A design method of FRP confined concrete filled steel tubular columns based on fiber element method

Abstract

The application discloses a design method of a FRP confined steel pipe concrete column based on a fiber unit method and belongs to the technical field of structural engineering. The steps are as follows: according to the material composition and the section size of the cross section of the FRP confined steel pipe concrete column, the cross section of the column at the middle of the span is divided into enough small fiber units along the height; the strain ε of each fiber unit is calculated by calculating the curvature of the cross section at the middle of the span i = φy i ; according to a set material constitutive relation model, the stress σ of each fiber unit is calculated i ; the internal force N in and the internal bending moment M in are calculated; u m is increased to u m + Δu m , and the above steps are repeated until u m reaches a terminal value u u , so that the corresponding N and M of each u m are obtained, the load-span deflection curve of the whole process is obtained, and the ultimate bearing capacity N u and the ultimate bending moment M u are determined.

Description

Technical Field

[0001] The invention relates to a design method for building structural components, in particular to a design method for FRP-constrained steel tube concrete columns based on a fiber element method, and belongs to the technical field of structural engineering. Background Art

[0002] Concrete-filled steel tubular columns leverage the strengths of both steel and concrete while compensating for their respective shortcomings. They offer high bearing capacity, good ductility, excellent seismic performance, and convenient construction, making them widely used in construction projects. Fiber-reinforced plastics (FRP) are lightweight, high-strength, corrosion-resistant, flexible, and easy to construct materials. They are rapidly developing and gaining widespread application in the construction industry. Some researchers have proposed attaching FRP to the surface of concrete-filled steel tubular columns to create FRP-constrained concrete-filled steel tubular columns, and preliminary research has been conducted.

[0003] In FRP-confined concrete-filled steel tube columns, the transverse FRP, similar to the steel tube, can constrain the concrete, improving its compressive strength and ductility under triaxial compression. It can also delay or even suppress localized outward buckling of the steel tube. The longitudinal FRP, in the tension zone, can utilize its high tensile strength to increase the column's load-bearing capacity. Therefore, FRP not only significantly improves the mechanical properties of concrete-filled steel tube columns, but also does so without increasing the column's cross-sectional area or deadweight, offering broad application prospects.

[0004] FRP-confined steel tube concrete columns involve a wide variety of materials, each with a complex interrelationship. The steel tube is an elastic-plastic material, maintaining a high bearing capacity even after reaching yield strain, while the FRP is an elastic-brittle material, losing its tensile strength once reaching fracture strain. The two materials exhibit distinct restraining effects on the core concrete. The steel tube is simultaneously supported by the inner core concrete and restrained by the external lateral FRP, resulting in a relatively complex stress state. Therefore, it is difficult to derive an analytical solution for its ultimate bearing capacity through theoretical derivation. While finite element models can accurately predict its ultimate bearing capacity and full-scale load-displacement curve, they are computationally expensive and highly dependent on the modeler's skill level, hindering practical engineering applications.

[0005] CN113565264A discloses a A concrete composite column uses an ultra-high performance fiber reinforced concrete (UHPFRC) tube to constrain a concrete column, and an FRP layer is bonded to the outside of the UHPFRC tube to form a composite concrete column. A liner is placed inside the UHPFRC tube to prevent damage to the UHPFRC tube during pouring. Steel bars can be configured in the concrete column as needed. CN101967853A discloses an FRP-rubber-steel composite tube concrete structure, which includes four parts: an FRP layer, a rubber layer, a steel tube, and core concrete, which are constructed from the outside to the inside; wherein the FRP layer, the rubber layer, and the steel tube are bonded by resin to form a composite tube as a whole, and the core concrete is filled inside the composite tube, and the composite tube exerts a constrained and reinforcing effect on the internal core concrete. CN101974958A discloses a steel tube concrete column with an I-shaped FRP profile, which relates to a building component. Composed of rectangular, square, or circular steel tubes, I-shaped FRP profiles placed within the tubes, and concrete filled between them, I-shaped FRP concrete-filled steel tube columns are composite columns made of three different materials: FRP, steel tube, and concrete. These columns are known as I-shaped FRP concrete-filled steel tube composite columns. However, these existing technologies all suffer from low prediction accuracy and high design or calculation costs.

[0006] Therefore, it is urgent to propose a convenient and fast design method for FRP-confined steel tube concrete columns that can maintain high prediction accuracy while effectively reducing calculation costs, so as to promote its application in practical engineering. Summary of the Invention

[0007] In view of the above-mentioned defects in the prior art, the present invention proposes a design method for FRP-confined steel tube concrete columns based on the fiber element method, which has a simple calculation process and accurate prediction results.

[0008] In order to achieve the above object, the technical solution adopted by the present invention is as follows:

[0009] A design method for FRP-confined concrete-filled steel tube columns based on a fiber element method includes the following steps:

[0010] S1. According to the material composition and cross-sectional dimensions of the FRP-constrained steel tube concrete column section, the column mid-span section is divided into sufficiently small fiber units along the height. The area of ​​each fiber unit is A. i ;

[0011] S2, given mid-span lateral deflection u m , the initial value is 0, according to the formula Calculate the curvature of the mid-span section at this time Where L0 is the calculated length of the column;

[0012] S3. Assuming the height of the compression zone is x, according to the plane section assumption, the strain ε of the midline of each fiber unit iThe distance y from the neutral axis i It is determined that when the fiber unit is divided into small enough, ε i It can represent the strain of the entire fiber unit, and thus the strain of each fiber unit ε is obtained i =φy i ;

[0013] S4. Calculate the stress σ of each fiber unit according to the set material constitutive relationship model i ;

[0014] S5. Calculate the internal force N by the following formula in and internal bending moment M in , h is the section height:

[0015]

[0016]

[0017] According to the equilibrium condition, the internal force N in and internal bending moment M in They should be equal to the axial force N and bending moment M applied in this state, that is, they satisfy the following formula:

[0018] M=N(e0+u m )=N in (e0+u m )

[0019]

[0020] Where δ is a minimum value and e0 is the eccentricity of the axial force. If the equilibrium condition is not met, it means that the assumed x is incorrect. According to M and M in The size of x is adjusted using the dichotomy method, and steps S3-S5 are repeated until the equilibrium condition is met;

[0021] S6. Increase u m =u m +Δu m , Δu m For each step u m Repeat steps S2-S5 until u m Reaching the end value u u , you can get each u m The corresponding N and M are obtained to obtain the full process load-span deflection curve and determine the ultimate bearing capacity N u and ultimate bending moment M u .

[0022] Furthermore, before step S1, the following assumptions are made:

[0023] First, there is no relative slip between the steel tube and the concrete, and between the steel tube and the FRP in the whole process of the FRP confined steel tube concrete column under the load, and the strain distribution on the section meets the plane section assumption;

[0024] Second, the lateral deflection of the FRP confined steel tube concrete column is in sinusoidal wave distribution, and the curvature meets where u m is the lateral deflection at the midspan, and L0 is the calculation length of the column;

[0025] Third, the tensile strength of the concrete in the tensile zone is ignored because the concrete cracks prematurely and stops working;

[0026] Fourth, for the longitudinal FRP, only the tensile strength is considered because the compressive strength is small; for the transverse FRP, only the influence of the confining effect is considered in the constitutive model of the concrete, and it is not calculated as a fiber element.

[0027] Further, in the step S1, the concrete, the steel tube and the longitudinal FRP are divided into three different fiber elements, and the areas of the fiber elements are A ci , A si and A fi , respectively, and the stresses of the fiber elements are σ ci , σ si and σ fi , respectively.

[0028] Further, in the step S3, because the thickness of the FRP is small, it is considered that the strain of the longitudinal FRP at the bottom of the tensile zone is approximately equal to the strain of the steel tube at the bottom of the tensile zone.

[0029] Further, in the step S4, the constitutive model of the concrete considers the combined confining effect of the steel tube and the transverse FRP on the basis of the constitutive model of the concrete in the steel tube concrete column, and is modified as follows:

[0030]

[0031] In the formula,

[0032] x = ε / ε0, y = σ / σ0, σ0 = f cc , ε0 = ε c + 800ξ 0.2 × 10 -6 , ε c = (1300 + 12.5f c ) × 10 -6 ,

[0033] η = 1.6 + 1.5x, ξ = ξ s + ξ f , Among them, ε, ε0 and ε c are concrete strain, peak strain of confined concrete and peak strain of unconfined concrete respectively; σ and σ0 are concrete stress and peak stress of confined concrete respectively; η is the fitting parameter; A c 、A s and A f are the cross-sectional areas of concrete, steel pipe and transverse FRP respectively; ξ s and ξ f are the constraint coefficients of steel pipe and transverse FRP respectively; f y is the yield strength of the steel pipe; f fu is the tensile strength of FRP; f c and f ck are the cylindrical compressive strength and axial compressive strength of concrete respectively; f cc The compressive strength of the confined concrete is calculated as follows:

[0034]

[0035]

[0036]

[0037]

[0038] Among them, f l is the lateral restraint stress provided by the steel pipe and FRP; d is the equivalent side length, which is determined by the concrete section length b c and width h c decision;k e is the effective constraint coefficient, which reflects the influence of the cross-sectional aspect ratio and the chamfer radius r on the constraint effect; t s and t f are the thickness of steel pipe and FRP respectively.

[0039] Furthermore, in step S4, the constitutive relationship model of the steel pipe adopts an ideal elastic-plastic model, which is expressed as follows:

[0040]

[0041] Among them E s and ε y are the elastic modulus and yield strain of the steel pipe, respectively.

[0042] Furthermore, in step S4, the constitutive relationship model of the longitudinal FRP is expressed as follows:

[0043]

[0044] Among them E fis the elastic modulus of the longitudinal FRP.

[0045] The invention discloses an application of a design method for FRP-confined steel tube concrete columns based on a fiber element method. The design method for FRP-confined steel tube concrete columns based on a fiber element method is applicable to FRP-confined steel tube concrete columns of various cross sections.

[0046] Furthermore, various types of cross-section FRP-confined steel tube concrete columns include FRP-confined steel tube concrete columns with rectangular cross-sections and circular cross-sections.

[0047] Furthermore, the design method is applicable to the steel tube concrete column partially wrapped with transverse FRP, which reduces the FRP constraint coefficient ξ according to the size of the wrapped area. f Make a reduction.

[0048] Compared with the prior art, the present invention has the following beneficial effects:

[0049] The proposed design method for FRP-confined concrete-filled steel tubular columns, based on the fiber element method, features a simple calculation process and accurate prediction results. It can generate the column's full-process load-to-midspan deflection curve, ultimate bearing capacity Nu, and ultimate bending moment Mu. Furthermore, this design method can be applied to concrete-filled steel tubular columns with any cross-section and various FRP confinement configurations, better meeting their development needs in practical engineering applications. BRIEF DESCRIPTION OF THE DRAWINGS

[0050] Figure 1 Calculate the flow chart of the design method of the present invention;

[0051] Figure 2 It is the fiber unit division diagram of the design method of the present invention;

[0052] Figure 3 The cross-sectional stress-strain distribution diagram of the design method of the present invention;

[0053] Figure 4 This is the constitutive model diagram of the steel pipe according to the design method of the present invention;

[0054] Figure 5 This is the FRP constitutive model diagram of the design method of the present invention;

[0055] Figure 6 Schematic diagram of a rectangular cross-section FRP-confined concrete-filled steel tube column according to an example of the present invention;

[0056] Figure 7 This is a comparison chart of the load-mid-span deflection curve predicted by the calculation example of the present invention and the test results.

[0057] In the figure, 1-concrete; 2-steel pipe; 3-FRP. DETAILED DESCRIPTION

[0058] The following is combined withFigures 1-7 The present invention will be further described in detail with specific implementations to facilitate a clear understanding of the present invention, but they do not constitute a limitation to the present invention.

[0059] Example 1

[0060] As attached Figures 1-5 As shown, the design method of FRP-confined concrete-filled steel tube columns based on the fiber element method in this embodiment first adopts the following assumptions:

[0061] First, during the entire process of the FRP-confined steel tube concrete column bearing load, there is no relative slip between steel tube 2 and concrete 1, or between steel tube 2 and FRP 3, and the coordinated deformation is good. The strain distribution on the cross section meets the plane section assumption.

[0062] Second, the lateral deflection of FRP-confined steel tube concrete columns is sinusoidal, and the curvature satisfies where u m is the mid-span lateral deflection, L0 is the calculated length of the column;

[0063] Third, the tensile strength of the concrete in the tensile zone is neglected because of premature cracking and work withdrawal.

[0064] Fourth, for longitudinal FRP, due to its low compressive strength, only its tensile strength is considered; for transverse FRP, only its constraint effect is considered in the constitutive model of concrete and it is not calculated as a fiber unit.

[0065] The design method of this embodiment includes the following steps:

[0066] S1. According to the material composition and cross-sectional dimensions of the FRP-confined steel tube concrete column section, the column mid-span section is divided into sufficiently small fiber units along the height, such as Figure 3 As shown, the area of ​​each fiber unit is A i ;

[0067] In step S1, the concrete 1, the steel pipe 2 and the longitudinal FRP 3 are divided into three different fiber units, and the area of ​​each fiber unit is A ci , A si and A fi , the stress of each fiber unit is σ ci , σ si and σ fi .

[0068] S2, given mid-span lateral deflection u m , the initial value is 0, according to the formula Calculate the curvature of the mid-span section at this time Where L0 is the calculated length of the column;

[0069] S3, such asFigure 3 As shown, assuming that the height of the compression zone is x, according to the plane section assumption, the strain ε of the midline of each fiber element is i The distance y from the neutral axis i It is determined that when the fiber unit is divided into small enough, ε i It can represent the strain of the entire fiber unit, and thus the strain of each fiber unit ε is obtained i =φy i ;

[0070] In step S3, since the thickness of the FRP is small, it is considered that the strain of the longitudinal FRP at the bottom of the tension zone is approximately equal to the strain of the steel pipe at the bottom of the tension zone.

[0071] S4. Calculate the stress σ of each fiber unit according to the set material constitutive relationship model i ;

[0072] In step S4, the constitutive model of concrete is based on the concrete model of the steel tube concrete column, taking into account the combined constraint effect of the steel tube and the transverse FRP, and is modified and expressed as follows:

[0073]

[0074] In the formula

[0075] x=ε / ε0,y=σ / σ0,σ0=f cc ,ε0=ε c +800ξ 0.2 ×10 -6 ,ε c =(1300+12.5f c )×10 -6 ,

[0076]

[0077] Among them, ε, ε0 and ε c are concrete strain, peak strain of confined concrete and peak strain of unconfined concrete respectively; σ and σ0 are concrete stress and peak stress of confined concrete respectively; η is the fitting parameter; A c 、A s and A f are the cross-sectional areas of concrete, steel pipe and transverse FRP respectively; ξ s and ξ f are the constraint coefficients of steel pipe and transverse FRP respectively; f y is the yield strength of the steel pipe; f fu is the tensile strength of FRP; f c and f ck are the cylindrical compressive strength and axial compressive strength of concrete respectively; fcc The compressive strength of the confined concrete is calculated as follows:

[0078]

[0079]

[0080]

[0081]

[0082] Among them, f l is the lateral restraint stress provided by the steel pipe and FRP; d is the equivalent side length, which is determined by the concrete section length b c and width h c decision;k e is the effective constraint coefficient, which reflects the influence of the cross-sectional aspect ratio and the chamfer radius r on the constraint effect; t s and t f are the thickness of steel pipe and FRP respectively.

[0083] In step S4, the constitutive model of concrete is based on the concrete model of the steel tube concrete column, taking into account the combined constraint effect of the steel tube and the transverse FRP, and is modified and expressed as follows:

[0084]

[0085] In the formula

[0086] x=ε / ε0,y=σ / σ0,σ0=f cc ,ε0=ε c +800ξ 0.2 ×10 -6 ,ε c =(1300+12.5f c )×10 -6 ,

[0087] η=1.6+1.5x,ξ=ξ s +ξ f , Among them, ε, ε0 and ε c are concrete strain, peak strain of confined concrete and peak strain of unconfined concrete respectively; σ and σ0 are concrete stress and peak stress of confined concrete respectively; η is the fitting parameter; A c 、A s and A f are the cross-sectional areas of concrete, steel pipe and transverse FRP respectively; ξ s and ξ f are the constraint coefficients of steel pipe and transverse FRP respectively; fy is the yield strength of the steel pipe; f fu is the tensile strength of FRP; f c and f ck are the cylindrical compressive strength and axial compressive strength of concrete respectively; f cc The compressive strength of the confined concrete is calculated as follows:

[0088]

[0089]

[0090]

[0091]

[0092] Among them, f l is the lateral restraint stress provided by the steel pipe and FRP; d is the equivalent side length, which is determined by the concrete section length b c and width h c decision;k e is the effective constraint coefficient, which reflects the influence of the cross-sectional aspect ratio and the chamfer radius r on the constraint effect; t s and t f are the thickness of steel pipe and FRP respectively.

[0093] In step S4, Figure 4 As shown in the figure, the constitutive relationship model of the steel pipe adopts the ideal elastic-plastic model, which is expressed as follows:

[0094]

[0095] Among them E s and ε y are the elastic modulus and yield strain of the steel pipe, respectively.

[0096] like Figure 5 As shown in Figure 2, the constitutive relationship model of longitudinal FRP is expressed as follows:

[0097]

[0098] Among them E f is the elastic modulus of the longitudinal FRP.

[0099] S5. Calculate the internal force N by the following formula in and internal bending moment M in , h is the section height:

[0100]

[0101]

[0102] According to the equilibrium condition, the internal force N in and internal bending moment M in They should be equal to the axial force N and bending moment M applied in this state, that is, they satisfy the following formula:

[0103] M=N(e0+u m )=N in (e0+u m )

[0104]

[0105] Where δ is a minimum value and e0 is the eccentricity of the axial force. If the equilibrium condition is not met, it means that the assumed x is incorrect. According to M and M in The size of x is adjusted using the dichotomy method, and steps S3-S5 are repeated until the equilibrium condition is met;

[0106] S6. Increase u m =u m +Δu m , Δu m For each step u m Repeat steps S2-S5 until u m Reaching the end value u u , you can get each u m The corresponding N and M are obtained to obtain the full process load-span deflection curve and determine the ultimate bearing capacity N u and ultimate bending moment M u .

[0107] Example 2

[0108] Select Figure 6 The load-span deflection curve of a FRP-confined steel tube concrete column is calculated according to the design method of Example 1. The ultimate bearing capacity N u and ultimate bending moment M u Calculation.

[0109] The column section size is a rectangular section of 120mm×180mm, the steel pipe wall thickness is 3mm, the calculated length is 560mm, the chamfer radius is 20mm, and it is wrapped with 2 layers of transverse and longitudinal FRP, each layer of FRP is 0.17mm thick, and the eccentricity is 40mm.

[0110] The material parameters of the column are shown in the following table:

[0111]

[0112] Follow these steps to calculate:

[0113] S1, the FRP confined concrete filled steel tubular column cross section in the middle of the height is divided into 100 layers of fiber units, a total of three, namely concrete, steel tube and longitudinal FRP fiber units;

[0114] S2, given the mid-span deflection u m , the initial value is 0, according to the formula to calculate the curvature of the cross section at this time.

[0115] S3, assuming that the compression zone height is x, according to the plane section assumption to determine the strain of each fiber unit ε i = φy i .

[0116] S4, according to the set material constitutive relation, the stress of each fiber unit σ ci , σ si and σ fi is calculated.

[0117] S5, the internal force N in and internal bending moment M in are calculated by the following formula:

[0118]

[0119]

[0120] According to the balance condition, the internal force N in and internal bending moment M in should be equal to the axial force N and bending moment M applied at this state respectively, that is, satisfy the following formula:

[0121] M = N (e0+u m ) = N in (e0+u m )

[0122]

[0123] In the formula, δ is a very small value, taking 0.001. If the balance condition is not satisfied, it means that the assumed x is incorrect at this time, according to the size of M and M in , the bisection method is used to adjust x, and steps 3-5 are repeated until the balance condition is satisfied.

[0124] S6, increase u m = u m + Δu m , repeat steps S2-S5 until u m reaches the termination value u u , in this example, Δu m takes 0.1mm, u u takes 15mm. Obtain each u mThe corresponding N and M are obtained to obtain the full process load-span deflection curve and determine the ultimate bearing capacity N u and ultimate bending moment M u .

[0125] According to the above steps, the load-mid-span deflection curve obtained by the design method is compared with the test results. Figure 7 The ultimate bearing capacity N obtained by the design method is shown as follows. u and ultimate bending moment M u 2017kN and 83kN·m respectively. The experimental results show that N u and M u The predicted results are 1891 kN and 79 kN·m, with errors of +6.7% and +5.1%, respectively. This shows that the predicted results have high accuracy and can provide a theoretical basis for practical engineering applications.

[0126] Example 3

[0127] The present embodiment is an application of a design method for FRP-confined steel tube concrete columns based on a fiber element method. The above-mentioned design method for FRP-confined steel tube concrete columns based on a fiber element method is applicable to FRP-confined steel tube concrete columns of various cross sections. Figure 2 、 3 As shown in FIG6 , the FRP-confined steel tube concrete column includes a rectangular or circular cross-section FRP-confined steel tube concrete column. The design method is applicable to a steel tube concrete column partially wrapped with transverse FRP, and the FRP confinement coefficient ξ is set according to the size of the wrapped area. f Make a reduction.

[0128] The above is merely a preferred embodiment of the present invention and does not constitute any formal limitation on the structure of the present invention. The layout and number of the present invention are not limited to this example and can be optimized according to actual engineering practices. Any modifications, equivalent changes, and decorations to the above embodiment based on the technical principles of the present invention that do not depart from the content of the technical solution of the present invention are still within the scope of the technical solution of the present invention.

Claims

1. A design method for FRP-confined concrete-filled steel tube columns based on the fiber element method, characterized in that: The steps include: S1. According to the material composition and cross-sectional dimensions of the FRP-constrained steel tube concrete column section, the column mid-span section is divided into sufficiently small fiber units along the height. The area of ​​each fiber unit is A. i ; S2, given mid-span lateral deflection u m , the initial value is 0, according to the formula Calculate the curvature of the mid-span section at this time Where L0 is the calculated length of the column; S3. Assuming the height of the compression zone is x, according to the plane section assumption, the strain ε of the midline of each fiber unit i The distance y from the neutral axis i It is determined that when the fiber unit is divided into small enough, ε i It can represent the strain of the entire fiber unit, and thus the strain of each fiber unit ε is obtained i =φy i ; S4. Calculate the stress σ of each fiber unit according to the set material constitutive relationship model i ; S5. Calculate the internal force N by the following formula in and internal bending moment M in , h is the section height: According to the equilibrium condition, the internal force N in and internal bending moment M in They should be equal to the axial force N and bending moment M applied in this state, that is, they satisfy the following formula: M=N(e0+u m )=N in (e0+u m ) Where δ is a minimum value and e0 is the eccentricity of the axial force. If the equilibrium condition is not met, it means that the assumed x is incorrect. According to M and M in The size of x is adjusted using the dichotomy method, and steps S3-S5 are repeated until the equilibrium condition is met; S6. Increase u m =u m +Δu m , Δu m For each step u m Repeat steps S2-S5 until u m Reaching the end value u u , you can get each u m The corresponding N and M are obtained to obtain the full process load-span deflection curve and determine the ultimate bearing capacity N u and ultimate bending moment M u .

2. The design method of FRP-confined concrete-filled steel tube columns based on the fiber element method according to claim 1, characterized in that: Before step S1, the following assumptions are made: First, during the entire process of bearing load, there is no relative slip between the steel tube and concrete, or between the steel tube and FRP, and the coordinated deformation is good. The strain distribution on the cross section satisfies the plane section assumption. Second, the lateral deflection of FRP-confined steel tube concrete columns is sinusoidal, and the curvature satisfies where u m is the mid-span lateral deflection, L0 is the calculated length of the column; Third, the tensile strength of the concrete in the tensile zone is neglected because of premature cracking and work withdrawal. Fourth, for longitudinal FRP, due to its low compressive strength, only its tensile strength is considered; for transverse FRP, only its constraint effect is considered in the constitutive model of concrete and it is not calculated as a fiber unit.

3. The design method of FRP-confined concrete-filled steel tube columns based on the fiber element method according to claim 1, characterized in that: In step S1, concrete, steel pipe and longitudinal FRP are divided into three different fiber units, and the area of ​​each fiber unit is A ci , A si and A fi , the stress of each fiber unit is σ ci , σ si and σ fi .

4. The design method of FRP-confined concrete-filled steel tube columns based on the fiber element method according to claim 3, characterized in that: In step S3, since the thickness of the FRP is small, it is considered that the strain of the longitudinal FRP at the bottom of the tension zone is approximately equal to the strain of the steel pipe at the bottom of the tension zone.

5. The design method of FRP-confined concrete-filled steel tube columns based on the fiber element method according to claim 4, characterized in that: In step S4, the constitutive model of concrete is based on the concrete model of the steel tube concrete column, taking into account the combined constraint effect of the steel tube and the transverse FRP, and is modified and expressed as follows: In the formula x=ε / ε0,y=σ / σ0,σ0=f cc ,ε0=ε c +800x 0.2 ×10 -6 ,he c =(1300+12.5f c )×10 -6 , Among them, ε, ε0 and ε c are concrete strain, peak strain of confined concrete and peak strain of unconfined concrete respectively; σ and σ0 are concrete stress and peak stress of confined concrete respectively; η is the fitting parameter; A c 、A s and A f are the cross-sectional areas of concrete, steel pipe and transverse FRP respectively; ξ s and ξ f are the constraint coefficients of steel pipe and transverse FRP respectively; f y is the yield strength of the steel pipe; f fu is the tensile strength of FRP; f c and f ck are the cylindrical compressive strength and axial compressive strength of concrete respectively; f cc The compressive strength of the confined concrete is calculated as follows: Among them, f l is the lateral restraint stress provided by the steel pipe and FRP; d is the equivalent side length, which is determined by the concrete section length b c and width h c decision;k e is the effective constraint coefficient, which reflects the influence of the cross-sectional aspect ratio and the chamfer radius r on the constraint effect; t s and t f are the thickness of steel pipe and FRP respectively.

6. The design method of FRP-confined concrete-filled steel tube columns based on the fiber element method according to claim 5, characterized in that: In step S4, the constitutive relationship model of the steel pipe adopts an ideal elastic-plastic model, which is expressed as follows: Among them E s and ε y are the elastic modulus and yield strain of the steel pipe, respectively.

7. The design method of FRP-confined concrete-filled steel tube columns based on the fiber element method according to claim 6, characterized in that: In step S4, the constitutive relationship model of the longitudinal FRP is expressed as follows: Among them E f is the elastic modulus of the longitudinal FRP.

8. An application of a design method for FRP-confined concrete-filled steel tube columns based on the fiber element method, characterized in that: The design method of FRP-confined steel tube concrete columns based on the fiber element method described in any one of claims 1 to 7 is applicable to FRP-confined steel tube concrete columns of various cross-sections.

9. Application of the design method according to claim 8, characterized in that: Various types of FRP-confined steel tube concrete columns include rectangular and circular cross-section FRP-confined steel tube concrete columns.

10. Application of the design method according to claim 9, characterized in that: The design method is applicable to steel tube concrete columns partially wrapped with transverse FRP, and the FRP constraint coefficient ξ is adjusted according to the size of the wrapped area. f Make a reduction.

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