A continuous bridge box girder dismantling method for keeping navigation

Abstract

This invention relates to the field of bridge demolition technology, and more particularly to a method for demolishing continuous bridge box girders while maintaining navigation. The method employs pre-construction of the new bridge to achieve zero road traffic interruption, and establishes a spatially coordinated force-bearing system by setting up a transverse connecting truss between the two old bridges. A foundation coupling settlement prediction model is established to monitor and dynamically match the demolition rate in real time to maintain dynamic equilibrium of the foundation forces. A four-floating crane spatial coupling synchronous control system is used for the navigation span, incorporating wave field interference effects to correct the lifting speed, achieving synchronous symmetrical lifting of the two bridge areas. The purpose of this invention is to solve the safety and efficiency problems encountered during the synchronous demolition and construction of parallel double continuous box girder bridges, including road and waterway traffic interruption, uneven settlement of the shared foundation of the two bridges, structural stress imbalance during demolition, and wave field interference during multi-floating crane coordinated lifting.

Description

Technical Field

[0001] This invention relates to the field of bridge engineering demolition technology, and in particular to a method for demolishing a continuous bridge box girder to ensure navigation. Background Technology

[0002] With the rapid development of inland waterway shipping and highway transportation in my country, a large number of parallel double continuous box girder bridges spanning navigable waterways need to be demolished and rebuilt due to reasons such as the expiration of their design life, the upgrading of waterway grades, and insufficient traffic capacity. These bridges are usually composed of two independent continuous box girder bridges arranged in parallel, jointly bearing the high-volume two-way highway traffic, and crossing high-grade inland waterways, serving as key nodes in the regional comprehensive transportation network. Their demolition and reconstruction projects are characterized by complex construction environments, high safety risks, significant impacts on traffic and navigation, and tight schedules. How to minimize the impact on social traffic and inland waterway shipping during the demolition and reconstruction process, while ensuring construction safety and efficiency, has become a crucial technical challenge that urgently needs to be solved in the field of bridge engineering.

[0003] In the actual dismantling and reconstruction process of parallel double continuous box girder bridges, the following prominent problems exist: First, traffic congestion is prominent. Traditional dismantling and reconstruction methods typically employ a single-bridge sequential dismantling approach, with only one bridge maintaining traffic flow during construction. This reduces traffic capacity by more than 50%, easily leading to regional and long-term traffic congestion, severely impacting the travel of residents along the route and regional economic development. Second, navigation is severely affected. The dismantling of navigation channels often employs independent floating crane operations on a single bridge, with each cutting unit being a single cantilever construction segment. A single closure of navigation can only complete the removal of a single small unit of a single bridge, resulting in numerous closures and long closure times, severely disrupting the normal operation of inland waterway shipping. Third, foundation coupling... The risks are out of control. The two parallel bridges share the same riverbed foundation. Dismantling the bridge first will cause the foundation to rebound and deform, which will lead to additional settlement and tilting of the piers of the bridge to be dismantled later. Existing technology lacks effective means to predict and control this coupling effect, which is prone to structural safety accidents. Fourth, the stress imbalance of the dismantled structure. During the independent dismantling process, each pier needs to bear huge unbalanced bending moments, which can easily lead to safety hazards such as box girder cracking and pier instability. Fifth, the difficulty of multi-floating crane coordinated hoisting is high. Under the complex hydrological conditions of inland rivers, the simultaneous operation of multiple floating cranes will produce wave field interference effects, causing load oscillation. Traditional synchronous control technology cannot effectively solve this problem, resulting in high hoisting safety risks.

[0004] A search revealed that patent CN115704206A, entitled "Method for Demolishing Bridges Adjacent to Existing Highways and Overpasses of Navigation Channels," utilizes a combined operation of a floating crane and a truck crane. The cut T-beams spanning the navigation channel are lifted using a reverse method to the undemolished bridge deck, then transported in sections to a temporary demolition site for crushing. This solves the problem of large floating cranes being unable to operate in confined construction areas. In addition, existing technologies include temporary auxiliary road methods, single-bridge floating crane overall demolition methods, and static blasting demolition methods. These existing methods offer certain advantages in specific engineering scenarios: the combined lifting method reduces the requirements for the water surface construction area and improves adaptability to confined spaces; the temporary auxiliary road method alleviates traffic pressure during construction to some extent; and the static blasting demolition method involves less vibration and has less impact on surrounding structures. However, these methods all have significant limitations: they are only applicable to the independent demolition of a single bridge, do not involve the parallel simultaneous demolition and reconstruction technology for two bridges, cannot solve the problem of a significant reduction in traffic capacity, and do not consider the coupling effect of the shared foundation of the two bridges, nor do they involve the synchronous control technology of multiple floating cranes; the temporary auxiliary road method has high construction costs and long construction periods, and cannot solve the problem of navigation impact; the single-bridge floating crane overall demolition method has small cutting units, requires multiple closures of navigation, and has serious navigation impact; the static blasting demolition method is inefficient, has a long construction period, and cannot meet the requirements of rapid demolition and reconstruction. In summary, the existing technologies cannot simultaneously ensure normal road traffic and uninterrupted waterway navigation, nor can they effectively solve the safety and efficiency problems of uneven settlement of the shared foundation of two bridges, stress imbalance of the demolition structure, and wave field interference during the coordinated lifting of multiple floating cranes. Therefore, there is an urgent need to develop a new method for the demolition of continuous box girders that ensures navigation to meet the engineering requirements of safe, efficient, and low-impact demolition and reconstruction of parallel double continuous box girder bridges. Summary of the Invention

[0005] This invention provides a method for dismantling continuous bridge box girders to ensure navigation, so as to balance normal road traffic and uninterrupted navigation in the process of simultaneous dismantling and reconstruction of parallel double continuous box girder bridges, while solving the safety and efficiency problems of uneven settlement of the shared foundation of the two bridges, stress imbalance of the dismantled structure, and wave field interference during the collaborative hoisting of multiple floating cranes.

[0006] To solve the above problems, the technical solution adopted by the invention is as follows: A method for dismantling continuous box girders to ensure navigational safety includes the following steps: S1. Construction of the new bridge in advance: A new bridge will be built outside one of the old bridges. After the new bridge is completed, inspected and ready for traffic, all traffic on the two parallel and independent continuous box girder old bridges will be closed. A transverse connecting truss will be installed between the two old bridges to connect the two independent box girder bridges into a spatially coordinated force-bearing system. The transverse connecting truss will be arranged at intervals along the longitudinal direction of the bridge at the top plate of the box girder between adjacent piers, so that the two bridges will form a cross-bridge load transfer path during the demolition process. S2. Foundation Coupling Determination: Based on the condition that the two bridges share the same riverbed foundation, a foundation coupling settlement prediction model for the bridge is established to predict the impact of unloading the first bridge demolition on the tilting and additional settlement of the piers of the second bridge demolition, and to determine the initial matching relationship of the demolition rates of the two bridges. S3. Collaborative Demolition of Non-Navigation Channels of Two Old Bridges: The structures within the non-navigation channel ranges of the two old bridges are simultaneously cut and lifted in sections, and the demolition work is carried out before the corresponding navigation channel demolition work. During the demolition process, the differential settlement of the corresponding pier tops of the two bridges is monitored in real time. When the settlement increment of any pier exceeds the preset settlement warning threshold, the demolition rate on both sides is dynamically adjusted to keep the foundation of the two bridges in dynamic equilibrium. S4. Coordinated Cutting and Synchronous Lifting of Two Existing Bridges' Navigation Channels: Within the navigation channel area, wire saws are used to cut sections along a pre-set cutting line; utilizing the load transfer capacity of the transverse connecting trusses, the length of the cutting unit is extended to include two adjacent cantilever construction segments and the intermediate closure segment; each existing bridge is equipped with two floating cranes symmetrically arranged on both sides of the segment to be demolished, and the four floating cranes adopt a spatial coupling synchronous control system. Based on traditional speed synchronization and load distribution control, the system takes into account the wave field interference effect generated by the operation of adjacent floating cranes and spatially transfers and corrects the lifting speed compensation of each floating crane; during the same intermittent navigation closure period, the two existing bridges simultaneously carry out symmetrical lifting operations, lifting the cut box girder segments away from the bridge position at the same time.

[0007] S5. Coordinated dismantling of main piers and abutments of the two bridges: The adjacent main piers of the two bridges, which are arranged on the same line, are dismantled alternately by cutting them in sections from top to bottom, so that the unloading of the foundation on both sides is symmetrical and balanced; the abutments in the water are cut and lifted off by underwater sections.

[0008] The principle and advantages of this scheme are as follows: The principle of this plan is to first completely resolve traffic issues during construction by building the new bridge in advance. Once the new bridge is passable, the two old bridges will be closed and demolished simultaneously. Then, by installing a transverse connecting truss between the two old bridges, the two originally independent box girder bridges are linked into a spatially coordinated load-bearing unit. This allows the loads generated during demolition to be transferred between the two bridges through the truss, fundamentally changing the stress pattern when demolishing a single bridge independently. Based on this, and considering the shared riverbed foundation of the two bridges, a cross-bridge foundation coupled settlement prediction model is established to quantify the impact of unloading the first bridge demolition on the subsequent bridge pier demolition, and to determine the initial matching relationship of the demolition rates of the two bridges. During the dismantling of non-navigable passages, the dismantling rate on both sides was dynamically adjusted by monitoring the differential settlement of the pier tops in real time, so that the foundation stress always remained in dynamic equilibrium. Finally, during the dismantling of navigable passages, the load transfer capacity of the bridge span provided by the transverse connecting trusses was fully utilized to extend the length of the cutting unit. At the same time, a spatial coupling synchronous control system that takes into account the wave field interference effect of adjacent floating cranes was adopted to achieve synchronous and symmetrical lifting operations of the two old bridges by four floating cranes during the same intermittent navigation closure period. This minimized the impact on navigation while ensuring the safety of the lifting. Finally, the alternating and coordinated dismantling of the main piers and abutments maintained the symmetrical balance of the foundation unloading, avoiding the risk of pier tilting and foundation instability in the later stage.

[0009] Compared with existing technologies, this solution solves the core problem that existing technologies cannot simultaneously accommodate both road traffic and waterway navigation. Existing technologies generally employ a sequential dismantling and reconstruction model for single bridges, resulting in a reduction of traffic capacity by more than 50% during construction and easily leading to long-term regional traffic congestion. This solution, however, achieves "zero traffic interruption" during dismantling and reconstruction by constructing the new bridge in advance, ensuring that road traffic capacity is completely unaffected during construction and fundamentally eliminating the interference of the dismantling and reconstruction project on social traffic. Existing technologies, when dismantling a single bridge independently, only require a single cantilever construction segment as the cutting unit. Each closure of navigation can only complete the removal of a single small unit of a single bridge, typically requiring 15-20 closures. This solution, however, utilizes transverse connecting trusses to extend the cutting unit to include two adjacent cantilever construction segments and the intermediate closure section. Furthermore, within the same navigation closure window, one extended unit from each of the two old bridges can be removed simultaneously, reducing the number of closures by approximately two-thirds and significantly reducing the impact on inland waterway shipping. Existing technologies completely ignore the coupling effect of the shared foundation between the two bridges. Dismantling the bridge first and unloading it can easily lead to excessive additional settlement and tilting of the piers when dismantling them later. This solution, for the first time, establishes a prediction mechanism for the coupled settlement of the bridge foundation. The model employs a dynamic rate control strategy, which can keep the pier tilt rate below 0.1%, far below the allowable value specified in the code. This effectively avoids structural safety accidents caused by foundation instability and prevents the dismantled old bridge from falling into the river and affecting the waterway. Existing technologies require each pier to withstand a huge unbalanced bending moment during independent dismantling of a single bridge, easily leading to box girder cracking and pier instability. This solution reduces the unbalanced bending moment borne by each pier by 40%-60% through load transfer across the bridge, improving the structural safety of the dismantling process. Regarding hoisting safety, existing technologies for multi-floating crane synchronous control only consider speed synchronization and load distribution, failing to address the load oscillation problem caused by wave field interference between floating cranes under complex inland waterway hydrological conditions. This solution incorporates the wave field interference effect into the synchronous control system, reducing the amplitude of floating crane load oscillations. During the main pier dismantling stage, this solution adopts an alternating segmented dismantling strategy, keeping the main pier tilt rate far below the code, solving the structural instability problem caused by asymmetrical unloading of the foundation later.

[0010] Furthermore, in S1, the transverse connecting truss adopts a detachable and assembled steel truss. Each truss is anchored at both ends to the pre-embedded anchor plates on the top plates of the box girders of the two old bridges through hinged nodes. The hinged nodes allow the two bridges to undergo limited relative displacement in the lateral direction to release temperature and shrinkage deformation, while restraining relative torsion and ensuring the reliability of load transfer across the bridge. The timing of the installation of the transverse connecting trusses is as follows: all truss installations are completed before the dismantling of non-navigation channels begins; the transverse connecting trusses corresponding to each section are dismantled after each cutting unit of the navigation channel is lifted out, so that the truss dismantling and box girder dismantling proceed alternately to avoid the trusses in the dismantled box girder sections becoming cantilever load-bearing components; during the construction of the new bridge, the old bridge on the right side maintains two-way two-lane traffic and normal navigation of the waterway; the new bridge adopts a prefabricated assembly construction process, with the construction of its substructure and the inspection of the old bridge structure carried out simultaneously, and the prefabrication of the superstructure and the preparation work for the dismantling of the old bridge carried out in parallel, using detachable assembly steel trusses and hinged node structures, and coordinating with the non-navigation channels. The sequential control of the installation of the entire bridge and the alternating dismantling of the navigation channels in stages not only ensures the stable and reliable transmission of the bridge load by releasing temperature and shrinkage deformation through the hinged nodes and restraining relative torsion, but also avoids the structural safety risks caused by the truss forming a cantilever stress state. At the same time, during the construction of the new bridge, the two-way two-lane traffic and normal navigation of the waterway on the right side of the old bridge are maintained. The construction of the substructure of the new bridge and the inspection of the old bridge, as well as the prefabrication of the superstructure of the new bridge and the preparation for the dismantling of the old bridge, are carried out in parallel. This can significantly reduce the overall construction period without interrupting traffic and navigation, improve the efficiency of construction organization and the safety of on-site operations, and achieve orderly connection of cross-operations between the new and old bridges and efficient use of resources.

[0011] Furthermore, in S4, the control strategy of the spatial coupling synchronous control system is as follows: Any floating crane located upstream of the main bridge is taken as the reference floating crane, and the other three are subordinate floating cranes. The reference floating crane operates according to a preset lifting speed curve. Each subordinate floating crane tracks the reference speed and then adds two corrections: The first is a wave field coupling correction between adjacent floating cranes. When the distance between any two floating cranes is less than a preset coupling distance threshold of 30-50m, the attenuation and phase difference of the wave field transmitted from one floating crane to another are calculated based on the water surface distance, river wave speed, and wave frequency between the two floating cranes. Based on this, the lifting speed of the subordinate floating cranes is compensated for either advance or lag, eliminating load oscillations caused by wave field interference. The second is a feedback correction of the internal force of the transverse connecting truss. The axial force of each member of the transverse connecting truss is collected in real time. When the rate of change of the axial force of any member exceeds the preset safe rate of change, it is determined that a load distribution imbalance has occurred during the lifting process of the two bridges. The axial force is automatically reduced, the lifting speed of the side floating crane is increased, and the axial force is increased. The lifting speed of the side-mounted floating cranes is reduced until the axial force of the members returns to a safe range. Through the above two corrections, the four floating cranes simultaneously meet the speed synchronization accuracy requirements and the safety requirements of the bridge structure during the synchronous lifting of the two bridges. By adopting a spatial coupling synchronous control strategy with the reference floating crane as the core and the subordinate floating cranes superimposed with wave field coupling correction and truss internal force feedback correction, it is possible to first eliminate the load oscillation caused by wave field interference generated by the operation of adjacent floating cranes and ensure the smooth and safe lifting operation. Then, by dynamically adjusting the lifting speed of each floating crane through real-time axial force monitoring of the transverse connecting truss, the load distribution imbalance problem during the lifting of the two bridges can be corrected in a timely manner. Thus, the four floating cranes can simultaneously achieve the dual goals of high-precision speed synchronization and the structural safety of the bridge during the synchronous lifting operation. This avoids the risk of lifting instability caused by hydrological disturbances and prevents damage to the connecting truss and box girder structure caused by uneven load distribution. It significantly improves the construction safety, operational stability and control reliability of the synchronous dismantling of large sections of the navigation channel.

[0012] Furthermore, in S1, the approach bridge is divided into a shore-side approach bridge section and a water-based approach bridge section, forming a comprehensive zoning system with the non-navigable and navigable openings of the main bridge; the construction method of the cross-bridge foundation coupling settlement prediction model is as follows: the foundation coupling coefficient between the two bridge piers is determined based on the ratio of the minimum center distance between the two bridge piers to the width of the pier foundation, and the ratio of the compression modulus of the pier bearing layer to the elastic modulus of the foundation concrete; the foundation coupling coefficient reflects the degree of impact of foundation rebound on the settlement of adjacent piers after the bridge is dismantled and unloaded, the softer the foundation and the closer the two bridges are, the larger the coupling coefficient; the shore-side approach bridge section is dismantled using a truck crane in conjunction with cutting, and the water-based approach bridge section is dismantled using a floating crane in conjunction with cutting. The dismantling of the approach bridge is carried out simultaneously with the dismantling of the corresponding main bridge section, and the dismantling rate is constrained by the foundation coupling coefficient; by... The entire bridge is divided into a comprehensive zoning system comprising approach bridges on the shore, approach bridges in the water, non-navigable spans, and navigable spans. A foundation coupling settlement prediction model is constructed based on bridge spacing, foundation dimensions, and geological and structural material parameters to determine the foundation coupling coefficient. This model accurately quantifies the additional settlement and tilting impact of foundation rebound caused by the initial bridge demolition and unloading on adjacent piers, enabling proactive risk assessment and control during demolition. Furthermore, differentiated demolition equipment, such as truck cranes and floating cranes, is adapted to the characteristics of each zone, allowing for simultaneous operation of the approach bridges and corresponding main bridge sections. The demolition rate is uniformly constrained by the foundation coupling coefficient, ensuring a balanced and coordinated demolition pace and foundation unloading across the entire area. This improves the targeted nature and efficiency of zoning construction while effectively suppressing uneven settlement and stress imbalance between parallel piers, ensuring a safe, controllable, and coordinated demolition process.

[0013] Furthermore, in S3, the cutting line is positioned using prestressed anchor head non-destructive testing technology to ensure a safe distance of no less than 0.5 meters between the cutting line and the nearest anchor head. The method for dividing the cutting unit is as follows: In the independent dismantling mode without the installation of the transverse connecting truss, the cutting unit only contains a single cantilever construction segment; in the collaborative dismantling mode with the installation of the transverse connecting truss, since the transverse connecting truss provides a load transfer path for the bridge span, the weight of the cut box girder segment is transferred to the adjacent bridge span through the truss section, reducing the unbalanced bending moment borne by a single pier. Therefore, the cutting unit can be extended to include two adjacent cantilever construction segments and the intermediate closure segment, and the total weight of the cutting unit does not exceed the combined rated lifting capacity of the two floating cranes. The cutting sequence is to proceed from the middle of the span towards both ends, first releasing the maximum positive bending moment stress at the middle of the span. Using prestressed anchor head non-destructive testing technology to locate the cutting line and maintain a safe distance of no less than 0.5 meters can effectively avoid damage to the prestressed system during cutting operations, ensuring structural safety and construction controllability during the demolition process. Under the condition of installing transverse connecting trusses to form a synergistic force, the cutting unit is expanded from a single cantilever segment to a large segment of two adjacent cantilever construction segments plus an intermediate closure segment. This can significantly reduce the unbalanced bending moment of a single pier by utilizing the load transfer path of the bridge, while maximizing the single lifting volume within the rated lifting capacity range of the double floating cranes. With the cutting sequence advancing from the middle of the span to both ends, the maximum positive bending moment stress at the middle of the span can be released first, further optimizing the structural stress state during the demolition process, and achieving comprehensive benefits of safer demolition operations, higher lifting efficiency, and more reasonable structural stress.

[0014] Furthermore, during non-closing periods, hoisting ropes are pre-installed on the sections of the two old bridges to be demolished and connected to the corresponding floating crane hooks. The ropes are laid along paths that avoid navigation clearance and are kept slack. The initial duration of intermittent closure windows is determined based on a positive correlation with the foundation coupling coefficient, and dynamically determined during construction based on real-time monitoring of the coupled settlement rate of the foundations of the two bridges: when the coupled settlement rate is lower than the preset stability threshold, the standard closure window duration is adopted; when the coupled settlement rate is higher than the preset stability threshold, the closure window duration is shortened and the closure frequency is increased to avoid excessive uneven settlement of the foundation during a single closure period, which could lead to excessive stress on the transverse connecting truss members. The work procedures within each closure window are optimized using the critical path method, and the total work time for a single closure does not exceed 180 minutes. By completing the installation of hoisting ropes and connection to hooks in advance during the navigation period, and ensuring that the ropes avoid the navigation clearance and remain slack, the preparation time after the closure of the waterway can be significantly reduced, improving the utilization rate of the closure window. The initial closure window duration is set in conjunction with the foundation coupling coefficient and dynamically adjusted according to the real-time coupling settlement rate. When the settlement is stable, the standard duration can be used to ensure operational efficiency, while the duration can be shortened and the frequency increased when the settlement is rapid. This effectively avoids the transverse connecting truss from exceeding the stress limit due to excessive uneven settlement of the foundation within a single closure. At the same time, by optimizing the procedures within the window using the critical path method and controlling the total duration of a single closure within 180 minutes, efficient coordination between demolition operations and navigation organization can be achieved while ensuring compliance with navigation control and structural safety, significantly improving the overall construction continuity and safety.

[0015] Furthermore, in S3, the non-navigable passage is divided into a bank-side section, a shallow water section, and a deep water section according to the terrain and water depth. These sections are respectively cut and dismantled using truck cranes, floating cranes, and floating cranes in conjunction with temporary support brackets beside the piers. The adaptive control strategy for the dismantling rate of the two bridges during the dismantling process of the non-navigable passage is as follows: The settlement difference between the corresponding piers of the two bridges is collected in real time. When the settlement difference exceeds a preset differential settlement threshold, a rate adjustment is triggered—the dismantling rate of the side with greater settlement is reduced to the product of the base rate and an attenuation coefficient linearly determined based on the settlement difference. The attenuation coefficient = 1 - (settlement difference / ... Differential settlement warning thresholds are set, ranging from 0.2 to 0.8. The demolition rate on the side with smaller settlement remains constant at the baseline rate until the settlement of the foundations on both sides becomes consistent before resuming synchronous demolition. Overlapping work areas are set up in adjacent sections to ensure continuous demolition operations for the two old bridges. Non-navigable channels are divided into bank-side sections, shallow water sections, and deep water sections, and differentiated demolition processes using truck cranes, floating cranes, and temporary support brackets beside piers are adopted. This ensures that equipment selection is highly matched with the site topography and water depth conditions, improving operational adaptability and demolition efficiency. A settlement-based approach is adopted. The adaptive control strategy for the demolition rate of the two bridges, which linearly determines the attenuation coefficient based on the settlement difference, can accurately reduce the demolition rate on the side with larger settlement when the settlement difference exceeds the threshold. This quickly converges the uneven settlement on both sides, avoids foundation stress imbalance and overload of the transverse connecting trusses, and ensures structural safety during the demolition process. The overlapping operation area set up in adjacent sections can ensure continuous connection of the demolition procedures of the two old bridges without any pauses or gaps. This enables efficient, safe, and coordinated progress of the demolition construction of non-navigable passages, while keeping the foundation stress and structural deformation under control.

[0016] Furthermore, in S3 and S4, a barge linked to the floating crane is arranged below each segment of the old bridge to be demolished; during the navigation closure period, the barge is moored below the cutting position, and cutting debris and dust are collected in real time through its onboard protective canopy and collection device; during the non-navigation closure period, the barge is moved to the side of the waterway and moored; the barge is also used as a temporary water operation platform for the transverse connecting truss: after each cutting unit is lifted out of the navigation channel, the construction personnel board the transverse connecting truss via the barge, dismantle the truss segment that has lost the support of the box girder, and the dismantled truss components are directly loaded onto the barge for transport away, so that the box girder lifting and truss dismantling are completed in a continuous flow within the same navigation closure window. Barges linked to floating cranes are deployed below the sections to be demolished. During navigation closures, these barges can dock at work sites and utilize protective canopies and collection devices to collect cutting debris and dust in real time, preventing waterway pollution and meeting environmental protection requirements. During non-navigation closures, they can be moved to the side of the waterway without affecting normal navigation. At the same time, the barges also serve as temporary floating work platforms for transverse connecting trusses. This allows for the completion of box girder lifting, truss section demolition, and transfer within the same navigation closure window, saving the cost and time of setting up dedicated work platforms, improving the utilization rate of navigation closure windows and construction efficiency, and achieving integrated and coordinated advancement of floating demolition operations, environmental protection and control, truss demolition, and component transfer.

[0017] Furthermore, in S5, the coordinated strategy for dismantling the main piers of the two bridges is as follows: For adjacent main piers arranged along the same line, the upper half of the main pier on the side with a larger foundation coupling coefficient is dismantled first, followed by the upper half of the main pier on the opposite side. This alternating top-down segmented cutting ensures symmetrical unloading of the foundation on both sides. During the dismantling process, the tilt rate of the remaining pier is continuously monitored. When the tilt rate exceeds the preset tilt warning threshold, the dismantling operation of the pier with faster unloading is suspended, and the dismantling of the pier on the opposite side is resumed synchronously after the pier on the opposite side has been dismantled to the same height. Before dismantling the underwater pier cap, the remaining steel wire ropes and temporary anchoring facilities are cleaned up, and then underwater segmented cutting and hoisting are used. The main pier dismantling is carried out by alternating segmented cutting according to the foundation coupling coefficient and coordinated dismantling from top to bottom. The strategy ensures that the unloading of the foundations of adjacent main piers on the same line of the two bridges remains symmetrical and balanced, effectively suppressing the tilting and additional settlement of the piers caused by excessively rapid unloading on one side. Real-time monitoring of the pier tilt rate during dismantling and dynamic pausing and synchronous progress ensure that the pier posture and stress remain within a safe and controllable range, avoiding the risk of structural instability. Before dismantling the underwater pier cap, residual steel wire ropes and temporary anchoring facilities are cleared, and then underwater segmented cutting and lifting are carried out. This not only prevents safety hazards such as hooking and jamming during the lifting process, but also ensures smooth underwater operations and thorough dismantling of the pier cap. The overall dismantling process of the main piers and pier caps is safe, orderly, coordinated and controllable, while minimizing disturbance to the riverbed foundation and surrounding structures.

[0018] Furthermore, prior to the execution of S3, water depth mapping was conducted along the operating routes of the four floating cranes. Sections with insufficient water depth (less than the crane's operating draft plus a 1.5m safety margin) were precisely dredged and re-measured for verification. A bridge structure health monitoring network was established on the two existing bridges. This network included settlement sensors positioned on the tops of each pier, axial force sensors positioned in the middle of each member of the transverse connecting truss, and stress sensors positioned near the box girder cutting sections. Data from all sensors was aggregated into a unified monitoring platform. When the value or rate of change of any monitored item exceeded the corresponding warning threshold, a dismantling operation pause command was automatically triggered, and the required adjustment to the dismantling rate or truss configuration was indicated. During the cross-operation of the new and old bridges, a rigid protective net with a height of no less than 6 meters was installed in the work area, and dedicated safety inspectors were assigned to monitor the operation status in real time. Alternating between different ground coupling coefficients was employed. The segmented cutting and top-down coordinated dismantling strategy for the main piers ensures that the unloading of the foundations of adjacent main piers on the same line of the two bridges remains symmetrical and balanced, effectively suppressing pier tilting and additional settlement caused by excessively rapid unloading on one side. Real-time monitoring of the pier tilt rate and dynamic pausing and synchronous progress during dismantling ensures that the pier posture and stress remain within a safe and controllable range, avoiding the risk of structural instability. Before dismantling the underwater abutment, residual steel wire ropes and temporary anchoring facilities are cleared, and then underwater segmented cutting and lifting are carried out. This not only prevents safety hazards such as hooking and jamming during the lifting process, but also ensures smooth underwater operations and thorough abutment dismantling. The overall dismantling process of the main piers and abutments is safe, orderly, coordinated, and controllable, while minimizing disturbance to the riverbed foundation and surrounding structures. Attached Figure Description

[0019] Figure 1 This is a flowchart of the method for dismantling continuous box girders to ensure navigation.

[0020] Figure 2 This is a schematic diagram of the overall structural elevation of the continuous box girder bridge spanning the waterway.

[0021] Figure 3 This is a schematic diagram showing the zoning of navigation channels and non-navigation channels, as well as the segment division of box girder sections.

[0022] Figure 4 This is a schematic diagram of the cutting unit grouping and demolition construction layout.

[0023] Figure 5 This is a schematic diagram of the symmetrical lifting elevation of the double floating cranes in the navigation channel. Detailed Implementation

[0024] Example 1 like Figure 1-5As shown in the figure, in this embodiment, a certain special large bridge is taken as the implementation object. This bridge is a double - span separated prestressed concrete continuous rigid - frame bridge. The main bridge span layout is 45m + 90m + 130m + 90m + 45m. The top plate width of the single - span box girder is 13.5m, the bottom plate width is 7m, the beam height at the main pier is 7m, and the beam height at the mid - span is 2.5m. The total length of the bridge is 1629.06m. It spans the third - level waterway of the main stream of the Huaihe River and passes through the first - level protected area of the drinking water source. During the construction period, the goals of zero road interruption, low impact on the waterway, and zero structural instability need to be achieved.

[0025] A new eight - lane expressway bridge in both directions is built on the downstream side of the double - span old bridge, and the prefabricated and assembled technology is adopted for rapid construction: The lower structure adopts φ1.5m rock - socketed cast - in - place bored piles + double - column piers. The center distance between the pile foundation and the main pier of the old bridge is kept at 15m to avoid direct coupling disturbance caused by sharing the river - bed foundation. The upper structure adopts 40m precast prestressed concrete small box girders. The layout of the prefabrication yard is carried out simultaneously with the structural inspection and demolition preparation work of the old bridge to speed up the cross - operation. After the new bridge passes the completion acceptance, the traffic of the entire G3 Beijing - Taipei Expressway is diverted to the new bridge for passage, and all lanes of the double - span old bridge are completely closed. During the diversion period, a rigid isolation protection net with a height not less than 6m is set up between the new and old bridges, and full - time safety inspectors are equipped to keep watch 24 hours a day to prevent unrelated personnel and vehicles from entering the old - bridge demolition area.

[0026] A detachable and assembled steel truss is installed between the top plates of the double - span old - bridge box girders to construct a cross - bridge space collaborative stress system: The main members of the truss adopt HN600×200H steel, the horizontal bracing and diagonal bracing adopt 20a channel steel. The span of a single truss is 10m, and one truss is arranged every 20m along the longitudinal direction of the bridge. The total input of steel is about 1200t. The two ends of the truss are anchored to the embedded anchor plates on the top plates of the old - bridge box girders by hinged joints. The anchor bars adopt φ25mm steel bars with a buried depth of 30cm. The hinged joints allow a relative displacement of ±5mm in the transverse direction of the double - span bridge to release temperature deformation, and at the same time restrain relative torsion to ensure reliable transfer of the cross - bridge load. All trusses are installed 7 days before the demolition of the non - navigable holes and are pre - loaded with a 1.2 - times design load for 24 hours. The maximum deformation of the truss ≤5mm is qualified. In the navigable - hole section, immediately after each box - girder unit is lifted off, the corresponding position truss is removed to avoid the safety risk caused by the truss forming a cantilever stress member.

[0027] Based on the condition of the granite residual soil foundation shared by the double - span old bridge, a cross - bridge foundation coupling settlement prediction model is established: According to the center distance of 10m between the main piers of the double - span bridge, the foundation width of 8m, the compression modulus of the bearing layer of 25MPa, and the elastic modulus of the foundation concrete of 3.45×10 4The calculated foundation coupling coefficient K = 0.35 was obtained from the MPa. It was predicted that after the bridge was dismantled and unloaded, the maximum additional settlement of the piers after dismantling would be 12mm, and the maximum tilt rate would be 0.08%, both lower than the allowable values ​​(settlement ≤ 20mm, tilt rate ≤ 0.2%) in the "Technical Specification for Highway Bridge Reinforcement Construction". Based on this, the initial dismantling rate was set as follows: two box girder segments per day for non-navigable sections, each segment weighing approximately 100t; one cutting unit for navigable sections, completed every three days. Simultaneously, warning thresholds were set: 15mm for pier top settlement, 8mm for differential settlement, 0.1% for pier tilt, and 100kN for axial force of the transverse connecting truss.

[0028] The non-navigable channel is divided into bank-side section, shallow water section, and deep water section according to water depth, and an appropriate construction process is adopted: Bankside section (water depth < 1m): Two 800t crawler cranes and a 55kW wire saw are used for cutting. The box girder is divided into 4m sections, with a maximum weight of 202.4t per section. Shallow water section (1m≤water depth<3m): One 500t floating crane is used in conjunction with a wire saw for cutting. The box girder is divided into sections of 3.5m each, with a single section weighing 177.1t. Deep water section (water depth ≥ 3m): Temporary support of φ820×10mm steel pipe piles is erected. The vibratory hammer drives the steel pipe piles 5m below the riverbed. The top of the pile is made of double HN600×200 steel as a distribution beam. The gap between the pile and the bottom plate of the box girder is filled with steel wedges. Four support piles are set for each pier. Then, the floating crane is used to cut and lift the piles.

[0029] The cutting line is located using non-destructive testing technology for prestressed anchor heads. The distance between the cutting line and the nearest prestressed anchor head is ≥0.5m to prevent damage to the prestressed system during cutting. The box girder is cut with a full-section wire saw. During cutting, the wire saw is placed above the box girder that has not been removed. The workers stand on the side of the cutting rope and are strictly prohibited from standing in the direction of the cutting rope's movement. After the cut is completed, the personnel and equipment must be immediately evacuated to a safe area.

[0030] The hoisting process utilizes a bottom-supported steel wire rope system. For non-navigable sections, four φ90mm fiber-core steel wire ropes (nominal tensile strength 1770MPa, breaking force 4730kN) are used, with a safety factor of 8. The angle between the steel wire rope and the box girder is controlled at 60°. Before hoisting, the floating / crawler crane is pre-lifted to 90% of the segment weight with lifting force before starting the wire saw cutting. After cutting, the section is smoothly lifted to the breakup zone on the bank for dismantling. During the dismantling process, GNSS settlement data of the corresponding pier tops of the two bridge spans are collected every 2 hours. When the settlement increment of a single pier is 5-8mm, the dismantling rate on that side is reduced to 0.7 times the baseline rate. When the settlement increment is >8mm or the differential settlement is >10mm, the dismantling operation on the side with the larger settlement is immediately suspended. Once the settlement difference between the two sides converges to within 5mm, synchronous dismantling operations are resumed to ensure dynamic balance of the foundation forces.

[0031] The cutting operation adopts wet construction method. Each wire saw is equipped with a high-pressure water gun to continuously spray water to reduce dust, and the dust emission concentration is ≤10mg / m³. A barge is arranged below the cutting position, and the cutting debris is collected in a fully enclosed protective shed. 100% of the debris is recycled and transported off-site, and it is strictly forbidden to fall into the Huaihe River drinking water source protection area. The construction area is fully enclosed with 2.5m high color steel tile fences, and 1.2m high guardrails and safety nets are set up for work near the edge. Workers must wear safety helmets, safety belts and life jackets.

[0032] Utilizing the load transfer capacity of the transverse connecting trusses, the navigation channel cutting unit is expanded into a large segment consisting of two adjacent cantilever segments plus a middle closure segment: the main span of 130m is divided into 3 extended units, with a maximum unit weight of 322.3t; the secondary span of 90m is divided into 2 extended units, with a maximum single unit weight of 286.5t; the cutting sequence proceeds from the middle of the span towards both ends, prioritizing the release of the maximum positive bending moment stress at the middle of the span to avoid cracking of the box girder.

[0033] Each section of the old bridge is equipped with two 500t floating cranes, with a total of four floating cranes symmetrically arranged on both sides of the section to be dismantled. The floating cranes have a rated lifting capacity of 460t with a boom length of 50m and an elevation angle of 65°, meeting the maximum lifting weight requirements. A spatial coupling synchronous control system is used to achieve precise and coordinated operation. The floating crane on the left side of the upstream side of the main bridge is the reference floating crane, and the other three are subordinate floating cranes. The system sampling frequency is 10Hz. Wave field coupling correction: Real-time acquisition of floating crane spacing, river wave velocity (1.2m / s), and wave frequency (0.8Hz). When the floating crane spacing is <50m, the load oscillation caused by wave field interference is calculated, and ±0.05m / s advance / lag compensation is applied to the lifting speed of the subordinate floating crane to eliminate the lifting sway caused by water surface fluctuations. Truss internal force feedback correction: Real-time acquisition of axial force of truss members. When the rate of change of axial force is >5kN / s, the axial force is automatically reduced and the lifting speed of the side floating crane is increased by 0.02m / s. The axial force is increased and the lifting speed of the side floating crane is decreased by 0.02m / s until the truss axial force returns to a safe range.

[0034] After communicating with the maritime authorities in advance, a 6-hour intermittent navigation closure will be implemented daily from 9:00 to 15:00, with each intermittent closure window lasting 180 minutes. The operational process has been optimized as follows: One hour before the closure of the waterway: the floating crane is in place, the barge is moored below the cutting position, and the maritime authorities deploy warning vessels and warning signs 500 meters upstream and downstream of the waterway; Navigation closure 0-30 minutes: 4 floating cranes simultaneously pre-lift, adjusting the wire rope tension to 90% of the segment weight; Navigation closure for 30-120 minutes: Start 16 55kW wire saws for simultaneous cutting, and monitor the stress of the box girder and the axial force of the truss in real time; Navigation closed for 120-150 minutes: After confirming that the cutting is completed, four floating cranes will lift the box girder segments simultaneously and place them into the broken area on the shore. Navigation closure for 150-180 minutes: The corresponding section of truss will be dismantled, the channel will be cleared, and the closure will be lifted. During non-closure periods, the floating crane will be moored on the south bank of the channel, with red and blue flashing lights installed on the edge of the pontoon. The hoisting wire rope will be pre-threaded and kept slack, avoiding the navigation clearance, so as not to affect normal navigation.

[0035] A barge linked to a floating crane is placed under each segment of the old bridge to be demolished. During the navigation closure period, it is moored under the cutting position and the cutting debris and wastewater are collected in a fully enclosed manner through the top protective canopy. During the non-navigation closure period, it is moved to the outside of the waterway and moored. The barge also serves as a platform for truss dismantling operations. Construction workers can board the truss via the barge, and the dismantled truss components are directly loaded and transported away, realizing a streamlined operation of box girder lifting and truss dismantling, and improving the utilization rate of the navigation closure window.

[0036] The main piers were cut into three sections using wire saws, each section being 3m high and weighing ≤150t, and were lifted by a 500t floating crane. The main piers of the two-span bridges were dismantled using an alternating strategy: the upper half of the left main pier was dismantled first, followed by the upper half of the right main pier, and the process was repeated from top to bottom to ensure symmetrical unloading of the foundation. During the dismantling process, the pier tilt rate was continuously monitored. When the tilt rate was >0.08%, the unloading operation on the faster side was immediately suspended, and synchronous dismantling was resumed after the heights of the two piers were equal.

[0037] The underwater pier was cut into sections, with each pier consisting of three sections, each weighing 307 tons. Before dismantling, divers used high-pressure water guns to clean the soil between the piles and any remaining steel wire ropes to prevent snagging during hoisting. The cutting was carried out using an underwater wire saw, and a floating crane lifted the cut pier sections off the water. All underwater operations were completed during the navigation closure period, and the waterway was cleared immediately after the operations to ensure that navigation conditions were restored.

[0038] Bridge structural health monitoring system Pier top settlement monitoring: GNSS sensors are deployed on the top of all main piers and transition piers, with a sampling frequency of 1Hz; Truss internal force monitoring: Axial force sensors are installed in the middle of each main member of the truss, with a sampling frequency of 5Hz; Box girder stress monitoring: Concrete strain gauges are installed near the cutting section, with a sampling frequency of 10Hz; all data are transmitted to the monitoring platform in real time. When any indicator exceeds the warning threshold, the system automatically triggers an audible and visual alarm and suspends the demolition work. Construction will resume after the hidden danger is eliminated.

[0039] Emergency support and construction during special seasons Emergency supplies: Equipped with 2 lifeboats, 60 life jackets, 20 dry powder fire extinguishers, 4 200kW backup generators, and 1 25t emergency crane; green rescue channels have been established with Huaiyuan County People's Hospital and Bengbu First People's Hospital. Emergency Drills: Organize one specialized emergency drill per month for electric shock, falls from heights, drowning, and lifting injuries to improve emergency response capabilities; Construction during the rainy season: Drainage ditches and sump pits are set up at the construction site, submersible pumps are provided for timely drainage, rain shelters are erected for mechanical equipment, and lightning protection and leakage prevention measures are taken for temporary power supply. Nighttime construction: The work area is fully covered by LED lighting, and the lighting voltage strictly follows the 220V / 36V / 24V classification standard. Red warning lights are set up at the edge of the opening. High-altitude and water-based operations are prohibited in severe weather.

[0040] After all demolition work is completed, the river channel will be cleaned to ensure that the water depth and clearance meet the navigation requirements; all construction waste at the construction site will be removed, and water and soil conservation and environmental protection measures will be implemented; and the demolition work will be completed and accepted only after joint inspection by the supervision unit, the owner unit, and the maritime department, and the demolition work meets the design requirements and safety standards.

Claims

1. A method for dismantling continuous box girders to ensure navigation, characterized in that, Includes the following steps: S1. Construction of the new bridge in advance: A new bridge will be built outside one of the old bridges. After the new bridge is completed, inspected and ready for traffic, all traffic on the two parallel and independent continuous box girder old bridges will be closed. A transverse connecting truss will be installed between the two old bridges to connect the two independent box girder bridges into a spatially coordinated force-bearing system. The transverse connecting truss will be arranged at intervals along the longitudinal direction of the bridge at the top plate of the box girder between adjacent piers, so that the two bridges will form a cross-bridge load transfer path during the demolition process. S2. Foundation Coupling Determination: Based on the condition that the two bridges share the same riverbed foundation, a foundation coupling settlement prediction model for the bridge is established to predict the impact of unloading the first bridge demolition on the tilting and additional settlement of the piers of the second bridge demolition, and to determine the initial matching relationship of the demolition rates of the two bridges. S3. Collaborative Demolition of Non-Navigation Channels of Two Old Bridges: The structures within the non-navigation channel ranges of the two old bridges are simultaneously cut and lifted in sections, and the demolition work is carried out before the corresponding navigation channel demolition work. During the demolition process, the differential settlement of the corresponding pier tops of the two bridges is monitored in real time. When the settlement increment of any pier exceeds the preset settlement warning threshold, the demolition rate on both sides is dynamically adjusted to keep the foundation of the two bridges in dynamic equilibrium. S4. Coordinated Cutting and Synchronous Lifting of Two Existing Bridges' Navigation Channels: Within the navigation channel area, wire saws are used to cut sections along a pre-set cutting line; utilizing the load transfer capacity of the transverse connecting trusses, the length of the cutting unit is extended to include two adjacent cantilever construction segments and the intermediate closure segment; each existing bridge is equipped with two floating cranes symmetrically arranged on both sides of the segment to be demolished, and the four floating cranes adopt a spatial coupling synchronous control system. Based on traditional speed synchronization and load distribution control, the system takes into account the wave field interference effect generated by the operation of adjacent floating cranes and spatially transfers and corrects the lifting speed compensation of each floating crane; during the same intermittent navigation closure period, the two existing bridges simultaneously carry out symmetrical lifting operations, lifting the cut box girder segments away from the bridge position at the same time. S5. Coordinated dismantling of main piers and abutments of the two bridges: The adjacent main piers of the two bridges, which are arranged on the same line, are dismantled alternately by cutting them in sections from top to bottom, so that the unloading of the foundation on both sides is symmetrical and balanced; the abutments in the water are cut and lifted off by underwater sections.

2. The method for dismantling continuous box girders to ensure navigation as described in claim 1, characterized in that: In S1, the transverse connecting truss adopts a detachable and assembled steel truss. Each truss is anchored at both ends to the pre-embedded anchor plates on the top plates of the box girders of the two old bridges through hinged nodes. The hinged nodes allow the two bridges to undergo limited relative displacement in the transverse direction to release temperature and shrinkage deformation, while restraining relative torsion and ensuring the reliability of load transfer across the bridge. The timing for installing the transverse connecting trusses is as follows: all trusses are installed before the dismantling of non-navigable passages begins; for each section of the navigable passage where a cutting unit is lifted, the corresponding transverse connecting truss is dismantled, allowing the dismantling of trusses to proceed alternately with the dismantling of box girders, thus preventing the trusses in the dismantled box girder sections from becoming cantilever load-bearing components; during the construction of the new bridge, the old bridge on the right side maintains two-way two-lane traffic and normal navigation of the waterway; the new bridge adopts a prefabrication and assembly construction process, with the construction of its substructure and the structural testing of the old bridge proceeding simultaneously, and the prefabrication of the superstructure and the preparation work for the dismantling of the old bridge carried out in parallel.

3. The method for dismantling a continuous box girder to ensure navigation as described in claim 1, characterized in that: In S4, the control strategy of the spatial coupling synchronous control system is as follows: any floating crane located upstream of the main bridge is taken as the reference floating crane, and the other three are subordinate floating cranes; the reference floating crane operates according to the preset lifting speed curve, and each subordinate floating crane tracks the reference speed and adds two corrections: the first is the wave field coupling correction between adjacent floating cranes. When the distance between any two floating cranes is less than the preset coupling distance threshold of 30-50m, the attenuation and phase difference of the wave field when it is transmitted from one floating crane to another are calculated according to the water surface distance between the two floating cranes, the river wave speed and the wave frequency. Based on this, the lifting speed of the subordinate floating cranes is advanced or delayed to compensate for the load oscillation caused by wave field interference. The second item is the feedback correction of the internal force of the transverse connecting truss. The axial force of each member of the transverse connecting truss is collected in real time. When the rate of change of the axial force of any member exceeds the preset safe rate of change, it is determined that there is an imbalance in the load distribution during the lifting process of the two bridges. The axial force is automatically reduced to increase the lifting speed of the side floating crane and the axial force is increased to reduce the lifting speed of the side floating crane until the axial force of the member is restored to the safe range. Through the above two corrections, the four floating cranes simultaneously meet the speed synchronization accuracy requirements and the safety requirements of the bridge structure during the synchronous lifting of the two bridges.

4. The method for dismantling a continuous box girder to ensure navigation as described in claim 1, characterized in that: In S1, the approach bridge is divided into the shore approach bridge section and the underwater approach bridge section, forming a full-area zoning system with the non-navigable and navigable openings of the main bridge; the construction method of the cross-bridge foundation coupling settlement prediction model is as follows: the foundation coupling coefficient between the two bridge piers is determined based on the ratio of the minimum center distance between the two bridge piers to the width of the pier foundation, and the ratio of the compression modulus of the pier bearing layer to the elastic modulus of the foundation concrete; The foundation coupling coefficient reflects the degree of impact of foundation rebound on the settlement of adjacent piers after the bridge is dismantled and unloaded. The softer the foundation and the closer the two bridges are, the larger the coupling coefficient is. The approach bridge section on the shore is cut and dismantled with the help of a truck crane, and the approach bridge section in the water is cut and dismantled with the help of a floating crane. The dismantling of the approach bridge is carried out simultaneously with the dismantling of the corresponding main bridge section, and the dismantling rate is constrained by the foundation coupling coefficient.

5. The method for dismantling a continuous box girder to ensure navigation as described in claim 1, characterized in that: In S3, the cutting line is located using prestressed anchor head non-destructive testing technology to ensure a safe distance of no less than 0.5 meters between the cutting line and the nearest anchor head. The method for dividing the cutting unit is as follows: In the independent dismantling mode without the installation of the transverse connecting truss, the cutting unit contains only a single cantilever construction segment; in the collaborative dismantling mode with the installation of the transverse connecting truss, since the transverse connecting truss provides a load transfer path for the bridge span, the weight of the cut box girder segment is transferred to the adjacent bridge span through the truss section, reducing the unbalanced bending moment borne by the single pier. Therefore, the cutting unit can be extended to include two adjacent cantilever construction segments and the intermediate closure segment, and the total weight of the cutting unit does not exceed the combined rated lifting capacity of the two floating cranes. The cutting sequence is to advance from the middle of the span to both ends, first releasing the maximum positive bending moment stress at the middle of the span.

6. The method for dismantling a continuous box girder to ensure navigation as described in claim 1, characterized in that: During non-closing periods, hoisting ropes are pre-installed on the sections of the two old bridges to be demolished and connected to the corresponding floating crane hooks. The ropes are laid in a slack state, avoiding the navigation clearance. The initial duration of intermittent closure windows is determined based on the positive correlation of the foundation coupling coefficient. During construction, the duration is dynamically determined based on the real-time monitoring of the coupled settlement rate of the foundation of the two bridges. When the coupled settlement rate is lower than the preset stability threshold, the standard closure window duration is adopted. When the coupled settlement rate is higher than the preset stability threshold, the closure window duration is shortened and the closure frequency is increased to avoid excessive uneven settlement of the foundation during a single closure period, which could lead to excessive stress on the transverse connecting truss members. The work procedures within each closure window are optimized using the critical path method, and the total operation time for a single closure does not exceed 180 minutes.

7. The method for dismantling a continuous box girder to ensure navigation as described in claim 1, characterized in that: In S3, the non-navigable passage is divided into bank-side section, shallow water section, and deep water section according to topography and water depth. These sections are dismantled using truck cranes, floating cranes, and temporary support brackets beside the piers, respectively. The adaptive control strategy for the dismantling rate of the two bridges during the non-navigable passage dismantling process is as follows: The settlement difference between the corresponding piers of the two bridges is collected in real time. When the settlement difference exceeds a preset differential settlement threshold, a rate adjustment is triggered—the dismantling rate on the side with greater settlement is reduced to the product of a baseline rate and an attenuation coefficient linearly determined based on the settlement difference. The attenuation coefficient = 1 - (settlement difference / differential settlement warning threshold), with a value range of 0.2-0.

8. The dismantling rate on the side with less settlement remains unchanged at the baseline rate, allowing the foundation settlement on both sides to become consistent before resuming synchronous dismantling. An overlapping work area of ​​5-10m in length is set up between adjacent sections to ensure continuous dismantling operations of the two old bridges.

8. The method for dismantling a continuous box girder to ensure navigation as described in claim 1, characterized in that: In S3 and S4, a barge linked to a floating crane is placed below each segment of the old bridge to be demolished. During navigation closure periods, the barge is moored below the cutting position, and its onboard protective canopy and collection device collect cutting debris and dust in real time. During non-navigation closure periods, the barge is moved to the side of the channel and moored. The barge also serves as a temporary floating platform for the transverse connecting trusses: after each cutting unit is lifted from the navigation channel, construction workers board the transverse connecting trusses via the barge, dismantle the truss segments that have lost their box girder support, and the dismantled truss components are directly loaded onto the barge for transport away, thus achieving a seamless process of box girder lifting and truss dismantling within the same navigation closure window.

9. A method for dismantling a continuous box girder to ensure navigation as described in claim 1, characterized in that: In S5, the coordinated strategy for dismantling the main piers of the two bridges is as follows: For adjacent main piers arranged on the same line, the upper half of the main pier on the side with a larger foundation coupling coefficient is dismantled first, and then the upper half of the main pier on the opposite side is dismantled. The dismantling is carried out alternately from top to bottom in segmented cutting to keep the unloading of the foundation on both sides symmetrical and balanced. During the dismantling process, the tilt rate of the remaining pier is continuously monitored. When the tilt rate exceeds the preset tilt warning threshold, the dismantling operation of the pier with faster unloading is suspended, and the dismantling operation of the pier on the opposite side is resumed synchronously after the pier on the opposite side is dismantled to the same height. Before dismantling the underwater pier cap, the remaining steel wire ropes and temporary anchoring facilities are cleaned up, and then underwater segmented cutting and hoisting are used.

10. A method for dismantling a continuous box girder to ensure navigation as described in claim 1, characterized in that: Before the execution of S3, water depth mapping was carried out along the operation route of the four floating cranes. Sections with insufficient water depth (less than the operating draft of the floating crane plus a safety margin of 1.5m) were precisely dredged and re-measured for verification. A bridge structure health monitoring network was established on the two old bridges. The monitoring network includes: settlement sensors arranged on the top of each pier, axial force sensors arranged in the middle of each member of the transverse connecting truss, and stress sensors arranged near the cut section of the box girder. Data from all sensors is aggregated into a unified monitoring platform. When the value or rate of change of any monitored item exceeds the corresponding warning threshold, a dismantling operation is automatically triggered, and the dismantling rate or truss configuration scheme that needs to be adjusted is prompted. During the cross-operation of the new and old bridges, a rigid isolation and protection net with a height of not less than 6 meters is set up in the work area, and a dedicated safety inspector is assigned to monitor the operation status in real time.

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