Concrete-filled steel tube structure is a composite structure formed by filling concrete in steel tube, which has the characteristics of both steel structure and concrete structure, effectively exerts the mechanical characteristics of concrete and steel, and greatly improves the compressive and deformation resistance of the structure by using the hoop effect of steel tube. Concrete-filled steel tubular tied arch bridge has been widely used in highway and urban bridge construction in recent years because of its beautiful shape, rigorous structure, scientific stress and reasonable economy. However, due to its high technical content, strict technology, numerous procedures and difficult construction, it is of practical guiding significance to summarize the construction management experience of such bridges in the future. Taking Nanhe Bridge as an example, this paper discusses the fabrication and construction control of steel pipe arch ribs.
2 Project overview
Nanhe Bridge, located in the west of liyang city, Jiangsu Province, is a newly-built bridge across the Nanhe River on the expressway of Nanjing and Hangzhou, with a total length of 659.44 meters ... Technical standard: the design load exceeds 20, and the load is-120; The clear width of the bridge deck is 2×15.25m; ; The channel level is planning level 5; Clearance standard is 50.0×5.0m；; ; The earthquake intensity is 7 degrees.
The main span of Nanhe Bridge is a through prestressed concrete tied arch structure with rigid tied arch. The calculated span is 126.28 m, the arch axis is a quadratic parabola, the rise is 25.256 m, and the rise-span ratio is 1: 5. The arch rib is made of dumbbell-shaped concrete filled steel tube, the outer diameter of upper and lower steel tubes is 1 lOem, and the wall thickness of steel tubes and webs is 1. 4em, filled with 40 # micro-expansive concrete (see figure 1). The wind strut is K-shaped and consists of welded steel pipes with outer diameters of 1 10cm and 80cm. The whole bridge is equipped with 4 arch ribs, 6 pairs of wind braces (see Figure 2), 38 cross beams and 76 suspenders.
3 steel pipe arch rib processing
3. 1 Main process flow
Inspection of raw materials → lofting → blanking → processing → assembly and welding → assembly and web welding of pyrotechnic micro-bending section → assembly and welding process inspection of related parts (accessories) of suspenders → installation of exhaust and slurry discharge holes → pre-assembly of arch ribs → painting and rust prevention.
3.2 Key points of processing scheme
Segmental division: In order to facilitate hoisting, arch rib steel pipes are fabricated in segments. According to the field hoisting capacity, each arch rib of this bridge is divided into two embedded segments of arch foot and seven intermediate hoisting segments, and each K-shaped wind-resistant column is a segment.
Manufacturing method: the steel plate is rolled into a circular tube by a plate rolling machine; Assemble and weld 6m and 17m arch rib pipes and wind bracing pipes with the designed foundation length; The design axes of the upper and lower arch rib pipes are slightly bent and formed by pyrotechnics, and then positioning, welding and segmental assembly are completed on the special jig. Each wind bracing pipe section is assembled on another plane fixture.
Allowance for large joints: In order to ensure that the construction scheme and technology of each step meet the design requirements and reach the specified deviation accuracy, the allowance of 80mm is added to the large joints of the upper and lower arch rib pipes, which is reserved when assembling segments, and the alignment is only carried out at the calculated length of segments. Increase welding compensation: Considering the segment assembly, web welding will affect the distance between the upper and lower pipes of each arch rib segment, and 5mm can be added along the radial line as welding compensation to ensure the design geometry size.
Scribing: Mark the radial lines of the arch rib tube at 0℃ and 180℃ as scribing lines for pyrotechnics and segment assembly inspection.
Installation marking: In order to facilitate on-site installation, before the arch rib is pre-assembled, the control points of each joint during on-site installation should be marked by measuring the radial line and the station number line, and certain protective measures should be taken during painting.
3.3 Key points of construction control
(1) According to the relevant acceptance specifications and process requirements provided by the design documents, the specific acceptance items and standards of each process are compiled.
(2) The lofting shall ensure the correctness and completeness of all supporting tables, nesting cards and blanking sketches, and indicate the angle, size, name, data, etc. The position of samples and sample strips in the subsequent process.
(3) Check the blanking of all parts, and the out-of-tolerance part shall not flow into the next working procedure; Parts cut by flame must be cleaned and polished, and those with thermal deformation must be corrected before use.
(4) The straightness and angle of the groove edge shall meet the tolerance requirements.
(5) The jig frame should have enough rigidity to control the structural deformation, and the center line, positioning datum line and auxiliary line of the jig frame should be marked when necessary.
(6) All assembly shall not be carried out by force, so as not to damage the parent material, and the pipeline shall be installed strictly to control the clearance. After welding is completed, it should be corrected in time.
(7) Strictly control the fireworks temperature of the arch rib tube, and it is forbidden to cool it with water.
(8) Develop a detailed professional measurement process, and the testing instruments must be inspected by the measurement department, and the environmental impact should be considered when operating.
(9) Improve safety measures.
Four key processes
4. 1 Weld welding
Welding is a professional and standardized work, which is very important in steel structure engineering construction. Therefore, it is the key to monitor the whole processing process of the steel tube arch rib of this bridge.
4. 1. 1 Preparation before welding
According to the design documents and referring to relevant standards, codes and regulations, the Construction Technology Department formulates the principles of welding technology, and defines the welding methods, process measures, quality standards and acceptance specifications. The welding methods of arch rib steel pipe of this bridge include manual arc welding, CO2 gas shielded welding and automatic submerged arc welding.
Process evaluation: Welding process evaluation is the basis of steel structure manufacturing, and the construction unit must complete the process evaluation document in combination with the actual project and keep it as a completed document. According to the weld forms of butt joint, lap joint and T-joint, the corresponding welding methods shall be determined and shall not be changed at will.
Within the scope of 25mm, it should be cleaned as required to remove oil, rust, scale and dust on the surface. Clean before welding, and the ceramic gasket must be constructed according to the operation procedures.
Production test plate: In order to ensure the weld quality, the arch rib structure of Nanhe Bridge is equipped with production test plate, which has the same material, thickness, groove and rolling direction as the corresponding weld. The mechanical properties are tested according to the corresponding technical standards to ensure that its parameters meet the specification requirements. Formulate the evaluation process of production test plates, clearly define the purpose, scope of application, reference standards and process content, and ensure the welding quality of welds.
4. 1.2 welding requirements
Factory welding: In the process of manufacturing and assembling the steel tube with arch rib of Nanhe Bridge, V-shaped grooves are used for longitudinal and circumferential joints, one side is welded, two sides are formed, and ceramic gaskets are stuck on the reverse side (inside the tube). The welding seam filling process is divided into four steps, namely, co2 gas shielded welding bottoming filling step, submerged arc welding filling step and covering step. Arc striking plate and arc extinguishing plate are installed at the beginning and end of longitudinal seam welding respectively, and the groove type is the same as that of longitudinal seam welding. The girth welding adopts rolling jig and overhead welding. The welding of each working procedure should be completed at one time. When welding is stopped for some reason and welding is continued, it is not allowed to strike an arc from the parent metal, and the arc striking place should be planed or polished into a slope lap of 1: 4, and the lap length should be not less than 50 mm ..
On-site installation: Manual arc welding is used for on-site installation, and symmetrical welding is used for the intersection line and segment butt joint of wind-resistant column and arch rib. After the arch rib closure section is positioned, welding can only be carried out after reaching the design closure temperature.
4. 1.3 Weld quality inspection
The weld grade of Nanhe Bridge meets the requirements of Grade I weld in GB50205. The appearance quality of the weld should be beautiful and tidy, and the size should meet the design and process requirements, so as to ensure that there are no welding defects such as cracks, pores, slag inclusion, flash and arc pits. Its internal quality requires that after 24 hours of welding, 100% of the weld length should be inspected by ultrasonic wave, and 10% should be inspected by X-ray.
4.2 piecewise linear machining
Arch rib segment is the basic unit of arch axis, and its linear shape depends on the slight bending of arch rib tube. Micro-bending method of initiating explosive device is a design curve formed by initiating explosive device and external force. According to the design arch axis equation, the coordinates of all control points of each upper and lower arch ribbed steel pipe are determined by computer calculation as the basis for micro-bending and measurement and control, and the specific acceptance standard process documents are compiled.
Implementation steps: design and build a model jig → mark its vertical and horizontal center lines and auxiliary lines → determine the position of each station line on the jig, accurately install the bracket and the equipment for applying external force → put the ribbed steel pipe in place, set the plane coordinates → draw out the heating area → heat in different regions and apply external force.
Control point:
(1) Ensure that the position of the steel pipe to be bent on the upper jig is correct, that is, the radial line and the center line of the steel pipe should be consistent with the longitudinal center line and the center line of the jig;
(2) heating should be carried out symmetrically from both ends to the middle;
(3) The external force should be applied symmetrically and reasonably, and the pressure should be gradually and appropriately;
(4) Emphasize the coordinated control of pressure and temperature;
(5) The heating belt should avoid the circumferential seam position, and the agreed bandwidth can be appropriately adjusted according to the actual situation;
(6) Observe and measure the coordinates of the arch axis at any time to control the bending.
4.3 Pre-splicing of arch ribs
Whether the arch rib molding conforms to the design line shape is the key to the completion of the bridge. Therefore, after the completion of the arch rib segment, the span, arch axis, levelness (lateral deviation of arch axis) and the accuracy of the suspender position must be comprehensively checked through pre-splicing, so as to prepare for on-site hoisting. The pre-assembly of arch rib factory of Nanhe Bridge adopts the horizontal whole pre-assembly scheme.
Pre-assembly method: it is carried out symmetrically from both ends, starting from the embedded section of the arch foot, in full accordance with the on-site sectional hoisting sequence. The manufacturing allowance of segments shall be left in the closed section (removed during site installation), and the manufacturing allowance of other segments shall be removed (only the girth allowance is left).
Control point:
(1) Choose a plane site and space with sufficient rigidity, and equip it with corresponding lifting equipment and jig tooling.
(2) The testing instruments shall be sent to special measuring units for calibration.
(3) Make a pre-assembly plan according to the pre-assembly process.
(4) Carry out pre-assembly according to the pre-assembly plan, properly save the pre-assembly measurement data, and take certain measures to protect the pre-assembly control points for use during installation.
Through factory pre-assembly, the machining accuracy of the arch rib segment was tested, the machining deviation of the arch rib segment was eliminated to the maximum extent, and all the preparations for on-site hoisting were completed.
5 concluding remarks
With the rapid development of national traffic construction, as a model of rational use of composite materials in bridge engineering, the development of concrete filled steel tubular arch bridge is in the ascendant. However, there are no qualitative and quantitative norms and procedures for the manufacture, arch forming technology, pre-assembly technology, quality control and supervision procedures of the steel tube arch rib, which is an urgent problem to be solved. I hope the above can have some reference value for this.
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