of the present invention.

[0026] According to the number of freeze-thaw cycles and fatigue loads to which the target combination specimens are subjected, the number of target combination specimens under N freeze-thaw cycles and n fatigue loads is determined. The number of freeze-thaw cycles in this experiment is 0, 90, and 150 times, and the number of fatigue loads is 0, 9000, and 15000 times. The number of specimens and the naming method are shown in Table 1. Table 1 Number of specimens and naming method

[0027] A total of 18 target composite specimens were designed for the experiment, including 9 composite structures of steel tube concrete without studs and 9 composite structures of steel tube concrete with studs. The target composite specimens were made, and the cross-sectional and longitudinal section dimension parameters of the specimens are shown in the figure. Figure 2 As shown, the bolt design is as follows Figure 3 As shown, in the target combination specimen, the diameter of the steel pipe is 144 mm, the length of the steel pipe is 250 mm, the embedded length is 300 mm, the distance between the stud and the bottom side of the steel pipe is 150 mm, the diameter of the stud is 10 mm, and the length is 40 mm.

[0028] Material property tests were conducted on the target combination specimens with and without studs. The concrete material property table is shown in Table 2: Table 2 Concrete material properties

[0029] The steel material properties are shown in Table 3: Table 3 Steel material properties

[0030] The target combination specimens were subjected to N freeze-thaw cycles and n fatigue load tests on the specimens without and with studs. The freeze-thaw and fatigue tests were carried out in accordance with the "Code for Test of Long-term and Durability Properties of Ordinary Concrete" (GB / T 50082-2009). The freeze-thaw test used a slow freezing method (air freezing and water thawing) and designed 0, 90, and 150 freeze-thaw cycle tests. During the freezing stage, the air temperature of the freeze-thaw test chamber was maintained at -20°C to -18°C. During the thawing stage, the concrete specimens were immersed in warm water at 18°C ​​to 20°C. A DW-40 freeze-thaw machine was selected as a dedicated equipment. The fatigue test was conducted on the target combination specimens using an electro-hydraulic servo dynamic and static testing machine. The target combination specimens were first loaded to a preset maximum load Pmax, and then a sinusoidal fatigue load was applied at a frequency of 4 Hz. The test was stopped when the predetermined number of cycles was reached or the free end slip reached 30 mm (whichever came first).

[0031] The push-out test was conducted on the target combination specimens that completed the freeze-thaw cycle test and fatigue load test. A 300KN universal servo testing machine was used to obtain the load-slip curve, and the test results were substituted into the following formula: The load obtained from the push-out test is converted into the target composite specimen bond strength. Where, τ is the bond strength; P is the load size; A is the total surface area of ​​the steel pipe anchor; C a is the cross-sectional perimeter of the internal steel tube concrete; L is the embedded length of the internal steel tube concrete.

[0032] After obtaining all the push-out test results of the target combination specimens, the load-slip curves of the target combination specimens under N freeze-thaw cycles and n fatigue loads are drawn. The load-slip curves of A00, A01, and A02 in the target combination specimens are shown in Figure 2. Figure 4 As shown in the figure, the load-slip curves of A10, A11, and A12 in the target combination specimen are as follows: Figure 5 As shown in the figure, the load-slip curves of A20, A21, and A22 in the target combination specimen are as follows: Figure 6 As shown in the figure, the load-slip curves of B00, B01, and B02 in the target combination specimens are as follows: Figure 7 As shown in the figure, the load-slip curves of B10, B11, and B12 in the target combination specimens are as follows: Figure 8 As shown in the figure, the load-slip curves of B20, B21, and B22 in the target combination specimens are as follows: Figure 9 shown.

[0033] The load-slip curve of the specimen without studs was analyzed, and the bond-slip characteristic values ​​were obtained by pressure control computer, which were recorded as peak bond strength τ u , residual bond strength τ r Peak slip S u and residual slip S r , a three-segment constitutive model is obtained. Figure 10 As shown in the figure, the three-segment constitutive model is described in sections according to the bond-slip characteristic value, and the model functions of the rising section, the falling section and the residual section are obtained. Among them, the rising section is a power function, the falling section is a linear function, and the residual section is a constant about τ. The three-segment constitutive model is expressed as follows:

[0034] Where, τ u is the peak bond strength, α, β, γ are the rising segment parameters affected by the number of freeze-thaw cycles and fatigue loads, K is the descending segment parameter affected by the number of freeze-thaw cycles and fatigue loads, δ is the residual segment parameter affected by the number of freeze-thaw cycles and fatigue loads, S is the slip amount corresponding to the bond strength, S u 、S r are the slip amount and residual slip amount corresponding to the peak bond strength, respectively.

[0035] The origin software was used to fit the rising section, descending section, and residual section of the three-section constitutive model of the specimen without studs under 0, 90, and 150 freeze-thaw cycles and 0, 9000, and 15000 fatigue loads. The parameters of the rising section, descending section, and residual section were obtained. The parameter fitting values ​​are shown in Table 4: Table 4 Parameter fitting values

[0036] Among them, since the experimental curve of A20 is too different from that of other specimens, it is not taken into account when calculating the average value of the ascending section. Substituting the ascending section parameters, descending section parameters, and residual section parameters into the three-segment constitutive model, a three-segment constitutive model affected by the number of freeze-thaw cycles and fatigue loads is established as follows: .

[0037] The obtained three-segment constitutive model is compared with the actual experimental data, as shown in the comparison figure. Figure 11 、 Figure 12 、 Figure 13 The results show that the three-segment constitutive model has a high degree of agreement with the experimental data in the ascending stage, and the prediction accuracy in the descending stage is also relatively reliable, which verifies the feasibility of the model.

[0038] The shear capacity of the specimen without studs was calculated by obtaining the relationship between the peak bond strength of the specimen without studs and the number of freeze-thaw cycles, freeze-thaw cycles, and fatigue loads of 0, 90, and 150 times, and 0, 9000, and 15000 times. The calculation formula of the shear capacity of the specimen without studs is as follows:

[0039] Among them, P u (N,n) is the shear capacity of the specimen without studs, τ0 is the initial bond strength of the specimen without studs in the target combination specimen under 0 freeze-thaw cycles and 0 fatigue loads, α1 is the influence parameter of N freeze-thaw cycles on the specimen without studs in the target combination specimen, that is, the freeze-thaw damage parameter, N is the number of freeze-thaw cycles applied to the target combination specimen, β1 is the influence parameter of n fatigue loads on the specimen without studs in the target combination specimen, that is, the fatigue gain parameter, n is the number of fatigue loads applied to the target combination specimen, and A is the total surface area of ​​the steel pipe anchorage.

[0040] The freeze-thaw damage parameter α1 and fatigue gain parameter β1 of the specimen without studs were quantified using the origin software, and the calculation formula for the shear capacity of the specimen without studs was obtained as follows: .

[0041] The reliability of the calculation formula for the shear resistance of the specimen without studs is verified by its bond strength. Figure 14 As shown, the calculated results are in good agreement with the experimental data, with a coefficient of determination (R²) of 0.98. The difference between the calculated and experimental values ​​was calculated, and the error analysis showed that the minimum error between the calculated and experimental values ​​for target combination specimen A01 was 0.2%, while the maximum error for target combination specimen A20 was 7.4%, with an overall mean absolute error of 1.88%. The relatively accurate results indicate that the obtained formula is feasible and further validate the reliability of the formula for calculating the shear capacity of studless specimens.

[0042] The shear capacity of the studs in the target combination specimens was obtained by the relationship between the peak load in the load-slip curve of the stud specimen and the number of freeze-thaw cycles of 0, 90, and 150 times and the number of fatigue loads of 0, 9000, and 15000 times. The calculation formula is as follows:

[0043] Among them, P s (N,n) is the shear capacity of the studs in the specimen with studs, P s0is the shear bearing capacity of the stud specimen in the target combination specimen under 0 freeze-thaw cycles and 0 fatigue loads, γ1 is the influence parameter of N freeze-thaw cycles on the stud specimen in the target combination specimen, that is, the freeze-thaw contribution factor, and δ1 is the influence parameter of n fatigue loads on the stud specimen in the target combination, that is, the fatigue attenuation factor.

[0044] The freeze-thaw contribution factor γ1 and fatigue attenuation factor δ1 of the specimen with studs were quantified by Origin software, and the calculation formula for the shear capacity of the studs under freeze-thaw cycles and fatigue loads was obtained, which is expressed as:

[0045] Based on the shear capacity calculation formula of the specimen without studs and the shear capacity calculation formula of the studs in the specimen with studs, the shear bearing capacity calculation formula of the steel pipe and the external concrete under freeze-thaw fatigue is established, which is expressed as:

[0046] Substituting the freeze-thaw damage parameter α1 and fatigue gain parameter β1 from the shear capacity formula of the specimen without studs, and the freeze-thaw contribution factor γ1 and fatigue attenuation factor δ1 from the shear capacity formula of the studs in the specimen with studs, we obtain the following formula: .

[0047] The calculation formula for the shear bearing capacity of steel pipe and external concrete under freeze-thaw fatigue is as follows: Figure 15 As shown, the model calculation results are in good agreement with the test data. The difference between the calculated and actual values ​​is calculated to obtain the calculated error. The error between the actual and calculated values ​​in B02 is a minimum of 3.9%, and the error between the actual and calculated values ​​in B20 is a maximum of 9.2%, with an average error of 5.37%. The calculation results are relatively accurate, indicating that the obtained formula is feasible. This also further verifies the reliability of the formula for calculating the shear bearing capacity of the steel pipe and the external concrete under freeze-thaw fatigue.

Claims

1. A method for calculating the shear bearing capacity of a steel pipe and external concrete under freeze-thaw fatigue, characterized in that: The following steps are included: Step 1: Determine the number of target composite specimens after N freeze-thaw cycles and n fatigue loads based on the number of freeze-thaw cycles and fatigue loads to which the target composite specimens are subjected, wherein the target composite specimens are a composite structure of an outer concrete structure and a steel tube concrete-filled structure with or without studs, including specimens with and without studs. Step 2: Conduct material property tests on the target combination specimens with and without studs; Step 3, performing N freeze-thaw cycles and n fatigue load tests on the specimens without studs and the specimens with studs in the target combination specimens; Step 4: Perform push-out tests on the target combination specimens with and without studs under freeze-thaw cycles and fatigue loads, and obtain load-slip curves of the specimens with and without studs under N freeze-thaw cycles and n fatigue loads; Step 5: Establish a three-segment constitutive model based on the load-slip curve of the specimen without studs; Step 6: Based on the load-slip curve and three-segment constitutive model of the specimen without studs under N freeze-thaw cycles and n fatigue loads, establish the shear capacity formula of the specimen without studs; Step 7: Obtain a shear capacity formula for the studs of the target composite specimen based on the load-slip curve of the stud specimen under N freeze-thaw cycles and n fatigue loads; Step 8: Based on the shear capacity formula of the specimen without studs and the shear capacity formula of the studs in the specimen with studs, establish a calculation formula for the shear bearing capacity of the steel pipe and the external concrete under freeze-thaw fatigue.

2. The method for calculating the shear bearing capacity of a steel pipe and external concrete under freeze-thaw fatigue according to claim 1 is characterized in that: For the specimens without studs, the load obtained in the push-out test is converted into the target composite specimen bond strength according to the ratio of the load obtained in the push-out test to the cross-sectional perimeter and embedded length of the internal steel tube concrete. The calculation method is as follows: Draw the load-slip curve of the specimen without studs under N freeze-thaw cycles and n fatigue loads. Where, τ is the bond strength; P is the load size; A is the total surface area of ​​the steel pipe anchor; C a is the cross-sectional perimeter of the internal steel tube concrete; L is the embedded length of the internal steel tube concrete.

3. The method for calculating the shear bearing capacity of a steel pipe and external concrete under freeze-thaw fatigue according to claim 2, characterized in that: The load-slip curve of the specimen without studs was analyzed, and the bond-slip characteristic values ​​were obtained by pressure control computer, which were recorded as peak bond strength τ u , residual bond strength τ r Peak slip S u and residual slip S r , a three-segment constitutive model is obtained; the three-segment constitutive model is described segmentally according to the bond-slip characteristic value, and the model functions of the rising segment, the descending segment and the residual segment are obtained; The rising segment is a power function, the falling segment is a linear function, and the residual segment is a constant about τ. The three-segment constitutive model is expressed as follows: Where, τ u is the peak bond strength, α, β, γ are the rising segment parameters affected by the number of freeze-thaw cycles and fatigue loads, K is the descending segment parameter affected by the number of freeze-thaw cycles and fatigue loads, δ is the residual segment parameter affected by the number of freeze-thaw cycles and fatigue loads, S is the slip amount corresponding to the bond strength, S u 、S r are the slip amount and residual slip amount corresponding to the peak bond strength, respectively.

4. The method for calculating the shear bearing capacity of a steel pipe and external concrete under freeze-thaw fatigue according to claim 3 is characterized in that: The origin software was used to fit the ascending segment, descending segment, and residual segment of the three-segment constitutive model of the specimen without studs, and the ascending segment parameters, descending segment parameters, and residual segment parameters were substituted into the three-segment constitutive model to establish the three-segment constitutive model affected by freeze-thaw cycles and fatigue load times as follows: 。 5. The method for calculating the shear bearing capacity of a steel pipe and external concrete under freeze-thaw fatigue according to claim 4 is characterized in that: The shear capacity of the specimen without studs is calculated by obtaining the relationship between the peak bond strength of the specimen without studs and the number of freeze-thaw cycles and fatigue loads. The calculation formula of the shear capacity of the specimen without studs is as follows: Among them, P u (N,n) is the shear capacity of the specimen without studs, τ0 is the initial bond strength of the specimen without studs in the target combination specimen under 0 freeze-thaw cycles and 0 fatigue loads, α1 is the influence parameter of N freeze-thaw cycles on the specimen without studs in the target combination specimen, that is, the freeze-thaw damage parameter, N is the number of freeze-thaw cycles applied to the target combination specimen, β1 is the influence parameter of n fatigue loads on the specimen without studs in the target combination specimen, that is, the fatigue gain parameter, n is the number of fatigue loads applied to the target combination specimen, and A is the total surface area of ​​the steel pipe anchorage.

6. The method for calculating the shear bearing capacity of a steel pipe and external concrete under freeze-thaw fatigue according to claim 5, characterized in that: The freeze-thaw damage parameter α1 and fatigue gain parameter β1 of the specimen without studs were quantified by Origin software, and the shear capacity of the specimen without studs was calculated. The calculation formula of the shear capacity of the specimen without studs is: 。 7. The method for calculating the shear bearing capacity of a steel pipe and external concrete under freeze-thaw fatigue according to claim 6, characterized in that: The shear capacity of the studs in the target combination specimen is obtained by the relationship between the peak load in the load-slip curve of the stud specimen and the number of freeze-thaw cycles (N) and fatigue load actions (n). The calculation formula is as follows: Among them, P s (N,n) is the shear capacity of the studs in the specimen with studs, P s0 is the shear bearing capacity of the stud specimen in the target combination specimen under 0 freeze-thaw cycles and 0 fatigue loads, γ1 is the influence parameter of N freeze-thaw cycles on the stud specimen in the target combination specimen, that is, the freeze-thaw contribution factor, and δ1 is the influence parameter of n fatigue loads on the stud specimen in the target combination, that is, the fatigue attenuation factor.

8. The method for calculating the shear bearing capacity of a steel pipe and external concrete under freeze-thaw fatigue according to claim 7, characterized in that: The freeze-thaw contribution factor γ1 and fatigue attenuation factor δ1 of the specimen with studs were quantified by Origin software, and the calculation formula for the shear capacity of the studs under freeze-thaw cycles and fatigue loads was obtained, which is expressed as: 。 9. The method for calculating the shear bearing capacity of a steel pipe and external concrete under freeze-thaw fatigue according to claim 8, characterized in that: Based on the shear capacity calculation formula of the specimen without studs and the shear capacity calculation formula of the studs in the specimen with studs, the shear bearing capacity calculation formula of the steel pipe and the external concrete under freeze-thaw fatigue is established, which is expressed as: Where P(N,n) is the shear bearing capacity of the steel pipe and the external concrete under freeze-thaw fatigue; the freeze-thaw damage parameter α1 and the fatigue gain parameter β in the shear capacity formula of the specimen without studs are 1， The freeze-thaw contribution factor γ1 and fatigue attenuation factor δ1 of the shear bearing capacity formula of the stud in the stud specimen are obtained as follows: 。

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