Design method of a steel truss beam-arch composite structure aqueduct

Abstract

The present invention relates to the field of water conservancy engineering design, and discloses a design method for a steel truss beam-arch combination structure aqueduct. The method selects an upper chord beam, a trough bottom beam and a lower arch beam in the aqueduct to form a beam component combination. The aqueduct structure is simplified into a multi-span continuous beam model with variable cross-sections. According to the geometric dimensions of the components, a first function is calculated to characterize the mapping relationship between the moment of inertia of the beam component combination and the longitudinal position of the aqueduct. According to the first function and based on the basic equation of the force method, all support reaction forces of the continuous beam model are calculated, and multi-order support reaction force vectors are constructed to calculate the internal force and deflection of the continuous multi-span steel truss beam-arch combination structure, and obtain the functional relationship between the most unfavorable deflection of the aqueduct and the beam height at the middle pier support point, so that the designer can adjust the overall stiffness of the entire aqueduct structure by adjusting the beam height at the support point, so that the structural deflection under the action of super-large water body live load can be quantitatively controlled to ensure the flow rate and water conveyance capacity.

Description

Technical Field

[0001] The invention relates to the field of water conservancy engineering design, in particular to a design method for a steel truss type beam-arch combined structure aqueduct. Background Art

[0002] Aqueducts are aerial water transfer structures that span rivers, roads, ravines, etc. They are widely used in water diversion and regulation projects. As these projects increase year by year, the construction of aqueducts tends to be large-flow, large-scale, and large-span. The traditional small-span concrete structure aqueduct has a limited scope of application, while the steel truss beam-arch composite structure aqueduct is more suitable for today's construction requirements due to its high strength, light weight, and large span.

[0003] The aqueduct structure directly bears the load of large tonnage water flow, and the live load base and amplitude are both large. The aqueduct needs to have a large beam height to ensure that the aqueduct has sufficient rigidity to avoid excessive deflection in the middle of the aqueduct span that affects the flow rate and water conveyance capacity; because the top road is connected to the existing embankment road, the height of the upper trough body is limited. For continuous beam aqueducts, generally a larger beam height is set at the aqueduct support, a smaller beam height in the middle of the span, and a variable cross-section in a parabolic change along the longitudinal direction. The mid-span deflection is controlled by adjusting the beam height at the support; the specific value of the mid-span beam height can be determined according to the water flow of the connected river channel, and the beam height at the support needs to be generated according to the overall force response of the structure under the load. The maximum deflection at mid-span is used to determine the limit value. However, there is no clear functional relationship between the beam height at the support and the mid-span deflection, and it is difficult to find a direct relationship for structural design. For the steel truss beam-arch composite structure aqueduct, it is a beam-arch composite structure composed of steel trusses, and it is a multi-span continuous beam with multiple hyperstatic structures. It becomes higher in the longitudinal direction and has a complex structure. There is currently no design method for this type of structure, and it is impossible to give a quantitative calculation method for adjusting the beam height at the support to control the maximum deflection. Engineers often use scaled experiments or finite element software modeling and analysis, but this will consume a lot of financial and human resources, and the parameter adjustment is difficult and not easy to operate, so it needs to be solved urgently. Summary of the invention

[0004] In order to solve the technical problems existing in the prior art, the present invention provides a design method for a steel truss beam-arch composite structure aqueduct, which can calculate the functional relationship between the most unfavorable deflection of the aqueduct and the beam height at the middle pier support, so that designers can adjust the overall stiffness of the entire aqueduct structure by adjusting the beam height at the support, thereby quantitatively controlling the structural deflection under the action of super-large live loads and ensuring the flow rate and water conveyance capacity.

[0005] To achieve the above object, the present invention provides the following technical solutions:

[0006] The invention discloses a design method for a steel truss beam-arch combined structure aqueduct, comprising steps S1 to S8.

[0007] S1. Select the upper chord beam, bottom beam and lower arch beam in the aqueduct to form a beam component combination.

[0008] S2. Simplify the aqueduct structure into a multi-span continuous beam model with variable cross-section.

[0009] S3. Calculate a first function for characterizing the mapping relationship between the moment of inertia of the beam component combination and the longitudinal position of the aqueduct according to the geometric dimensions of the components.

[0010] S4. According to the first function and based on the basic equation of the force method, all support reactions of the continuous beam model are calculated, thereby constructing a multi-order support reaction vector.

[0011] S5. Calculate a second function for characterizing the mapping relationship between the bending moment of each span of the beam component combination and the longitudinal position of the aqueduct based on the uniformly distributed water load in the aqueduct and the support reaction vector.

[0012] S6. According to the second function, the mid-span deflection of each span of the continuous model is calculated by the virtual work method, and the maximum value is taken as the most unfavorable deflection of the aqueduct, so as to obtain the third function used to characterize the mapping relationship between the most unfavorable deflection and the lower girder height at the middle pier support point; wherein, the lower girder height is the vertical distance from the bottom of the bottom beam to the bottom of the lower arch beam at the same longitudinal position of the aqueduct.

[0013] S7. Based on the third function, the fourth function is calculated using Xie Cai's formula to characterize the mapping relationship between the average design flow velocity of the most unfavorable deflection span and the lower girder height at the middle pier support.

[0014] S8. According to the minimum limit value of the aqueduct design flow velocity, the value range of the lower girder height at the middle pier support point is calculated from the fourth function, thereby obtaining the lower limit value of the lower girder height at the middle pier support point.

[0015] As a further improvement of the above scheme, the aqueduct consists of an upper trough body, a lower arch beam and an intermediate web member; wherein, the upper trough body consists of two longitudinally extending upper chord beams, a trough bottom beam and a U-shaped corrugated steel plate, which are combined to form a U-shaped water passage with equal height along the longitudinal direction; the lower arch beam is a box beam that is arched upward in each span, and the arch line conforms to the shape of a quadratic parabola. The arch foot is located at the middle pier support of the aqueduct, and the arch top is located in the middle of each span of the aqueduct. The lower arch beam of the side span is a half arch, and the lower arch beam of the middle span is a full arch.

[0016] As a further improvement of the above scheme, in step S2, the aqueduct structure is simplified into an N-span continuous beam model formed by N+1 supports; wherein, the transverse midpoint of the top of the upper chord beam at the head end of the aqueduct, i.e. the first support, is taken as the coordinate origin, the x-axis is along the longitudinal direction of the aqueduct and pointing to the tail end of the aqueduct, the y-axis is along the transverse direction of the aqueduct, and the z-axis is along the vertical direction of the aqueduct, thereby establishing a three-dimensional coordinate system.

[0017] As a further improvement of the above solution, step S3 includes the following specific steps, namely S31~S33.

[0018] S31. Calculate the function expression of the lower girder height with respect to the longitudinal position of the aqueduct :

[0019] ;

[0020] In the formula, H 1 is the height of the lower girder at the middle pier support point; L p It is the distance from the starting point of the arch beam under the side span to the support point of the side pier on the x-axis; L i is the span of the ith span, L 1 is the span of the first span; k is the longitudinal position of the aqueduct along the positive direction of the x-axis, i.e. The span number, k∈[1,N] where k is an integer;

[0021] L k is the span of the kth span.

[0022] S32. Calculate the vertical coordinates of the centroid of the combined cross section of the beam member z c :

[0023] ;

[0024] In the formula, A bi’ represents the cross-sectional area of ​​the i'th beam member; when i'=1,2,3,4, A bi’ They represent the cross-sectional areas of the left upper chord beam, the right upper chord beam, the bottom beam of the trough and the lower arch beam respectively; z i’ Represents the vertical coordinate of the centroid of the i'th beam component.

[0025] S33. Calculate the first function, that is, the moment of inertia of the beam component combination about The expression :

[0026] ;

[0027] In the formula, I i’ is the section moment of inertia of the i'th beam member, a i’ is the centroid of the i'th beam component and z c The vertical distance between them, when i'=1,2,3, a i’=| z i’ - z c |; When i'=4, ; H 0 is the height of the aqueduct, that is, the vertical distance from the top of the upper chord beam to the bottom of the bottom beam of the aqueduct; h 4 is the cross-sectional height of the lower arch beam.

[0028] As a further improvement of the above solution, in step S32, the vertical coordinate of the centroid of the lower arch beam is z 4 is related to the lower truss height, and the calculation formula is: .

[0029] As a further improvement of the above solution, step S4 includes the following specific steps, namely S41~S47.

[0030] S41. In the N-1-order hyperstatic structure continuous beam model, release the constraint forces of the 2nd to Nth supports to make the structural system statically determinate and use it as the basic system of the force method. The released support reaction force is used as the unknown force. The unknown force vector Defined as:

[0031] ;

[0032] In the formula, X 1 is the support reaction of the second support, is the support reaction of the third support, is the support reaction of the Nth support, n =N-1; superscript T is the transpose symbol.

[0033] S42. Calculate the flexibility coefficients of the continuous beam model; where the flexibility coefficient The unknown force vector X Middle j The unknown force is the unit force, The time caused by Unknown force Direction displacement, ; The calculation formula is:

[0034] ;

[0035] In the formula, E is the elastic modulus of dry steel; is the equivalent section moment of inertia of the k-th span of the aqueduct, ; is the cumulative length of all spans before the kth span, ; for Aqueduct bending moment at ; for The bending moment of the aqueduct at .

[0036] S43. Constructing a flexibility matrix from multiple flexibility coefficients , the expression is:

[0037] ;

[0038] S44. Calculation of unknown forces along the axis due to external water loads Displacement in direction :

[0039] ;

[0040] In the formula, Uniformly distributed water load q The bending moment of the aqueduct under the action of .

[0041] S45. Constructing the aqueduct displacement vector , the expression is:

[0042] ;

[0043] S46. According to the deformation coordination condition that the vertical displacement at each support point of the continuous beam model is 0, the basic equation of the force method is established. The matrix equation is as follows:

[0044] ;

[0045] S47. The flexibility matrix of step S44 is and the aqueduct displacement vector of step S45 Substituting into the matrix equation, the unknown force vector is solved , further calculate the support reaction of the aqueduct structure from the 1st to the N+1th support, and construct the N+1-order support reaction vector , the expression is ; Among them, the reaction force of the tth support is The expression is:

[0046] .

[0047] As a further improvement of the above scheme, in step S5, according to the uniformly distributed water load in the aqueduct q and the support reaction vector Calculate the second function :

[0048] ;

[0049] As a further improvement of the above solution, step S6 includes the following specific steps, namely S61~S63.

[0050] S61. Apply a vertical virtual unit force Z=1 to the mid-span of each span of the aqueduct in turn, and use the force method to calculate the aqueduct bending moment in each span under the action of the virtual unit force by referring to steps S4-S5; among which, the general expression of the bending moment of the kth span caused by the virtual unit force of the sth span is for:

[0051] ;

[0052] In the formula, is the cumulative length of all spans before the sth span, ; is the span of the sth span.

[0053] S62. According to the actual working condition bending moment calculated and the bending moment of the aqueduct under the action of virtual unit force , the virtual work method is used to calculate the mid-span deflection of each span, and the deflection vector is obtained by combining ;in, is the mid-span deflection of the s-th span of the aqueduct, and the calculation formula is:

[0054] .

[0055] S63. Select the maximum mid-span deflection in the deflection vector As the most unfavorable deflection of the aqueduct structure, the most unfavorable deflection is obtained. Height of lower girder at middle pier support H 1 The third function related to the expression is: .

[0056] As a further improvement of the above solution, step S7 includes the following specific steps, namely S71~S74.

[0057] S71. Calculate the hydraulic slope of the most unfavorable deflection span J , the calculation formula is:

[0058] ;

[0059] In the formula, is the span where the most unfavorable deflection occurs.

[0060] S72. Calculation of hydraulic radius in aqueduct R , the calculation formula is:

[0061] ;

[0062] In the formula, H wIt is the net height of water level in the aqueduct. B It is the net width of the aqueduct.

[0063] S73. Calculate the average design velocity of the most unfavorable deflection span using Xie Cai's formula V :

[0064] ;

[0065] In the formula, is the roughness coefficient of the aqueduct body.

[0066] S74. Construct the average design flow rate of the most unfavorable deflection span V About the height of the lower girder at the middle pier support H The fourth function of 1 :

[0067] .

[0068] As a further improvement of the above solution, in step S8, the lower girder height at the middle pier support point is H The calculation formula for the value range of 1 is:

[0069] ;

[0070] In the formula, v -1 (·)for v The inverse function of (·), whose independent variable is the design flow rate [ V ].

[0071] Compared with the prior art, the present invention has the following beneficial effects:

[0072] 1. The present invention discloses a method for designing a steel truss beam-arch composite structure aqueduct, provides a method for calculating the internal force and deflection of a continuous multi-span steel truss beam-arch composite structure, and provides a functional relationship between the most unfavorable deflection of the aqueduct and the beam height at the middle pier support point, so that designers can adjust the overall stiffness of the entire aqueduct structure by adjusting the beam height at the support point, thereby quantitatively controlling the structural deflection under the action of super-large water body live load and ensuring the flow rate and water conveyance capacity.

[0073] 2. The equivalent processing method for complex variable-section truss beams disclosed in the present invention simplifies complex problems and provides a mechanical calculation method for complex spatial structures such as large-span trusses.

[0074] 3. The calculation method disclosed in the present invention is a general formula for multi-span continuous beams. It is not limited by the number of spans and spans, has a wide range of applicable scenarios, and is easy to develop and apply digitally.

[0075] 4. The aqueduct of the present invention is composed of an upper water conveyance channel body of equal height and a lower arched beam, and the overall shape is flat on the top and arched on the bottom. It can not only ensure a water conveyance channel with equal flow in the upper part, but also achieve the effect of a variable-section continuous beam through the lower arch, thereby increasing the clear navigation height below the aqueduct as much as possible, reducing the deadweight, and ensuring that the overall rigidity of the structure meets the requirements. BRIEF DESCRIPTION OF THE DRAWINGS

[0076] Figure 1 It is a schematic diagram of the longitudinal section of a steel truss beam-arch combined structure aqueduct in an embodiment of the present invention.

[0077] Figure 2 It is a schematic cross-sectional view of a steel truss beam-arch combined structure aqueduct in an embodiment of the present invention.

[0078] Figure 3 The present invention is a flowchart of a design method for a steel truss beam-arch composite structure aqueduct in an embodiment of the present invention. DETAILED DESCRIPTION

[0079] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0080] Example 1

[0081] See also Figure 1 and Figure 2 , this embodiment provides a steel truss type beam-arch combined structure aqueduct, which is composed of an upper trough body, a lower arch beam 2 and an intermediate web member 3;

[0082] The upper trough body is composed of two longitudinally extending upper chord beams 1.1, a trough bottom beam 1.2 and a U-shaped corrugated steel plate 1.3, which are combined to form a U-shaped water passage with equal height along the longitudinal direction to ensure sufficient water flow.

[0083] The lower arch beam 2 is a box beam that is arched upward in each span. The arch line conforms to the shape of a quadratic parabola. The arch foot is located at the middle pier support of the aqueduct, and the arch top is located in the middle of each span of the aqueduct. The lower arch beam 2 of the side span is a half arch, and the lower arch beam 2 of the middle span is a full arch.

[0084] See also Figure 3 This embodiment provides a design method for the above-mentioned steel truss beam-arch composite structure aqueduct, including steps S1 to S8.

[0085] S1. The components of the steel truss beam-arch composite structure aqueduct are disassembled and classified, and the longitudinal full-length components that mainly bear the bending moment in the overall stress of the structure are selected, including the upper chord beam 1.1, the trough bottom beam 1.2 and the lower arch beam 2, which form a beam composite component as a whole.

[0086] S2. Simplify the aqueduct structure into a multi-span continuous beam model with variable cross-section.

[0087] In step S2, the aqueduct structure is simplified into an N-span continuous beam model formed by N+1 supports; wherein the transverse midpoint of the top of the upper chord beam 1.1 at the head end of the aqueduct, i.e. the first support, is taken as the coordinate origin, the x-axis is along the longitudinal direction of the aqueduct and pointing to the tail end of the aqueduct, the y-axis is along the transverse direction of the aqueduct, and the z-axis is along the vertical direction of the aqueduct, thereby establishing a three-dimensional coordinate system.

[0088] S3. According to the geometric dimensions of the components, calculate the first function used to characterize the mapping relationship between the moment of inertia of the beam component combination and the longitudinal position of the aqueduct. Since the aqueduct structure is a continuous beam with a variable cross-section, the moment of inertia of each point on the x-axis is different, so it is defined as x Function expression .

[0089] Step S3 includes the following specific steps, namely S31~S33.

[0090] Calculate the height of the lower girder at any point on the longitudinal coordinate axis with respect to the longitudinal position of the aqueduct, i.e., the coordinate x Function expression , the lower beam height refers to the same x The vertical distance from the bottom of the trough bottom beam 1.2 to the bottom of the lower arch beam 2 at the coordinate is expressed as follows:

[0091] ;

[0092] In the formula, H 1 is the height of the lower girder at the middle pier support point; L p It is the distance from the starting point of the arch beam under the side span to the support point of the side pier on the x-axis; L i is the span of the ith span, L 1 is the span of the first span; k is the longitudinal position of the aqueduct along the positive direction of the x-axis, i.e. The span number, k∈[1,N] where k is an integer; L k is the span of the kth span.

[0093] S32. Calculate the vertical coordinates of the centroid of the combined cross section of the beam member z c :

[0094] ;

[0095] In the formula, A bi’ represents the cross-sectional area of ​​the i'th beam member; when i'=1,2,3,4, A bi’ represent the cross-sectional areas of the left upper chord beam 1.1, the right upper chord beam 1.1, the trough bottom beam 1.2 and the lower arch beam 2 respectively; z i’ represents the vertical coordinate of the centroid of the i'th beam component. Among them, the vertical coordinate of the centroid of the lower arch beam 2 is z 4 is related to the lower truss height, and the calculation formula is: ; H 0 is the upper channel height of the aqueduct, i.e. the vertical distance from the top of the upper chord beam 1.1 to the bottom of the channel bottom beam 1.2; h 4 is the cross-sectional height of the lower arch beam 2.

[0096] S33. Calculate the first function, that is, the moment of inertia of the beam component combination about The expression :

[0097] ;

[0098] In the formula, I i’ is the section moment of inertia of the i'th beam member, a i’ is the centroid of the i'th beam component and z c The vertical distance between , i'=1,2,3; .

[0099] S4. According to the first function and based on the basic equation of the force method, all support reactions of the continuous beam model are calculated, thereby constructing a multi-order support reaction vector.

[0100] Step S4 includes the following specific steps, namely S41 to S47.

[0101] S41. In the N-1-order hyperstatic structure continuous beam model, release the constraint forces of the 2nd to Nth supports to make the structural system statically determinate and use it as the basic system of the force method. The released support reaction force is used as the unknown force. The unknown force vector Defined as:

[0102] ;

[0103] In the formula, X 1 is the support reaction of the second support, is the support reaction of the third support, is the support reaction of the Nth support,n =N-1; superscript T is the transpose symbol.

[0104] S42. Calculate the flexibility coefficients of the continuous beam model; where the flexibility coefficient The unknown force vector X Middle j The unknown force is the unit force, The time caused by Unknown force Direction displacement, ; The calculation formula is:

[0105] ;

[0106] In the formula, E is the elastic modulus of dry steel; is the equivalent section moment of inertia of the k-th span of the aqueduct, ; is the cumulative length of all spans before the kth span, ; for Aqueduct bending moment at ; for The bending moment of the aqueduct at .

[0107] S43. Constructing a flexibility matrix from multiple flexibility coefficients , the expression is:

[0108] ;

[0109] S44. Calculation of unknown forces along the axis due to external water loads Displacement in direction :

[0110] ;

[0111] In the formula, Uniformly distributed water load q The aqueduct bending moment under the action of the force is expressed as follows (1):

[0112] .

[0113] S45. Constructing the aqueduct displacement vector , the expression is:

[0114] ;

[0115] S46. According to the deformation coordination condition that the vertical displacement at each support point of the continuous beam model is 0, the basic equation of the force method is established. The matrix equation is as follows:

[0116] ;

[0117] S47. The flexibility matrix of step S44 is and the aqueduct displacement vector of step S45 Substituting into the matrix equation, the unknown force vector is solved , further calculate the support reaction of the aqueduct structure from the 1st to the N+1th support, and construct the N+1-order support reaction vector , the expression is ; Among them, the reaction force of the tth support is The expression (2) is:

[0118] .

[0119] S5. Calculate a second function for characterizing the mapping relationship between the bending moment of each span of the beam component combination and the longitudinal position of the aqueduct based on the uniformly distributed water load in the aqueduct and the support reaction vector.

[0120] In step S5, according to the uniformly distributed water load in the aqueduct q and the support reaction vector Calculate the second function That is, expression (3) is:

[0121] .

[0122] S6. According to the second function, the mid-span deflection of each span of the continuous model is calculated by the virtual work method, and the maximum value is taken as the most unfavorable deflection of the aqueduct, so as to obtain the third function used to characterize the mapping relationship between the most unfavorable deflection and the lower girder height at the middle pier support point; wherein, the lower girder height is the vertical distance from the bottom of the bottom beam to the bottom of the lower arch beam at the same longitudinal position of the aqueduct.

[0123] Step S6 includes the following specific steps, namely S61 to S63.

[0124] S61. Apply a vertical virtual unit force Z=1 in the middle of each span of the aqueduct in turn, and use the force method to calculate the bending moment of the aqueduct in each span under the action of the virtual unit force, referring to steps S4-S5. The specific process is: the above formula (1) is adjusted to formula (4), formula (2) is adjusted to formula (5), and formula (3) is adjusted to formula (6).

[0125] Among them, formula (4) is:

[0126] ;

[0127] In the formula, ; , .

[0128] Formula (5) is:

[0129] .

[0130] General expression for the bending moment in the kth span caused by the fictitious unit force in the sth span That is, formula (6) is:

[0131] ;

[0132] In the formula, is the cumulative length of all spans before the sth span, ; is the span of the sth span.

[0133] S62. According to the actual working condition bending moment calculated and the bending moment of the aqueduct under the action of virtual unit force , the virtual work method is used to calculate the mid-span deflection of each span, and the deflection vector is obtained by combining ;in, is the mid-span deflection of the s-th span of the aqueduct, and the calculation formula is:

[0134] .

[0135] S63. Select the maximum mid-span deflection in the deflection vector As the most unfavorable deflection of the aqueduct structure, the most unfavorable deflection is obtained. Height of lower girder at middle pier support H 1 The third function related to the expression is: .

[0136] S7. Based on the third function, the fourth function is calculated using Xie Cai's formula to characterize the mapping relationship between the average design flow velocity of the most unfavorable deflection span and the lower girder height at the middle pier support.

[0137] Step S7 includes the following specific steps, namely S71~S74.

[0138] S71. Calculate the hydraulic slope of the most unfavorable deflection span J , the calculation formula is:

[0139] ;

[0140] In the formula, is the span where the most unfavorable deflection occurs.

[0141] S72. Calculation of hydraulic radius in aqueduct R , the calculation formula is:

[0142] ;

[0143] In the formula,H w It is the net height of water level in the aqueduct. B It is the net width of the aqueduct.

[0144] S73. Calculate the average design velocity of the most unfavorable deflection span using the hydraulic formula V :

[0145] ;

[0146] In the formula, is the roughness coefficient of the aqueduct body.

[0147] S74. Substitute the formulas of steps S63 and S71 into the formula of step S73 to construct the average design flow rate of the most unfavorable deflection span V About the height of the lower girder at the middle pier support H The fourth function of 1 :

[0148] .

[0149] S8. According to the minimum limit value of the aqueduct design flow velocity, the value range of the lower girder height at the middle pier support point is calculated from the fourth function, thereby obtaining the lower limit value of the lower girder height at the middle pier support point.

[0150] In step S8, the lower beam height at the middle pier support point is H The calculation formula for the value range of 1 is:

[0151] ;

[0152] In the formula, v -1 (·)for v The inverse function of (·), whose independent variable is the design flow rate [ V ].

[0153] By calculating the lower beam height The lower limit value of is used to complete the design of a variable-section steel truss beam aqueduct with a flat top and arched bottom that meets the deflection requirements.

[0154] The above description is only a preferred specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any technician familiar with the technical field can make equivalent replacements or changes according to the technical scheme and inventive concept of the present invention within the technical scope disclosed by the present invention, which should be covered by the protection scope of the present invention.

Claims

1. A design method for a steel truss beam-arch composite structure aqueduct, characterized in that: Includes steps: S1. Selecting an upper chord beam, a bottom beam and a lower arch beam from the aqueduct to form a beam component combination; S2. Simplify the aqueduct structure into a multi-span continuous beam model with variable cross-section; S3. Calculate a first function for characterizing the mapping relationship between the moment of inertia of the beam component combination and the longitudinal position of the aqueduct according to the geometric dimensions of the component; S4. Calculate all support reactions of the continuous beam model according to the first function and based on the basic equation of the force method, thereby constructing a multi-order support reaction vector; S5. Calculate a second function for characterizing the mapping relationship between the bending moment of each span of the beam member combination and the longitudinal position of the aqueduct according to the uniformly distributed water load in the aqueduct and the support reaction vector; S6. According to the second function, the mid-span deflection of each span of the continuous model is calculated by using the virtual work method, and the maximum value is taken as the most unfavorable deflection of the aqueduct, and a third function is obtained for characterizing the mapping relationship between the most unfavorable deflection and the lower girder height at the middle pier support point; wherein the lower girder height is the vertical distance from the bottom of the bottom beam of the trough to the bottom of the lower arch beam at the same longitudinal position of the aqueduct; S7. Based on the third function, the fourth function for characterizing the mapping relationship between the average design flow velocity of the most unfavorable deflection span and the lower girder height at the middle pier support point is calculated using the Xie Cai formula; S8. According to the minimum limit value of the aqueduct design flow velocity, the value range of the lower girder height at the middle pier support point is calculated from the fourth function, thereby obtaining the lower limit value of the lower girder height at the middle pier support point.

2. The design method of a steel truss beam-arch composite structure aqueduct according to claim 1, characterized in that: The aqueduct consists of an upper trough body, a lower arch beam and an intermediate web member; wherein, the upper trough body consists of two longitudinally extending upper chord beams, a trough bottom beam and a U-shaped corrugated steel plate, which are combined to form a U-shaped water passage with equal height along the longitudinal direction; the lower arch beam is a box beam that is arched upward in each span, and the arch line conforms to the shape of a quadratic parabola. The arch foot is located at the middle pier support of the aqueduct, and the arch top is located in the middle of each span of the aqueduct. The lower arch beam of the side span is a half arch, and the lower arch beam of the middle span is a full arch.

3. The design method of a steel truss beam-arch composite structure aqueduct according to claim 1, characterized in that: In step S2, the aqueduct structure is simplified into an N-span continuous beam model formed by N+1 supports; wherein, the transverse midpoint of the top of the upper chord beam at the head end of the aqueduct, i.e. the first support, is taken as the coordinate origin, the x-axis is along the longitudinal direction of the aqueduct and pointing to the tail end of the aqueduct, the y-axis is along the transverse direction of the aqueduct, and the z-axis is along the vertical direction of the aqueduct, thereby establishing a three-dimensional coordinate system.

4. The design method of a steel truss beam-arch composite structure aqueduct according to claim 3 is characterized in that: Step S3 includes the following specific steps: S31. Calculate the function expression of the lower girder height with respect to the longitudinal position of the aqueduct : ； In the formula, H 1 is the height of the lower girder at the middle pier support point; L p It is the distance from the starting point of the arch beam under the side span to the support point of the side pier on the x-axis; L i is the span of the ith span, L 1 is the span of the first span; k is the longitudinal position of the aqueduct along the positive direction of the x-axis, i.e. The span number, k∈[1,N] where k is an integer; L k is the span of the kth span; S32. Calculate the vertical coordinates of the centroid of the combined cross section of the beam member z c : ； In the formula, A bi’ represents the cross-sectional area of ​​the i'th beam member; When i'=1,2,3,4, A bi’ They represent the cross-sectional areas of the left upper chord beam, the right upper chord beam, the bottom beam of the trough and the lower arch beam respectively; z i’ represents the vertical coordinate of the centroid of the i'th beam member; S33. Calculate the first function, that is, the moment of inertia of the beam component combination about The expression : ； In the formula, I i’ is the section moment of inertia of the i'th beam member, a i’ is the centroid of the i'th beam component and z c The vertical distance between them, when i'=1,2,3, a i’ =| z i’ - z c |; When i'=4, ; H 0 is the height of the aqueduct, that is, the vertical distance from the top of the upper chord beam to the bottom of the bottom beam of the aqueduct; h 4 is the cross-sectional height of the lower arch beam.

5. The design method of a steel truss beam-arch composite structure aqueduct according to claim 4, characterized in that: In step S32, the vertical coordinate of the centroid of the lower arch beam is z 4 is related to the lower truss height, and the calculation formula is: .

6. The design method of a steel truss beam-arch composite structure aqueduct according to claim 4, characterized in that: Step S4 includes the following specific steps: S41. In the N-1-order hyperstatic structure continuous beam model, release the constraint forces of the 2nd to Nth supports to make the structural system statically determinate and use it as the basic system of the force method. The released support reaction force is used as the unknown force. The unknown force vector Defined as: ； In the formula, X 1 is the support reaction of the second support, is the support reaction of the third support, is the support reaction of the Nth support, n =N-1; Superscript T is the transpose symbol; S42. Calculate the flexibility coefficients of the continuous beam model; where the flexibility coefficient The unknown force vector X Middle j The unknown force is the unit force, The time caused by Unknown force Direction displacement, ; The calculation formula is: ； In the formula, E is the elastic modulus of dry steel; is the equivalent section moment of inertia of the k-th span of the aqueduct, ; is the cumulative length of all spans before the kth span, ; for Aqueduct bending moment at ; for Aqueduct bending moment at ; S43. Constructing a flexibility matrix from multiple flexibility coefficients , the expression is: ； S44. Calculation of unknown forces along the axis due to external water loads Displacement in direction : ； In the formula, Uniformly distributed water load q The bending moment of the aqueduct under the action of ; S45. Constructing the aqueduct displacement vector , the expression is: ； S46. According to the deformation coordination condition that the vertical displacement at each support point of the continuous beam model is 0, the basic equation of the force method is established. The matrix equation is as follows: ； S47. The flexibility matrix of step S44 is and the aqueduct displacement vector of step S45 Substituting into the matrix equation, the unknown force vector is solved , further calculate the support reaction of the aqueduct structure from the 1st to the N+1th support, and construct the N+1-order support reaction vector , the expression is ; Among them, the reaction force of the tth support is The expression is: 。 7. The design method of a steel truss beam-arch composite structure aqueduct according to claim 6, characterized in that: In step S5, according to the uniformly distributed water load in the aqueduct q and the support reaction vector Calculate the second function : 。 8. The design method of a steel truss beam-arch composite structure aqueduct according to claim 7, characterized in that: Step S6 includes the following specific steps: S61. Apply a vertical virtual unit force Z=1 to the mid-span of each span of the aqueduct in turn, and use the force method to calculate the aqueduct bending moment in each span under the action of the virtual unit force by referring to steps S4-S5; among which, the general expression of the bending moment of the kth span caused by the virtual unit force of the sth span is for: ； In the formula, is the cumulative length of all spans before the sth span, ; is the span of the sth span; S62. According to the actual working condition bending moment calculated and the bending moment of the aqueduct under the action of virtual unit force , the virtual work method is used to calculate the mid-span deflection of each span, and the deflection vector is obtained by combining ;in, is the mid-span deflection of the s-th span of the aqueduct, and the calculation formula is: ； S63. Select the maximum mid-span deflection in the deflection vector As the most unfavorable deflection of the aqueduct structure, the most unfavorable deflection is obtained. Height of lower girder at middle pier support H 1 The third function related to the expression is: .

9. The design method of a steel truss beam-arch composite structure aqueduct according to claim 8, characterized in that: Step S7 includes the following specific steps: S71. Calculate the hydraulic slope of the most unfavorable deflection span J , the calculation formula is: ； In the formula, is the span of the most unfavorable deflection; S72. Calculation of hydraulic radius in aqueduct R , the calculation formula is: ； In the formula, H w It is the net height of water level in the aqueduct. B It is the net width of the aqueduct. S73. Calculate the average design velocity of the most unfavorable deflection span using Xie Cai's formula V : ； In the formula, is the roughness coefficient of the aqueduct body; S74. Construct the average design flow rate of the most unfavorable deflection span V About the height of the lower girder at the middle pier support H The fourth function of 1 : 。 10. The design method of a steel truss beam-arch composite structure aqueduct according to claim 9, characterized in that: In step S8, the lower beam height at the middle pier support point is H The calculation formula for the value range of 1 is: ； In the formula, v -1 (·)for v The inverse function of (·), whose independent variable is the design flow rate [ V ].

Images