Pressure steel pipe and concrete combined stress analysis method

Abstract

The invention discloses a pressure steel pipe and concrete combined stress analysis method, which belongs to a hydropower station pressure steel pipe stress analysis technology, is used for analyzing the stress of a pressure steel pipe by considering the combined stress condition of the hydropower station pressure steel pipe and the concrete of a pouring gap, and comprises the following steps of: determining that when the hydropower station is operated, determining the pressure of the pressure steel pipe; the water pressure P borne by the interior of the pressure steel pipe; the water pressure P1 when the interior of the pressure steel pipe is subjected to water pressure deformation in the exposed pipe state is obtained; the stress process of the pressure steel pipe is simplified into two stages under the real operation state, and in the first stage, before the pressure steel pipe makes contact with concrete, the pressure steel pipe is independently stressed; in the second stage, after the pressure steel pipe makes contact with concrete, the pressure steel pipe and the concrete are jointly stressed; finally, the stress states of all discrete units of the pressure steel pipe in the discrete state are obtained, the stress and strain states of the pressure steel pipe in the state are evaluated, the evaluation result is more accurate, and the size design of the pressure steel pipe can be optimized conveniently.

Description

Technical Field

[0001] The invention relates to the technical field of force analysis of a penstock of a hydropower station, and in particular to a combined force analysis method of a penstock and concrete. Background Art

[0002] Water from the upstream of the hydropower station enters the turbine room through the penstock, driving the runner to rotate and generate electricity. Usually, the water level upstream of the hydropower station is high, and the penstock and other water diversion pipelines (hereinafter collectively referred to as penstocks) have to withstand greater water pressure, especially the location of the steel pipe openings, requiring the penstock to have a sufficient safety factor. In recent years, with the vigorous development of pumped-storage power stations, especially the construction of ultra-high head pumped-storage power stations, it is necessary to consider that the penstock has a sufficient safety margin. Otherwise, the damage of the penstock will further cause the damage of the concrete, which will have a huge impact on the safety of the power station.

[0003] At present, the stress analysis of penstock is mainly carried out by means of open pipes, that is, when analyzing the penstock and openings, the penstock and its branches are completely exposed to the air, the outside of the pipe is not subject to any constraints, and then water pressure is applied to the inside of the pipe for analysis. This method does not take into account that when the penstock expands under water pressure, when the deformation of the penstock is greater than the casting gap between the pipe and the concrete, the concrete will be subjected to stress together with the penstock, causing the stress on the penstock to be much higher than the actual state, resulting in a waste of steel.

[0004] In addition, a small number of researchers have conducted joint stress analysis on pressure steel pipes and concrete, but did not consider the stage when the steel pipe deformation is smaller than the gap between the steel pipe and the concrete pouring when the steel pipe is subjected to stress alone. Instead, the pressure steel pipe is allowed to be in direct contact with the concrete, which causes the concrete to bear more force, resulting in the stress on the pressure steel pipe being far lower than the actual state.

[0005] The first method of calculating and judging the stress on the penstock is too conservative, and the second combined stress method results in insufficient design safety factor, and neither can accurately analyze and evaluate the actual stress on the penstock of a hydropower station. Summary of the invention

[0006] The purpose of the present invention is to provide a method for analyzing the combined stress of a penstock and concrete, which takes into account the combined stress of the penstock and concrete in the casting gap of a hydropower station, and more accurately analyzes and evaluates the actual stress of the penstock of the hydropower station.

[0007] The present invention is achieved through the following technical solutions:

[0008] A combined stress analysis method for a pressure steel pipe and concrete comprises the following steps:

[0009] S1. Determine the water pressure P inside the penstock when the hydropower station is in operation;

[0010] S2. After the pouring is stable, the pouring gap between the penstock and the concrete is recorded as Δ. Through the parametric analysis model and the inverse calculation method, the water pressure P1 when the penstock is in the open pipe state and the internal water pressure deformation is Δ is obtained;

[0011] S3. The stress process of the penstock under the actual operation state is simplified into two stages. The first stage: before the penstock contacts the concrete, the penstock is subjected to stress alone; the second stage: after the penstock contacts the concrete, the penstock and the concrete are subjected to stress together, and a calculation model of the final stress state σ3 of the penstock is established;

[0012] S4. Finally, the stress state of all discrete units of the penstock under discrete state is obtained, and the stress and strain state of the penstock under this state is evaluated to optimize the size design of the penstock.

[0013] Furthermore, in step S2, the gap Δ between the concrete and the pressure steel pipe is usually 0.5 to 1.5 mm.

[0014] Furthermore, in step S2, the relationship between the internal water pressure deformation Δ and the water pressure P1 of the penstock in the open pipe state is obtained as follows:

[0015] S2.1. After the pouring is stable, a separate model of the penstock to be analyzed is established, i.e., a parametric analysis model of the penstock;

[0016] S2.2. Based on the parametric analysis of response surface, a numerical replacement model of the correlation between internal water pressure and pressure steel pipe deformation is obtained;

[0017] S2.3, by inverse calculation, obtain the water pressure P1 when the pressure steel pipe is in the open pipe state and the internal water pressure deformation is Δ. Further, in step S3, the method for obtaining the stress state of the pressure steel pipe under different states includes the following steps:

[0018] S3.1. Establish a joint model of the penstock and concrete to be analyzed. The joint model does not consider the gap between the penstock and concrete.

[0019] S3.2, discretize the penstock and concrete model according to the control equation of the following formula (1);

[0020]

[0021] Where, [M] is the structural mass matrix; [C] is the structural damping matrix; [K] is the structural stiffness matrix; {x} is the displacement vector; {F(t)} is the load vector;

[0022] S3.3, set the constraints and material properties of the penstock and concrete respectively;

[0023] S3.4. Set up two analysis steps, control the contact properties between the penstock and concrete through the contact switch, simulate different contact states between the penstock and concrete, and obtain the stress state of the penstock.

[0024] Furthermore, in step S3, the radial dimension of the concrete surrounding the penstock should be at least 8 times greater than the outer diameter of the penstock.

[0025] Furthermore, in step S3.4, the contact properties between the penstock and concrete are controlled by the contact switch CSS, and the stress state of the penstock is obtained as follows:

[0026] S3.4.1. Define CSS = NO, that is, the penstock is disconnected from the concrete. At this time, the penstock is subjected to the P1 water pressure alone, and the concrete is not subjected to stress. Calculate the stress state σ1 of the current penstock;

[0027] S3.4.2. Define CSS = YES, that is, the penstock is in contact with the concrete. At this time, the penstock and the concrete jointly bear the P-P1 water pressure. Calculate the current stress state σ2 of the penstock;

[0028] S3.4.3. Define the combination relationship Comb to calculate the final stress state σ3 of the penstock under discrete state, that is, σ3=σ2+σ1. The σ3 state is the actual state of joint stress of the penstock and concrete when the casting gap between the penstock and concrete is taken into account during the normal operation of the hydropower station.

[0029] Furthermore, in step S3.4.3, a calculation model for the final stress state σ3 of the pressure steel pipe is established to satisfy the following relationship (2):

[0030] Where, δ is the wall thickness of the pressure steel pipe, in mm;

[0031] P is the internal pressure of the pressure pipe, in MPa;

[0032] D is the inner diameter of the pressure steel pipe, in mm;

[0033] Δ is the pouring gap between the penstock and concrete, in mm;

[0034] E is the elastic modulus of the pressure steel pipe, in MPa.

[0035] Furthermore, in step S4, the method for evaluating the stress and strain state of the pressure steel pipe is:

[0036] S4.1. Extract the relationship between stress and strain of the penstock under the real state of joint stress of the penstock and concrete when simulating the normal operation of the hydropower station, that is, when considering the pouring gap between the penstock and concrete;

[0037] S4.2. If the evaluation results meet the design requirements, the analysis ends; if the evaluation results do not meet the design requirements, the structural parameters of the pressure steel pipe need to be redesigned and the analysis and calculation are re-performed according to the process of steps S2 to S4.

[0038] Furthermore, in step S4.2, the evaluation of the results is mainly divided into the following two aspects:

[0039] S4.2.1. For a penstock with constant cross-sectional dimensions and no branches, after the casting gap and the penstock are pressurized, in addition to calculating the final stress state σ3 of the penstock according to the technical solution provided by the present invention, the final stress state of the penstock after pressurization can also be directly calculated according to the following formula. Or use formula (2) to check the accuracy of the results calculated by other methods. Preferably, the error of the results calculated by the two methods shall not exceed 3%.

[0040]

[0041] S4.2.2 When the cross-sectional dimensions of the penstock vary and there are complex situations such as multiple branches, the penstock can be placed at a location that is geometrically uniform and far away from the stress concentration area, and the correctness of the calculation results can be verified by the following formula (3):

[0042]

[0043] For complex geometric models, σ3 can be expressed as:

[0044]

[0045] Among them, α and β are the stress concentration factors of the first and second stages respectively.

[0046] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0047] 1. In the present invention, the casting gap Δ (Δ is usually 0.5-1.5 mm) between the penstock and concrete is considered, and the water pressure when the internal water pressure deformation of the penstock in the open pipe state is just Δ is obtained through parametric analysis model and calculation inverse technology. Then, the force-bearing process of the penstock under the actual operation state is simplified into two stages. The first stage: before the penstock contacts the concrete, the penstock is subjected to force alone; the second stage: after the penstock contacts the concrete, the penstock and the concrete are subjected to force together. Finally, the force state of all discrete units of the penstock in the discrete state is obtained, and the stress and strain state of the penstock in this state is evaluated.

[0048] Second, in the present invention, a method is proposed for defining the contact properties of the penstock and concrete in two analysis stages through a CSS contact switch, so that the penstock and concrete are located in the same coordinate space, which facilitates obtaining reliable results.

[0049] 3. In the present invention, a method for obtaining a numerical replacement model of the correlation between internal water pressure and pressure steel pipe deformation based on response surface parametric analysis is proposed. Then, by reverse calculation, the water pressure P1 when the pressure steel pipe is exposed and the internal water pressure deformation is just Δ can be obtained.

[0050] Fourth, in the present invention, the actual gap between the pressure steel pipe and the concrete does not need to be considered in the second stage, which greatly simplifies the complexity of the analysis model, can multiply the convergence of the analysis model, and greatly shorten the calculation time. BRIEF DESCRIPTION OF THE DRAWINGS

[0051] Figure 1 It is a schematic diagram of the combined model of penstock and concrete.

[0052] Figure 2 It is a flow chart of the combined force analysis of the pressure steel pipe and concrete in the present invention.

[0053] Figure 3 It is a curve chart showing the relationship between pressure steel pipe stress and concrete outer diameter.

[0054] Figure 4 It is a schematic diagram of the stress on the exposed pipe state of the pressure steel pipe in method 1 in embodiment 1.

[0055] Figure 5 It is a schematic diagram of the pressure steel pipe, gap and concrete in method 3 in embodiment 1. DETAILED DESCRIPTION

[0056] The present invention is further described in detail below in conjunction with examples, but the embodiments of the present invention are not limited thereto.

[0057] The present invention proposes a joint stress analysis method for the penstock and concrete of a hydropower station taking into account the casting gap. Considering the casting gap Δ (Δ is usually 0.5-1.5 mm) between the penstock and the concrete, the water pressure when the internal water pressure deformation of the penstock in the open pipe state is just Δ is obtained through parametric analysis model and calculation inverse technology. Then, under the actual operating state, the stress process of the penstock is simplified into two stages. The first stage: before the penstock contacts the concrete, the penstock is subjected to stress alone; the second stage: after the penstock contacts the concrete, the penstock and the concrete are subjected to stress together. Finally, the stress state of all discrete units of the penstock in a discrete state is obtained, and the stress and strain state of the penstock in this state is evaluated. To facilitate the public to understand this scheme, it is further explained below with a specific implementation method.

[0058] Example 1

[0059] A combined force analysis method for a pressure steel pipe and concrete, relating to the technical field of force analysis of pressure steel pipes in hydropower stations, Figure 1 , comprising the following steps:

[0060] Step 1:

[0061] Determine the water pressure P inside the penstock when the hydropower station is in operation.

[0062] Step 2:

[0063] After the pouring is stable, the pouring gap between the penstock and the concrete is recorded as Δ. The gap between the concrete and the penstock is Δ, which is usually 0.5-1.5 mm. Through the parametric analysis model and the inverse calculation method, the water pressure P1 is obtained when the penstock is in the open pipe state and the internal water pressure deformation is Δ.

[0064] In this step, the relationship between the internal water pressure deformation Δ and the water pressure P1 of the penstock in the open pipe state is obtained as follows:

[0065] S2.1. After the pouring is stable, a separate model of the penstock to be analyzed is established, i.e., a parametric analysis model of the penstock;

[0066] S2.2. Based on the parametric analysis of response surface, a numerical replacement model of the correlation between internal water pressure and pressure steel pipe deformation is obtained;

[0067] S2.3. By inverse calculation, the water pressure P1 is obtained when the pressure pipe is in an open state and the internal water pressure deformation is Δ.

[0068] Step 3:

[0069] Under the actual operating state, the stress process of the pressure steel pipe is simplified into two stages. The first stage: before the pressure steel pipe contacts the concrete, the pressure steel pipe is subjected to stress alone; the second stage: after the pressure steel pipe contacts the concrete, the pressure steel pipe and the concrete are subjected to stress together.

[0070] More specifically, the method for obtaining the stress state of the penstock under different states includes the following steps:

[0071] S3.1. Establish the joint model of the penstock and concrete to be analyzed, refer to Figure 2 ,This joint model does not consider the gap between the penstock and the concrete;

[0072] S3.2, discretize the penstock and concrete model according to the control equation of the following formula (1);

[0073]

[0074] Where, [M] is the structural mass matrix; [C] is the structural damping matrix; [K] is the structural stiffness matrix; {x} is the displacement vector; {F(t)} is the load vector;

[0075] S3.3, set the constraints and material properties of the penstock and concrete respectively;

[0076] S3.4. Set up two analysis steps, control the contact properties between the penstock and concrete through the contact switch, simulate different contact states between the penstock and concrete, and obtain the stress state of the penstock.

[0077] In step S3.4, the contact properties between the penstock and concrete are controlled by a contact step switch (CSS), and the stress state of the penstock is obtained as follows:

[0078] S3.4.1. Define CSS = NO, that is, the penstock is disconnected from the concrete. At this time, the penstock is subjected to the P1 water pressure alone, and the concrete is not subjected to stress. Calculate the stress state σ1 of the current penstock;

[0079] S3.4.2. Define CSS = YES, that is, the penstock is in contact with the concrete. At this time, the penstock and the concrete jointly bear the P-P1 water pressure. Calculate the current stress state σ2 of the penstock;

[0080] S3.4.3. Define the combination relationship Comb to calculate the final stress state σ3 of the penstock under discrete state, that is, σ3=σ2+σ1. The σ3 state is the real state of joint stress of the penstock and concrete when the hydropower station is operating normally, considering the casting gap between the penstock and concrete. In this step, the casting gap between the penstock and concrete is considered, and the real stress state of all discrete units of the penstock under discrete state is considered, rather than the numerical addition and subtraction of the results of independent analysis in two stages, and the result is closer to the actual value.

[0081] Special attention should be paid to the following: To ensure the authenticity of the force on the penstock, the radial dimension of the concrete surrounding the penstock should be at least 8 times greater than the outer diameter of the penstock. Figure 3 , Figure 3 This is a curve diagram showing the relationship between the stress borne by the pressure steel pipe and the outer diameter of concrete in this embodiment.

[0082] Depend on Figure 3 It can be seen that when the ratio of the radial outer diameter of concrete / the outer diameter of the pressure steel pipe is greater than 8, the stress of the pressure steel pipe hardly changes, indicating that the model size of the concrete no longer has any effect on the stress calculation of the pressure steel pipe. Therefore, in order to improve the calculation accuracy, the radial outer diameter of concrete / the outer diameter of the pressure steel pipe should be greater than 8.

[0083] Step 4:

[0084] Finally, the stress state of all discrete units of the penstock under discrete state is obtained, and the stress and strain state of the penstock under this state is evaluated to optimize the size design of the penstock.

[0085] The specific operation method is:

[0086] S4.1. Extract the relationship between stress and strain of the penstock under the real state of joint stress of the penstock and concrete when simulating the normal operation of the hydropower station, that is, when considering the pouring gap between the penstock and concrete;

[0087] S4.2. If the evaluation results meet the design requirements, the analysis ends; if the evaluation results do not meet the design requirements, the structural parameters of the pressure steel pipe need to be redesigned and the analysis and calculation should be re-performed according to the process of steps S2 to S4. The evaluation of the results mainly distinguishes the following two aspects:

[0088] S4.2.1 For cross-sectional dimensions that are constant, refer to Figure 5 , and there is no branched penstock, considering the pouring gap and after the penstock is pressurized, in addition to calculating the final stress state σ3 of the penstock according to the technical solution provided by the present invention, the final stress state of the penstock after pressurization can also be directly calculated according to the following formula. Or use this formula to check the accuracy of the calculation results of the technical solution provided by this patent, and the error of the results calculated by the two shall not exceed 3%.

[0089]

[0090] S4.2.2, when the cross-sectional dimensions of the penstock vary and there are multiple branches and other complex situations, such as Figure 2 As shown, the pressure steel pipe can be taken in a geometrically uniform and far away from the stress concentration area, and the correctness of the calculation results can be verified by the following formula.

[0091]

[0092] For complex geometric models, σ3 can be expressed as:

[0093]

[0094] Among them, α and β are the stress concentration factors of the first and second stages respectively, which can be calculated by the finite element method.

[0095] In order to further determine the feasibility of this solution, the inventor of this solution conducted further research and verification in combination with existing technologies.

[0096] In the prior art, the following two methods, method 1 and method 2, are often used to analyze the stress of the pressure steel pipe:

[0097] Method 1: This method is also the main calculation method at present, that is, in the open pipe state, all internal pressures are borne by the pressure steel pipe. Figure 4 shown.

[0098] Method 2: This method does not consider the gap between the pressure steel pipe and the concrete, and all internal pressure is borne by the pressure steel pipe and the concrete together.

[0099] The method of the present invention (hereinafter referred to as "method 3") is to consider the gap between the penstock and the concrete. In the first stage, the penstock bears the internal pressure of P1 alone, and the stress state σ1 is obtained. In the second stage, the penstock and the concrete bear the internal pressure of (P-P1) together, and the stress state σ2 is obtained. The calculation result obtained by method 3 is σ3=σ2+σ1. The simplified diagram of the initial state of the penstock, gap and concrete is shown in FIG. Figure 5 In this embodiment, the above three methods are used to analyze the same experimental object, and the stresses of the pressure steel pipe are calculated respectively. The results are shown in Table 1. The inner diameter D and wall thickness δ of the pressure steel pipe used in the experiment, as well as the internal pressure P borne by the pressure steel pipe in the experiment are shown in Table 1.

[0100] Table 1

[0101]

[0102] As can be seen from Table 1, the calculation results of method 1 are much larger than those of method 3 (the method of this scheme), which shows that the data obtained by using the method has caused a lot of material waste in engineering applications. The calculation results obtained by method 2 are smaller than those of method 3. This is because the calculation model is distorted, and the concrete bears a large part of the load, resulting in insufficient design safety margin for the penstock, which causes inaccurate analysis and evaluation of the actual stress of the penstock of the hydropower station.

[0103] To facilitate the public's understanding of the evaluation process in this plan, the process of obtaining the aforementioned mathematical model is briefly described below, and the relationship between factors such as the pressure steel pipe wall thickness δ, inner diameter D and the stress on the pressure steel pipe is further examined.

[0104] In Table 2 below, the parameters of the penstock used in the experiment are: Δ is 0.5mm, D is 2500mm, and δ is 30mm. Among them, σ1 represents the stress state of the penstock when method 3 is adopted, when the penstock is subjected to P1 water pressure alone and the concrete is not stressed; σ2 represents the stress state of the penstock when the penstock is in contact with concrete and the penstock and concrete are jointly subjected to P-P1 water pressure; σ3 is σ2+σ1; It indicates the stress generated inside the penstock when it is subjected to P-P1 internal pressure alone; It represents the result obtained by calculating the model established by finite element method, which takes into account the casting gap and is completely consistent with the actual situation.

[0105] Table 2

[0106]

[0107]

[0108] In Table 3 below, the parameters of the pressure steel pipe used in the experiment are: Δ is 0.5 mm, D is 2500 mm, and δ is 45 mm.

[0109] Table 3

[0110]

[0111] From Table 2 and Table 3, we can see that σ3 and The errors are almost very small, which further proves the accuracy of the technical solution theory proposed in the present invention. In addition, it can be seen from Tables 2 and 3 that under different internal pressures P, under the same pipe wall δ Almost the same ratio.

[0112] In Table 4 below, the parameters of the pressure steel pipe used in the experiment are: Δ is 0.5 mm, D is 3000 mm, and δ is 30 mm.

[0113] Table 4

[0114]

[0115] In Table 5 below, the parameters of the pressure steel pipe used in the experiment are: Δ is 0.5 mm, D is 3000 mm, and δ is 40 mm.

[0116] Table 5

[0117]

[0118] In Table 6 below, the parameters of the pressure steel pipe used in the experiment are: Δ is 0.5 mm, D is 2800 mm, and δ is 25 mm.

[0119] Table 6

[0120]

[0121] In Table 7 below, the parameters of the pressure steel pipe used in the experiment are: Δ is 0.75 mm, D is 2800 mm, and δ is 25 mm.

[0122] Table 7

[0123]

[0124] From Table 6 and Table 7, we can see that under the same D and δ, The same, regardless of the gap value Δ. Therefore, in this scheme, it is defined as: ε can represent the rigidity level of the pressure steel pipe itself. According to the analysis data in Tables 2 to 7, through the analysis, statistics and mathematical modeling of the above data, we can get:

[0125]

[0126]

[0127]

[0128] Wherein, E is the elastic modulus of the pressure steel pipe, in MPa.

[0129] Substituting formula (4) and (5) into (6) yields the calculation model of formula (2):

[0130]

[0131] That is, a mathematical calculation model of the final stress on the pressure steel pipe is obtained when the pressure steel pipe and concrete are subjected to joint stress without geometric nonlinearity (taking the casting gap into consideration).

[0132] The above description is only a preferred embodiment of the present invention and does not limit the present invention in any form. Any simple modification or equivalent change made to the above embodiment based on the technical essence of the present invention shall fall within the protection scope of the present invention.

Claims

1. A method for analyzing the combined stress of a pressure steel pipe and concrete, characterized in that: The steps include: S1. Determine the water pressure P inside the penstock when the hydropower station is in operation; S2. After the pouring is stable, the pouring gap between the penstock and the concrete is recorded as Δ. Through the parametric analysis model and the inverse calculation method, the water pressure P1 when the penstock is in the open pipe state and the internal water pressure deformation is Δ is obtained; S3. The stress process of the penstock under the actual operation state is simplified into two stages. The first stage: before the penstock contacts the concrete, the penstock is subjected to stress alone; the second stage: after the penstock contacts the concrete, the penstock and the concrete are subjected to stress together, and a calculation model of the final stress state σ3 of the penstock is established; S4. Finally, the stress state of all discrete units of the penstock under discrete state is obtained, and the stress and strain state of the penstock under this state is evaluated to optimize the size design of the penstock.

2. A combined force analysis method of a pressure steel pipe and concrete according to claim 1, characterized in that: In step S2, the gap Δ between the concrete and the pressure steel pipe is usually 0.5 to 1.5 mm.

3. A combined force analysis method of a pressure steel pipe and concrete according to claim 1, characterized in that: In step S2, the relationship between the internal water pressure deformation Δ and the water pressure P1 of the penstock in the open pipe state is obtained as follows: S2.

1. After the pouring is stable, a separate model of the penstock to be analyzed is established, i.e., a parametric analysis model of the penstock; S2.

2. Based on the parametric analysis of response surface, a numerical replacement model of the correlation between internal water pressure and pressure steel pipe deformation is obtained; S2.

3. By inverse calculation, the water pressure P1 is obtained when the pressure pipe is in an open state and the internal water pressure deformation is Δ.

4. A combined force analysis method of a pressure steel pipe and concrete according to claim 1, characterized in that: In step S3, the method for obtaining the stress state of the penstock under different states includes the following steps: S3.

1. Establish a joint model of the penstock and concrete to be analyzed. The joint model does not consider the gap between the penstock and concrete. S3.2, discretize the penstock and concrete model according to the control equation of the following formula (1); Where, [M] is the structural mass matrix; [C] is the structural damping matrix; [K] is the structural stiffness matrix; {x} is the displacement vector; {F(t)} is the load vector; S3.3, set the constraints and material properties of the penstock and concrete respectively; S3.

4. Set up two analysis steps, control the contact properties between the penstock and concrete through the contact switch, simulate different contact states between the penstock and concrete, and obtain the stress state of the penstock.

5. A combined force analysis method of a pressure steel pipe and concrete according to claim 4, characterized in that: In step S3, the radial dimension of the concrete surrounding the penstock should be at least 8 times greater than the outer diameter of the penstock.

6. A method for combined force analysis of a pressure steel pipe and concrete according to claim 4, characterized in that: In step S3.4, the contact properties between the penstock and concrete are controlled by the contact switch CSS, and the stress state of the penstock is obtained as follows: S3.4.

1. Define CSS = NO, that is, the penstock is disconnected from the concrete. At this time, the penstock is subjected to the P1 water pressure alone, and the concrete is not subjected to stress. Calculate the stress state σ1 of the current penstock; S3.4.

2. Define CSS = YES, that is, the penstock is in contact with the concrete. At this time, the penstock and the concrete jointly bear the P-P1 water pressure. Calculate the current stress state σ2 of the penstock; S3.4.

3. Define the combination relationship Comb to calculate the final stress state σ3 of the penstock under discrete state, that is, σ3=σ2+σ1. The σ3 state is the actual state of joint stress of the penstock and concrete when the casting gap between the penstock and concrete is taken into account during the normal operation of the hydropower station.

7. A combined force analysis method of a pressure steel pipe and concrete according to claim 6, characterized in that: In step S3.4.3, a calculation model for the final stress state σ3 of the penstock is established to satisfy the following relationship (2): Where, δ is the wall thickness of the pressure steel pipe, in mm; P is the internal pressure of the pressure pipe, in MPa; D is the inner diameter of the pressure steel pipe, in mm; Δ is the pouring gap between the penstock and concrete, in mm; E is the elastic modulus of the pressure steel pipe, in MPa.

8. A combined force analysis method of a pressure steel pipe and concrete according to claim 7, characterized in that: In step S4, the method for evaluating the stress and strain state of the penstock is: S4.

1. Extract the relationship between stress and strain of the penstock under the real state of joint stress of the penstock and concrete when simulating the normal operation of the hydropower station, that is, when considering the pouring gap between the penstock and concrete; S4.

2. If the evaluation results meet the design requirements, the analysis ends; if the evaluation results do not meet the design requirements, the structural parameters of the pressure steel pipe need to be redesigned and the analysis and calculation are re-performed according to the process of steps S2 to S4.

9. A combined force analysis method of a pressure steel pipe and concrete according to claim 8, characterized in that: In step S4.2, the evaluation of the results mainly includes the following two aspects: S4.2.1 For a penstock with constant cross-sectional dimensions and no branches, the final stress state of the penstock after pressurization can be directly calculated using formula (2); or the model of formula (2) can be used to verify the accuracy of the stress state results of the penstock under this state calculated by other methods; S4.2.2 When the cross-sectional dimensions of the penstock vary and there are complex situations such as multiple branches, the penstock can be placed at a location that is geometrically uniform and far away from the stress concentration area, and the correctness of the calculation results can be checked using the following formula (3):

10. A combined force analysis method of a pressure steel pipe and concrete according to claim 9, characterized in that: In step S4.2.1, when the model of formula (2) is used to check the accuracy of the stress state results of the pressure steel pipe under this state calculated by other methods, the error between the results of the two methods shall not exceed 3%.

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