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【Abstract】In recent years, flat steel box girders with superior wind resistance have been widely used as stiffening girders for long-span cable support structures (suspension bridges and cable-stayed bridges). From a manufacturing perspective, the steel box girder is an all-welded plate system structure. That is, the steel box girder is divided into several types of plate unit components with vertical and horizontal stiffeners, which are prefabricated in the factory, and then assembled and welded into the box girder in sections, and then hoisted and welded section by section on site to form a whole. Based on this manufacturing and erection feature, the geometric accuracy of steel box girders is extremely high. The geometric accuracy mainly depends on the control of welding shrinkage deformation. Taking the Second Nanjing Yangtze River Bridge as an example, one standard beam section is 15m long. The total length of the weld is more than 5,000 meters, and there are more than 40 types of welding joints. Various welding methods such as CO2 gas shielded welding, submerged arc automatic welding, and manual arc welding are used. The control of welding deformation is a very complex issue. This article briefly introduces the test results of welding deformation under various conditions, as well as the systematic control method of welding deformation in steel box girder assembly welding. 【Key words】Steel box girder welding residual deformation and welding transverse shrinkage 1. The mechanism and influencing factors of welding residual deformation 1. The welding of welding residual deformation steel usually uses the fusion welding method, which is to locally heat the joints to melt the welded material and the added welding material into liquid metal to form a molten pool, which is then cooled and solidified into solid metal, so that the originally separated steel materials are connected into a whole. Due to welding heating, the base metal outside the fusion line expands, and then cools, and the molten pool metal and the base metal near the fusion line shrink. Due to the thermal changes of heating and cooling, the thermal changes occur rapidly in a local area, and the expansion and contraction deformations are restrained to produce plastic deformation. In this way, the plastic deformation remains after welding is completed and cooled to normal temperature. Table 1 shows the basic forms of welding residual deformation. In actual structures, welding residual deformation presents a complex state composed of these basic forms. 2． Factors affecting welding deformation The main factors affecting welding deformation are as follows: (l)Welding method: The welding connections of steel bridges usually use welding methods such as manual arc welding, CO2 gas shielded welding, and submerged arc automatic welding (including welding process parameters selected for different welding joint forms). Because the heat input of these welding methods is different, the amount of welding residual deformation caused is also different. (2) Connector form: Steel bridge joints usually include butt joints, T-joints, cross-type joints, corner joints, lap joints and assembled plate joints. Generally, fillet welds of butt welds are used, including plate thickness, weld size, groove form and root gap, penetration or non-penetration, etc. That is, various factors that constitute the cross-sectional area of the weld and affect heat dissipation (cooling rate). (3) Welding conditions: Factors such as preheating and tempering, as well as ambient temperature, influence the temperature gradient during cooling of steel. (4) Welding sequence and constraints: For a three-dimensional structure, the parts welded first will have different degrees of restraint on the parts welded later, and their welding deformations will also be different. To prevent distortion, a symmetrical welding sequence should be used. 2. Structural characteristics of the Second Nanjing Bridge 1. Structural characteristics of steel box girder The steel box girder of the Second Nanjing Bridge has a total length of 1238m and a main span of 628m. It is currently the third largest long-span steel box girder cable-stayed bridge in the world and the first in China. The steel box girder of the whole bridge is divided into 93 sections. The standard girder section is 15m long, 38.2m wide and 3.5m high. Figure 1 is a cross-sectional view of the steel box girder. A total of 55 plate unit components with vertical and horizontal stiffeners were prefabricated in the factory, and then assembled and welded into steel box girder segments at an assembly site near the bridge. They were then shipped to the bridge and hoisted in place, and welded together to form the entire bridge. 2. Control method for geometric accuracy of steel box girder. From the manufacturing and installation sequence described in the previous section, the control points and control measures for the geometric dimensions of steel box girder are listed in Table 2. 3. Welding shrinkage measurement test From the structural characteristics of the steel box girder and its geometric size control items, it can be known that except for the width and U-shaped rib matching of adjacent beam segments that are closely related to the welding lateral shrinkage, other items can achieve accuracy requirements through post-welding treatment measures. Therefore, accurately grasping the welding lateral shrinkage of beam segment plates is the key to controlling the geometric dimensional accuracy of steel box girder segment manufacturing. Therefore, this article only conducts detailed measurements of the welding lateral shrinkage under various conditions. 1. Plate welding process parameters (Table 3) 2. Measurement test introduction In order to reduce the welding workload on the tire frame assembly, 2.4m wide plate unit components are first spliced into 4.8m wide blocks on the platform on the tire frame side, referred to as panels, which reduces the splicing workload on the tire frame by about half. Due to the different restraint conditions under the tire and on the tire, the welding shrinkage was measured separately according to different plate thicknesses. In addition, the root partition is about 33m long. Considering the transportation conditions, it is divided into three pieces and is welded vertically on the tire frame. Because the lower end has been welded to the bottom plate and the inclined bottom plate, it is in a strongly constrained state, and the upper end is in a free state, and its lateral shrinkage deformation was also measured. The measurement gauge length is 300mm. In order to reduce the influence of temperature difference, the measurement time is set during a time when the temperature is relatively constant. 3． Measurement structure and analysis For the same plate thickness, the same welding process, and the same constraint conditions, the lateral shrinkage values are grouped according to the gap at the root of the weld. The data distribution histogram of each group is shown in Figure 2. In the figure, G is the root gap of the weld, and Δ is the lateral shrinkage of the welding. Figure 3 shows the relationship between the average transverse shrinkage caused by the longitudinal butt welding in Figure 2 and the root gap. The welding method and its process parameters are the same. The transverse shrinkage caused by the longitudinal butt weld can be summarized as a function of the weld cross-sectional area, plate thickness and groove root gap, expressed in the form of formula (1): t——Plate thickness, unit mm ； G--Weld root gap, unit mm ； a, b--experience coefficient, which changes with the change of welding conditions. Regress each group of values in Figure 3 according to formula (1), and the regression parameters can be obtained as shown in Table 5. The oblique lines in each diagram in Figure 3 are straight lines calculated according to formula (l). It can be seen from the correlation coefficient that the average lateral shrinkage of welding under given welding conditions can be more accurately predicted by using the coefficients a and b values obtained from regression (Table 5) and formula (1). Through the mean and its standard deviation, the range of lateral shrinkage can be predicted. From Figure 3 and analysis, we can draw the following conclusions:: (1) The welding process is the same, the plate thickness is the same, and the constraints are the same. The lateral shrinkage increases with the increase in the gap at the groove root, showing a linear relationship. ； (2) Comparing Figure 3 (a), (b) and (c), it can be seen that the welding process is the same, the restraint conditions are the same, the gap at the groove root is the same, and the lateral shrinkage increases with the increase of the plate thickness. ； (3) Constraint conditions have a significant impact on the lateral shrinkage. In Figure 3(a) and (d), because the plate thickness is the same and the constraints are the same, the lateral shrinkage is very close. In Figure 3 (b) and (e), although the plate thickness is the same, the constraint conditions are different, so the measured value during the total assembly time is smaller than the measurement value during the assembly time. It shows that the restraint of the diaphragm on the top plate during assembly is stronger than the restraint of the flexible horse board during assembly. Figure 4 shows the arrangement of measuring points on the diaphragm and the changes in the average lateral shrinkage of welding with the position of the measuring points. After the lower end of the diaphragm and the bottom plate are welded, the adjacent diaphragm units are butt welded, and the welding sequence is from bottom to top. Figure 4 shows that the closer to the strongly constrained end of the bottom plate, the smaller the shrinkage, and the closer to the upper free end, the greater the shrinkage. 4. Compensation for welding transverse shrinkage deformation According to the test results and analysis, certain measures were taken to compensate for the welding transverse shrinkage during the manufacturing process of South Second Bridge: ①The cutting width of the top plate, bottom plate, inclined bottom plate and other units is 3mm wider than the design size, that is, 1.5mm on each side of the longitudinal baseline. The length of the single unit of the transverse partition is increased by 2.0mm. ； ②Considering the discrete nature of welding shrinkage deformation and the accumulation of shrinkage deformation errors caused by multiple welds when the top plate and bottom plate are assembled, a single plate unit is left at the edge of the panel and bottom plate to match the cutting width (see Figure 1). 5. Conclusion This article measured and analyzed the lateral shrinkage of plate unit welding under various conditions during the manufacturing process of the Nanjing Second Yangtze River Bridge steel box girder, established corresponding empirical formulas, and took compensation measures based on the analysis results. At the same time, a series of measures were also taken to effectively control the length, height, camber, flatness and matching between beam segments, so that the outer dimensions of the steel box girder met the design accuracy requirements.