Digital pre-assembly fabrication control method for long-span arch bridge

Abstract

Provided is a digital pre-assembly fabrication control method for a large-span arch bridge including: planning and arranging scanning sites, arranging laser scanners and spherical targets, obtaining a

Description

TECHNICAL FIELD The present invention relates to the technical field of bridges, and in particular to a digital pre-assembly fabrication control method for a long-span arch bridge. BACKGROUND Due to access to free prestress under the action of its own weight, an arch bridge maintains a good compression state across a cross section thereof, and has advantages of reasonable stress distribution, high rigidity, strong seismic resistance, and good durability. Therefore, arch bridges are preferred in mountainous areas and applied in an increasingly wider range of fields. In order to ensure the precision of on-site installation of the arch rib and discover and solve potential problems that may arise during the on-site installation in advance, preassembly of arch rib components is required before large-scale installation of an arch bridge. Due to geometry errors in fabrication of arch rib segments, a height of a jig frame needs to be repeatedly adjusted during physical pre-assembly of arch rib segments to ensure a 3D posture of the arch rib segments with fabrication errors, which results in low construction efficiency, as well as a significant increase in costs including those of machinery, labor, site and time. Therefore, use of a digital pre-assembly fabrication control method is of great engineering application significance to improve the precision and efficiency of adjusting the 3D posture of the arch rib during the physical pre-assembly of a large-span arch bridge and avoid incurring high site and labor costs. In the prior art, during the physical pre-assembly of arch rib segments, an arch rib is usually marked correspondingly in advance to form an arch rib control point, a height thereof is adjusted step by step through a support, and the 3D posture of the arch rib is adjusted until meeting specification requirements of the physical pre-assembly. However, the following problems exist in the physical pre-assembly of large-span arch bridge segments: 1. a large-span arch bridge has a large span and numerous segments, thereby incurring huge costs of site, labor and machinery; and 2. for the arch rib control point, a theoretical feature position of the arch rib is usually marked correspondingly in advance. Centering errors of marking the arch rib control point in a fabrication stage, and significant discrepancies between a feature position of the arch rib (such as a flange position) and a theoretical position, easily result in substantial errors of control points relative to design references. When the 3D posture of the arch rib subjected to the physical pre-assembly is determined based on the control point, there exists a problem of insufficient precision in the 3D posture of the arch rib subjected to the physical pre-assembly; and when a method of the prior art is used, factors of adjustment precision and measurement means result in failure of precise posture adjustment with a flange, thereby causing lack of posture precision; and 3. since a final posture of the arch rib subjected to the physical pre-assembly cannot be determined before final adjustment, repeated adjustment through use of a support is required during the pre-assembly process, thereby resulting in an increase in the cost and construction period. Therefore, those skilled in the art are in an urgent need to provide a digital pre-assembly fabrication control method for a long-span arch bridge to solve the problems of the prior art including huge costs of site, labor and machinery, insufficient precision in the flange welding posture and the 3D posture of the arch rib subjected to the physical pre-assembly, repeated adjustment through use of a support required during the pre-assembly process, and a resultant longer construction period. SUMMARY In view of this, the present invention provides a digital pre-assembly fabrication control method for a large-span arch bridge, which can achieve an effect of precise evaluation. To achieve the above objective, the present invention adopts the following technical solution: a digital pre-assembly fabrication control method for a large-span arch bridge, including the following steps: SI, planning and arranging scanning sites, arranging laser scanners and spherical targets according to locations of the sites, using the laser scanner to scan an arch rib to obtain actual point cloud data, and constructing an actual point cloud model of arch rib segments according to the actual point cloud data; S2, obtaining a theoretical BIM model of the arch rib, and inputting the theoretical BIM model of the arch rib into professional point cloud processing software to obtain a point cloud model by sampling, so as to generate a theoretical point cloud model of the arch rib; S3, matching and aligning the theoretical point cloud model of the arch rib with the actual point cloud model of arch rib segments constructed in the SI, obtaining an optimal preassembly posture of the arch rib, and generating a point cloud model of the optimal pre assembly posture of the arch rib in an actual scanning stage; S4, based on the point cloud model of the optimal pre-assembly posture of the arch rib in an actual scanning stage, extracting axis features of a lower chord arch rib to obtain an arch rib centerline, and installing and adjusting an adjustment support according to the arch rib centerline; S5, calculating 3D coordinates of a jig frame through the adjustment support and the arch rib centerline, placing the arch rib according to the 3D coordinates of the jig frame, and roughly adjusting a posture of the arch rib subjected to physical pre-assembly, to obtain a posture of the arch rib placed; S6, using the laser scanner in the SI for second scanning on the posture of the arch rib placed to obtain an actual posture of the arch rib placed, using the theoretical point cloud model of the arch rib in the S2 to generate a 3D posture of a target arch rib, and fine-adjusting the posture of the arch rib subjected to physical pre-assembly according to the actual posture of the arch rib placed and the 3D target posture of the arch rib to obtain a finely adjusted posture of the arch rib; S7, installing a flange adjustment wall according to the precisely adjusted posture of the arch rib, and welding the flange, to achieve digital pre-assembly of the arch rib. In the above method, optionally, the planning and arranging scanning sites in the SI includes: S1.1, four basic sites are arranged for each arch rib segment; SI.2, each of the sites is located on a side away from a beam body, and a distance dz between the site and a corner point of the arch rib should meet the requirement of 1 Om <dz< 2Gm anc| SI.3, an angle a between a line connecting the site and the corner point of the arch rib and an axial direction of the arch rib should meet the requirement of 40° <a< 60° In the above method, optionally, the arranging laser scanners and spherical targets in the SI includes: erecting a 3D laser scanner at each of the sites, and arranging a spherical target between every two scanning sites. In the above method, optionally, the theoretical BIM model of the arch rib in the S2 is constructed when the flange is in a vertical posture. In the above method, optionally, the matching and aligning in the S3 are performed through a point cloud data matching algorithm. In the above method, optionally, the obtaining a centerline in the S4 includes: S4.1, linearly fitting a point cloud data set of the arch rib to obtain a point cloud of the arch rib parallel to an x-z plane; S4.2, fitting the point cloud of the arch rib using the n-order Fourier series to obtain a curved cylindrical point cloud with a step size d; S4.3, projecting the curved cylindrical point cloud onto the x-y plane to obtain a microsegment control point; and S4.4, generating a centerline of a circular tube through interpolation of the micro-segment control point. In the above method, optionally, the adjustment support in the S4 includes a support frame, a 3D adjustment jack and a crescent plate, where the 3D adjustment jack can be adjusted in three directions. In the above method, optionally, the fine-adjusting the posture of the arch rib subjected to physical pre-assembly in the S6 includes obtaining an elongation in each direction of the 3D adjustment jack according to parameters R' and T' of transformation between the posture of the arch rib placed and the 3D target posture of the arch rib, including: a matrix of influence of change of an elongation of the 3D jack in a unit X direction on the transformation 7 parameter J?' of the arch rib is 1A7?, a matrix of influence of change of an elongation thereof 7 in a unit Y direction on the transformation parameter R' of the arch rib is 1IK , and a matrix of influence of change of an elongation thereof in a unit Z direction on the transformation parameter R' of the arch rib is ^1ZR ; a matrix of influence of change of an elongation of a 3D jack I in the unit X direction on the Z transformation parameter T' of the arch rib is 1XT , a matrix of influence of change of an elongation thereof in the unit Y direction on the transformation parameter T' of the arch rib is Z , and a matrix of influence of change of an elongation thereof in the unit Z direction on the transformation parameter T' of the arch rib is ; d. d. d d where xns is an elongation of an nth 3D jack in a longitudinal bridge direction; yns is an d elongation of the n111 3D jack in a transverse bridge direction; is an elongation of the nfc 3D jack in a vertical direction; similarly, for a 3D jack 2, an influence matrix is as follows: similarly, for a 3D jack 3, an influence matrix is as follows: similarly, for a 3D jack 4, an influence matrix is as follows: Z4IR 7 ^4YT 7 ^4ZR 7 ^4ZT based on the above, the following influence matrix is obtained: and an elongation of each unit of the jack is obtained. In the above method, optionally, the welding the flange in the S7 includes: installing the flange adjustment wall in a vertical state, and fixing the flange on the adjustment wall, such that the flange automatically keeps vertical. It can be seen from the above technical solution that, compared with the prior art, the digital pre-assembly fabrication control method for a large-span arch bridge provided by the present invention, has the following beneficial effects: 1) the present invention uses 3D laser scanning technology to acquire 3D precise fabrication posture data of arch rib segments in a pre-assembly site, and achieves an optimal alignment with the theoretical point cloud model by using the acquired 3D precise fabrication posture data so as to obtain a target adjustment posture of arch rib segments subjected to physical pre-assembly, which improves the precision of the physical pre-assembly and solves the problem of insufficient precision in the 3D posture of the arch rib subjected to the physical pre-assembly; 2) after obtaining the target adjustment posture of arch rib segments, the present invention provides a method for quickly calculating a 3D support height of an arch rib, which achieves formation of a rough 3D target posture of the arch rib, and accelerates adjustment of the 3D target posture of the arch rib; adjustment devices such as the flange adjustment wall and the arch rib adjustment support are invented to achieve the pre-assembly adjustment of a large-span arch bridge and the high-precision adjustment of a flange welding posture, which solves the problem of insufficient precision in the flange welding posture; and 3) the arch rib adjustment support of the present invention, on the basis of fast formation of a rough 3D target posture of the arch rib, achieves fast precise adjustment, which solves the problem of a longer construction period caused by repeated adjustment through use of a support required during the preassembly process, achieves low-cost, fast and high-precision pre-assembly of the large-span arch bridge, greatly reduces construction costs, and improves construction efficiency. BRIEF DESCRIPTION OF THE DRAWINGS In order to more clearly describe the technical solutions in the examples of the present invention or in the prior art, a brief introduction to the accompanying drawings required for the description of the examples or the prior art will be made below. Apparently, the accompanying drawings in the following description are merely some examples of the present invention, and those of ordinary skill in the art would also be able to derive other drawings from these drawings without making creative efforts. FIG. 1 is a flowchart of a digital pre-assembly fabrication control method for a large-span arch bridge disclosed in the present invention. FIG. 2 is a schematic diagram of site planning disclosed in the present invention. FIG. 3 is a schematic diagram of arrangement of scanning spheres disclosed in the present invention. FIG. 4 is a schematic diagram of a full-site assembly model disclosed in the present invention. FIG. 5 (a) is a schematic diagram of an actual arch rib segment scanning model disclosed in the present invention. FIG. 5 (b) is a schematic diagram of a theoretical point cloud model of the arch rib obtained after matching disclosed in the present invention. FIG. 6 is a schematic diagram of an actual point cloud model of the arch rib matching a theoretical point cloud model of the arch rib disclosed in the present invention. FIG. 7 (a) is a front view of an adjustment support disclosed in the present invention. FIG. 7 (b) is a side view of an adjustment support disclosed in the present invention. FIG. 7 (c) is a top view of an adjustment support disclosed in the present invention. FIG. 8 is a schematic diagram of a flange adjustment wall disclosed in the present invention. DETAILED DESCRIPTION OF EMBODIMENTS The technical solutions in the examples of the present invention will be clearly and completely described below in combination with the accompanying drawings in the examples of the present invention. Apparently, the examples described are merely some rather than all of the examples of the present invention. Based on the examples of the present invention, all other examples obtained by those of ordinary skill in the art without making creative efforts shall fall within the protection scope of the present invention. In the present invention, relational terms herein such as first and second are only used to distinguish one entity or operation from another entity or operation without necessarily requiring or implying any such actual relation or order between such entities or operations. The terms “include”, “including” or any other variants thereof are intended to cover nonexclusive inclusion, such that a process, method, article or device including a series of elements not only includes those elements, but also includes those elements that are not explicitly listed, or also includes elements inherent to the process, method, article or device. Without more restrictions, the elements defined by the sentence “including a ...” do not exclude the existence of other identical elements in the process, method, article, or device including the elements. With reference to FIG. 1, a digital pre-assembly fabrication control method for a large-span arch bridge, including the following steps: SI, plan and arrange scanning sites, arrange laser scanners and spherical targets according to locations of the sites, use the laser scanner to scan an arch rib to obtain actual point cloud data, and construct an actual point cloud model of arch rib segments according to the actual point cloud data; S2, obtain a theoretical BIM model of the arch rib, and input the theoretical BIM model of the arch rib into professional point cloud processing software to obtain a point cloud model by sampling, so as to generate a theoretical point cloud model of the arch rib; S3, match and align the theoretical point cloud model of the arch rib with the actual point cloud model of arch rib segments constructed in the SI, obtain an optimal pre-assembly posture of the arch rib, and generate a point cloud model of the optimal pre-assembly posture of the arch rib in an actual scanning stage; S4, based on the point cloud model of the optimal pre-assembly posture of the arch rib in an actual scanning stage, extract axis features of a lower chord arch rib to obtain an arch rib centerline, and install and adjust an adjustment support according to the arch rib centerline; S5, calculate 3D coordinates of a jig frame through the adjustment support and the arch rib centerline, place the arch rib according to the 3D coordinates of the jig frame, and roughly adjust a posture of the arch rib subjected to physical pre-assembly, to obtain a posture of the arch rib placed; S6, use the laser scanner in the SI for second scanning on the posture of the arch rib placed to obtain an actual posture of the arch rib placed, use the theoretical point cloud model of the arch rib in the S2 to generate a 3D posture of a target arch rib, and fine-adjust the posture of the arch rib subjected to physical pre-assembly according to the actual posture of the arch rib placed and the 3D target posture of the arch rib to obtain a finely adjusted posture of the arch rib; S7, install a flange adjustment wall according to the precisely adjusted posture of the arch rib, and weld the flange, to achieve digital pre-assembly of the arch rib. Further, as shown in FIG. 2, the planning and arranging scanning sites in the SI includes: S1.1, four basic sites are arranged for each arch rib segment; SI.2, each of the sites is located on a side away from a beam body, and a distance dz between the site and a corner point of the arch rib should meet the requirement of 10m <dz <20m ■ an(j SI.3, an angle a between a line connecting the site and the corner point of the arch rib and an axial direction of the arch rib should meet the requirement of 40° <a< 60° Specifically, by locations, four sites for each arch rib segment are divided into a shortmileage left site, a short-mileage right site, a long-mileage left site, and a long-mileage right site. Further, a 3D laser scanner is erected at the corresponding planned site, and before data collection for a beam on site, relevant parameters need to be set for scanning of each site. According to this method, a "distance" of the scanner is 120 m, with a target resolution of 3.1 mm / 10 m. Further, the arranging laser scanners and spherical targets in the SI includes: erect a 3D laser scanner at each of the sites, and arrange a spherical target between every two scanning sites. Specifically, as shown in FIG. 3, scanning of spherical targets needs to meet the following requirements: 1. at least two spherical targets for scanning are arranged between sites; 2. a projection distance dyl of an inter-site 1# scanning sphere in an axial direction of an arch rib center should meet the requirement of a projecpOn distance dyl of an inter-site 2# scanning sphere in the axial direction of the arch rib center should meet the requirement of ^0m <dyl <25m. a projection distance dx\ of the inter-site 1# scanning sphere in the mileage direction of the arch rib center should meet the requirement of 5m <dxl <10m ■ and 3 a projection distance dx2 of the inter-site 2# scanning sphere in the mileage direction of the arch rib center should meet the requirements of 5m dx2 <10m and dx] + dx2 >8m Further, as shown in FIG. 4, the constructing an actual point cloud model of arch rib segments in the SI includes matching and aligning point cloud data obtained by scanning the spherical targets. Specifically, 1) assuming coordinates of a circle center for fitting are a spherical equation is expressed as: (x-x0)2 +(y^o)2 +(^zo)2 =-^ Results of expansion and transposition are as follows: x2 +j2 +z2 = 2xa + 2yb + 2zc+ r2 -xf -yf -z^ 2 2 2 2 x v z r — x — y — z assuming there are / / spherical points to be estimated, and °, °, 0 and 0 0 0 are parameter x,+y,+^ ^22 ^+yl+zl x2 + y2 +z2 _ n s n n _ 5 tc be estimated, then: 2x, 2^ 2zx f 2x2 2y 2z 1 2x„ 2yn 2zn 1 m A n n X x0 y0 zo 2222 [r -x0-y0-z0\ r a x 7 where K is an observation vector, A is a coefficient matrix, and X is a parameter vector to be estimated, a loss function is constructed as follows: loss = fc-Axffy-Ax) an expansion formula is as follows: loss = YYt - YTAX - XT A1'Y + X1'Ar AX 7 a result of derivation is as follows: = -AtY -yfr + 2At AX dX X is solved when zero is set: ^atax = aty x = (a! a)' a'y coordinates of a center of the sphere are calculated to be 2) solving a transformation matrix p In the following formula, a second-phase corresponding circle center set ’ is denoted as p c, a rotation matrix is denoted as R, and a translation matrix is denoted as T: PA = R* PB +T 1 average coordinates of and are calculated as follows: centroidA = (XA,YA,ZA) centroid, = (xb,YB,ZB) 5 2 unitary matrix calculation: H = ^(pa- centroidA pB - centroidB J i=i singular value decomposition: It r q tt I_ (ATXTu T Ft 1(7 , aJ , r I — Ak jL4 / 1 J the following is obtained according to a method of least squares: R = VUt the following is further obtained: T = R * centroidA + centroid,, 5 through the above steps, a coordinate system transformation matrix between point clouds is obtained to complete matching between point clouds of each site. The theoretical BIM model of the arch rib in the S2 is constructed when the flange is in a vertical posture. The matching and aligning in the S3 are performed through a point cloud data matching algorithm. Specifically, as shown in FIGs. 5(a), 5(b) and 6, a point cloud N is transformed into a coordinate system of a point cloud M through rigid transformation, and M and N are accurately matched in shape, where the point cloud M is a theoretical arch rib point cloud, and the point cloud N is an actual arch rib segment point cloud. A point in M is denoted as mi, M={mi mi ER3, i=l:n}, and a point in Nis denoted as n,, N={nj ni ER3t i=l:m}. Point cloud matching is a completely rigid transformation, that is, the point cloud N can be aligned with M in a 3D space only by rotating a, / 3 and y and translating tx, ty and tz around axes of X, Y and Z. a, ft and y constitute a rotation matrix R, and tx, ty and 1- constitute a translation vector T. f(R, 7) is a function of error between the transformed point cloud A and the point cloud M, and a transformation parameter that minimizes / is an optimal solution for matching. Mathematical modeling and construction off directly determine the precision of the actual point cloud matching under optimal numerical conditions. An error function is a sum of squares of a distance between a point nt in N and a nearest point m, in M, which is expressed as follows: f = Tf -mt •> In order to minimize the error function, an iterative algorithm needs to be executed, with specific steps as follows: 1. sample from the point cloud N to obtain a point set , and search a nearest point set W inM; 2 calculate an average distance between *H'*and through the following formula: 3. when d is less than a given convergence threshold, calculate a convergence; when d is not less than the given convergence threshold, calculate f (R, T ), determine whether f (R, T ) reaches a minimum; when both the above convergence determination conditions are met, estimate new transformation parameters R and / based on and . 4. obtain a new point set n\ = { / / ’, \n\ ~NR} + 7’}, and repeat the step (1). Further, the obtaining an arch rib centerline in the S4 includes: S4.1, linearly fit a point cloud data set of the arch rib to obtain a point cloud of the arch rib parallel to an x-z plane; S4.2, fit the point cloud of the arch rib using the / / -order Fourier series to obtain a curved cylindrical point cloud with a step size d, S4.3, project the curved cylindrical point cloud onto the x-y plane to obtain a micro-segment control point; and S4.4, generate a centerline of a circular tube through interpolation of the micro-segment control point. Specifically, the S4.1 includes: an initial posture of the arch rib is generally a spatial posture, and to reduce complexity of spatial data processing, a spatial point cloud is firstly flattened. Assuming that a point cloud data set of the arch rib is |(x°, jf,zz°),z =1,2,...,m}. perform linear fitting on |(x°,y°, ), z = 1,2,..., , that is, y = kx + b , which conforms to a principle of least squares: m IK+*’- / ] 1=0 According to the above formula, obtain a projection slope of the arch rib on an x-y plane, and calculate an angle 0 = arctan (C) against an x-axis. Rotate 6clockwise around a z-axis to obtain a point cloud of the arch rib parallel to the x-z plane. X ^cos(^) -sin(-0) 0 0 x° y sin(—6*) cos(-0) 0 0 y° z 0 0 10 z° 1 0 0 0 1 1 the S4.2 includes: / = +... +anx\ fit the point cloud of the arch rib using n-order Fourier series, where n is determined based on experimental data. Divide a point cloud [(xk,zk),i = 1,2,...,into m segments with a step size d = (xzmax m, where dis usually (5cm<d<10cm) . The S4.3 includes: based on the idea of "replacing curves with straight lines", a curved cylindrical point cloud with the step size d can be treated as a cylinder. A conventional cylinder fitting algorithm is easily affected by outliers, and the outliers need to be removed in order to avoid the influence thereof and further improve fitting precision. Centering is performed through an iterative least squares method, and each segment of curved circular tube is projected onto the x-y plane to obtain a circular shape, where a control point is ■ A geometric relationship indicates that since a center of a projected circle is o= x,y , the center of the circle can be solved with reference to details of circle fitting, where a micro-segment control point is o = x,y,z , z = ---—— - x tan(a) y J I cos(a) x’ 1 0 0 0 X cos(a) 0 sin(a) 0 y’ 0 i 0 0 y 0 1 0 0 z’ 0 0 1 0 z -sin(cr) 0 cos(a) 0 1 rxk 0 -¾ 1 1 0 0 0 1 The S4.4 includes: after obtaining a control point of each micro-segment cylinder, restore same to original spatial positions corresponding to respective rotation and translation parameters, with no details described herein; and finally interpolate all the control points through a cubic spline curve to obtain the centerline of the circular tube. Further, with reference to FIGs. 7(a), 7(b) and 7(c), the adjustment support in the S4 includes a support frame, a 3D adjustment jack and a crescent plate, where the 3D adjustment jack can be adjusted in three directions. The crescent plate has a width of 600 mm, a thickness of 20 mm, and a height of 250 mm, and a radius R is a radius of a steel pipe to be assembled. The support frame includes a base, stiffening ribs of a column base structure, steel pipes of the support frame, transverse struts of the support frame, and steel plates at a top of the support frame. A height Hl of the support frame is derived from a specific arch rib height and remains constant, and a spacing between two support frame bases is D, which is the same as a distance from the arch rib center. The circular tube and the base of the support frame need to be fixed through the column base structure with stiffening ribs. Herein Z>=2500 mm, and / 71=700 mm. The 3D adjustment jack can be adjusted in three directions, and a lateral spacing between two 3D jacks is QY, which is the same as a lateral distance of the centerline of the circular tube. A distance QX of the 3D jack in a mileage direction is a fixed value, which is determined based on a length of the arch rib. A height QZ of the 3D jack is calculated as follows. Other dimensions of the support provided in the present invention, such as a thickness of the steel plate and a general size of a material, need to be verified according to the importance of a specific arch rib before use, and can be appropriately adjusted based on verification results. Further, the roughly adjusting a posture of the arch rib subjected to physical pre-assembly includes setting a lateral distance and a vertical height of the jack. Specifically, after the centerline of the circular tube is generated, an axial distance of the center of the lower-chord arch rib against a pre-assembly coordinate system is denoted as Actual. Therefore, the lateral spacing QY of the 3D jack is expressed as: I - / -'actual Taking an adjustment support frame 1 as an example, after a mileage of the adjustment support frame 1 is determined, obtain 3D coordinates ( v \ of a center of the steel pipe corresponding to the mileage of the adjustment support frame 1 according to an existing axis, and then calculate 3D coordinates z -D / 2) a contact P°int between the adjustment support frame 1 and the steel pipe according to a diameter I) of the circular tube corresponding to the mileage of the adjustment support frame 1, therefore, the vertical height QZ of the 3D jack is expressed as: QZ = z,-DJ2-25$-H\ Coordinates of remaining jig frames are calculated according to the above ideas. Then the arch rib is placed to complete rough posture adjusting of the arch rib subjected to physical pre-assembly. In this case, QY = Actual = 2501mm; and QZ = z} -1400 / 2 - 250 - 700 = 1500 - 700 - 250 - 700 = 250mm . Further, the fine-adjusting the posture of the arch rib subjected to physical pre-assembly in the S6 includes obtaining an elongation in each direction of the 3D adjustment jack according to parameters of transformation between the posture of the arch rib placed and the 3D target posture of the arch rib. Further, the fine-adjusting the posture of the arch rib subjected to physical pre-assembly in the S6 includes obtaining an elongation in each direction of the 3D adjustment jack according to parameters R' and T' of transformation between the posture of the arch rib placed and the 3D target posture of the arch rib, including: a matrix of influence of change of an elongation of the 3D jack in a unit X direction on the transformation parameter R' of the arch rib is Z1XR, a matrix of influence of change of an elongation thereof in a unit Y direction on the transformation parameter R' of the arch rib is ZXYR ; and a matrix of influence of change of an elongation thereof in a unit Z direction on the transformation parameter R' of the arch rib is Z12R ; a matrix of influence of change of an elongation of a 3D jack 1 in the unit X direction on the transformation parameter T' of the arch rib is Z1JT , a matrix of influence of change of an elongation thereof in the unit Y direction on the transformation parameter T' of the arch rib is Zm, and a matrix of influence of change of an elongation thereof in the unit Z direction on 5 the transformation parameter T' of the arch rib is ZVZT; Z, "IYT z1ZR 7 ^IZT J where xns is an elongation of an «th 3D jack in a longitudinal bridge direction; dyns is an elongation of the / / "' 3D jack in a transverse bridge direction; d,ns is an elongation of the 10 3D jack in a vertical direction; similarly, for a 3D jack 2, an influence matrix is as follows: 2YR v2s '2YT ^2,S’ similarly, for a 3D jack 3, an influence matrix is as follows: ,. -, d , X3S ^3XR ^3YR ^3ZR i i H d , 777 y^s ^3A7 ^3 }7 ^3ZT J RA T / 3 J similarly, for a 3D jack 4, an influence matrix is as follows: 15 ■^420? Z£ z z . 7 4 dxAs 417? ^4ZR j 7 '4YT J y4s d:4s RA based on the above, the following influence matrix is obtained: 13 J , and an elongation of each unit of the jack is obtained. Specifically, 7^+7^+7^ + 7’ rj~! rri 1 । 2 । $ I 1 can be expressed as: R' •< r k J where 7 ^\XR 7 ^\XT 7 ^IXR < 7 1^137 7 ^117? 7 'XZR 7 ^2XR 7 ^2YR 7 ^2ZR 7 ^3XR 7 ^\YT 7 ZT 7 ^2 XT 7 ^2YT 7 ^2ZT 7 ^3 XT 7 ^YlR 7 ^\ZR 7 ^2XR 7 ^2YR 7 ^2ZR 7 ^3XR 7 ^\YT 7 ^\ZT 7 ^2 XT 7 ^2YT 7 lZT 7 ^3XT dx\s 77 77 77 77 77 1 ^317? XzR XxR XyR XzR H 7 7 7 7 7 ^3YT ^3ZT ^4XT 4YT 4ZT J 7 7 7 7 7^ ^3YR ^3ZR ^4XR ^4YR ^4ZR ^yls d , z]s dX3S dy2s dZ2S dX3S d , y3s d:3s d . Uy4s 7 7 7 7 7 ^3YT ^3ZT ^4XT ^4YT ^4ZT J is known, an elongation of each 3D jack in each direction can be solved according to the above formula, and after adjustment, the fine-adjusting of the posture of the arch rib subjected to physical pre-assembly can be completed. Results of solving according to the above formula are as follows: d , xk 1 ' dyXS 2 dzxs -1 dX2s 1 dy2s 2 dZ2S > — < -1 > d x3s 1 d , y3s 2 d , z3s 3 dX4s 1 dy4s 2 dZ4Sj 3 L J Further, as shown in FIG. 8, the welding the flange in the S7 includes: install the flange adjustment wall in a vertical state, and fix the flange on the adjustment wall, such that the 10 flange automatically keeps vertical. Specifically, the actual posture of the arch rib has met the posture accuracy requirements for welding connection with the vertical flange, without need of additional adjustment. The flange adjustment wall is a steel plate with a length of 10 m, a height of 15 m and a thickness of 20 cm. A verticality precision of the flange adjustment wall is less than 1 mm. The steel plate needs to be fixed to the ground, and a thickness of the flange should be verified before use such that a deformation size of the flange during flange welding does not exceed 1 mm. A middle of the flange adjustment wall is hollowed out to form a vertical adjustment track for the flange. The number of the tracks is related to arrangement of bolts for the flange, and a width of the track is the same as a hole diameter of the bolts for the flange. After the height of the flange is adjusted, the flange is fixed to the adjustment wall in a way of tightening the bolts. In this case, the width of the track is A = 30mm . After the flange adjustment wall is installed, the flange and the arch rib segment adjusted in posture are welded, to complete the pre-assembly of one arch rib segment. The rest of the arch rib are assembled sequentially in the same way. Each example in the description is described in a progressive manner, the same and similar parts between the examples may refer to each other, and each example focuses on the differences from other examples. In particular, a system or system example is basically similar to the method example, such that the description thereof is relatively simple, and a partial description of the method example can be referred to for details. The system and system examples described above are merely schematic, the unit described as a separable component may be physically separated or not, and a component shown as a unit may be a physical unit or not, that is, may be located at one place or may also be distributed on a plurality of network units. Part or all of the modules may be selected according to actual needs to achieve the objective of the solution of the examples of the present invention. Those skilled in the art can understand and implement the present disclosure without making creative efforts. The above description of the disclosed examples enables any person skilled in the art to implement or use the present invention. Various modifications to these examples will be readily apparent to those skilled in the art, and the generic principles defined herein may be implemented in other examples without departing from the spirit or scope of the present invention. Thus, the present invention is not intended to be limited to the examples shown herein, but is to be accorded with the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A digital pre-assembly fabrication control method for a large-span arch bridge, characterized by comprising the following steps:SI, planning and arranging scanning sites, arranging laser scanners and spherical targets according to locations of the sites, using the laser scanner to scan an arch rib to obtain actual point cloud data, and constructing an actual point cloud model of arch rib segments according to the actual point cloud data;S2, obtaining a theoretical BIM model of the arch rib, and inputting the theoretical BIM model of the arch rib into professional point cloud processing software to obtain a point cloud model by sampling, so as to generate a theoretical point cloud model of the arch rib;S3, matching and aligning the theoretical point cloud model of the arch rib with the actual point cloud model of arch rib segments constructed in the SI, obtaining an optimal pre-assembly posture of the arch rib, and generating a point cloud model of the optimal pre-assembly posture of the arch rib in an actual scanning stage;S4, based on the point cloud model of the optimal pre-assembly posture of the arch rib in an actual scanning stage, extracting axis features of a lower chord arch rib to obtain an arch rib centerline, and installing and adjusting an adjustment support according to the arch rib centerline;S5, calculating 3D coordinates of a jig frame through the adjustment support and the arch rib centerline, placing the arch rib according to the 3D coordinates of the jig frame, and roughly adjusting a posture of the arch rib subjected to physical preassembly, to obtain a posture of the arch rib placed;S6, using the laser scanner in the SI for second scanning on the posture of the arch rib placed to obtain an actual posture of the arch rib placed, using the theoretical point cloud model of the arch rib in the S2 to generate a 3D posture of a target arch rib, and fine-adjusting the posture of the arch rib subjected to physical pre-assembly according to the actual posture of the arch rib placed and the 3D target posture of the arch rib to obtain a finely adjusted posture of the arch rib;S7, installing a flange adjustment wall according to the precisely adjusted posture of the arch rib, and welding the flange, to achieve digital pre-assembly of the arch rib; the planning and arranging scanning sites in the SI comprises:S1.1, four basic sites are arranged for each arch rib segment;SI.2, each of the sites is located on a side away from a beam body, and a distance dz between the site and a corner point of the arch rib should meet the requirement of 10m <dz <20m an(jSI.3, an angle a between a line connecting the site and the corner point of the arch rib and an axial direction of the arch rib should meet the requirement of 40° <«<60°.the obtaining a centerline in the S4 comprises:S4.1, linearly fitting a point cloud data set of the arch rib to obtain a point cloud of the arch rib parallel to an x-z plane;S4.2, fitting the point cloud of the arch rib using the / / -order Fourier series to obtain a curved cylindrical point cloud with a step size d;S4.3, projecting the curved cylindrical point cloud onto the x-y plane to obtain a micro-segment control point; andS4.4, generating a centerline of a circular tube through interpolation of the microsegment control point;the adjustment support in the S4 comprises a support frame, a 3D adjustment jack and a crescent plate, wherein the 3D adjustment jack can be adjusted in three directions;the fine-adjusting the posture of the arch rib subjected to physical pre-assembly in the S6 comprises obtaining an elongation in each direction of the 3D adjustment jack according to parameters R' and T' of transformation between the posture of the arch rib placed and the 3D target posture of the arch rib, comprising: a matrix of influence of change of an elongation of a 3D jack in a unit X direction on a transformation2parameter R' of the arch rib is ™, a matrix of influence of change of an elongationZthereof in a unit Y direction on the transformation parameter R' of the arch rib is, and a matrix of influence of change of an elongation thereof in a unit Z direction onZthe transformation parameter R' of the arch rib is 1ZR ;a matrix of influence of change of an elongation of a 3D jack 1 in the unit X directionZon the transformation parameter T' of the arch rib is XXI, a matrix of influence of change of an elongation thereof in the unit Y direction on the transformation parameter T of the arch rib is ^UT , and a matrix of influence of change of an elongation thereof in the unit Z direction on the transformation parameter T' of the7arch rib is lzr;7^\XR <7^\YR7^\YTd dwherein xm is an elongation of an nth 3D jack in a longitudinal bridge direction; >BSis an elongation of the nth 3D jack in a transverse bridge direction; dzns is an elongation of the nth 3D jack in a vertical direction;similarly, for a 3D jack 2, an influence matrix is as follows:similarly, for a 3D jack 3, an influence matrix is as follows:similarly, for a 3D jack 4, an influence matrix is as follows:7 7^4XR ^4YR7 7^4XT ^4YT7^4ZTdx4s '\dy^r4based on the above, the following influence matrix is obtained:\Ry+R2+R3+R^< rj 1y; + 72, and an elongation of eachunit of the jack is obtained.

2. The digital pre-assembly fabrication control method for a large-span arch bridge according to claim 1, characterized in thatthe arranging laser scanners and spherical targets in the SI comprises: erecting a 3D laser scanner at each of the sites, and arranging a spherical target between every two scanning sites.

3. The digital pre-assembly fabrication control method for a large-span arch bridge according to claim 1, characterized in thatthe theoretical BIM model of the arch rib in the S2 is constructed when the flange isin a vertical posture.

4. The digital pre-assembly fabrication control method for a large-span arch bridge according to claim 1, characterized in thatthe matching and aligning in the S3 are performed through a point cloud data 5 matching algorithm.

5. The digital pre-assembly fabrication control method for a large-span arch bridge according to claim 3, characterized in thatthe welding the flange in the S7 comprises: installing the flange adjustment wall in a vertical state, and fixing the flange on the adjustment wall, such that the flange 10 automatically keeps vertical.

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