The approach bridge of Yijia Fuel Oil Wharf is 537 meters long, straight and 9.45 meters wide. The span of the bridge is 56 5×85 56m prestressed concrete variable cross-section single-box single-cell continuous box girder, with the middle fulcrum beam 5 meters high and the end beam 2.5 meters high. The bottom line of the beam changes according to a quadratic parabola. The side span is cast-in-situ with full-house support, with a length of 12.5m, and the closure length of each span is 2m. The fulcrum on the pier is 2.5m long and the beam is 5m high. Each pier is divided into 1 1 beam section for cantilever cast-in-place, in which the length of block 0 is 9 meters, that of block 1 ~ 6 is 3.5 meters, and that of block 7 ~ 10 is 4 meters. The width of the box girder top plate is 9.45 meters, the width of the bottom plate is 5.05 meters, the cantilever plates on both sides are 2.2 meters long, the end of the cantilever plate is 0.2 meters thick, and the root is 0.53 meters thick. The top plate thickness of box girder is 0.28 m, the bottom plate thickness is 0.28 ~ 0.8 m, the middle pier is locally thickened to 1.4 m ... The middle fulcrum is provided with a partition plate with a thickness of 2.5m, and a manhole of 1.2× 1.0m is reserved.

The beam adopts longitudinal prestress system, and the prestress is arranged in the way of top plate, bottom plate and web plate, and is arranged as close as possible to the web plate. What is the use of longitudinal prestressed tendons? S 15.24mm high strength and low relaxation steel strand, standard strength fpk= 1860MPa, tension control stress under anchor 0.72fpk= 1339.2MPa, elastic modulus Ep= 1.95× 105MPa.

The lower part is gravity caisson foundation. After caisson installation is completed, cast-in-place sealing cover plate and pier top concrete are used to form pier columns.

The section form and block of box girder are shown in figure 1.

2. Introduction of overall construction technology During pier construction, fine steel bars are embedded vertically, and after the pouring of 0 pieces of box girder is completed, the fine steel bars are tensioned and anchored on the top surface of box girder as a temporary reinforcement measure.

Each pier is poured symmetrically from the middle block 0 to both sides in turn. Block 0 in the middle and the beam section with side span near the beam end are cast-in-situ with full-house support, and the two sides of the pier are symmetrically suspended by hanging baskets. After each section is poured, dismantle the hanging basket, and pour the closed section in the order of first side span and then middle span. After the pouring of each closure section is completed, the temporary consolidation is removed and the structural system is transformed until the whole bridge is closed into a continuous beam.

3. Temporary reinforcement and load-bearing structure

3. 1 Temporary consolidation: In order to prevent uneven load at both ends during construction, temporary anchorage measures are set on the pier to stabilize the cantilever system;

3.2 Temporary consolidation scheme: vertical fine steel bars are pre-embedded before pouring pier concrete, and metal spiral sleeves are placed outside the fine steel bars contained in 0 pieces of concrete of box girder, and pores are formed in the concrete. After pouring concrete of No.0 block, and the strength and elastic modulus reach above 90%, tension fine steel bars on the top surface of No.0 block, and then anchor them with spiral sleeves;

3.3 After the concrete pouring of the pier and abutment is completed, conduct positioning measurement, respectively locate the center lines of the temporary pier and the bearing, and then install the bearing for concrete pouring of the temporary pier (sulfur mortar interlayer with embedded resistance wires is used in the middle of the temporary pier, which is removed by conduction during system conversion).

4. Cast-in-place section construction

4. 10 support erection. The construction of box girder section 0 adopts full-hall bowl-buckle scaffold to support and pour. Support adoption? 48×3.5mm seamless steel pipes are combined into fastener-type steel pipe frame, which is supported on the pier below the vertical pole with adjustable jacking above the vertical pole. The bottom plate bracket adopts jack-up and laid 10cm× 10cm longitudinal square timber, and the internal formwork bracket adopts combined scaffold. When installing the bracket, the initial installation position should be determined first, and the height of the vertical pole should be determined according to the elevation of the bottom supporting surface. Box girder formwork shall be leveled with adjustable jacking to avoid the vertical pole hanging or uneven stress caused by local unevenness.

4.20 pieces of support preloading. In order to test the strength and stability of the support, eliminate the overall plastic deformation of the support, calculate the elastic deformation deflection of the support, and provide a basis for adjusting the elevation of the bottom die, the 0 # section support should be pre-pressed by stacking sand bags and concrete precast blocks before use. The preloading load is 1.2 times the unit area pressure of the formwork. (See Figure 3 for the schematic diagram of preloading) Figure 30# bracket preloading schematic diagram.

4.30 Concrete pouring.

4.3. 10 block of concrete is poured at one time, and the step pouring process is adopted, from low to high, from middle to both ends, and the pouring thickness of each layer is controlled at about 40cm. When pouring the bottom plate, two pouring grooves are symmetrically arranged on the top plate;

4.3.20 Beam section is the center of continuous box girder and the key beam section of cantilever casting, with complex structure and great construction difficulty. According to the design bearing capacity and construction operability of the support, section 0 must be cast at one time. After the completion of the construction of Block 0 and Section 0, the center point of the pier will be restored to the top of Section 0, and fixed signs will be buried. At the same time, the point is rechecked with the bridge axis through measurement. When the beam body extends forward, each beam body is buried with the mark that the left, middle and right parallel lines at its end point intersect with the mileage line as the control point of the segmented casting template;

4.3.3 Designated persons shall observe and check the stability of reinforcement and formwork during casting, and handle the problems in time when found;

4.3.4 During pouring, the loose concrete attached to the top steel bar and wing plate shall be cleaned in time;

4.4 Tension temporary consolidated reinforcement and longitudinal steel strand.

4.4. 1 tensioning equipment: YGM-350 hydraulic jack;

4.4.2 Tension sequence: firstly, temporarily consolidated steel bars are tensioned, and then the longitudinal steel strands are symmetrically tensioned in both directions;

4.4.3 Graded tensioning: 0→ initial stress, measured value → 100%σcon, measured value, anchoring;

4.4.4 In-hole grouting: when the hole length is less than 20m, one end grouting is adopted, and when the hole length is longer than 20m, tee vacuum grouting is adopted;

4.5 The cast-in-place section of side span is basically the same as that of block 0.

5. Hanging basket construction

5. 1 Structure and characteristics of the hanging basket. Commonly used hanging basket structures include composite truss, triangle and diamond. This paper takes rhombic hanging basket as an example. Hanging basket consists of five parts: main truss system, back anchor and walking system, suspension system, formwork system and working platform. Together with all formwork and construction machinery, the load shall not exceed 33 tons.

The hanging basket has the following characteristics: light weight, large overall stiffness, small deformation, clear force, quick assembly and enough walking space under the basket. The hanging basket moves forward with the template as a whole, which is convenient for processing and operation.

5.2 Design requirements of hanging basket. The ratio of hanging basket quality to beam concrete quality should be controlled between 0.3 and 0.5, and should not exceed 0.7 under special circumstances; The total weight of the hanging basket is controlled within the design weight limit; The maximum allowable deformation (including the sum of sling deformation) is 20mm, and the overturning safety factor during construction and walking is 2; The safety factor of self-anchored system is 2; The safety factor of cable-stayed horizontal limit system is 2; The upper limit of the horizontal limit safety factor is 2;

5.3 Installation and positioning of hanging basket.

5.3. 1 The hanging basket shall be positioned and installed after concrete pouring, tensioning and grouting of No.0 beam section are completed.

5.3. 1. 1 The positioning of the hanging basket mainly depends on the accurate lofting and positioning of the slideway where the hanging basket moves forward, so that it is parallel to the bridge axis, and the change of slope is consistent with that of the design beam, so that the synchronous walking of the hanging basket can be monitored while walking. At the same time, whether the plane and elevation of the slideway change during the construction process can be observed, and the relationship between the hanging basket and the bridge axis before, during and after construction can be checked. After the hanging basket is in place, immediately carry out the walking and hoisting positioning of the hanging basket, and control the mileage and elevation of each segment. The positioning monitoring measurement of each cantilever beam should be checked with the whole bridge control network;

5.3. 1.2 The installation sequence of the hanging basket is as follows: put the slideway in the set position, fix it with a compactor to prevent overturning → install the main beam and column with tower crane (the bottom beam of rhombic truss is anchored on the slideway) and connect them in parallel → install the front upper beam at the end of the main beam and connect it with the main beam → hoist the front lower beam, the rear lower beam and the bottom web lower longitudinal beam in turn, and install the suspender →

5.3.2 Precautions in the process of hanging basket assembly:

5.3.2. 10 # beam section is completed, and the hanging basket is assembled after formwork removal;

When 5.3.2.2 is installed, two hanging baskets are symmetrically assembled at the same time on the 0 # beam section at both ends of the T-shaped structure to ensure the stress balance at both ends of the T-shaped structure;

5.3.2.3 hanging basket assembly for aerial work, each process must be carefully checked before the next process, to ensure construction safety;

5.4 Pre-pressing of hanging basket. In order to test the bearing capacity and actual deformation of the hanging basket and accurately reserve the deflection value of the hanging basket, after the hanging basket is installed, the hanging basket should be loaded and preloaded. The preloading method is as follows:

5.4. 1 The maximum preloading load of the hanging basket is 1.2 times of the total concrete weight and construction load;

5.4.2 Sand bags and precast concrete blocks (1.2× 1.5m, each block weighs 1.5t) are used for preloading, and the loading position is the same as that of block 0;

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