1 project overview
Liziping Bridge 1 is located in Liziping Township, Shimian County, Sichuan Province. It winds in the Nanya Valley and crosses the Nanya River. Nanya River is a mountain river, which has obvious characteristics of rapid flow and violent fluctuation. 1 The right side of the bridge crosses Nanya River. Pier 2#, 14# and 15# of this bridge were originally designed as left and right separated capping beams, and two square piers were used in each half of the pile foundation. Therefore, two of the three piers were originally designed in the middle of the river, which affected the normal flood discharge of Yaoheba Power Station and the safety of the bridge. Therefore, the substructure of Liziping Bridge 1 No.3 pier adopts integral prestressed capping beam, and the pile foundation and piers are close to the steep rocks on both sides of the river, and there is no pier in the center of the river. Due to the special terrain, the pile foundation and pier are located on steep rocks, and the steep terrain near the pile site can not provide the working site for construction, especially the construction of integral capping beam is more difficult.
2. Selection of capping beam construction scheme
According to the preliminary engineering calculation, the maximum clear distance between 14# capping beam and 15# capping beam is 30m, and 396.7m3 of concrete needs to be poured. In the actual construction process, combined with the construction load, the overall load exceeds1000 t. In order to save construction cost and improve construction safety, the project department analyzed and discussed the construction scheme of the capping beam according to the specific conditions of the project, and preliminarily determined the following construction scheme. 1) Treat the river foundation under the bridge, then build two temporary concrete piers, assemble two columns on the temporary piers with universal members, and the height of the columns shall be subject to reaching the bottom of the capping beam. Steel members shall be embedded in the concrete square piers, and the bearing surface of the long capping beam shall be combined with Bailey and I-steel horizontally [1], and then the reinforced concrete construction shall be carried out on it. 2) Considering that the height of the river bottom capping beam is 40m, in order to ensure the stability of the temporary pillar, members can be embedded at regular intervals on the concrete pier, and I-beams can be horizontally connected into a whole. By setting temporary pillars, the span of capping beam can be gradually changed from 30m to 10m, which can effectively reduce the construction difficulty. Using this method, the construction components are relatively small, and the crane can be used to drive below the river surface (dry season) for construction. 3) Steel members are embedded in the side of the reinforced concrete square pier, and then Bailey pieces and I-beams are combined into a supporting surface, and concrete construction is carried out on the supporting surface. However, when this scheme is used for construction, due to the limitation of terrain conditions, construction machinery is often not in place, and it is difficult and expensive to lay gantry cables at both ends for disassembly during construction [2]. 4) After comprehensive comparison, the project decided to adopt a scheme with higher safety and lower cost. (1) The design pile foundation of pier 2 # is 4m×8.3m square pile, the pile length is 17.5m, C40 concrete, the pier column is a thin-walled hollow pier, the pier column height is 36.43m, the clear distance between two piers is 25m, and the clear height from the bottom of capping beam to the riverbed is 42.948m The total height of Bailey frame, I-beam and unloading device is The bottom of the support is a temporary expansion foundation of flaky concrete, with a plane size of 5.0m long, 3.0m wide and 8.0m high (the buried depth is about 5.0m, depending on the geological conditions, and the bedrock excavated to the hard ground of the river bed shall prevail). The temporary expansion foundation is determined according to the actual excavation situation on site. Φ14 steel mesh is laid within 50cm at the top of the foundation, with a vertical and horizontal spacing of 20cm, and then eight supporting steel plates (2cm thick and 80cm×80cm in size) are embedded. (2) 2. The design pile foundation of pier10m # is 4m×3.5m square pile with a length of 22.5m and C40 concrete. The pier column is a thin-walled hollow pier, with a pier height of 34.26m and a clear distance of 30m between two piers. The clear height from the bottom of the capping beam to the riverbed is 45.5 15m, in which the total height of Bailey frame, transverse I-beam and unloading device is 2.655. The bottom of the bracket is a temporary expansion foundation of flaky concrete, with a plane size of 5.0m long, 3.0m wide and 8.0m high. The height of temporary foundation is determined according to the actual situation on site. (3) The design pile foundation of pier15 # is 4m×3.5m square pile with a length of 27.50m and C40 concrete. The pier column is a thin-walled hollow pier, with a pier height of 29.70m, a clear distance of 30m between two piers and a clear height of 46.053m from the bottom of the capping beam to the river bed, in which the total height of Bailey frame, transverse I-beam and unloading device is 2.65438+. The bottom of the support is a temporary expansion foundation made of flaky concrete, and the plane size of the foundation is 5.0m long, 3.0m wide and 8.0m high (the buried depth is about 5.0m, depending on the geological conditions, and the bedrock excavated to the hard ground of the river bed shall prevail). The height of temporary foundation is determined according to the actual situation on site.
Cast-in-situ construction technology of pier cap beam 3
3. 1 Support construction
Combined with the specific conditions of this project, during the construction process, the capping beam will extend out of the pier by 2m, so it is necessary to consider that the support will extend out by 2m. Therefore, during the construction of this project, φ 150mm through-hole is reserved on the pier. In addition, considering that the hanging end of the puncture rod needs to bear pressure, a bracket [2] is arranged below it. The bracket adopts 400 I-beam, and the steel plate embedded in the fulcrum of the bracket is embedded in the pier during the construction of the pier. Berets are evenly arranged on both sides of the pier and placed on the sand unloading cylinder above the piercer rod. The original design 1 1 berets are evenly distributed and are odd. In order to ensure the structural safety, 1 beret was added, with 6 berets on each side of the pier. Details of the formwork and bracket assembly are shown in Figure 1. The formwork support combination at the support (from bottom to top) is: temporary pier (placed on the bedrock of river bed, 8m high and 3m)+ high)+exposed)+steel pipe support (38m)+ sand unloading box (30cm, one for each main pipe, two supports, a total of16)+two main beams 56b (56cm high and 6m long, a total of four). 45m)+ length) +30a transversely distributed beam (height 30cm, length 6m, spacing 50cm)+ bottom formwork (width 2m+ thickness 10cm)+ side formwork (height 4 10cm, side formwork with bottom formwork, back frame height 420m).
3.2 template construction
1) bottom formwork construction. The bottom formwork construction adopts the old steel formwork used in the original square pier column construction. The design height of the original pier is 2m, and the bottom formwork is inclined downwards1m. As the Bailey frame is used as the supporting beam in this project, it is necessary to disconnect the transverse horizontal distribution beam, set a beam supported by the bottom die in the middle of the Bailey frame, and arrange the horizontal distribution beam and vertical I-beam on the beam for fixing. 2) Side form. The side formwork is also supported by the flat formwork built by the original pier, which is fixed with a back frame with a spacing of 50cm. All the back frames are fixed on the transverse distributed beams, and the side formwork is fixed with tie rods, with the horizontal and vertical distance of 80cm, and the top of the back frame is provided with top tie rods. The horizontal and vertical spacing of the tie rods is 80cm, and the top of the back frame is equipped with a day tie rod.
3.3 capping beam construction
Before capping beam construction, it is necessary to accurately determine the position of the center point of the pier on the top surface of the pier and the corresponding edge point, and take this as the installation control point of the formwork and reinforcement. In the process of construction, the elevation of the bottom of the capping beam is released with a level as the control line for the installation of scaffolding formwork. Chisel and clean the pier surface, then clean the cement slurry attached to the anchor bar at the pier top, and check whether the anchor bar at the pier top meets the design requirements. For capping beams with a height of more than 30m, steel pipe supports can be used for support and reinforcement [3]. For the capping beam with higher height, Bailey beam bracket supported by steel corbel is needed for vertical formwork cast-in-place. During capping beam construction, the bracket should be installed first, and then the bottom die should be laid. After the capping beam reinforcement is installed, the side formwork can be installed, then the transverse support and crosspiece can be installed, and finally the diagonal brace and tie rod can be installed.
3.4 Installation of reinforcement
Put out the position of the middle steel mesh and the steel mesh on both sides on the bottom die, and weld the steel stirrups on the anchor bars at the top of the pier. It is required that the key position and edge position of the installation stud be consistent with the released control line. Mark the position of the main reinforcement on the steel bar, then weld the main reinforcement to the steel bar, mark the position of the oblique bending reinforcement and stirrup on the column reinforcement, and install the bending reinforcement and stirrup [4]. Weld the embedded parts with the main reinforcement firmly to avoid the movement or deformation of concrete during vibrating. After installation, the specification, quantity, model and welding quality of steel bars must be checked, especially the installation position of bent steel bars.
3.5 pouring concrete
After centralized mixing of concrete, it will be delivered into the mould by a delivery pump, and then poured in layers, with the thickness of each layer not exceeding 50cm. When using internal vibrator to vibrate, attention should be paid to strengthening the vibrating work of embedded parts and concrete under embedded parts. In order to ensure the accuracy of the exposed part and the top elevation of the capping beam embedded parts, it is necessary to arrange obvious control lines on the top elevation of the capping beam.
4 conclusion
To sum up, in bridge engineering construction, the cast-in-place construction of pier capping beam is the key point in construction. Taking the actual project as an example, this paper analyzes and discusses the support construction, formwork construction, capping beam construction, steel bar construction and concrete pouring construction in pier construction. The adopted scheme successfully completed the project construction and achieved good construction effect, which is worth learning from similar projects.
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