I. Introduction of the Project

The superstructure of the main bridge of the North Branch Bridge of the Second Nanjing Yangtze River Bridge is a 90+3*l65+90(m) five-span prestressed concrete continuous box girder. Its main span165m is the first in China. According to the design requirements, the construction method needs to adopt cantilever pouring construction technology, and then it is transformed into continuous beam through closure and formation system. Each pier has 23 cantilever casting sections-5 * 2.5m+5 * 3.0m+5 * 3.5m+8 * 4.0m. The longest beam section is 4.0m and the maximum weight is 156t. Box girder section: roof width 15.42m, web width 7.5m, beam height gradually changed from 8.8m in block 0 to 3.0m in block 23 according to quadratic parabola. Prestress design is three-dimensional prestress: 27 strands and 25 strands of φj 15.24 steel strand are used longitudinally; Four φjl5.24 steel strands are used horizontally; Vertically, φ32 fine-rolled threaded steel bars are used. Tension control 0.75Ryb. E2 is marked as half of the main bridge of Beicha, which will be constructed by China Railway 19th Bureau. According to the characteristics of the bridge and referring to various types of hanging baskets at home and abroad, the rhombic truss hanging basket is selected as the main equipment for cantilever casting. The hanging plate is designed by the affiliated design institute of China Railway Construction Group Corporation, and has been applied to continuous beam suspension cable construction many times in China, but it is the first time to apply it to such a long-span bridge construction.

Second, the structure of rhombic truss hanging basket

The rhombic truss hanging basket consists of main frame, walking and anchoring device, bottom formwork, inner and outer formwork, front hanging device, rear hanging device and front upper beam (Figure 1).

1. Main Frame The main frame consists of two truss connection systems and a portal frame. Two trusses are connected into a rhombus with 2 [40 bolts, and connected with the gantry into a whole through the connection system, forming the main stress structure of the hanging basket.

2. The underframe and the underframe of the bottom die are composed of six longitudinal beams and front and rear beams. The longitudinal beam adopts truss structure, with truss height 12m and truss length of 5.43m The front and rear beams are welded by 2[40 groups. The bottom die is made of bamboo veneer with 100mm* 100mm square wood underneath. The bamboo glue template and the square timber are fixed on the longitudinal beam with U-shaped clips and demoulded and fixed.

3. Front upper crossbeam The front upper crossbeam is made of two [40] I-beams, which are connected to the nodes at the front end of the main frame to connect the two main frame flow frames into a whole.

4. The front and rear suspenders of steel suspenders are made of 150mm*32mm 16Mn steel plates, which are connected by pins. Adjust holes with the interval of 100mm, and adjust the required length with LQ30 jack, shoulder pole and bolster.

5. The external formwork of box girder combined with internal and external formwork is welded by 5mm steel plate and steel frame. Two external formworks are supported on two walking beams respectively, and the walking beams are hung on the front upper cross beam and the poured box beam wing plate through suspenders. The walking beam consists of two groups [30a] of welding surfaces. The internal model is composed of internal model truss, vertical belt, vertical belt and composite steel formwork. The internal formwork truss is hung on two internal formwork walking beams, and the internal formwork walking beams are hung on the top plates of the front upper beam and the pouring beam section. After demoulding, the internal mold can move forward along the walking beam. The walking beam consists of two groups [30a] of welding surfaces.

6. The traveling device of the hanging basket and anchoring system consists of rails, steel (wood) pillows, front and rear supports, and manual hoists. The track is welded by [[ 16a and δ 10] steel plates into a Ⅱ-shaped section, and the bottom plate is provided with elliptical long holes every 500mm for vertical prestressing tendon anchorage. The vertical prestressing tendon is QL32 fine-rolled threaded tendon, with an exposure of 0.3m The track is divided into 3.0m and 1.0m according to the length of the beam section.

The hanging basket is equipped with front and rear bearings, the front bearing is supported on the top surface of the track, and the PVC skateboard is below, which can slide along the track. The rear support slides along the lower edge of the track in the form of anti-buckle wheel without adding counterweight. When the hanging basket moves forward, pull the front support with the manual hoist to drive the whole hanging basket to move forward. When the hanging basket is concreted, the back end of the hanging basket is anchored on the completed beam section with 12φ32 finishing rolled steel bar, and the track is anchored on the vertical prestressed steel bar of the completed beam section. When anchoring, remove the hook plate of the rear bracket from the track with a jack, and then anchor.

Three, hanging basket strength and deformation calculation

Calculation load: Check according to the self-weight of the hanging basket, the self-weight of the beam section, the load generated by construction machinery and personnel, and the load combination generated during concrete vibrating, in which the bulk density of concrete is 2.6t//m3, the load of construction machinery and personnel is 1.0KPa, and the vibrating load of concrete is 0.0kpa..

The main truss, bottom formwork truss and the main stressed components of each part are checked at the most unfavorable load position, and the corresponding checking is carried out by using the comprehensive calculation program of Tongji University bridge structure.

The calculation shows that the maximum stress of a single top chord is 72kN, which is less than the allowable stress of a single top chord of 252kN. The maximum stress of a single diagonal bar is 39kN, which is less than the allowable stress of a single bar129 kn; . The maximum stress of a single vertical pole is 27kN, which is less than the allowable stress of a single upper chord 170kN.

The deformation of the hanging basket is also checked, and the calculation results show that the maximum deformation of the bridge section at the north branch occurs at 16, and the elastic deformation value of the hanging basket is 2 1mm at this time.

Four, hanging basket before use test

In order to check the calculated deformation value of the hanging basket and eliminate the inelastic deformation after the first installation, the ground loading test of the hanging basket was carried out in the factory. At the same time, after the installation of hanging baskets, in order to check the ground test results, a pair of hanging baskets were selected for on-site weighing test.

The main truss, pin seat, pin, front and rear suspenders, front upper beam and other components were tested in the factory.

Main truss: Back-to-back method is adopted, the rear anchorage position is anchored with fine-rolled threaded steel bars, and the YC60 jack is horizontally loaded at the front hoisting point every 10t, and the deformation curves of each pair of main analytical frames are obtained.

The ground simulation loading test of pin seat, pin, front and rear suspenders and front upper beam was carried out by similar method, and the deformation curve of each set of hanging basket was obtained by combining the test results of main truss.

After the first pair of hanging baskets are assembled, according to the actual load of the beam section, the simulated load test is carried out on the spot with sandbags, and the results are compared with the calculated deformation value and the factory test value, so that the deformation value of each group of hanging baskets during the preparation of the section can guide the formwork construction of each beam section.

Referring to the design and test results of the hanging basket, the deformation formula of the hanging basket is: y = 0.4x+2 (where x is the stress value at the front sling of each segment).

Verb (abbreviation of verb) assembly process

1. Assembly preparation

Assemble the rhombic truss of the main frame in the factory, transport it to the site for hoisting after assembly, compile the assembly process, prepare the assembly tools and various connecting bolts, and train the assembly workers.

Lay a pillow

Use 1: 2 cement mortar to level beam top sleepers and lay steel sleepers.

Mounting track

Install two 2.5m-long rails from the center of No.0 section to both sides, insert vertical prestressing tendons into the rails, level the top surface of the rails, measure the center distance of the rails, and then lock the rails with nuts.

4. Install the front and rear brackets

First, insert the rear bearing from the front end of the track, and then place the front bearing after the rear bearing is in place. Before placing the front bearing, place a tetrafluoroethylene slide plate (300mm*500mm) at the corresponding position on the top surface of the track, and then place the front bearing.

5. Hoist the main frame

The main frame is assembled into a rhombus in the factory, and then transported to the site to be hoisted in sections, fitted with front and rear brackets, and tightened with connecting bolts. In order to prevent toppling, scaffolding is used as temporary support.

6. Install the connection system and interlayer between the main frames and tighten the connecting bolts.

7. Anchor the rear end of the main frame on the completed beam section with a long screw (φL32 finish rolled rebar) and a pole frame, and fix the lower chord of the main frame on the track at the front support position with a pole frame.

8. Lift the front upper beam

Assemble four jacks, upper and lower cushion beams and two steel slings to the front upper cross beam, and then lift and install them as a whole.

9. Install the rear sling

Install the rear sling in the reserved hole of the bottom plate of beam section 1, first place the cushion block, jack and upper cushion beam, and then connect the rear sling with the underframe through the bottom plate.

10. Hoist the bottom die and bottom die.

1 1. Lift the internal model walking beam and install the rear boom. The front suspender adopts steel wire rope and inverted chain.

12. Install the external formwork

The external formwork used for hanging basket is first used for the construction of beam section 1. Before the above assembly procedure, put the walking beam of the external formwork on the vertical frame of the external formwork, and insert the rear end into the rear hanger. The front ends of the two walking beams are hung on the front upper beam with inverted chains and steel ropes.

Drag the external formwork to the position of No.2 beam section with chain chain, install the rear hanger of the external formwork walking beam on both sides of the middle section of 1, and dismantle the rear hanger of No.0 section. Six, cantilever casting construction technology

Each T-shaped structure starts from the second section, and cantilever pouring construction can be carried out after the hanging basket is symmetrically assembled.

1. Hoist the bottom plate and web plate, conduct reinforcement construction, and place prestressed pipes.

2. Pull out the beam section 1 internal model.

3. Adjust the lower die according to the height of No.2 beam section.

4. Leave a hole in the position where the blanking string pipe is installed on the top plate and the web plate, and tamp it on the tamping position of the web plate.

5. Install the end die and connect it with the inner and outer dies.

6. Tie the roof reinforcement.

7. Place prestressed pipes, anchor plates, etc.

8. Symmetrically pour the concrete of No.2 beam section.

9. Concrete curing.

10. Prestressed tendons are tensioned.

1 1. Grouting.

12. Hanging basket walking

Leveling the top surface of the beam, laying steel (wood) sleepers and tracks.

Loosen the front and rear straps of the bottom frame.

10t chain is installed between the lifting lugs on both sides of the back beam of the bottom die and the walking beam of the external die, that is, the bottom die is hung on the walking beam.

Remove the connection between the sling and the bottom die.

Remove the anchor screw at the rear end of the hanging basket.

Two 5t inverted chains (each group of hanging baskets) are installed on the top surface of the track, and the front support position (the support center is 50cm away from the beam end) is marked.

The rear chain pulls the front bracket to make the hanging basket, underframe and outer formwork move forward together. When moving, a 10t safety chain should be set at the rear of the hanging basket.

Install the rear sling and lift the bottom frame.

Install the back hanger of the side formwork walking beam on the No.2 beam section, first remove the back hanger on the 1 beam section and move it to the No.2 beam section, and then remove the other back hanger and move it to the No.2 beam section.

Pull out the inner mold and the hanging basket is finished.

Six, the removal of the hanging basket

Before the construction of the closure section, the hanging basket can be removed, and the removal sequence is as follows:

1. Install a hoist on the top surface of the beam, hang up the front and rear suspenders of the external formwork (the bottom formwork is hung on the walking beam), slowly lower it, and then disembark.

Let's go Or put the bottom mold first, and then put the outer mold.

2. The internal formwork and walking beam that are not used in the closing section shall be removed before the construction of the closing section, and the rest can be removed from the exits of the beams at both ends.

3. Remove the front upper cross member.

4. The main frame can be moved to the range where the tower crane can be hoisted and dismantled.

5. Remove the track and steel (wood) sleepers.

Eight, the main technical parameters of the rhombic hanging basket of the North Branch Bridge of the Second Nanjing Yangtze River Bridge

1. Applicable maximum girder weight: 160 tons.

2. Maximum girder length: 40m

3. Beam height: 8.8 ~ 3.0m

4. Applicable to beam width: 16m.

5. Walking mode: unbalanced walking.

6. Hanging basket weight: 49 tons

7. Stability coefficient of hanging basket overturning

No load: 28

Perfusion period: 2.48

8. Suspension coefficient; 3.0

Nine, the main experience of using diamond hanging basket for suspension casting construction.

1. Advantages

The rhombic hanging basket is light in weight, and its utilization coefficient in the construction of this bridge has reached 3.0, which should be the lightest one in China except the sliding cable-stayed hanging basket, thus saving the labor for processing, transportation, assembly, moving and dismantling, and further achieving the purpose of saving money. In a word, the application of this hanging basket construction can have great economic benefits.

The inner form and outer form of rhombic hanging basket are hung on the bridge deck or hanging basket by longitudinal sliding beam, which is very convenient to walk longitudinally and the sliding beam is also used for reinforcement.

The rhombic hanging basket has simple structure and definite force, and the actual elastic deformation after loading is not much different from the theoretical calculation value. If the hanging basket has been used, the trial installation procedure can be omitted.

Because the hanging basket is designed as a diamond, the lifting points are all located in the air above the beam surface, which provides a large operating space for the construction personnel and is beneficial to the construction.

2. Shortcomings and suggested modification measures

The design of external formwork is slightly thin, and there are too many tie rods between internal and external formwork during reinforcement, which is not conducive to improving the appearance quality of box girder. In the next design, the stiffness of the external formwork truss can be increased and the number of tie rods can be reduced.

At present, there are no effective measures to correct the lateral deviation of the hanging basket. It would be better if a hydraulic lateral device could be installed on the front sling.