IntroductionSince the twentieth century, large-span spatial structures have been widely used in public buildings, such as stations, exhibition centers, and stadiums, because of their light weight, large internal space, high flexibility, low damping, low natural frequency, and good seismic performance (Chen et al. 2011). However, because large-span spatial structures are susceptible to the effects of environmental conditions and uncertain factors during construction, they have higher quality requirements for the safety performance of the structure during construction compared to that of other structures. In an actual construction project, incidents such as casualty-producing structural collapse and inadequate building quality occasionally occur, and construction methods and technologies are the key factors affecting construction quality. In 1994, the roof of the workshop of Dongqiao Glass Chemical Factory in Dongsheng City collapsed because of overloading following a rainstorm. This incident stemmed from the misuse of steel pipes of different materials and the poor weld quality of some rods during construction. In the same year, when the coal shed of Laiyang Power Plant in Hunan was constructed using a sliding method, a temporary tie rod was removed too early, resulting in the displacement of 0.9 m, causing severe damage to the shed eventually. In 2004, the steel of the roof of Transvaal Water Park in Moscow, Russia, underwent rapid fatigue owing to the residue of the condensing agent in the welding process. Under a 51°C indoor and outdoor temperature differential, the roof could not bear the load and collapsed. In 2011, the steel structure canopy of the Naadam Fair in the Ordos, Inner Mongolia Autonomous Region, China caused a partial collapse due to quality defects in the construction stage (Liu et al. 2017). Thus, to improve construction quality, it is very important to monitor the real-time stress state of structures during the construction process and compare the impact of different construction methods on the final state of the structures.In the past few years, experts and scholars have conducted in-depth studies on mechanical analysis, construction equipment, and construction methods in the construction process. In terms of mechanical analysis, Southwell (1941), Arutyunyan (1977), and Arutyunyan and Naumov (1984) have successively studied the process of force change and mechanical mechanism of different time-varying structural systems; Choi and Kim (1985), Choi et al. (1992) applied time-varying mechanics to engineering structure and discussed the changes of internal force and deformation of spatial structure and high-rise building components in the construction process; Cao et al. (2004) proposed the topological FEM method for the analysis of time-varying structures, which reduced the workload of simulation calculation and avoided the difficulty of changing the solution domain.Considering the interaction characteristics of forces at each construction stage, Braun et al. (2015) combines BIM model with photogrammetry to study the concept of automatic construction schedule control. Tian and Hao (2015) introduced a forward iteration of the node rectification method and the element birth and death of node rectification to solve the problem of element floating in the simulation of the construction process of the main stadium for the University Games Sports Center. In construction equipment and construction methods, Zheng et al. (2012) analyzed the process of raising a steel roof considering the displacement difference and determined the effect of the difference in the displacement at the lifting points. Mi et al. (2013) developed an electrohydraulic system that could perform the consecutive motion of lifting or lowering of hydraulic strand jacks. Xu et al. (2015) improved the uniformity and continuity of a large hydraulic jack-up system for continuous beam bridges. Tian et al. (2016) analyzed and monitored the synchronous integral lifting construction of the steel roof of one of China Eastern Airlines hangars; in addition, the asynchronous lifting process of the structure was simulated and analyzed. Shao et al. (2010) Taking the large-span cantilever steel roof of space tube truss structure in Helan Mountain Stadium in Yinchuan, Ningxia Hui Autonomous Region as an example, the unloading process was simulated by numerical simulation to obtain the bearing characteristics of each support point, and the structural performance under two different unloading conditions was analyzed and evaluated, and then the optimized unloading scheme was obtained. Wan et al. (2019) Numerical simulation and comparative analysis of the construction unloading of a four-point supported orthogonal steel truss structure with fixed ends by the gap element method, the equivalent rod end displacement method and the support displacement method respectively, and it was found that the gap element method was more suitable for the analysis of orthogonal space trusses. Chen et al. (2021) during the construction unloading of the gymnasium of Fule International School in Mianyang, Sichuan, the deflection deformation of the large span floor and the field splicing node are monitored. The results show that the prefabricated construction method and the steel columns-scaffold combination support system are suitable for the large-span and high-load floor structure.As described earlier, analyses of structures during construction processes have been widely carried out. However, research in this field concentrates more on the construction process of structural lifting with a long construction time and large displacements. There is a dearth of studies on the unloading process of a structure, which is when the stress state of a structure is more complex and more likely to result in damage. Therefore, the unloading process of construction is equally important compared with the construction process of structure hoisting. Because the construction process is not repeatable, comparison of different construction methods can only be realized through the simulation of actual large structures. However, when a construction method is selected, the monitoring data can only reflect the stress state of the concerned structure, and it is difficult to compare the advantages and disadvantages of different construction methods. However, in the second reconstruction phase of the Harbin railway station, two similar truss structures adopt different unloading methods. Therefore, in this paper, the different unloading processes of the two truss structures are simulated numerically based on the finite-element analysis software ANSYS, and the strain variation rules of the structures under synchronous unloading and asynchronous unloading are analyzed. The results of the numerical simulation are compared with actual monitoring results whose strain trends are in good agreement. The results of the study reflect the characteristics of different unloading construction methods and provide a reference for the construction of other large-scale trusses.Description of Structure and Construction MethodsBasic Information of Truss StructureThe Harbin railway station is located in Harbin, Heilongjiang Province, which is known as one of the largest transportation hubs in Northeast China. The roof of the elevated train station is constructed using a steel structure roof with dimensions of 162.15×72×13.25 m. The construction of the steel structure roof is divided into two phases, and the completed second-phase structure has dimensions of 93.9×72×13.25 m.As shown in Fig. 1, the steel structure roof is composed of transverse principal trusses and transverse secondary trusses arranged along the longitudinal direction, which are connected by longitudinal trusses. The completed second-phase structure consists of 13 transverse arch trusses and 4 longitudinal vertical trusses. As shown in Fig. 2, the trusses are composed of top chords, bottom chords, and web members, which are all constructed with steel hollow pipes of different section dimensions. The section dimensions are listed in Table 1. The steel used in the truss structure is Q345B, and its mechanical properties are listed in Table 2. The entire structure is mounted on the concrete structure of the lower part with horizontal elastic supports and vertical rigid supports.Table 1. Summary of section dimensions of truss structural membersTable 1. Summary of section dimensions of truss structural membersSpeciesSection dimensions (mm) (outer diameter×wall thickness)Installation positionTop chord400×14Principal truss450×18Principal truss245×8Secondary truss299×10Longitudinal trussBottom chord650×18Principal truss700×20Principal truss273×8Secondary truss299×10Longitudinal trussTop chord straight web member377×10Principal truss180×8Principal truss299×10Secondary truss114×8Secondary truss, longitudinal trussTop chord diagonal web member203×8Principal truss, longitudinal trusses140×8Secondary truss, longitudinal trusses180×8Principal truss, longitudinal trusses114×8Secondary trussBottom chord straight web member114×8Longitudinal trusses114×8Longitudinal trussesDiagonal web member219×8Principal truss140×8Principal truss377×10Principal truss203×8Secondary truss114×8Secondary truss, longitudinal trussesTable 2. Mechanical properties of Q345BTable 2. Mechanical properties of Q345BPropertyValueE (GPa)206ν0.25ρ (kg/m3)7,800σs (MPa)345Rm (MPa)510–600Truss Structure Construction MethodsAs shown in Fig. 3, the second-phase truss structure is divided into three parts, which are assembled and welded on the floor of the 9-m-high waiting hall of the station. Each part contains four, seven, and two transverse trusses. During the hoisting stage, the first and second parts are lifted from the floor of the waiting hall to the roof height of 22 m by fixed hydraulic jacks, as shown in Fig. 4. The third part of the truss is lifted by a crane truck. The three parts are successively elevated to the same height and mounted on the concrete structure and supported by welded steel pipes. The connections at the supports are shown in Fig. 5. Finally, the three parts are connected to each other with the longitudinal trusses.During the unloading stage, the methods of unloading large trusses in practical projects are mainly divided into synchronous and asynchronous unloading methods. For a large-span steel structure with high integrity and symmetry, the synchronous unloading method of unloading the whole structure at the same time is usually chosen. For a large-span steel structure with clear regional structural division and a relatively independent structure or more supporting points before the completion of the whole structure, the partition synchronous unloading method is usually used to carry out synchronous unloading of each part of the structure separately. Asynchronous unloading is a construction method that dismantles and unloads the same structure at different unloading points in a certain sequence, which is seldom used in engineering (Pan 2015).In the study presented in this paper, the synchronous unloading method is adopted for the first part of a truss structure, and the asynchronous unloading method is adopted for the second part. Because the first and second parts of truss structures are relatively similar, the finite-element analysis of synchronous unloading of the second part of the truss structure is carried out. And the results show that the stress of each member of the main and secondary truss is similar to that of the first part of the truss in synchronous unloading, and the strain variation trend is the same, as is the strain state in the final stress state. Therefore, this paper directly compares the first part of the truss structure, where synchronous unloading is used, with the second part of the truss structure, where asynchronous unloading is used.The first part of the truss structure is connected to the hydraulic jacks by four groups of steel strands. The connection positions are located on the bottom chords of the two principal trusses. Each group has eight strands. After all the principal and secondary trusses are welded to the supports through connecting rods, the steel strands in each lifting position are cut to complete the process of fixing the trusses. The connections between the strands and the bottom chords are shown in Fig. 6.The second part of the truss structure is connected to hydraulic jacks by six groups of steel strands, and each group has eight steel strands. After the truss structure is elevated to a predetermined height, the rods connected to the supports are welded to each of the principal trusses in a certain sequence, as shown in Fig. 7. The determination of the welding sequence should meet the following principles. First, ensure that the structure in the construction process of the overall deformation is small. Second, ensure that the internal forces of key components are always within the elastic range during the construction process so as to avoid any structural strength damage or instability caused by the destruction of local components. Finally, ensure that the internal force and displacement of the structure change slowly during construction. When the welding of the connecting rods at certain positions is completed, the steel stands are cut off. After all the principal trusses are fixed to the supports and the steel strands are cut, the connecting rods are welded to the secondary trusses, and the process of fixing the truss structure is complete.In terms of environmental factors, the effect of the slight wind on the truss structure in Harbin during the construction process may be ignored. The construction processes of the first and second parts of the truss structure were performed on August 19, 2018 and August 27–28, 2018, respectively. The daytime temperature varied from 19°C to 28°C and 20°C to 26°C. Owing to the slight temperature difference during the construction process, the effects of temperature on the structure could be ignored. Therefore, the effects of the construction methods became the main factors that altered the internal force of the truss members.FEM Analysis of StructureFor the completed first and second parts of the truss structure, the FEM model was established in ANSYS, as shown in Fig. 8. It contains 203,989 Beam 188 elements, which are established to simulate all of the top chords, bottom chords, and web members of the truss structure. A Beam 188 element is a three-dimensional linear or quadratic beam element, considering the effect of shear deformation, suitable for the analysis of linear, large angle rotation, and nonlinear large strain problems. Ten Link 180 elements are established to simulate the steel strands by setting the stress property as compression-free and not considering the bending deformation. The diameter of the steel strand is 17.8 mm. A Link 180 element, which does not bear the bending moment, is a tension and compression element in the direction of the rod axis. Each node has three degrees of freedom: the translational movement along the x-, y-, and z-directions of the node coordinate system, which is just like a hinged structure. In addition, the element has the functions of plastic creep, rotation, large deformation, and large strain. To be consistent with the actual structure, only the aisles on the secondary trusses of the second part are simulated by 160 Mass 21 elements. A Mass 21 element is a structural mass element with six degrees of freedom: that is, movement in the x-, y-, and z-directions and rotation around the x-, y-, and z-axes. Each direction can have a different mass and moment of inertia. The birth and death attributes of the elements are used to simulate the welding sequences of the connecting rods that connect the supports and the trusses. As for the constraint setting at the support, the upper end of the Link 180 element of the simulated steel strand is fully constrained when simulating the suspension condition of the truss structure. When simulating the unloading process of the truss structure, the lower end of the beam element with simulated steel support is subjected to full constraint, and then the same vertical downward displacement constraint is applied simultaneously to the upper end of four Link 180 elements. However, for the connection mode of transverse truss and longitudinal truss, the finite-element model adopts a fixed connection mode.For the FEM model, the entire structure, load, and restraint of the first part are completely symmetrical in the transverse and longitudinal directions. The second part is only transversely symmetrical. Thus, the stress of the first part of the truss exhibits a high degree of symmetry. Figs. 9 and 10 show the strains of the truss only restrained by steel strands under gravity.Strain State of Trusses in Suspended ConditionFor arch truss structures, axial forces are the main form of bearing and transmitting loads. For the first part of the truss structure, all the bottom chords of the principal trusses are compressed. The trusses deform in the transverse direction under gravity, and the steel strands deviate by small angles from a vertical position. Therefore, the axial pressures on the bottom chords of the principal trusses are generated by the steel strands. Since the effect of axial pressure on strain is stronger than that of the bending moment caused by upward shearing forces, the entire bottom chords are compressed. Under the constraints of the steel strands, the weight of the top chords is transferred to the bottom chords through the compressed web members. Because of the bending moments caused by the downward shearing force from the web members, the parts partially located on the bottom chords are obviously compressed, as shown in Fig. 9. The axial pressures at the ends of the bottom chords are stronger than those at midspan. The bending moments caused by the upward shearing forces from the steel strands are more obvious in the midspan of the truss. Thus, the compression degree on the upper surface of the bottom chords increases gradually from the middle of the bottom chords to the end. The position with the largest compressive strain appears in the lifting position. For the top chords of the principal trusses, the connections between the web members and the top chords in the midspan part are clearly in compression. The weights of the secondary trusses are transferred to the principal trusses through the longitudinal trusses. Since the longitudinal trusses are directly connected to the top chords of the principal trusses, the bending moments in the midspan of the top chords caused by the downward shearing force are relatively large. Consequently, for the midspan part of the top chords, the compressive strains on the upper surface are large. Owing to the small axial pressure of the top chords and the absence of external constraints, the compression degree of the members is gradually weakened from the middle to the end of the span.For the secondary trusses of the first part of the truss, the weight is transferred to the principal trusses by four longitudinal trusses. Since the secondary trusses and the longitudinal trusses are not connected to the steel strands, their deformations are more obvious than those in the first part of the truss structure. Fig. 11 shows the vertical displacements of the secondary trusses under the effect of the self-weight (the unit is meters). Consequently, the bottom chords are tensioned in the midspan area, and the entire top chords are compressed.Trusses 17 and 23 are on the same sides of the structure in the longitudinal direction as Trusses 13 and 16. Therefore, the strain states of the top and bottom chords of Trusses 17 and 23 are the same as those of Truss 16. As shown in Fig. 3, owing to the slight difference in the distances between the adjacent principal trusses of the second part, the strains of Trusses 17 and 23 exhibit a slight asymmetry. Truss 20 is further loaded owing to the middle position of the structure. The FEM analysis shows that the steel strands connecting Truss 20 are subjected to the largest tension. Owing to the stronger axial pressure caused by the strands, the compressive strains of the bottom chords are larger than those of the other principal trusses. The longitudinal trusses transfer more load to Truss 20, and the bending moments caused by the downward shearing forces of the top chords are obvious. Consequently, the degree of compression of the top chords of Truss 20 is more obvious than that of the other principal trusses, as shown in Fig. 10.For the secondary trusses of the second part, the strain states are the same as those of Truss 15. Owing to the construction aisles on the four secondary trusses, the weights of the secondary trusses increase; thus, the compression strains on the upper surface of the bottom and top chords are larger than those of Truss 15. The vertical displacements of the secondary trusses are shown in Fig. 12 (unit: meteres).Analysis of Synchronous Unloading ProcessIn this subsection, a finite-element model simulation and time history analysis are carried out for the truss structure during the unloading process. The mechanical behavior of the structure in the construction process belongs to the category of slow time-varying structural mechanics. The time history analysis of the structure can be selected based on the slow change of material properties, geometric shape, and boundary conditions of the structure with time. The changing structure can be divided into a series of static structures according to the time node, and the most unfavorable state of the structure in the construction process can be analyzed, or the relationship between the structure itself and the load in the time-varying process of the structure can also be considered and the strength of the structure in the construction process analyzed. In the construction stage of a long-span steel structure, the internal force and displacement of the structure in the previous construction stage will inevitably affect the next stage. It is necessary to superimpose the internal force and displacement of each stage to reflect the final stress state and displacement of the structure accurately.The structural state of any stage in the construction process can also be obtained considering the mechanical state of each construction stage (Hu 2016).Based on the preceding discussion, influential factors, such as the wind and ambient temperature, are assumed to be invariable owing to their insignificant effects. Since the eight strands in each group are cut off one after another, the weight of the truss structure is gradually transferred to the supports. The entire construction process is analyzed by the numerical simulation software.Since boundary conditions change during the construction process, the internal structural forces are redistributed and can be reflected by the strains. Fig. 14 shows the strain variations for typical elements of members with a time step. It can be seen from the figure that the force of each member of the truss structure is stabilized after a redistribution of internal forces. In the 0.25th time step, the Link 180 unit exits work, and the strain of each selected unit tends to stabilize. The positions of the elements are shown in Fig. 13. Since the main function of the web members is to transmit the forces, the strain variations of the web members are not discussed.In the time step, the load of the structure is transferred from the steel strands to the supports. With the continuous shearing of the steel strands, the web members gradually transfer the weight of the bottom chords to the top chords and finally to the supports. Owing to the change in the boundary conditions, the axial pressures of the bottom chords decrease. The direction of the bending moments caused by the shearing forces of the web members reverses. For the bottom chords, owing to the more obvious effects of the bending moments caused by the shearing forces, the degree of compression is weakened. Noticeably, the trends of the strain variations are similar for different positions on the bottom chords. For the top chords, the two ends are constrained horizontally and longitudinally. According to Fig. 14, the degree of compression for the top chords increases with increasing axial pressure and bending moment caused by supports. Moreover, owing to the greater axial pressures caused by the supports at the end of the top chords, the degree of compression increases from the midspan of the top chords to the ends.The supports of the secondary trusses have the same form as those of the principal trusses. After the boundary conditions of the principal and secondary trusses change, the top chords bear their main weight. Therefore, the degree of compression on the top chords of the principal and secondary trusses increases. However, before the hydraulic jacks are unloaded, the secondary trusses are supported by four longitudinal trusses, and the supporting positions are dispersed. Thus, compared with the principal trusses, the strain variations of the top chords of the secondary trusses are smaller than those of the principal trusses. For the bottom chords, the supports enhance the transverse stiffness of the secondary trusses and inhibit the transverse deformation of the bottom chords. Therefore, the degree of compression of the bottom chords of the secondary trusses increases.The strain variations of the rods that connect the trusses to the supports are shown in Fig. 14(c). Since the connecting rods bear the total weight of the structure after the hydraulic jacks are unloaded, the strains clearly change. According to the preceding discussions, the rods connecting the top chords become part of the top chords after being loaded; thus, the rods are obviously compressed. The rods connecting the bottom chords transfer part of the weight of the bottom chords to the supports; thus, the connecting rods are obviously in tension.Based on the foregoing analysis, it can be concluded that strain variations of structural members are related to the construction methods and structural forms. The strains can directly or indirectly reflect changes in the structural internal forces. During the static construction process of the first part, the strains of the truss members vary uniformly and symmetrically. Since the stresses of the truss reflected by the strains are far less than the allowable stress of the structural steel, the structure remains in a stable state during the construction process.Analysis of Asynchronous Unloading ProcessFor the second part of the truss structure, the strain variations of the members are complex owing to the asynchronous unloading method and the longitudinal asymmetry of the structure. Fig. 16 shows the strain variations for typical elements with time steps. During the six time steps, hydraulic jacks are unloaded in sequence. The positions of the elements are shown in Fig. 15. As shown in Fig. 16, the loaded supports have substantial effects on the trusses connected to them. Moreover, there are significant strain variations of the members near the unloading position. In the first and fourth time steps, the hydraulic jacks on both sides of Truss 20 are unloaded successively. For the elements on the bottom chords of Truss 20, Positions 20-1 and 20-5 exhibit maximum strain variations in the first and fourth time steps, respectively. The strain variation of Position 20-1 (82.403 με) in the first time step is lower than that of Position 20-5 (85.024 με) in the fourth time step. This is because the constraints of the structure in the first time step are less than those in the fourth time step.For the principal trusses of the second part, the strain change tendencies of the bottom chords in the midspan are different from those of the nonmidspan positions and the bottom chords of the principal trusses of the first part. Since the secondary trusses of the second part are not connected to the supports during the unloading process, the weight of the second part is supported by the longitudinal trusses. Thus, the loads on the principal trusses increase. Because the two longitudinal trusses located in the middle of the transverse direction transfer most of the loads, there is a slight compression phenomenon in the midspan of the bottom chords. The tendencies of the strain variation of the top chords of the second part are the same as those of the top chords of the first part. It is worth mentioning that because Truss 20 is located in the middle of the second part and bears more load, the top chords of Truss 20 show the most significant strain variations compared to those of other trusses. In addition, since Trusses 17 and 23 are not completely symmetrical in the longitudinal direction, there are slight differences between their strain variations.With the change in the boundary conditions, the internal forces of the principal truss members change substantially. Since the secondary trusses are only supported by the principal trusses through the longitudinal trusses and are not connected to the supports during the construction process, the strain of the secondary trusses varies slightly in value. Therefore, the secondary trusses are not analyzed in detail. Compared with Truss 15, the strain variations of all the members in each time step are relatively small, and there are no obvious differences, as shown in Fig. 16(b).For the connecting rods, the trends of the strain variation are the same as those of the first part, as shown in Fig. 16(e). Since the second part has a larger weight and the secondary trusses are not supported during the construction process, the connecting rods are subjected to larger loads than those of the trusses. Consequently, the strain values of the connecting rods change more than those of the first part.Analysis of the strain data lead one to conclude that the strains of different members have different variation degrees during the unloading process. When the hydraulic jacks on both sides of a truss are unloaded, the strain variations of the members are more obvious, and the tendencies of the variations are different. For Truss 23, the hydraulic jacks on both sides are unloaded in the third and sixth time steps, and the strains of the members change significantly during the two time steps.For the second part of the truss structure, owing to the frequent changes of the structural constraints at different positions, the redistributions of the internal forces occur repeatedly in the structural members. The structure stays in an asymmetrical state, and the strains of the members exhibit complicated changes. Moreover, compared with the first part, the maximum strains of some members appear during the construction process rather than at the beginning or the end of the state.Monitoring ResultsBasic Information on Structural Strain Monitoring and SensorsDuring the unloading process, in addition to the nonobvious environmental factors, the internal forces of the truss structure are mainly affected by the construction factors. To monitor the real-time state of trusses to ensure that the structure is in a stable state, the axial strains on typical top chords and bottom chords are monitored. The layout of the monitoring positions is shown in Fig. 17. The selected typical sensors are located at the same positions as the partial elements chosen in the FEM results.As shown in Fig. 18, optical fiber Bragg grating (OFBG) surface strain sensors were installed to collect the axial strains of the typical chords. The properties of the OFBG strain sensors are listed in Table 3. The sensors are installed during the process of the assembly and welding of the truss structure. The state of the entire structure is evaluated by collecting and analyzing the values of the strain variation of the members during each construction stage.Table 3. Properties of OFBG surface strain sensorsTable 3. Properties of OFBG surface strain sensorsParameterSurface strain sensorsMeasurement range−1,500 to +1,000 μεWavelength range1,510–1,590 nmPrecision1‰ FSResolution0.5‰ FSVariations in Axial StrainThe monitoring results of the two unloading construction processes on August 19, 2018, are displayed in Fig. 19. For the four monitoring positions of Truss 16, before the unloading construction at 13:20, the strain basically remains in a stable state. The fluctuation of the strain is caused by construction factors, such as the welding of members on site. When the unloading construction begins, the four groups of strands are cut synchronously, and the strands of each group are cut one after another. The internal force of the first part changes abruptly, and the strains shown in Fig. 19 show obvious step differences. For Positions 16-1 to 16-4, the degree of compression at each location is weakened. Among the four monitoring positions, the strains of Positions 16-1 and 16-3 show the most and least significant changes. The strain variations of transversely symmetrical Positions 16-2 and 16-4 are different. This might be due to actual structural differences. After the unloading construction is complete, structural strains tend to stabilize. Since the surface strains of the members are greatly affected by construction and personnel factors, the monitoring results clearly fluctuate. The strain of Position 16-3 shows the most significant fluctuation, while that of Position 16-1 shows the least fluctuation. This indicates that the members farther away from the supports are more affected by construction factors than those close to the supports.The overall variation trends of the strains for the bottom chords of Truss 16 are the same as the results of the FEM analysis. The monitored strain change is less than the simulated strain change. For a truss structure in the stable state before and after unloading construction, the measured strain change value of each sensor and the finite-element simulated strain change value are shown in Table 4.Table 4. Comparison of the monitoring and simulated strain difference results during the unloading of Truss 16Table 4. Comparison of the monitoring and simulated strain difference results during the unloading of Truss 16Sensor16-116-216-316-4Average valueMonitoring strain difference (με)−34−273−23−20.25Simulated strain difference (με)−55−382−38−32.25Percentage difference (%)47.233.84049.242.55It can be seen from Table 4 that the overall monitoring strain change is smaller than the numerical simulation strain change result, and the causes of this result are analyzed. In addition to the impact of site construction factors and actual truss structure differences on the monitoring data, the limitations of finite-element simulation also have a certain impact on the simulation results. In actual engineering, when a truss is hovering, it is only constrained by the vertical upward pulling force of the steel strand, and there is no restriction in the horizontal direction. The truss and the steel strand system are called mechanisms in engineering mechanics. In the finite-element simulation, it is impossible to make the structure reach a statically indeterminate state by applying only vertical constraints to the lifting points of the model, so the analysis can only be performed by applying corresponding constraints to the truss in the horizontal direction. Therefore, compared with the actual structure, the strain value in the initial state in the numerical simulation increases with the increase of the model’s constraints, so the strain change in the unloading process in the simulation result is slightly larger than the monitoring result. However, considering that the force distribution law of the truss are the same as the simulation analysis result, the strain change amplitude and trend of each monitoring position during the unloading process are consistent with the simulation conclusions, and after the truss is stabilized, the strain approaching the phenomenon of each monitored member is similar to the simulation result. Therefore, the finite-element simulation of simultaneous unloading construction is reliable to a certain extent. During the construction process, the strain variations of truss members are explicit, and the strain changes in the same direction. During unloading construction, the strains of the truss members have significant variations. Before and after unloading construction, environmental factors have little effect. The small fluctuations in strain are mainly caused by construction factors. Before unloading, construction personnel on the truss check the weld, spray steel anticorrosion paint on it, and repair the specific weld, which will cause small strain fluctuation. After unloading, the construction personnel cut the residual steel strand at the lifting position of the end of the bottom chord and the steel clamps welded there, which will also cause small strain fluctuation. Therefore, the collected data are reliable. Generally, the first part of the truss structure is in a stable state during the monitoring process.The monitoring results for the two unloading construction processes on August 27–28, 2018, are displayed in Fig. 20. According to the figure, when the hydraulic jacks are unloaded, the welding construction of the connecting rods and the unloading construction at each support position are carried out continuously without a construction interval. Moreover, the construction processes at different support positions are independent. Thus, the monitored strain fluctuates during the total monitoring process. For Positions 17-1 and 17-2, when the closest hydraulic jack of Truss 17 is unloaded, the strain exhibits a large step mutation. When the hydraulic jack on the other side of Truss 17 is unloaded, the strain exhibits a small step mutation. Nevertheless, when the hydraulic jacks of other trusses are unloaded, the strain is basically unchanged. Since Position 17-3 is at midspan, the unloading of the hydraulic jacks on both sides of Truss 17 has similar effects on the strains, which are not significant. Similarly, the strain fluctuations of the members farther from the supports are more obvious than those of the first part.Table 5 shows a comparison of the measured strain difference of each monitoring position of Truss 17 at the initial and final steady states of the overall unloading construction and the final strain difference of the numerical simulation results.Table 5. Comparison of monitoring and simulated strain difference results at initial and final states of Truss 17Table 5. Comparison of monitoring and simulated strain difference results at initial and final states of Truss 17Sensor17-117-217-3Average valueMonitoring strain difference (με)−25−252−16Simulated strain difference (με)−50.121−37.0876.553−26.885Percentage difference (%)66.938.910670.6Table 5 shows that the overall monitored strain difference is less than the simulated difference, for the same reasons as given previously.As in the preceding discussion, according to Fig. 21, for the bottom and top chords of Truss 20, the strain differences are obvious when the hydraulic jacks on both sides of Truss 20 are unloaded. Since Positions 20-1 and 20-2 are relatively close, the tendencies and values of the strain variation of the two positions are similar when the jacks are unloaded. It is worth mentioning that the strain variation of Position 20-1 is larger because Position 20-1 is closer to the support. For Position 20-5, which is located in the transversely symmetric position of 20-1, the value of the strain variation is larger when the jack on its side is unloaded than when it is loaded, as discussed earlier. Like Position 17-3, Position 20-3 exhibits obvious strain variations only when the hydraulic jacks on both sides are unloaded and the compression degree increases. For Positions 20-6 and 20-7, Position 20-6 show the maximum value of the strain variation of the seven monitoring positions when the jacks of Truss 20 are unloaded. When the boundary conditions change, the top chords near the support positions are subjected to substantial pressure, and the internal force changes significantly. Compared with Truss 17, the strain variation of Truss 20 is greater because it is located in the middle of the structure.As shown in Table 6, the measured strain difference for the sensors of Truss 20 before and after unloading construction is always smaller than the final strain difference in the numerical simulation, except for the limitations imposed by the constraints. Compared with the material and construction defects of the finite-element model, the actual structure is also related to the wire stiffness simulation of the steel strand in the simulation process, so the monitoring results cannot fully meet the results of the finite-element analysis. However, during the unloading process, the strain change trend of each monitoring position during the unloading process is consistent with the simulation results, the main force-bearing members and force areas are the same as the simulation analysis, and the force distribution of each member before and after unloading basically conforms to the simulation force analysis results. Therefore, the finite-element simulation can be used to obtain relatively accurate strain trend and force characteristics of the truss structure under synchronous and nonsynchronous unloading and then compare the effects of the two unloading methods on the mechanical state of the truss structure.Table 6. Comparison of monitoring and simulated strain difference results for initial and final states of Truss 20Table 6. 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