Shi Jiansheng
Xinfeng Branch of Ganzhou Highway Development Center
Abstract: A bridge project spans 1 highway and 1 railway, and its spanning beam adopts lateral cast-in-situ steel gate pier traversing construction technology. Focusing on the construction technology of its single-line cast-in-situ ballasted simply supported box girder, this paper combs and introduces the relevant technical points from the aspects of foundation treatment, steel gate pier assembly, beam pouring, and traversing in place. The results show that the lateral cast-in-situ traversing in-place process can complete the operation in a limited construction window, which can minimize the impact on the crossing line and have less operational risk.
Keywords: spanning beam; ballasted simply supported box girder; lateral cast-in-place; steel gate piers; traversing construction; technical research;
About author:SHI Jiansheng (1979—), male, undergraduate, engineer, engaged in highway and bridge construction management.
0 Introduction
In the context of rapid traffic development, a considerable number of new lines intersect with existing lines. In order to successfully complete the construction of these spanning projects, the construction personnel designed the cantilever construction method, the bracket cast-in-place method, the rotating cross-line method, the longitudinal pushing top method, and the lateral cast-in-place gate pier transverse displacement in-place method. The lateral cast-in-situ portal pier traversing in-place method passes the lateral forming girder, and the transverse movement pushes in place, especially suitable for the box girder engineering that crosses the existing line at a small angle, the lateral prefabrication of the construction method is the transverse crossing beam, and the transverse shift in-place operation is quickly completed in the window period, and the construction conditions are relatively simple, and the construction hidden danger is prevented to a greater extent, and the influence on the normal operation of the crossing line is minimized. The case project adopts this construction method, and successfully completes the lateral cast-in-situ and traversal displacement of the single-line cast-in-situ ballasted simply supported box girder. Here, the relevant technical points are sorted out and introduced from the aspects of foundation treatment, steel gate pier assembly, beam pouring, and traversal displacement in place, so as to provide technical reference for similar construction applications.
1 Project Overview
The case is a prestressed concrete simply supported box girder road and bridge, with a length of 1 650.2 m, in the 10~16# pier section, the steel gate pier is adopted to cross 1 highway and 1 railway at an angle of 8 degrees. The spanning beam is a single-line cast-in-situ ballasted simply supported box girder, with a width of 7.6 m and a length of 32.6 m, and the main beam in the middle span is 3.00 m wide and 2.64 m high. The top surface of the box girder is a double-sided herringbone slope, with a thickness of 0.70 m for the mid-span web and a thickness of 0.65 m for the fulcrum section. The steel cap beam of the pier is 28.5 m× 3.5 m× 2.5 m in length× width × high, and there is no shrinkage C40 concrete cast-in-the-wall pad stone. The steel gate pier column adopts a concrete-filled steel tube structure, and the column is slightly expanded with C40 concrete pouring. The height of the steel gate pier column is 11 m, the wall thickness is 2 cm, the outer diameter is Φ3 m, the steel pipe and the steel cap beam are firmly connected by welding, and the cross-beam is constructed by lateral cast-in-situ traversing method.
2. Lateral cast-in-situ transverse movement in place process
Operation plan: above the existing line, the steel cover beam and the steel portal pier column are assembled by hoisting. The cast-in-situ lateral support adopts the temporary steel pipe pier on the pier side and the full hall support of the disc buckle to realize the high-altitude cast-in-place operation next to the pier, in which the temporary steel pipe pier is consolidated in the adjacent pier body. The slideway is formed by H-shaped steel double-piece welding, and is installed on the steel portal pier and the temporary steel pipe pier. After the on-site pouring of the box girder was completed, the 32 m cast-in-situ beam was moved to the design position from the temporary steel pipe pier to the limited skylight point in a draggered manner with the help of a traversing slideway. After completing the lateral movement, the main beam is dropped by two jacks at each end at one end, and the deviation correction is carried out in combination with the jack and the stepping stone reaction support, and its operation process is shown in Figure 1.
Figure 1 Process flow diagram Download the original diagram
3. Technical points of lateral cast-in-situ transverse movement in place
3.1 Foundations and steel portal piers
(1) Ground treatment. The foundation bearing capacity of the steel pier column should not be less than 200 k Pa. The foundation bearing capacity of the laterally moving temporary pier should not be less than 200 k Pa. The base load under the buckle bracket should not be less than 150 k Pa. If the bearing capacity is insufficient, it should be treated by bored piles or replacement methods.
(2) Installation of steel portal pier. Install steel gate pier, treat the foundation, after completing the door pier column foot installation, install the door pier column and pour C40 concrete, then carry out steel cap beam assembly, then the steel cap beam is hoisted in place with the crawler crane of large tonnage, and finally weld and fix the steel gate pier column and the steel cap beam.
3.2 Temporary steel pipe piers and cast-in-place supports
3.2.1 Temporary steel pipe piers
The pipe pier takes the form of pipe pile, and the steel pipe specification is Φ630×10 mm, and after the installation of the steel pipe column is completed, the 20 b channel steel is double-welded to ensure its stability. The steel pipe pile is erected on the strip foundation, and the latter is reinforced with steel pipe pile, the steel pipe specification is Φ630×10 mm, the pile spacing is set to be 1.5 m, and the design position is driven into the design position by hydraulic vibratory hammer. After the installation of the temporary steel pipe pier is completed, a traversing slideway is installed at the top of the slideway, the standard section of the slideway is 2.5 m, and the slideway base is welded on the steel pipe pile. Install the slide above the slide and apply butter in advance on the contact surface between the slide and the slide [1].
3.2.2 Cast-in-situ beam buckle bracket
Table 1 shows the parameter configuration of the Φ60×3.2 mm disc buckle bracket.
Table 1 Parameter configuration of disc buckle bracket of cast-in-situ beam Download the original drawing
3.2.3 Stent preload
In this project, 1 m×1 m×1 m precast concrete blocks are used to carry out span preloading, and the maximum load weight is controlled at 1.2 times of the weight of the formwork and beams. When preloading, the load should be symmetrical to avoid uneven load affecting the support. During the preloading construction, step by step loading is adopted, and after the load reaches 60%, 80% and 100%, the loading is stopped and the settlement and displacement support are stopped, and the settlement and displacement of the bearing are continuously observed. The next level of load application is only allowed when the load is applied at each stage and there is no anomaly observed [2].
3.3 Traversing the main beam
3.3.1 Cascade girder cast-in-situ
Before the box beam cast-in-situ, a rubber plate is placed on the contact surface of the bottom of the box beam and the top plate of the slide, and after the erection support, the assembly template, the assembling steel bar, and the prestressing system of the cast-in-place beam are systematically assembled and meet the cast-in-place conditions, the beam body pouring construction can be carried out. During cast-in-place construction, it should be poured in layers, compacted, and watered and maintained after the completion of cast-in-situ.
3.3.2 Tensioning and grouting
After the beam is poured, the curing age expires, and the strength conditions meet the requirements, the beam tensioning operation can be carried out. Based on the jack calibration equation and the tensile stress design parameters, the applied tensile force and the expected gauge readings can be calculated, from which the prestress tension can be carried out. After the tensioning operation was completed, the end capping and grouting construction were carried out within 48 h. After completing the above-mentioned process of cast-in-place beam, the bottom die and the disc buckle bracket that affect the traversing operation can be disassembled.
3.4 Beam traversing system
3.4.1 Slides
The slides are welded from steel plates, and the individual slides are 760 mm wide and 660 mm long, and are assembled above the slideway located at the bottom of the box beam. In order to ensure that the bottom of the box beam can be evenly stressed, a rubber plate is placed on the surface of the steel plate at the top of the slide. The steel plate at the bottom is made into a groove and embedded in the MGE slide plate with a thickness of 25 mm. In order to facilitate the retention of lubricating oil, the contact surface between the top plate of the slideway and the slide plate should have a small round hole. The side of the slide is equipped with a 55 mm high anti-release groove plate to play the role of deviation correction and guidance, and the inner side also needs to be inlaid with a slide plate, and the two slides are connected by two connecting rods made of 10 cm channel steel.
3.4.2 Slideway structure
The slideway is composed of two 60 cm wide sections of steel and 0.4 m thick stainless steel plates, and the slideway is composed of non-standard sections of 1.87 m, 1.63 m and 0.58 m and standard sections of 2 m, 2.5 m and 5 m, which are spliced by M20 high-strength bolts to form the required length. If there is a longitudinal slope on the line, the height of the steel pipe pier can be temporarily adjusted to prevent longitudinal movement when the cast-in-place beam moves laterally [3].
3.4.3 Reaction seat
It is constructed of welded 1.20 cm and 1.60 cm steel plates. In order to facilitate the passage of the steel strand, a round hole of Φ8 cm is required in the pressure plate. The reaction seat is connected to the traction equipment by 4 M20 bolts. The steel plate below the reaction seat should be welded firmly with the roof plate of the steel cap beam, and the reaction seat at the top of the concrete pier can be welded with chemical anchors.
3.4.4 Traction strands
The traction reaction seat force transmission system is formed through the slide and five bundles of 1.52 cm left-handed low-relaxation steel strands. The tensioning fixed end of the traversing system is arranged on the reaction seat, and the movable end is arranged on the slide, and the slide is connected by a clamp and a five-hole anchor. The use of left-handed low relaxation steel strand can effectively prevent left and right stress imbalance.
3.4.5 Towing equipment
The traversing traction equipment is an intelligent continuous jacking system, which is composed of components such as pumping station and main control console. The pumping station can control the flow rate and infinitely control the speed with variable frequency. The continuous jack collects the elongation and displacement with the help of the rope displacement sensor, with an accuracy of 0.02 mm, and forms a closed-loop control with the inverter of the oil pump to achieve accurate synchronization and realize the dual control of pressure and displacement synchronization.
3.5 Main beam traverse
3.5.1 Pre-work preparation for traversing operations
(1) Determine the coefficient of friction. The weight of the single-span box girder of this project is about 500 t. Experiments show that the μ value of the static friction coefficient is 0.096 when the lateral motion is started, and the μ value of the dynamic friction coefficient in the lateral motion is 0.064. If a lubricant is blended with butter and molybdenum disulfide-based grease applied to the sliding surface, the coefficient of friction can be reduced by 0.02 [4,5].
(2) Assembling the slideway. Before assembling the slideway, the axis of the pay-off slideway needs to be measured, and the slideway is connected with the top plate of the cap beam through the gusset plate. Bolts are welded on the top plate of the cap beam every 50 cm, and 25 cm× 10 cm×2 cm steel plates are used as gussets. The side of the slideway is provided with a limiting baffle, which should be welded firmly. The gap between the top plate of the cap beam and the slideway can be filled with a cushion seat and a pad steel plate to ensure uniform force.
(3) Assemble steel strands and jacks. After completing the assembly of the slideway, connect the steel strand, and the steel strand passes through the reserved hole of the reaction seat at the bottom of the slideway from the slideway and is connected with the pre-installed jack. The case adopts a 5-bundle steel strand configuration, and the safety factor is about 3.78. Once all strand assembly has been completed, the slides are cleaned and the preparation is complete.
3.5.2 Lateral Shift Operation
(1) Start traversing. When the traverse movement is started, the jack is started at the 2nd end of the beam at the same time, and the traction force is controlled by 241.5 k N on one side, and when the calculated dynamic friction resistance is reached, the force is applied step by step according to the level of 10 k N, until the horizontal displacement of the slide is started, and the static friction resistance of the start is observed and recorded, and the unilateral side shall not be higher than 300 k N.
(2) Cast-in-place beam traverse. In the process of lateral movement, the speed of 13 cm/min is maintained to slide at a constant speed, and the technician applies butter to the slide and repeatedly observes the changes of the slide seat and slide. In order to ensure that both ends maintain the same sliding distance and speed, the laser sensor combined with the slide scale is used to carry out synchronous detection and control. Whenever one side slides 10 cm, the two ends of the box beam are checked once. If the slip distance at one end is exceeded, the two ends can be balanced by reducing the traction force.
(3) Traverse movement in place. The distance from the design target is 0.2 m, that is, when the lateral movement is basically in place, the combined operation of 30 s, 20 s, 10 s, 5 s and 2 s is adopted, and the technical personnel detect and confirm, and then continue the jog operation. After the lateral movement is in place, the surveyor should re-check the planar position of the box girder. In order to ensure the smooth development of traversing construction, it is necessary to monitor the stress and deflection values of steel portal pier cover beams, slideways and temporary steel pipe piers in real time during construction to ensure the safety and stability of the operation process.
3.6 Falling beams and deviation correction
3.6.1 Assemble the jack and backing plate
After the beam body is moved laterally in place, the beam falling operation is carried out with four jacks, and the top beam jacks are arranged at the bottom plate lower part of the two ends of the box body beam. The single-sided roof beam is composed of a box girder supporting steel plate and 2 jacks. The jack is assembled in two stepping stones, and on the outside of the beam end, near the stepping stone position, the box girder is assembled to support the over-cushioned steel plate. Before the construction operation, the position of the jack and the supporting steel plate is detected and marked. Two jacks on each side, controlled by 1 pumping station, ensure the same oil pressure, synchronous control of lifting. After the jack and supporting steel plate are in place, the box beam is jacked up 1~2 cm, the slideway, slide and steel strand are removed, and the jack is installed [6].
3.6.2 Support assembly
First, put the anchor bolts of the support seat into the reserved hole of the stepping stone. After the box girder is moved laterally into place, the support seat and the trolley are adjusted to the two stepping stones on the top surface of the steel cap beam, and the cross bridge is pushed to the side of the stepping stones. Ink lines are marked on the pads in advance, and the support seat is placed in the design position with a crowbar. After the bearing cushion is covered with 10 cm of I-shaped steel, four galvanized steel sheets of 10 cm× 10 cm × 2 cm are assembled on the top of the pad. Use a wrench to connect the supporting anchor bolt and the steel plate at the bottom of the support seat, adjust the support coordinate to the design position, and operate the falling beam. When the bottom of the beam is 2 cm away from the top plate of the support seat, the steel plate on the top of the support seat and the steel plate of the preset support seat at the bottom of the beam are connected with bolts, and the bolts are not completely tightened at this time.
3.6.3 Beam Drop Operation
When the beam body moves to the design position laterally, the beam body is jacked up with a jack, and the slide seat is pulled out of the slide along the slideway surface with a chain block; the slideway is pulled to the vacant position along the bridge direction, and finally it is retracted by the crane. The maximum jacking height of the jack is 25 cm. For operational safety reasons, each beam drop operation moves 3 cm. In the operation of falling beams, in order to avoid side slip caused by rigid contact, rubber plates are placed in the contact area between the steel gasket at the top of the jack and the bottom of the beam. The thickness of the steel backing plate is selected from 0.5 cm, 1 cm, 2 cm, and 3 cm. The last 1 steel backing plate is welded with 2~3 short steel bars perpendicular to the surface of the steel plate, and the other end is connected to the jack. The rebar can position the jack and ensure that it can jack the beam perpendicular to the horizontal plane. Because the longitudinal slope of the box girder of the case project is large, and there are different beam drop heights at the high and low ends, the beam drop operation is mainly carried out at both ends. After the initial assembly of the bearing is completed, the jacks at both ends of the bottom of the box beam are lifted synchronously. The lifting process should maintain a synchronous lifting height and speed, and pull out the beam support plate with a thickness of 30 mm. The jack returns oil, continues to maintain the same speed and height, and drops the beam by 30 mm. After the box beam is stabilized, continue to jack, pull off the beam support, and lower the beam by 30 mm. The cycle repeats 30 mm at a time until the low end is in place. After the low-end beam body falls into place, connect the temporary support seat, and then operate the high-end falling beam. The box girder is jacked up, the steel backing plate is pulled out, and the jack returns oil to control the falling beam by 30 mm. After the box beam is stabilized, continue to jack the box beam, pull out the backing plate, and control the falling beam by 30 mm. This is done 30 mm at a time, until the high end is in place [7].
3.6.4 Biased operation
When only the last 1 backing plate is left, the M30 nut is welded on the top plate of the pier in advance, the hole is punched in the bottom plate of the reaction support, and the high-strength bolt is fixed and pressed. Assemble the reaction bracket around the stepping stone to ensure the strength of the reverse support, use the jack to operate the deviation correction, push the jack back to the box beam offset direction, fine-tune and correct. Before installing the support, the surveyor checks whether the axis of the box beam meets the design requirements according to the bridge design, and the bearing can be installed only after it is qualified.
4 Conclusion
To sum up, based on the engineering application, the lateral cast-in-situ steel gate pier traversing construction technology was studied. The characteristics of the case project were introduced, the applied lateral cast-in-situ traversing in-place process flow was introduced, and the technical points of the lateral cast-in-situ traversing in-place process were introduced in detail from the aspects of foundation treatment, steel gate pier, temporary buttress pier, bracket, lateral prefabrication of the main beam, traversal in-place operation, beam drop and deviation correction. The results of case application and combing show that the construction conditions of the lateral cast-in-situ traversal in-situ construction method are relatively simple, and the impact on the normal operation of the crossing line is low, which can control the construction hazards to a greater extent, and can quickly complete the transverse in-place operation in the window period.
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