With the development of external prestressing technology and the appearance of problems such as corrosion of internal prestressing tendons, assembled construction methods are becoming more and more popular in China. Why not learn it quickly?

Construction of assembled pier and abutment

The assembled pier is made by dividing the tall pier into several components along the vertical and horizontal directions according to a certain modulus, pouring them on the prefabricated site around the bridge site, transporting them to the site by car and boat, and hoisting and assembling them.

The main feature of prefabricated piers and abutments is that they can be prefabricated in the prefabrication yard, which is less disturbed by the surrounding environment, but relatively speaking, they require higher transportation and hoisting machinery and equipment. The assembled column pier system decomposes the pier into several components, such as cap, column, capping beam (pier cap) and so on. , prefabricated in the factory or site, and then transported to the site to assemble the pier. Its construction procedures are mainly component prefabrication, installation and connection and concrete joint filling. Among them, assembling joints is the key process, which should be firm and safe, simple in structure and convenient in construction.

Commonly used assembly joints are as follows:

(1) adopts bonded post-tensioned prestressed tendon connection structure.

The connection structure with bonded post-tensioned prestressed tendons is often combined with mortar cushion or epoxy adhesive joint structure to realize the construction of segmental precast piers. The prestressed reinforcement in this scheme can be high-strength reinforcement such as steel strand or finish-rolled rebar. The structure is characterized by prestressed tendons passing through the joints, which is widely used in practical projects and has mature experience in design theory, calculation analysis and construction technology. The disadvantages are that the cost of pier body is much higher than that of traditional cast-in-place concrete pier, and the prestressed tendons need to be tensioned and grouted in site construction, so the construction technology is complicated and the construction time is long.

(2) Grouting sleeve connection

Precast pier segments are connected with protruding steel bars through grouting connecting sleeves, and the contact surface between pier and capping beam or bearing platform often adopts mortar cushion, and epoxy glue joints are adopted between pier segments. Its structural features are high construction precision, short construction time, and no need to tension prestressed tendons, which greatly reduces the field workload. Its mechanical properties under normal use conditions are similar to those of traditional cast-in-place concrete piers, so it has certain economic advantages. From the application experience abroad, the low seismic risk area has been widely used, and the application and scientific research of high seismic risk area are still in progress.

(3) Grouted metal corrugated pipe connection

This connection structure is often used to connect the pier shaft with the bearing platform or the pier shaft with the capping beam. The precast pier shaft is connected with the steel bars extending out of the pier shaft through the grouting metal corrugated pipe embedded in the capping beam or the bearing platform. Mortar cushion is often used as the contact surface between pier shaft and capping beam or cap, and epoxy glue joints are used between pier shaft segments, as shown in Figure 5. The construction time of this structure is short, but it needs to meet the sufficient anchorage length of longitudinal reinforcement, and its mechanical properties are similar to those of traditional cast-in-place concrete piers. At present, a few bridges abroad have adopted this connection structure for construction, but it is rarely used in areas with high earthquake risk, and its seismic performance is still under study.

(4) Slot connection

Trough connection structure, as shown in Figure 6, has been used in some bridge projects, mainly for the connection between pier shaft and capping beam, and between pile and cap. Compared with grouting casing and metal bellows, it has the advantages that the required construction tolerance can be larger and some concrete needs to be poured on site.

(5) Steel bars shall be welded or lapped, and wet joints shall be adopted.

A certain number of steel bars are pre-extended from the prefabricated pier, which overlap with the reserved steel bars of adjacent components and need temporary support. The connection parts of steel bars need to be connected by post-pouring concrete (wet joint), which is also the design idea of segmental assembled pier widely used in China at present. The mechanical properties of piers constructed with this structure are often similar to those of traditional cast-in-place concrete piers, but the existence of wet joints will increase the construction time and the workload of overlapping pouring of steel bars on site. From the point of view of rapid construction, this scheme has some shortcomings.

(6) Socket connection

The connection structure of socket joint is to insert the precast pier shaft into the corresponding reserved hole of the foundation, and the insertion length is generally 1.2 ~ 1.5 times of the section size of the pier shaft. The bottom is paved with mortar with a certain thickness, and the periphery is filled with semi-rigid concrete. The advantages are simple construction technology and less on-site workload; The disadvantage is that the mechanical performance of joints, especially the seismic performance, needs further study. Beijing Jishuitan Bridge was built with this connection structure, and some bridges in the United States were also built with this connection structure. As shown in the figure.

Seismic performance of prefabricated columns

The seismic performance of precast columns is a technical problem that hinders the application of full precast assembly technology in bridges in high earthquake risk areas. In order to realize the comprehensive popularization and application of fully prefabricated assembly technology, it is necessary to study the seismic performance of precast columns in depth. Based on the typical practical engineering pier structure, the scale test of rectangular solid segment precast column under low cyclic horizontal load was carried out by selecting three precast assembly connection methods: connector, conduit and bonded prestressed reinforcement. Through quasi-static test and finite element numerical analysis of segmental precast columns with different structural details, the hysteretic characteristics, ductile deformation, nonlinear mechanical behavior of joints and damage mechanism of segmental precast columns under different structural modes are studied.

The test results show that compared with the traditional cast-in-place concrete pier, the pier with prefabricated connecting pipe and corrugated pipe has similar seismic performance and can meet the expected seismic performance requirements. Prefabricated piers connected with bonded prestressed tendons have similar deformation capacity to cast-in-place concrete piers, but their energy consumption capacity is weak. In addition, through calculation and analysis, connection device test, and the study of the whole batch of specimens, it is shown that the overall stress, structural connection, seismic performance and detailed structure of the whole construction process of the column can meet the requirements of the current design and construction, and can be used in engineering practice.

Application example:

The following points should be paid attention to in the construction of assembled column pier:

(1) Number the pier column members and the rod-shaped foundation reserved on the top surface of the foundation, and check whether the height of each pier and the coordinate height of the foundation meet the design requirements; The gap between the base mouth and the column edge shall not be less than 2cm.

(2) When the pier and abutment column is put into the pedestal cup, longitudinal and transverse measurements should be made to make the verticality or inclination and plane position of the column meet the design requirements; For heavy and slender piers, they must be fixed with cable wind rope or stay wood before releasing hooks.

(3) Before installing the capping beam at the top of the pier, check whether the position of the reserved groove on the capping beam meets the design requirements, otherwise it should be repaired first.

(3) After the column body and capping beam (pier cap) are installed and meet the requirements through inspection, thin mortar can be poured at the gap between the foundation cup and capping beam groove. After it hardens, take out the wedge, bracket or wind cable, and then fill the wedge hole with mortar.

With the maturity and development of prestress technology, prestress has been applied to piers, especially post-tensioned prestressed reinforced concrete assembled piers. Its construction method is similar to the construction method of assembled column pier, except for the connection joint treatment technology during installation, the connection mode between segmental prefabricated members mainly depends on prestressed steel beams.

There are two kinds of prestressed steel bars used in post-tensioned prestressed reinforced concrete assembled piers and abutments: high-strength low-relaxation steel wire and cold-drawn grade ⅳ coarse steel bars. There are two kinds of prestressed tensioning methods for post-tensioned prestressed reinforced concrete assembled pier: the tensioning position can be tensioned at the top of pier cap; It can also be tensioned in the solid part of the pier bottom. In general, the top of pier cap is tensioned.

The main features of the prestressed steel beam with the pier cap tensioned (1) are as follows:

(1) Tensioning operators and equipment are working at high altitude, although tensioning operation is convenient, but the safety is poor; (2) The anchorage end of the prestressed steel beam can be directly buried in the pile cap without setting a transition section;

(3) At the position with the greatest stress on the pier bottom section, the strong bending capacity of prestressed steel tendons can be exerted.

(2) The main features of the prestressed steel beam tensioned at the pier bottom are as follows:

(1) The tensioning operators and equipment are on the ground, which is safe and convenient;

(2) A transition section should be set at the pier bottom, which should not only meet the requirements of placing prestressed steel tendon tensioning jacks, but also arrange more stressed steel bars to meet the stress requirements of this section in the operation stage;

(3) The relationship between the tension position of prestressed reinforcement and the vertical stress reinforcement in the transition member is complicated. The tensioning requirements of prestressed steel tendons and grouting requirements in prestressed pipes are consistent with those of prestressed concrete beams.

Application example:

Prefabrication process flow:

The total weight of the integral formwork truss system of the pier body of the bearing platform is about 420t, and it is designed with all-steel formwork, which is mainly divided into four parts: formwork system, truss system, sliding system and adjustment system. Precise positioning of the whole at one time.

Formwork system: outer mould of bearing platform, bottom mould of bearing platform, outer mould of pier body, template of reserved hole of bearing platform, inner mould of pier body and mould pressing of the top surface of bearing platform.

Truss system: platform truss and dock truss are supported by sliding system.

Sliding system: slideway and sliding device.

Adjustment system: fine adjustment measures after the formwork is in place, including horizontal adjustment of jack and wedge.

Concrete pouring of pile caps and bottom piers:

After the concrete pouring of the bearing platform and the bottom pier shaft is completed, the formwork shall be removed;

Fully prefabricated foundation by buried bed method;

A set of suspension system integrating hoisting, guiding, positioning and installation is adopted. In addition to the hoisting and hanging functions of precast piers, the system also has the positioning functions including verticality, vertical elevation and in-plane position of piers, and the positioning accuracy reaches 0.5 mm

Double cable tower structure

Concrete tower:

(1) engineering difficulties

The cable tower is high and flexible, which is easily influenced by sunshine, temperature change and wind, and its linear control is difficult.

The steel anchor box is used for the first time in the anchorage zone, which requires high precision and is difficult to control in construction.

The tower is high and far from the shore, so the measurement accuracy is difficult to guarantee;

There are many windy days and few effective working days.

(2) Key technologies

Control technology of concrete pylon;

Installation control technology of steel anchor box.

Cable tower:

Cantilever construction of three steel beams

Cantilever assembly method is to use a special cantilever crane supported on the completed cantilever to hang the prefabricated segment on the beam for assembly. After anchoring with shaped segments, the assembly length is generally 2-5m.

According to different hoisting methods, hanging components can be divided into floating crane hanging components, traction pulley block hanging components, continuous jack hanging components, cable crane hanging components and cantilever crane. The core of cantilever assembly is the transportation and assembly of beams, and the prefabrication of beam segments is the basis of cantilever assembly. The cantilever construction process mainly includes prefabrication, displacement, storage and transportation of beam segments; Hoisting the beam section; Transformation of cantilever beam system: Construction of closure section.

Four concrete segmental beam construction

Construction method of assembling precast segments (reinforced concrete segments and steel beam segments): the beam body of the bridge is divided into several segments longitudinally, prefabricated in a factory or a prefabrication yard near the bridge site, transported to the bridge site, and then assembled into a whole structure by applying prestress. It is a construction method developed in recent forty years and widely used in the construction of reinforced concrete and steel bridges.

Segment prefabrication mainly includes long-line pedestal method and short-line pedestal method.

Long-line pedestal method is to prefabricate all blocks on a long pedestal one by one according to the designed beam line type, so that a natural mating surface is formed between two blocks. The advantage of this method is that it is easy to control the geometric shape and no deviation is accumulated in the prefabrication process. The deviation of the manufactured block can be adjusted in time through the next block, and more points can be matched and prefabricated at the same time to speed up the construction progress. In addition, after the prefabricated members are demoulded in this way, they do not need to be transported to the storage place immediately.

But this method also has the following shortcomings: ① large area; (2) The pedestal must be built on a solid foundation. (3) Curved bridges need to form the required curvature. (4) Equipment such as pouring and curing can be moved.

Short-term pedestal construction means that each section is poured in the same formwork, with one end fixed and the other end poured first. The formwork is only one section in length, and the formwork is not moved, but the beam section is moved from the pouring position to the matching position, and then transported to the beam storage yard. This method occupies a small area, can form assembly line operation, improve the construction speed, and is suitable for engineering requirements with many changes in segment types and frequent formwork inversion. Its deficiency mainly lies in the requirement that the matching section must be placed very accurately, so it requires precise measuring instruments and equipment and accurate measurement and control methods.

Precast concrete segmental beam:

Handling of segmental beams: The lifting points for reasonable handling of segmental beams should be selected appropriately, and the stress of the segmental beams should be controlled within the allowable range.

Stacking of segmental beams: the stacking method of segmental beams must avoid warping and secondary stress, and the floor can provide good support for segmental beams; The lower part and a section near the web should be supported. When stacking, the weight of the upper segment should be directly transferred by the web to avoid the bending of the top plate and bottom plate to transfer force.

Segmented beam transport: Due to the heavy self-weight of concrete beam segments, the prefabrication site of concrete beam segments is generally selected near the bridge site, and transported to the bridge site by tire carriers, beam carriers or track carriers for assembly.

Segmented beam assembly:

(1) Full house support mode:

(2) Less support method (mostly used for steel beams):

Because the full-house support needs to deal with the whole foundation under the bridge, the cost is large and the construction period is long. Therefore, if conditions permit, segments are generally lengthened and assembled in the form of fewer brackets.

(3) step-by-step assembly method:

The superstructure is erected in one direction, one span at a time. Before prestressing, the prestressed section is temporarily supported by the lower cantilever beam (below the main beam) or the bridge erecting machine (above the main beam). The erected main girder can be simply supported on the pier, or several holes can be connected into a continuous structure through post-tensioning prestress.

It is suitable for small and medium span prestressed concrete simply supported beam bridges and span-by-span simply supported continuous beam bridges. The main equipment of this construction method is composed of steel truss guide beam whose length is less than (or more than) twice the standard span length, lifting flat car, guide beam movement and adjustment.

If the segments are assembled by hanging under the guide beam, the bridge erecting machine is called the upper guide beam bridge erecting machine; If the segments are assembled by supporting method above the guide beam, the bridge erecting machine is called the lower guide beam bridge erecting machine; The guide beam will bear the weight of the precast segment span.

(4) Balanced cantilever method:

Assemble the segments in symmetrical order with the pier as the center. Each fragment is integrated with the previous assembled fragment and self-balanced, and serves as the assembly basis of the next fragment. For each construction step, the cantilever structure ensures its safety and stability by tensioning the prestressed tendons arranged in the box girder segments. Segments can be hoisted by bridge deck supporting cranes, guide beams or ground cranes.

It is suitable for cantilever construction and prestressed concrete bridges with medium and long spans.

(5) Cantilever assembly method

Simple devices moving on the cantilever of the assembly structure are used to ensure the lifting, translation and assembly of the segments; The lifting equipment is installed on the flat car and runs on the bridge deck track.

Large and medium span bridges suitable for cantilever construction.

(6) Cable hoisting method

Construction method of suspended cable transportation installation component.

It is generally suitable for the construction of arch ribs or beams of arch bridges and main beams of suspension bridges.

Installation of external prestressed tendons: HDPE pipe is usually used for the jacket of external prestressed steel strand bundle. In order to make the steel strands arranged neatly in the outer sleeve and realize the adjustment and replacement of single steel strand as much as possible, the method of installing single steel strand in turn should be adopted.

Adjustment and replacement of external prestressing tendons;

1) When it is necessary to adjust the tensioning of external prestressing tendons, the protective sleeve can be removed, the anticorrosive grease can be removed, the steel strand can be tensioned or loosened with a special jack, and then the protective sleeve can be reinstalled and the anticorrosive grease can be injected;

2) When the external prestressing tendons need to be replaced, connect the new and old steel strands with special connectors, pull out the old steel strands, pull in the new steel strands, then stretch them, reinstall the protective sleeve, and inject anticorrosive grease.

Segmental joint splicing method usually adopts three methods:

1) dry joint is not processed;

2) Adhesive joint coated with epoxy resin;

3) Wet joints shall be poured with cement concrete.

Wet joints are mainly used to close segments or connect segments; Internal and external mixed prestressed bridges and circulating freeze-thaw areas should be bonded, and sealing washers should be set before assembling the joints of internal prestressed tendons.

I believe that after the above introduction, everyone has a certain understanding of prefabricated bridge construction, from pier to cable tower. Welcome to Zhong Da for more information.

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