Synchronous Jacking Bearing Replacement

With the increasing demands of modern traffic, the frequency of reinforcement work for highway bridges and overpasses also rises. Traffic capacity can be improved by adding additional piers and distributing the bridge load more evenly across the existing ones.

A commonly used solution in bridge maintenance and stabilization involves using ultra-thin hydraulic jacks or self-locking hydraulic jacks to lift the bridge superstructure, replace the bearing pads, and finally lower it onto the new bearing positions.

When lifting a large object, multiple hydraulic jacks need to be placed at various support points underneath the load for synchronous lifting. This is particularly crucial when the center of gravity is not central or the object is irregularly shaped, to prevent differential lifting and resulting eccentric loading. Strict requirements for synchronous hydraulic jacks are essential in such cases.

Effectively utilizing synchronous lifting hydraulic jacks is key knowledge required during operation.

To achieve synchronous lifting with multiple hydraulic jacks, the primary step is to use jacks of the same model. Our hydraulic jacks, used for bridge lifting, are safe and convenient.

1. Using a Flow Divider Valve: The flow from the electric pump is distributed to multiple hydraulic jacks via a divider valve.

During lifting, if one jack is observed to be moving faster than others, the flow control for that specific jack on the divider valve is adjusted, reducing its flow supply. This slows down the faster jack, achieving synchronization with the others.

Operators need to continuously observe the lifting status of each jack and adjust the flow divider valves accordingly based on these observations.

Method 1: Single-Point Control, Multi-Point Collaboration Synchronous Lifting Solution:

One pump controls one jack, using identical pump models. The number of such setups corresponds to the number of lifting points required. Each point requires one operator.

Since identical hydraulic jacks and pumps are used, the flow provided to each jack over the same time period is equal, resulting in identical lifting speeds.

Method 2: PLC-Controlled Synchronous Lifting Hydraulic Jack Solution:

A displacement sensor is installed on each hydraulic jack. The sensor collects displacement data and feeds it back to a computer. The computer program calculates the required oil supply for each jack based on this data. Through continuous calculation over time, precise oil supply to each jack is achieved, ensuring uniform lifting speeds. Utilizing program control with PLC and sensors eliminates human operational uncertainties, typically achieving synchronization errors within 1mm. This method is suitable for applications requiring high synchronization precision like bridge lifting.

Technical Principle of Synchronous Lifting

The synchronous lifting system consists of an electric motor, high-pressure hydraulic pump, fuel tank and operating control system, jacks and hoses, flow dividers, etc.

The flow from one pump station is output through the flow divider and supplied to each actuating cylinder. The displacement control system, based on feedback signals from different measurement points, controls the load-bearing lifting speed of each cylinder, achieving the synchronous lifting action of the entire concrete continuous girder.

Key Technical Points for Synchronous Lifting

(1) The configuration of the new bearings must comply with design requirements and relevant industry regulations.
(2) The construction scheme for overall bearing replacement should be calculated to determine the sequence of replacement, the displacement for lifting/lowering the girder, and the construction steps.
(3) Temporary supports for lifting the girder must meet strength, stiffness, and stability requirements.
(4) Lifting and lowering of the girder must be performed according to design requirements.
(5) During bearing replacement, the bearing offset should be calculated based on ambient temperature, and construction should preferably be carried out under favorable temperature conditions.
(6) Measure the height difference between the original and new bearings. Adjust construction to ensure the girder and deck elevation meet the reinforcement design requirements.

Precautions for Synchronous Lifting Bearing Replacement Construction

1. Precautions for Bridge Bearing Replacement

Based on the design and field survey of the bridge structure, mechanical model calculations must be performed before synchronous lifting. Calculate the total lifting tonnage based on the mechanical model, considering an absolute weight coefficient and a safety factor for the jacks, to reasonably configure and combine the jacks.

During bridge lifting construction, comprehensive monitoring of the hydraulic jacks is essential. The purpose is to ensure the synchronization of the jacks, guarantee the lifting displacement meets design requirements, thus ensuring uniform force distribution and the safety of the bridge, construction personnel, and traffic.

Use appropriate lifting equipment determined through calculation to simultaneously lift several girders longitudinally and transversely to a certain height, ensuring all lifting equipment bears the load. When the girder is lifted to the design height, install temporary supports before placing the new bearings into position according to the design. The lowering process should be simultaneous, uniform, and slow, restoring the bearing reaction force.

A dedicated person should be assigned for unified command. Lifting or lowering must be simultaneous and uniform. During lifting, assign personnel to monitor the top of the slab, deck, and negative moment regions for any cracking. If cracking occurs, stop lifting immediately and take appropriate measures.

2. Key Points for Bridge Bearing Replacement Construction

The bridge bearing replacement must comply with current specifications. The bearing configuration shall follow design specifications. When replacing and installing various bearings, calculate and verify the bearing displacement.

Overall bearing replacement construction generally requires structural analysis to determine: the allowable distance from the jacking position to the original bearing; whether longitudinal connections between girders need to be released and the release locations; the sequence and height for lifting/lowering batches of girders; calculate the installation offset of the bearings based on the temperature during replacement.

Due to potential temperature differences between the original bearing installation and the replacement, the girder length may change, requiring corresponding adjustment of the new bearing position. Accurate calculation of the offset value is essential. If the difference is significant, it often necessitates increasing the area of the bearing pad stone.

Temporary supports for lifting the girder must be designed, and their strength, stiffness, and stability must meet code requirements. Preloading tests may be conducted if necessary.

Synchronous lifting of the girder superstructure is a crucial principle for ensuring safety during construction operations. The lifting height at each bearing location transversely across the girder slab should be the same. Longitudinally, the lifting height between different piers and abutments should not differ excessively.

Below, the construction process is detailed using a specific project case.

I. General Requirements

Considering the structural characteristics of this project and the actual traffic flow on the bridge deck, and to reduce capital investment and shorten the construction period, the proposed method is "synchronous lifting of the girder transversely, pier-by-pier longitudinally, without interrupting traffic." After lifting to the required position, the bearings and pad stones are replaced and repaired, and limiters are installed according to the design drawings. This means that during the bearing replacement process, synchronous lifting (and lowering) is performed sequentially for the bearings requiring replacement on each pier/abutment along the longitudinal direction. To ensure the structural safety of the girder and traffic safety during bearing replacement, all support points on the same pier/abutment must be lifted (or lowered) synchronously.

II. Main Material Requirements

Epoxy Structural Adhesive: Primarily used for leveling and local repairs. The performance indicators of the structural adhesive shall meet the requirements for Grade A adhesive in the "Specifications for Strengthening Design of Highway Bridges" (JTGT J22-2008), with tensile strength ≥40 MPa and compressive strength ≥70 MPa.

Bearings: Determined according to the specific bearing specifications for the slab girders. The positioning and installation of bearings shall follow the relevant requirements of the project's specific design drawings and also comply with standards such as "Highway Bridge Elastomeric Pad Bearings" (JT/T4-2004), "Series of Specifications for Highway Bridge Elastomeric Pad Bearings JT/T663-2006", "General Code for Design of Highway Bridges and Culverts JTG D60-2004", etc.

III. Synchronous Lifting (Dual Control) Equipment Requirements

① Bridge synchronous lifting is a precise and meticulous construction process. Besides a scientific and rational process flow, reliable and advanced lifting equipment is essential to ensure the safety of the bridge structure during lifting (and lowering). During bridge lifting, the force application and synchronization of the lifting system are crucial. The lifting system must strictly control the lifting force and displacement to ensure bridge structural safety; it must also strictly control the synchronization of each jack to ensure smooth lifting and avoid structural damage due to non-synchronization.

② To ensure the structural safety of the bridge during lifting in this project, according to the design requirements and the bridge's specific characteristics, the system will control the oil pressure of individual jacks by setting flow divider valves, and control displacement via displacement sensors (percentage).

IV. Synchronous Lifting Safety Requirements

Bridge synchronous lifting is a complex technical project where construction safety must be guaranteed. Safety during lifting includes: structural safety, equipment reliability, and the safety of construction personnel.

Ensure Girder Safety: Given the characteristics of the composite box girders, simply-supported T-beams, and deck continuity, significant forced displacements should be avoided during lifting to prevent inducing additional internal forces (e.g., increased forces at continuous deck sections), which could affect structural safety.

Ensure Local Safety at Lifting Points: During lifting, concentrated forces exist at the contact points on the girder and pier cap/abutment. These areas need reinforcement to avoid local damage during lifting, which could compromise the safety of the bridge span structure.

Ensure Construction Safety During Lifting: If a jack leaks oil or fails, it may cause uneven lifting of the girder longitudinally or transversely, potentially leading to girder overturning. Therefore, use safety steel plates for protection during lifting. Employ locking devices and alarm systems for the oil supply system to prevent overall girder overturning. Additionally, lifting should not be done in one step but in multiple stages.

V. Synchronous Lifting Construction Feasibility Requirements

Bridge lifting is a maintenance activity on an existing bridge with many constraints. Proposed schemes might be unfeasible in practice. Therefore, the lifting scheme must have good operability. Factors constraining operability include: jack size, lifting capacity, maximum lifting height, feasible jack placement locations, construction operating space, and requirements regarding traffic interruption.

Regarding jack size, capacity, height, placement, and construction space, flexible selection of jack types and support positions can be made based on on-site component dimensions and spatial constraints.

Vehicle impact crossing the bridge can damage jack cylinders and expansion joints (especially at abutments where lifting can create step misalignments). Therefore, to ensure lifting safety and normal equipment operation, consider restricting severely overloaded vehicles during this bearing replacement process. Implement appropriate traffic control if necessary.

VI. Jack and Temporary Support Layout

Before synchronous lifting construction for bearing replacement, determine the jack positions first according to the design drawings, and clean the areas. When installing jacks, place steel plates at the contact surfaces between the jack and the bottom of the box girder/pier cap to increase the local bearing area. The jack axis must be vertical, and the bottom surface of the plates must be horizontal. Use a level during installation for calibration, and grind the contact surfaces smooth.
If the jack or auxiliary support height is insufficient, use level and flat steel plates to shim between them, ensuring tight contact without gaps.

Install one tension-type displacement sensor per slab girder to achieve displacement synchronization control. During the load-holding phase, use steel plates of various specifications combined as temporary supports. These temporary supports must possess long-term stability to ensure bridge structural safety during this phase.

VII. Lifting Method

Adopt the method of "synchronous lifting of the girder transversely, pier-by-pier longitudinally, without interrupting traffic."

During lifting construction, restrict the passage of overloaded vehicles or implement speed limits to prevent accidents caused by jacks exceeding their design load.

According to design requirements, the main implementation stages are divided into four phases: Preparation Stage → Lifting Stage → Load-Holding Stage → Lowering Stage. The Load-Holding Stage includes three parts: Removing original bearings → Repairing pad stones/steel plates → Installing new bearings and leveling.

Use a synchronous lifting control system (e.g., EPS-24 point system), select suitable jacks. The lifting system monitors and dynamically regulates the pressure and displacement of each hydraulic cylinder in real-time.

According to design requirements, repair or replace severely damaged bearings.

Before formal lifting, perform trial weighing and trial lifting to check system operation and compare theoretical vs. actual lifting forces.

Lift in stages using micro-incremental, step-by-step lifting; lower in stages using micro-incremental, step-by-step lowering.

During the load-holding stage, use adjustable temporary supports with mechanical locking to prevent loosening. These supports must be stable for long durations to ensure structural safety.

Start lifting from the 0# Abutment, sequentially repair/replace severely damaged bearings, and finish at the other abutment.

VIII. Main Construction Process and Key Points

(1) Site Survey

Before formal construction, conduct a survey of the bridge requiring bearing replacement. Confirm basic information such as access routes, work platform setup, scaffold height, and the clear distance between girder and pier cap at piers/abutments.

(2) Construction Preparation

① Determine materials and equipment needed based on the design and site conditions. All incoming materials must have factory certificates and product quality inspection documentation. All incoming equipment should be within their valid calibration period. Key equipment like jacks and hoses should have backups.
③ Jack performance and dimensions must meet design and on-site installation requirements. Considering uneven load transfer during lifting, the effective lifting capacity of the jacks should be not less than 200 tons. The lifting system should have weighing capability, able to monitor and dynamically regulate the pressure and displacement of each hydraulic cylinder. In case of cylinder or circuit failure, it should automatically lock and maintain constant pressure (displacement). The system should achieve synchronous lifting/lowering in increments as small as 0.5mm, with synchronization error for jacks on the same circuit controlled within 0.5mm.
④ Before construction, organize technicians to thoroughly study the design drawings and synchronous lifting scheme, fully understanding the design and lifting intent. Collect relevant data and conduct a detailed site survey. Then, adjust and modify the specialized lifting scheme as necessary. Organize expert review if required before full-scale construction.
⑤ Conduct technical briefings for all construction personnel, ensuring everyone understands their roles and responsibilities are assigned.
⑥ Check if all components of the lifting system and subsystems are in normal, good condition. Check all components of auxiliary systems (temporary supports, grating scales, hydraulic oil, etc.). Resolve any issues promptly to avoid equipment failure during lifting.
⑦ The on-site command team consists of 1 Chief Commander and 1 Deputy Commander, responsible for overall on-site command. Under the command team, there are 5 functional groups: Scaffolding/Support Group, Monitoring Group, Control Group, Hydraulic Group, and Replacement Operation Group, each responsible for relevant tasks. Each functional group has a leader. These leaders, together with the Chief Commander, Deputy Commander, and Equipment Engineer, form the on-site command team.
⑧ Cooperate with the client for drawing review, communicating fully with the designer to understand the design intent.
⑨ Conduct random sampling inspection of newly arrived bearings with the supervision unit. Bearings can only be used after passing inspection.
⑩ After scaffold setup, clean debris around the bearings on the pier/abutment tops to ensure space for jack placement. Perform necessary leveling and repairs.

(3) Erecting Work Platforms

Use two methods for work platforms: 1) Using a bridge inspection vehicle as a platform, and 2) Erecting scaffolds on-site for work on pier tops, bearing replacement/repair, and monitoring. Determine scaffold height and plan dimensions based on actual pier height and operator convenience.

After personnel arrive on site, conduct initial inspections. Inspections are also needed during the bearing replacement process. Site inspection includes four aspects:

① Pier Column, Pier Cap, Abutment Check: Check for cracks or other defects to avoid adverse effects from existing issues on lifting safety. Compare conditions before and after construction to determine if new damage occurred.
② Bearing Check: Mainly inspect the defective bearings, but also check bearings on other piers. If other defects are found, notify the client and designer promptly. Mark the centerline position of the pad stone as a reference for restoration.
③ Girder Check: Mainly check webs, bottom slabs, and flange slabs for cracks. Record and mark existing defects in detail. Monitor crack width changes during replacement if necessary to ensure structural safety.
④ Expansion Joint Check: Ensure cleanliness within the expansion joints at lifting locations to prevent damage to rubber seals during lifting. Check for height differences across joints. If there are sudden changes in the cast-in-place concrete at transition piers, use shim height adjustments during bearing replacement to achieve a smooth transition.

(4) On-site Measurement

① After girder lifting and old bearing removal, first mark the bearing's plan position. Inspect the mortar bed under the bearing. If minor defects exist, repair and level directly with structural adhesive. Measure the clear distance Fd between the repaired mortar bed and the girder's embedded steel plate bottom (deducting the lifted height). If major defects exist that cannot be repaired/leveled with adhesive alone, report to the designer.
② If the repaired mortar bed level is below the target (Fd > F1), add stainless steel plates on top to ensure the thickness of the bearing pad under the girder's embedded plate is Hg = Fd - F1.
③ Before lifting, conduct a comprehensive inspection of the girder and its related upper/lower components to understand the current structural state and avoid safety hazards from existing defects. Similarly, conduct a comprehensive inspection after lifting, comparing it with the pre-lifting condition to assess if lifting caused damage. Implement repair measures if damage is found.

(5) Jack Placement

① The specific arrangement of lifting jacks shall follow the relevant design drawings.
② Place jacks according to design requirements. Use suitable jacks determined by calculation, with an effective lifting capacity ≥200 tons, possessing weighing and auto-locking functions.
Level the jack base at the designated positions on the girder bottom and pier cap using structural adhesive as required. Paste 1cm thick steel plates to distribute stress (ensure upper and lower plates are level). The jack base must be level. Use epoxy mortar to adjust for girder longitudinal/transverse slopes, ensuring jacks are installed vertically and securely for vertical force transmission.
Install one tension-type displacement sensor near each jack to enable system monitoring and control.
Install grating scales on the pier columns, with the reading head fixed at the top of the pier. Grating scale range should be ≥1200mm. This, combined with the sensors, forms a closed-loop displacement control system.

(6) Girder Lifting

① Lifting Equipment Installation:
A. Install the entire lifting system: control console, controller, displacement sensors, computer, pump station, oil circuits, jacks, grating scales, etc. Debug the synchronous lifting system, control system, hydraulic system, and monitoring system to ensure all are in good condition for normal operation during lifting.
B. Measure the elevation above and below the bearings as initial values for subsequent relative measurements, ensuring monitoring system accuracy.
C. The synchronous lifting system controls oil supply for synchronization. Each support point has solenoid valve groups and hydraulic control valve groups to ensure normal oil pressure, prevent pressure drop from leaks (hydraulic control valves lock to maintain pressure).

② Monitoring Equipment Installation:
Install displacement sensors, dial gauges, etc., to monitor and control the entire lifting process, ensuring safety.

③ Trial Lifting and Inspection:
A. Check all temporary structures, instruments, equipment, and personnel are in place. Record initial readings for all monitored objects.
B. Perform a trial lift before formal lifting. The trial lift mainly eliminates non-elastic deformation/settlement of the supports themselves. Stop when the main girder just begins to lift (e.g., 1mm). Hold pressure for 20 minutes, observing the girder and all jacks. Proceed only if no changes occur.

④ Formal Lifting:
A. Formal lifting uses graded loading, e.g., 1mm per step. Hold pressure for 5 minutes at the first step, 10 minutes for others. The final step's pressure is controlled by the target lift height. Keep longitudinal and transverse differential lifting within design limits.
B. After reaching the target height, if jacks lack self-locking, install temporary supports. Adjust support height to closely contact the girder bottom, maintaining jack oil pressure.
C. During lifting and load-holding, assign personnel specifically to monitor hinge joints and deck continuity sections. If cracking is observed, stop immediately. Use manual jack adjustment, reduce loading rate, slow pressure release, or slow lowering if needed.

(7) Load-Holding Stage Construction

Main tasks during this stage: Remove original bearings, repair defects in the supporting area of the girder bottom, repair bearing pad stones, repair original steel plates, install new bearings, and level bearings.

① Avoid impacting jacks during this stage. Strictly avoid mechanical collisions with the support system. Prefer small tools or manual chiseling for removing loose concrete. Avoid damaging original pier cap main/rebar during chiseling, minimizing vibration impact. Expose sound aggregate at the chiseled interface and remove dust. Derust and apply anti-rust treatment to exposed rebar.
② During concrete removal, avoid damaging other parts of the slab girder. Repair any local spalling with structural adhesive.
③ Protect the original bearing pad stone and steel plates during old bearing removal.
④ After lifting, paste stainless steel plates onto the girder bottom at abutment bearings. Minimize the adhesive layer thickness between the new plate and the original embedded plate, ensuring effective bonding.
⑤ If the pad stone is defective, the embedded plate is uneven with concrete, or the steel plate is defective, perform corresponding treatment after lifting and removing the old bearing. Derust, treat rust, or replace the original plate. Uneven embedded plates can cause bearing edge damage; remove protruding concrete to align with the plate. Level the pad stone below the bearing with high-grade epoxy mortar. Calculate the required additional height and use steel plates of suitable thickness for adjustment. After adjustment, install the new bearing. Before installation, measure the clear height at four points around the bearing perimeter, ensuring height difference ≤1mm between any two points.
⑥ After repair and leveling, the original girder bottom surface, pad stone top, or pier cap top should be level. Flatness variation in the bearing installation area should not exceed 1mm.
⑦ Before and after lifting, measure the girder bottom elevation at each lifted slab girder's bearing location. Ensure the final elevation matches the pre-lifting elevation after replacement.
⑧ Before replacement, mark the original position of the bearing on the pier cap. Install the new bearing as close to the original position as possible. If the original bearing was significantly misplaced, determine the new position based on the girder's embedded plate location after lifting.
⑨ Bearings in the same row on the same pier/abutment should align transversely, with equal perpendicular distance to the pier's transverse centerline. If originally misplaced, reposition based on the embedded plate.
⑩ Bearings in two rows on the same pier must have equal perpendicular distances to the pier's transverse centerline to avoid eccentric compression.
11. The new bearing model should match the as-built drawing model. If the actual bearing differs from the design, consider the actual situation and select the new model after comparative analysis.
12. The preferred ambient temperature for bearing installation is 20°C to prevent eccentric compression or excessive initial shear deformation. Ensure tight contact between the bearing and the upper/lower structures after installation, no voids. This design follows the original as-built drawings, which typically show the girder's embedded plate bottom (bearing top) as horizontal. If the site condition differs (plate not level), add leveling wedge-shaped stainless steel plates, bonded with epoxy to the embedded plate/concrete, strictly ensuring a level bearing top and dense installation. Spot-weld the perimeter of the new wedge plate after epoxy bonding.
13. Construction Sequence: After cleaning debris from the original voided bearing, measure the required thickness, subtract the bearing's compression amount, use combinations of 1.5mm, 2mm stainless steel plates, embed from the bottom, use a >2kg hammer on a special tool to drive the plates tight until the bearing closely contacts the girder bottom and plates can't be driven further. Then, use a >2kg hammer to strike the bearing horizontally; if no loosening or displacement, it's qualified. Document with photos.
14. After the leveling repair material reaches design strength, install the bearing. Other specific requirements for elastomeric bearing installation follow "Highway Bridge Elastomeric Pad Bearings" (JT/T4-2004).

(8) Lowering the Girder

To minimize error, lower the girder in one continuous operation for this project. Before lowering, complete all pad stone repairs and leveling, ensuring the clear distance between the repaired pad stone top and the girder bottom accommodates the bearing thickness. Then lower simultaneously. Key points for precision:

① Determine the total bearing compression – obtain from the manufacturer or refer to the manufacturer's compression test curve.
② Determine the repair标高 for the pad stone based on bearing compression, girder bottom elevation, bearing thickness, and leveling wedge thickness.
③ Lower the girder only after the pad stone repair material has fully cured. Utilize the bearing compression to restore the girder as close as possible to its original elevation.
④ During lowering, synchronously retract all jacks, strictly controlling simultaneous lowering at all support points on the same pier/abutment, managed by the computer-controlled synchronous lowering system. Lower in steps, e.g., 1mm per level, adhering to the dual-control principle (force & displacement), ensuring the girder returns to its pre-lifting elevation.
⑤ After bearing installation, remove shim plates and other temporary supports. Lower the girder synchronously using the same stages as lifting. Employ the same monitoring measures as during lifting to observe if bearing loads are uniform.
⑥ After lowering, measure the girder bottom elevation at all lifted points, ensuring it matches the pre-lifting elevation post-replacement.
⑦ After all work is complete and new bearings are verified correctly installed, wait for structural adhesive to reach strength. Clear debris from the girder. Activate the synchronous lifting system to load the jacks. Remove steel shims/temporary supports, then lower the girder. Maintain overall balance during lowering via the computer-controlled system. Perform synchronous lowering in multiple stages, opposite to the lifting sequence (e.g., 1mm/step), using the same monitoring measures. Control the descent per step until the girder is fully seated. Re-check for full contact between bearings and girder bottom; re-lift if necessary until requirements are met. Conduct a final bridge-wide inspection and measurement; address any issues promptly. After acceptance by relevant parties, finally dismantle the lifting system, support structures, limiters, scaffolds, etc.

(9) Quality Acceptance

According to "Technical Specification for Construction of Highway Bridge Strengthening" (JTG/T J23-2008), specifics are as follows:

① Basic Requirements:
A. Bearing material, quality, and specifications must meet design and code requirements. Install only after acceptance.
B. Leveling mortar under the bearing base plate shall meet design requirements, be densely poured without voids.
C. The vertical axes of all bearing components must align. If installation temperature differs from design, calculate and set the longitudinal pre-offset.
D. Bearings must not tilt, be unevenly loaded, or void. The PTFE sliding surface and stainless steel plate must be free of scratches/bruises, positioned correctly, and coated with silicone grease before installation.
② The position deviation for replaced bearings shall comply with the following table:

Table 4.1 Bearing Installation Measured Items

③ Appearance Assessment:
The bearing surface should be clean. Remove debris and dust near the bearings.