Fengjiazhuang main line overpass bridge, superstructure is15+18+18+15m reinforced concrete cast-in-place continuous slab, substructure: abutment is column abutment and pile foundation, pier is thin-walled pier, cap and pile foundation. The total length of the bridge is 69.78m, and the total width of single deck is10.3m.. According to the reconstruction design requirements, replacing the bridge bearing requires synchronous jacking of the bridge superstructure about 1.5-2cm, and the estimated weight of the superstructure is 1750 tons.

2. Selection, arrangement and stress calculation of jacks

The original abutment and the original thin-walled column are used as the reaction foundation of jacking, and the whole bridge jacking is realized. The jacking control system adopts hierarchical and step-by-step hydraulic synchronous jacking.

(1) jack selection. According to the on-site investigation, the spacing between the bottom of abutment beam No.0 and No.4 and the top of abutment is generally 4.5-5.5cm, and the spacing between pier 1 pier, pier 2 and pier 3 is 9.5- 13.4cm. The jack thickness should meet the requirements of spacing and jacking process. Through on-the-spot investigation and research with professional jack manufacturers, two kinds of thin-plate jacks, 100 ton and 150 ton, were customized according to this project. Their specifications are as follows: 100 ton jack, 12 jack, with a height of 50mm, a diameter of 198mm and a stroke of 12mm, which are used for abutment on both sides. 24 jacks, 150 tons, 95mm high, size 198*245mm, stroke 20mm, used for thin-walled columns. All jacks are equipped with hydraulic locks to prevent the system and pipeline from losing pressure and ensure the effective support of the load.

(2) jack arrangement. According to the calculation, the weight of the whole bridge is 1750 tons, the bearing reaction of the middle abutment is the largest, which is 557.9 tons, and the bearing reaction of the abutments on both sides is the smallest, which is about 232.4 tons. The number of jacks planned to be arranged from south to north is 0:6 * 1 ton; /kloc-pier 0/,pier 2 and pier 3: 8 sets each * 1 50 tons; Platform 4: 6 *100t.

The jack layout is as shown in the figure:

After the bottom of the whole thin-walled pier beam is roughly ground, the high-strength epoxy standard mortar is coated, and then the steel backing plate is laid, so that the steel backing plate is closely combined with the bottom of the beam. Within the placement range of jack on the top of thin-walled pier, after rough leveling, high-strength epoxy standard mortar is coated, and then steel backing plate is laid to make the steel backing plate closely combined with the top of thin-walled pier.

According to the checking calculation, the bottom edge of jack is not less than 10 cm from the edge of abutment cap beam and thin-walled pier. In strict accordance with the inspection requirements, steel pads are laid at the bottom of the jack. The thickness of the steel backing plate shall not be less than 1 cm, the corners shall be obtuse, and the angle shall be kept between 100 degrees and 120 degrees.

(3) Force analysis of jack. Check the original bridge completion drawing and verify on the spot that the bearing reaction force of the heaviest section is R3=557900kg. Eight jacks are installed in the hole, and the effective bearing capacity of each jack is 150000kg. The sum of the effective bearing capacity of one jack on the bridge is r =150000× 8 =1200000 kg. Safety factor RS=R branch /R load =1200000/557900 = 2.15, and the jack setting scheme fully meets the bearing requirements.

(4) Stress analysis and checking calculation of thin-walled pier and beam slab. Through stress analysis and checking calculation, the bearing requirements of jack-up foundation are met.

(5) Support system. The upper surface of the original abutment or thin-walled column is used as the bearing surface of the jack as its main bearing system.

3, construction preparation

(1) personnel arrangement. In order to ensure the smooth progress of jack-up work, a special command and leadership organization was set up on the construction site. On-site commander 1, deputy commander 1, with beam monitoring group, hydraulic control group and operation observation group. Among them, there are 4 people in the beam monitoring group, 3 people in the hydraulic control group and 5 people in the operation observation group/kloc-0 (3 people in each support).

(2) Arrangement of equipment and materials. The number of equipment and materials used must meet the jack-up requirements of four holes and one section.

(3) Working platform. The support is erected near the abutment or around the column with steel pipes as the working platform.

Because the whole bridge jacking scheme is adopted, the bridge deck road needs to be closed during the construction.

4. Bridge jacking

(1) Installation and debugging of jack-up system. The jacking equipment of this bridge adopts 5 hydraulic pump stations and 36 hydraulic jacks; The hydraulic pump station is placed on the bridge deck. After the installation, debug the jack-up system.

When everything is ready, start the jack at the same time to jack up. Jack-up control will be carried out according to the jack-up stroke, the jack-up height is 2mm, and the load will be unloaded after more than 5min. After unloading, carefully check the obvious phenomena of their respective control systems and systems; At the same time, carefully check whether there is any deformation on the upper and lower steel plates of the jack, and adjust the thickness of the steel plates when necessary to meet the jack-up requirements; Carefully check whether the structure under the jack placement position is different from that before jacking, and it is forbidden to continue jacking when the situation is unknown. After the trial jack-up, the overall attitude and structural displacement of the bridge are provided, which provides the basis for the formal jack-up.

(2) jack-up preparation. Complete the following preparations before jack-up: ① The reliability inspection and test of jack-up system have been completed without exception; (2) observation points have been determined; ③ There is no connection around the bridge expansion joint; (4) All personnel involved in jack-up construction have received technical training and strict division of labor; ⑤ The transfer point has been determined (each pier has five sections on the bridge deck, and each section has three monitoring points, two on both sides of the bridge deck, and the center line is 1, which is used to monitor the changes of bridge elevation and center line position during jacking, and is used as the transfer point after jacking); ⑥ Set limit devices (the bridge is limited by anti-seismic anti-skid blocks in the transverse direction, and the expansion joints are limited by steel wedges in the longitudinal direction).

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