Method for calculating creep effect of steel-concrete composite structure

Abstract

The invention discloses a method for calculating the creep effect of a steel-concrete composite structure. The method comprises the following steps: S1, calculating the stress states of concrete and a steel beam of a composite section of a steel-concrete composite beam under the action of external force when the steel-concrete composite beam falls for one time and does not creep; s2, calculating constraint reaction caused by concrete creep; s3, applying a constraining force on the creep calculation section in a reverse direction, applying a reaction force to the whole section, calculating stress change values of the upper edge and the lower edge of the concrete slab after the creep effect acts on the creep calculation section, and calculating stress change values of the upper edge and the lower edge of the steel beam of the steel beam section; and S4, calculating the stress state corresponding to the creep effect generated by the combined section by adopting an accumulation method. The method can solve the problem that a calculation result obtained by a traditional calculation method for the creep effect of the composite beam has a large deviation from an actual situation.

Description

Technical Field

[0001] The invention relates to the technical field of bridge engineering in the transportation industry, and in particular to a method for calculating creep effect of a steel-concrete composite structure. Background Art

[0002] Steel-concrete composite bridge is an important structure in the field of bridge engineering. It has been widely used in highway bridges because of its good economy and reasonable stress performance.

[0003] The traditional method for calculating the creep effect of steel-concrete composite structures in the field of bridges is generally based on the "Highway Steel Structure Bridge Design Code". The creep effect of composite beam bridges can be calculated using the effective elastic modulus of concrete. To approximate the calculation, the calculation formula is as follows:

[0004]

[0005] Where: E c Elastic modulus of concrete; ψ L The creep factor is determined according to the load type, and the permanent action is taken as 1.1; is the creep coefficient of concrete and is adopted according to Table 3.1.5 of the “Design Code for Highway Reinforced Concrete and Prestressed Concrete Bridges and Culverts”.

[0006] However, there is a big problem with the traditional calculation method of creep effect of composite beams. The calculation results are greatly deviated from the actual situation. This is because the traditional calculation method is relatively rough in calculating the creep effect of steel-concrete composite structures. It does not conduct a complete stress analysis on the creep effect of concrete creep from the mechanical principle. This calculation result causes confusion. Therefore, it is necessary to conduct a full-process stress analysis on the concrete creep of the composite structure and calculate the creep effect of the steel-concrete composite structure more accurately. Summary of the invention

[0007] The purpose of the present invention is to provide a method for calculating the creep effect of a steel-concrete composite structure, so as to solve the problem that there is a large deviation between the calculation results obtained by the traditional method for calculating the creep effect of composite beams and the actual situation.

[0008] The purpose of the present invention can be achieved through the following technical solutions.

[0009] A method for calculating creep effect of a steel-concrete composite structure comprises the following steps:

[0010] S1. Calculate the stress state of concrete and steel beams in the composite section of the "steel-concrete" composite beam under the action of external forces when the frame is dropped once and no creep occurs;

[0011] S2. Calculate the restraint reaction caused by concrete creep:

[0012] The constraint on the concrete is regarded as a special virtual rigid arm. After the concrete creeps, the constraint reaction forces -ΔN(t) and -ΔM(t) of the composite section due to the creep of the concrete are calculated.

[0013] S3. Apply restraint forces -ΔN(t) and -ΔM(t) in the opposite direction on the creep calculation section. The reaction force acts on the entire section. Calculate the stress change values ​​at the upper and lower edges of the concrete slab after the creep effect acts on the creep calculation section. Calculate the stress change values ​​at the upper and lower edges of the steel beam section.

[0014] S4. Use the accumulation method to calculate the stress state corresponding to the creep effect of the composite section.

[0015] The present invention also has the following preferred calculation method:

[0016] Step S1 of the present invention may include:

[0017] S101. Convert the section of the "steel-concrete" composite section, using the top edge of the concrete beam on the section as the reference axis for analysis to convert the area A, area moment S, and moment of inertia I of the composite section:

[0018] A=bd c +n ss A ss (1)

[0019]

[0020] In the formula, b is the width of the concrete slab, d is c is the thickness of the concrete slab, n ss is the ratio of the elastic modulus of steel to the elastic modulus of concrete, A ss is the cross-sectional area of ​​the steel beam, d ss is the distance between the centroid axis of the steel beam section and the analysis reference axis, I ss is the moment of inertia of the steel beam part about its own neutral axis;

[0021] S102. Calculate the strain ε1 at the top edge of the composite section and the deflection ψ at any height point of the section when the composite section is subjected to axial force N and bending moment M. i , the strain at any height y of the combined section from the reference axis is expressed as:

[0022] ε i =ε1-ψ i y (4),

[0023] σ i =E c ε i=E c (ε1-ψ i y) (5),

[0024] In the formula, y is the point in the combined section at a distance y from the reference axis, ε i is the strain corresponding to the height point at a distance y from the reference axis in the cross section, σ i is the stress corresponding to the height point at a distance y from the reference axis in the cross section, E c is the elastic modulus of concrete;

[0025] According to the relationship between stress and force of the combined section:

[0026]

[0027] Substituting into formula (4) and formula (5), we can obtain:

[0028]

[0029]

[0030] S103. Calculate the stress of the concrete or steel beam at a distance y from the reference axis in the section, and obtain the stress state of the composite beam under the action of axial force and bending moment after the composite beam is dropped once:

[0031] σ ci =E c (ε1-yψ i ),

[0032] σ si =E s (ε1-yψ i ),

[0033] where σ ci is the stress at different heights on the concrete slab, σ si is the stress at different heights on the steel beam, E s is the elastic modulus of steel, and the stress state at any height on the combined section can be calculated.

[0034] The step S2 of the present invention comprises:

[0035] S201. According to formula (10), calculate the effective coefficient γ(t,t o ):

[0036]

[0037] in is the final value of creep coefficient;

[0038] S202. Calculate the effective elastic modulus of concrete according to formula (11):

[0039] S203. Assuming that the concrete slab of the composite beam is constrained by a special virtual rigid arm, the constraint is turned on and the creep strain Δε(t) of the concrete bridge deck is calculated according to formula (12):

[0040]

[0041] S204. Calculate the restraint stress Δσ1(t) corresponding to the restrained creep deformation of the concrete slab at time t according to formula (13):

[0042]

[0043] S205. According to formula (14) and formula (15), the constraint stress in step S204 can be simplified to the required constraint reaction force -ΔN(t) and -ΔM(t) about the reference axis of the analysis:

[0044]

[0045] Among them A c is the cross-sectional area of ​​the concrete bridge deck, S c is the area moment of the concrete bridge deck section, I c is the moment of inertia of the concrete bridge deck section, ε oi is the strain at the top edge of the cross section.

[0046] The step S3 of the present invention comprises:

[0047] S301. Calculate the geometric properties of the creep calculation section and the creep calculation cross-sectional area Calculate the cross-sectional area moment using the Schuh transformation Calculate the moment of inertia of the section using the Slug transformation

[0048]

[0049] E is the ratio of the elastic modulus of steel to the elastic modulus of concrete after creep. ss is the elastic modulus of steel corresponding to the creep of concrete;

[0050] S302. Calculate the strain Δε1(t) at the upper edge of the concrete slab and the corresponding section deflection Δψ(t) when the constraint force of the creep effect acts in reverse on the creep calculation section according to formulas (20) and (21):

[0051]

[0052] S303. According to formula (22), the stress increment corresponding to the point in the cross section at a distance y from the reference axis is calculated as Δσ2(t):

[0053]

[0054] Under the influence of concrete shrinkage creep, the total stress change value of the concrete slab at the distance y from the top edge of the section is Δσ3(t), and the total stress change value of the steel beam at the distance y from the top edge of the section is Δσ ss (t) is:

[0055] Δσ3(t)=Δσ1(t)+Δσ2(t) (23),

[0056] Δσ ss (t) = E ss (Δε1(t)-yΔψ(t))(24), the stress state at any height on the composite section can be calculated.

[0057] The step S4 of the present invention comprises:

[0058] S401. Calculate the stress σ on the upper edge of the concrete slab when the frame is dropped once according to steps S1 to S3. c1 and the stress σ at the lower edge of the concrete slab c2 , stress on the upper edge of the steel beam σ s1 and the stress at the lower edge of the steel beam σ s2 ;

[0059] S402: Calculate the total stress change Δσ at the top edge of the concrete slab after concrete creep occurs. c1 , and the total stress change Δσ at the bottom edge of the concrete slab c2 ; Total stress change value Δσ of the upper edge stress of the steel beam s1 , and the total stress change value Δσ of the stress at the lower edge of the steel beam s2 ;

[0060] S403: According to formula (25), formula (26), formula (27) and formula (28), the force superposition principle is used to calculate the final stress value σ1 at the upper edge of the concrete slab; the final stress value σ2 at the lower edge of the concrete slab; the final stress value σ3 at the upper edge of the steel beam; and the final stress value σ4 at the lower edge of the steel beam:

[0061] σ1=σ c1 +Δσ c1 (25),

[0062] σ2=σ c2 +Δσ c2 (26),

[0063] σ3=σ s1 +Δσ s1(27)

[0064] σ4=σ s2 +Δσ s2 (28).

[0065] The present invention needs to satisfy the plane section assumption of the composite section. When the composite beam is in normal use, no slip occurs between the concrete slab and the steel beam, and the composite section of the composite beam satisfies the plane section assumption.

[0066] In the present invention, when there is no slip between the composite beam concrete slab and the steel beam, the concrete slab is strongly constrained by a special virtual rigid arm. It is considered that after creep occurs at both ends of the concrete slab under the constraint condition of the virtual rigid arm, the concrete slab is still in the state of the composite beam being dropped once as described in step S1 under the constraint condition of the virtual rigid arm.

[0067] The present invention does not make special considerations on the shear nails between the concrete slab and the steel beam of the composite beam. The shear nails serve to connect the concrete slab and the steel beam. The concrete slab is considered to be an ideal slab: it meets the requirements of continuity, uniformity and isotropy of the analysis object in material mechanics.

[0068] The creep effect calculation method of the "steel-concrete" composite structure of the present invention can be used to calculate the creep effect of the "steel-concrete" composite beam, and can also be used to calculate the creep effect of the "steel-concrete" composite slab.

[0069] Compared with the prior art, the present invention has the following beneficial effects:

[0070] The present invention conducts a full-process mechanical analysis based on the influence of the internal force generated by the creep of the concrete slab on the composite structure. Compared with the traditional calculation method, the calculation method of the present invention can calculate the creep effect of steel-concrete composite structures such as steel-concrete composite beams more clearly and accurately, and the calculation result is closer to the actual situation.

[0071] The invention has strong practicability and can be used to guide and check the bending calculation of the positive section of the composite beam. It is of great significance to the design calculation of steel-concrete composite structures such as "steel-concrete" composite beams in bridge engineering. BRIEF DESCRIPTION OF THE DRAWINGS

[0072] Figure 1 It is a flow chart of a method for calculating creep effect of a steel-concrete composite structure according to the present invention;

[0073] Figure 2 is a composite cross-sectional view of a steel-concrete composite beam in an embodiment;

[0074] Figure 3 is a three-dimensional schematic diagram of a steel-concrete composite beam in an embodiment;

[0075] Figure 4 is a schematic diagram of calculation results of an embodiment;

[0076] Figure 5 It is a schematic diagram of the calculation results of the prior art for comparison.

[0077] Explanation of the reference numerals: 1. Concrete slab; 2. Steel beam; 3. Reference axis. DETAILED DESCRIPTION

[0078] In order to make the purpose, technical solution and effect of the present invention clearer and more specific, the present invention is further described in detail below in combination with specific embodiments and drawings. Among them, the drawings are only used for exemplary descriptions, and only represent schematic diagrams, not physical drawings, and cannot be understood as limitations on this patent; in order to better illustrate the embodiments of the present invention, some parts and device connections in the drawings are omitted, enlarged or reduced, and do not represent the size of the actual product; for those skilled in the art, it is understandable that some well-known structures and their descriptions in the drawings may be omitted.

[0079] In order to fully understand the present invention, a detailed structure will be proposed in the following description to illustrate the technical solution proposed by the present invention. The optional embodiments of the present invention are described in detail as follows, but in addition to these detailed descriptions, the present invention may also have other implementations.

[0080] A method for calculating creep effect of steel-concrete composite structures, such as Figure 1 As shown, the following steps are included:

[0081] S1. Calculate the stress state of concrete and steel beams in the composite section of the "steel-concrete" composite beam under the action of external forces when the frame is dropped once and no creep occurs;

[0082] S2. Calculate the restraint reaction caused by concrete creep:

[0083] The constraint on the concrete is regarded as a special virtual rigid arm. After the concrete creeps, the constraint reaction forces -ΔN(t) and -ΔM(t) of the composite section due to the creep of the concrete are calculated.

[0084] S3. Apply restraint forces -ΔN(t) and -ΔM(t) in the opposite direction on the creep calculation section. The reaction force acts on the entire section. Calculate the stress change values ​​at the upper and lower edges of the concrete slab after the creep effect acts on the creep calculation section. Calculate the stress change values ​​at the upper and lower edges of the steel beam section.

[0085] S4. Use the accumulation method to calculate the stress state corresponding to the creep effect of the composite section.

[0086] Combination Figure 2 and Figure 3As shown, in this embodiment, the concrete is C40, and the elastic modulus of concrete is E c =3.25×10 4 MPa; the elastic modulus of steel is E s =2.0×10 5 MPa, creep coefficient in the embodiment The composite beam is subjected to a bending moment of M=1500 kN·m, N=0. Step S1 comprises:

[0087] S101. Convert the section of the "steel-concrete" composite section, using the top edge of the concrete beam on the section as the reference axis for analysis to convert the area A, area moment S, and moment of inertia I of the composite section:

[0088] A=bd c +n ss A ss =5.95235×10 5 (1)

[0089]

[0090] In the formula, b = 2000 mm is the width of the concrete slab 1, d c =150mm is the thickness of concrete slab 1, n ss =5.7971 is the ratio of the elastic modulus of steel to the elastic modulus of concrete, A ss =50928mm 2 is the cross-sectional area of ​​steel beam 2, d ss =1030.66mm is the distance between the centroid axis of the steel beam 2 section and the analysis reference axis 3, I ss =15686000553mm 4 is the moment of inertia of the second part of the steel beam about its own neutral axis;

[0091] S102. Calculate the strain ε1 at the top edge of the composite section and the deflection ψ at any height point of the section when the composite section is subjected to axial force N and bending moment M. i , the strain at any height y of the combined section from the reference axis is expressed as:

[0092] ε i =ε1-ψ i y (4),

[0093] σ i =E c ε i =E c (ε1-ψ i y) (5),

[0094] In the formula, y is the point in the combined section at a distance y from the reference axis, εi is the strain corresponding to the height point at a distance y from the reference axis in the cross section, σ i is the stress corresponding to the height point at a distance y from the reference axis in the cross section, E c is the elastic modulus of concrete;

[0095] According to the relationship between stress and force of the combined section:

[0096]

[0097] Substituting into formula (4) and formula (5), we can obtain:

[0098]

[0099]

[0100] S103. Calculate the stress of the concrete or steel beam at a distance y from the reference axis in the section, and obtain the stress state of the composite beam under the action of axial force and bending moment after the composite beam is dropped once:

[0101] σ ci =E c (ε1-yψ i ),

[0102] σ si =E s (ε1-yψ i ),

[0103] where σ ci is the stress at different heights on the concrete slab, σ si is the stress at different heights on the steel beam, E s is the elastic modulus of steel, and the stress state at any height on the combined section can be calculated.

[0104] The calculated stress on the upper edge of the concrete slab is:

[0105] σ c1 =E c (ε1-yψ i )=3.583MPa,

[0106] The stress at the lower edge of the concrete slab is:

[0107] σ c2 =E c (ε1-yψ i )=2.630MPa,

[0108] The stress at the upper edge of the steel beam is:

[0109] σ s1 =E s (ε1-yψi )=15.246MPa,

[0110] The stress at the lower edge of the steel beam is:

[0111] σ s2 =E s (ε1-yψ i )=-36.325MPa.

[0112] The step S2 comprises:

[0113] S201. According to formula (10), calculate the effective coefficient γ(t,t o ):

[0114]

[0115] in is the final value of creep coefficient;

[0116] S202. Calculate the effective elastic modulus of concrete according to formula (11):

[0117] S203. Assuming that the concrete slab of the composite beam is constrained by a special virtual rigid arm, the constraint is turned on and the creep strain Δε(t) of the concrete bridge deck is calculated according to formula (12):

[0118]

[0119] S204. Calculate the restraint stress Δσ1(t) corresponding to the restrained creep deformation of the concrete slab at time t according to formula (13):

[0120]

[0121] The calculated restraint stress on the upper edge of the concrete slab is:

[0122]

[0123] The restraint stress at the lower edge of the concrete slab is:

[0124]

[0125] S205. According to formula (14) and formula (15), the constraint stress in step S204 can be simplified to the required constraint reaction force -ΔN(t) and -ΔM(t) about the reference axis of the analysis:

[0126]

[0127] Among them A c =3×105 mm 2 is the cross-sectional area of ​​the concrete bridge deck,

[0128] S c =2.25×10 7 mm 3 is the area moment of the concrete bridge deck section, I c =2.25×10 9 is the moment of inertia of the concrete bridge deck section, ε oi is the strain at the top edge of the combined section, in this embodiment = ε oi =ε1=1.039×10 4 .

[0129] The step S3 comprises:

[0130] S301. Calculate the geometric properties of the creep calculation section and the creep calculation cross-sectional area Calculate the cross-sectional area moment using the Schuh transformation Calculate the moment of inertia of the section using the Slug transformation

[0131]

[0132] E is the ratio of the elastic modulus of steel to the elastic modulus of concrete after creep. ss is the elastic modulus of steel corresponding to the creep of concrete;

[0133] S302. Calculate the strain Δε1(t) at the upper edge of the concrete slab and the corresponding section deflection Δψ(t) when the constraint force of the creep effect acts in reverse on the creep calculation section according to formulas (20) and (21):

[0134]

[0135] S303. According to formula (22), the stress increment corresponding to the point in the cross section at a distance y from the reference axis is calculated as Δσ2(t):

[0136]

[0137] The calculated stress increment at the upper edge of the concrete slab is:

[0138] Δσ c2上 (t) = E φ (t,τ0)·(Δε1(t)-yΔψ(t))=1.838MPa,

[0139] The stress increment at the lower edge of the concrete slab is:

[0140] Δσ c2下 (t) = Eφ (t,τ0)·(Δε1(t)-yΔψ(t))=1.637MPa,

[0141] Under the influence of concrete shrinkage creep, the total stress change value of the concrete slab at the distance y from the top edge of the section is Δσ3(t), and the total stress change value of the steel beam at the distance y from the top edge of the section is Δσ ss (t) is:

[0142] Δσ3(t)=Δσ1(t)+Δσ2(t) (23),

[0143] The total stress change value of the upper edge of the concrete slab is calculated as follows:

[0144] Δσ c1 =Δσ c1上 (t)+Δσ c2上 (t) = -1.451 MPa,

[0145] The total stress change value of the lower edge of the concrete slab is:

[0146] Δσ c2 =Δσ c1下 (t)+Δσ c2下 (t) = -0.777 MPa,

[0147] Δσ ss (t) = E ss (Δε1(t)-yΔψ(t)) (24),

[0148] The total stress change value at the upper edge of the steel beam is:

[0149] Δσ s1 (t) = E ss (Δε1(t)-yΔψ(t))=25.841MPa

[0150] The total stress change at the lower edge of the steel beam is:

[0151] Δσ s2 (t) = E ss (Δε1(t)-yΔψ(t))=-3.794MPa.

[0152] The step S4 comprises:

[0153] S401. Calculate the stress σ on the upper edge of the concrete slab when the frame is dropped once according to steps S1 to S3. c1 and the stress σ at the lower edge of the concrete slab c2 , stress on the upper edge of the steel beam σ s1 and the stress at the lower edge of the steel beam σ s2 ;

[0154] S402: Calculate the total stress change Δσ at the top edge of the concrete slab after concrete creep occurs. c1 , and the total stress change Δσ at the bottom edge of the concrete slab c2 ; Total stress change value Δσ of the upper edge stress of the steel beam s1 , and the total stress change value Δσ of the stress at the lower edge of the steel beam s2 ;

[0155] S403: According to formula (25), formula (26), formula (27) and formula (28), the force superposition principle is used to calculate the final stress value σ1 at the upper edge of the concrete slab; the final stress value σ2 at the lower edge of the concrete slab; the final stress value σ3 at the upper edge of the steel beam; and the final stress value σ4 at the lower edge of the steel beam. The calculation results are as follows: Figure 4 As shown:

[0156] σ1=σ c1 +Δσ c1 =3.583-1.451=2.132MPa(pressure)(25),

[0157] σ2=σ c2 +Δσ c2 =2.630-0.777=1.853MPa(pressure)(26),

[0158] σ3=σ s1 +Δσ s1 =15.246+25.841=41.087MPa(pressure)(27),

[0159] σ4=σ s2 +Δσ s2 =-36.325-3.794=-40.119MPa(under tension)(28).

[0160] As a comparison with this embodiment, the following lists the traditional calculation methods for Figure 2 , Figure 3 Calculation method for creep effect of composite sections in composite beams.

[0161]

[0162] in: is the effective elastic modulus of concrete after creep; The concrete creep coefficient when the loading age is τ and the calculation age is t; α Es is the ratio of elastic modulus of steel to concrete, ψ L is the creep factor.

[0163] The distance between the centroid of the steel beam section and the lower flange of the steel beam:

[0164] The distance between the flange and the neutral axis of the concrete slab is: The distance from the lower flange of the concrete slab to the neutral axis of the converted section is y cu下 ,

[0165] The converted section moment of inertia is:

[0166]

[0167] Among them I ol is the converted section moment of inertia, I s is the effective section moment of inertia of the steel beam, I cs is the effective section moment of inertia of the concrete part, A s is the cross-sectional area of ​​the steel beam, A cs is the converted area of ​​the concrete slab, y s y is the distance between the centroid of the steel beam section and the bottom edge of the lower flange plate of the steel beam, cs It is the distance between the centroid of the concrete slab and the bottom surface of the lower flange plate of the steel beam.

[0168] The normal stress at the upper edge of the concrete slab in the mid-span section of the composite beam is:

[0169]

[0170] The normal stress at the lower edge of the concrete slab in the mid-span section of the composite beam is:

[0171]

[0172] The normal stress at the upper edge of the steel beam in the mid-span section of the composite beam is:

[0173]

[0174] The normal stress at the lower edge of the steel beam in the mid-span section of the composite beam is:

[0175] The calculation results are as follows Figure 5 shown.

[0176] By comparing the calculation results of this embodiment with the calculation process of the traditional calculation method, it can be found that there are certain differences in the stress of the concrete slab part, and the calculation results of the stress of the steel beam part are not much different. The traditional calculation method is relatively simple for the stress analysis of concrete after creep, and there is no mechanical basis. The creep effect is only reflected by the creep coefficient, and there is a lack of explanation of the mechanical mechanism. The calculation method of the present invention analyzes the entire process of concrete creep and has a mechanical basis. Therefore, the force results of the concrete part are different from the existing general calculation methods, and are more in line with the actual situation of the creep effect force model of the composite beam.

[0177] The present invention needs to satisfy the plane section assumption of the composite section. When the composite beam is in normal use, no slip occurs between the concrete slab and the steel beam, and the composite section of the composite beam satisfies the plane section assumption.

[0178] In the present invention, when there is no slip between the composite beam concrete slab and the steel beam, the concrete slab is strongly constrained by a special virtual rigid arm. It is considered that after creep occurs at both ends of the concrete slab under the constraint condition of the virtual rigid arm, the concrete slab is still in the state of the composite beam being dropped once as described in step S1 under the constraint condition of the virtual rigid arm.

[0179] The present invention does not make special considerations on the shear nails between the concrete slab and the steel beam of the composite beam. The shear nails serve to connect the concrete slab and the steel beam. The concrete slab is considered to be an ideal slab: it meets the requirements of continuity, uniformity and isotropy of the analysis object in material mechanics.

[0180] The creep effect calculation method of the "steel-concrete" composite structure of the present invention can be used to calculate the creep effect of the "steel-concrete" composite beam, and can also be used to calculate the creep effect of the "steel-concrete" composite slab.

[0181] Finally, it should be noted that the above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art can still modify the technical solutions described in the aforementioned embodiments or replace some of the technical features therein by equivalents. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A method for calculating creep effect of steel-concrete composite structure, characterized in that: The following steps are involved: S1. Calculate the stress state of concrete and steel beams in the composite section of the "steel-concrete" composite beam under the action of external forces when the frame is dropped once and no creep occurs; S2. Calculate the restraint reaction caused by concrete creep: The constraint on the concrete is regarded as a virtual rigid arm. After the concrete creeps, the constraint reaction forces -ΔN(t) and -ΔM(t) of the composite section due to the creep of the concrete are calculated. S3. Apply restraint forces -ΔN(t) and -ΔM(t) in the opposite direction on the creep calculation section. The reaction force acts on the entire section. Calculate the stress change values ​​at the upper and lower edges of the concrete slab after the creep effect acts on the creep calculation section. Calculate the stress change values ​​at the upper and lower edges of the steel beam section. S4. Use the accumulation method to calculate the stress state corresponding to the creep effect of the composite section.

2. The method for calculating creep effect of steel-concrete composite structure according to claim 1, characterized in that: The step S1 comprises: S101. Convert the section of the "steel-concrete" composite section, using the top edge of the concrete beam on the section as the reference axis for analysis to convert the area A, area moment S, and moment of inertia I of the composite section: A=bd c +n ss A ss (1), In the formula, b is the width of the concrete slab, d is c is the thickness of the concrete slab, n ss is the ratio of the elastic modulus of steel to the elastic modulus of concrete, A ss is the cross-sectional area of ​​the steel beam, d ss is the distance between the centroid axis of the steel beam section and the analysis reference axis, I ss is the moment of inertia of the steel beam part about its own neutral axis; S102. Calculate the strain ε1 at the top edge of the composite section and the deflection ψ at any height point of the section when the composite section is subjected to axial force N and bending moment M. i , the strain at any height y of the combined section from the reference axis is expressed as: e i =ε1-ψ i y (4), s i =E c e i =E c (ε1-ψ i y) (5), In the formula, y is the point in the combined section at a distance y from the reference axis, ε i is the strain corresponding to the height point at a distance y from the reference axis in the cross section, σ i is the stress corresponding to the height point at a distance y from the reference axis in the cross section, E c is the elastic modulus of concrete; According to the relationship between stress and force of the combined section: Substituting into formula (4) and formula (5), we can obtain: S103. Calculate the stress of the concrete or steel beam at a distance y from the reference axis in the section, and obtain the stress state of the composite beam under the action of axial force and bending moment after the composite beam is dropped once: s ci =E c (ε1-yψ i ), s si =E s (ε1-yψ i ), where σ ci is the stress at different heights on the concrete slab, σ si is the stress at different heights on the steel beam, E s is the elastic modulus of steel.

3. The method for calculating creep effect of steel-concrete composite structure according to claim 2, characterized in that: The step S2 comprises: S201. According to formula (10), calculate the effective coefficient γ(t,t o ): in is the final value of creep coefficient; S202. Calculate the effective elastic modulus of concrete according to formula (11): S203. Assuming that the concrete slab of the composite beam is constrained by the virtual rigid arm, the constraint is turned on and the creep strain Δε(t) of the concrete bridge deck is calculated according to formula (12): S204. Calculate the restraint stress Δσ1(t) corresponding to the restrained creep deformation of the concrete slab at time t according to formula (13): S205. According to formula (14) and formula (15), the constraint stress in step S204 can be simplified to the required constraint reaction force -ΔN(t) and -ΔM(t) about the reference axis of the analysis: Among them A c is the cross-sectional area of ​​the concrete bridge deck, S c is the area moment of the concrete bridge deck section, I c is the moment of inertia of the concrete bridge deck section, ε oi is the strain at the top edge of the cross section.

4. The method for calculating creep effect of steel-concrete composite structure according to claim 3 is characterized in that: The step S3 comprises: S301. Calculate the geometric properties of the creep calculation section and the creep calculation cross-sectional area Calculate the cross-sectional area moment using the Schuh transformation Calculate the moment of inertia of the section using the Slug transformation E is the ratio of the elastic modulus of steel to the elastic modulus of concrete after creep. ss is the elastic modulus of steel corresponding to the creep of concrete; S302. Calculate the strain Δε1(t) at the upper edge of the concrete slab and the corresponding section deflection Δψ(t) when the constraint force of the creep effect acts in reverse on the creep calculation section according to formulas (20) and (21): S303. According to formula (22), the stress increment corresponding to the point in the cross section at a distance y from the reference axis is calculated as Δσ2(t): Under the influence of concrete shrinkage creep, the total stress change value of the concrete slab at the distance y from the top edge of the section is Δσ3(t), and the total stress change value of the steel beam at the distance y from the top edge of the section is Δσ ss (t) is: Δσ3(t)=Δσ1(t)+Δσ2(t) (23), Board ss (t)=E ss (Δε1(t)-yΔψ(t)) (24).

5. The method for calculating creep effect of steel-concrete composite structure according to claim 4, characterized in that: The step S4 comprises: S401. Calculate the stress σ on the upper edge of the concrete slab when the frame is dropped once according to steps S1 to S3. c1 and the stress σ at the lower edge of the concrete slab c2 , stress on the upper edge of the steel beam σ s1 and the stress at the lower edge of the steel beam σ s2 ; S402: Calculate the total stress change Δσ at the top edge of the concrete slab after concrete creep occurs. c1 , and the total stress change Δσ at the bottom edge of the concrete slab c2 ; Total stress change value Δσ of the upper edge stress of the steel beam s1 , and the total stress change value Δσ of the stress at the lower edge of the steel beam s2 ; S403: According to formula (25), formula (26), formula (27) and formula (28), the force superposition principle is used to calculate the final stress value σ1 at the upper edge of the concrete slab; the final stress value σ2 at the lower edge of the concrete slab; the final stress value σ3 at the upper edge of the steel beam; and the final stress value σ4 at the lower edge of the steel beam: σ1=σ c1 +Ds c1 (25), σ2=σ c2 +Ds c2 (26), σ3=σ s1 +Ds s1 (27), σ4=σ s2 +Ds s2 (28)。 6. The method for calculating creep effect of steel-concrete composite structure according to any one of claims 1 to 5, characterized in that: When the composite beam is in normal use, there is no slip between the concrete slab and the steel beam, and the composite section of the composite beam satisfies the plane section assumption.

7. The method for calculating creep effect of steel-concrete composite structure according to claim 6, characterized in that: When there is no slip between the composite beam concrete slab and the steel beam, the concrete slab is strongly constrained by the virtual rigid arm. It is considered that after creep occurs at both ends of the concrete slab under the constraint conditions of the virtual rigid arm, the concrete slab is still in the state of the composite beam being dropped once as described in step S1 under the constraint conditions of the virtual rigid arm.

8. The method for calculating creep effect of steel-concrete composite structure according to claim 6, characterized in that: No special consideration is given to the shear studs between the concrete slab and the steel beam of the composite beam. The shear studs serve to connect the concrete slab and the steel beam. The concrete slab is considered to be an ideal slab: it meets the requirements of continuity, uniformity and isotropy of the analysis object in material mechanics.

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