3. 1.2.3.3. Construction of side span cast-in-place section
(1) side span cast-in-place scheme
The cast-in-place section of the side span adopts bowl-button scaffold pole to build a full-house scaffold for formwork casting, and then it is closed with the suspension casting section. After scaffold erection, before formwork erection, static preloading must be carried out to observe the deformation, so as to eliminate foundation settlement and elastic compression deformation of scaffold, check the bearing capacity of scaffold, and pour the side span cast-in-place section after preloading is stable.
2 template settings
The external formwork of cast-in-place box girder segment adopts large steel formwork, the internal formwork adopts composite steel formwork, and the bottom formwork adopts composite steel formwork to stick phenolic resin coated plywood. Before the bottom die is installed, the lower longitudinal distribution beam is placed first, and then the transverse distribution beam is placed. After adjusting the elevation according to the reserved deformation and pre-camber, lay the bottom formwork and support. After the bracket installation is completed, install the box girder side formwork. The straight part of the side formwork adopts large steel formwork, which is connected with the steel support. Steel formwork should be based on beams.
The promotion of this department has received firm support. If the template has a big gap, it needs special treatment.
The formwork adopts angle steel longitudinal ribs, which are connected by bolts, and the joints are plastered with cement epoxy resin slurry. Set enough tie rods and supports, strictly control the deformation of formwork, and ensure that the poured concrete conforms to the specified size and shape.
(3) Reinforcement engineering
Construction of the same suspended irrigation section.
(4) beam concrete pouring
Concrete pouring starts from one end, the whole section is inclined and segmented, and it is continuously poured horizontally in layers. The pouring distance of the front and rear upper and lower layers shall not be less than 1.5m, and the layering thickness shall not be more than 30cm. Plug-in vibrators are used to vibrate with connected vibrators.
5] Prestressed construction
The same hanging casting part.
[6] Removal of support
Non-load-bearing side formwork can be removed only after the compressive strength of concrete reaches 2.5Mpa, and the bottom formwork and bracket can be removed only after the concrete strength meets the design requirements and is closed, tensioned and compacted.
Process block diagram of side span cast-in-place section (see Table 2).
3. 1.2.3.4. Construction of closure section
(1) construction sequence of closure section
The bridge section is a continuous beam with 48+80+48m three holes, and the middle hole is closed after the side span is poured in place.
2 Closed cross-section hanging beam
The closure section is directly constructed by cast-in-place hanging basket method. When the last suspension casting section is hoisted, holes shall be reserved in advance according to the position of the hanger under the hanging basket. During the construction of the closure section, the bottom formwork is hung on the poured concrete beam section by using the hanging basket bottom formwork hanger, and then the concrete of the closure section is poured by using the hanging basket external formwork. See the next page for the construction scheme of closure section.
(3) Temporary restraint and stability of the closure.
There are two main problems in the construction of closure section:
(1) The fresh concrete in the closure section will shrink during the hardening process, and the cantilever beams at both ends of the closure will shrink with the decrease of humidity, so the connection performance between the closure section and the cantilever beams at both ends cannot be guaranteed.
⑵ The closed section is different from other cantilever construction beam sections. With the increase of humidity, the cantilever beam will extend prematurely to participate in the pressure of the system, and the newly poured concrete will have a certain strength in time.
Premature loading will destroy the cemented structure inside concrete and affect its strength.
In order to solve the above two problems, the following measures were taken in the construction of the closure section:
Firstly, steel members are added to the cross sections of cantilever beams at both ends of the closure opening, and temporary prestressed elements are relatively tensioned at the ends of the two cantilever beams at the closure opening. When the closure mouth is busy, it should be restrained and stabilized when the concrete of the closure section is about to be poured. The pouring of the closure section should be carried out at the lowest temperature in a day and completed in the shortest possible time.
(4) Preparation before concrete pouring in the closure section.
(1) Installation of steel bars and prestressed pipes
There are non-prestressed steel bars and three-way prestressed pipes in the closure section, so it is difficult for the beam to pass after closure, so the steel strand is inserted into the corrugated pipe before closure.
When the steel grain line passes through the beam, the wave tube should be supported by a bracket to prevent the wave tube from being damaged and cracked. Both ends of the corrugated pipe joint are fixed ends, and there are steel strands inside, which makes the joint connection difficult. Corrugated pipe joint can be a common sleeve at one end and an iron sleeve at the other end, which is coated with epoxy resin and wrapped with adhesive tape.
(2) Construction survey and observation of closed section
The setting of observation points in the closure section is the same as the linear control requirements. In the process of pouring, precise level should be used for measurement and observation, so as to deal with problems in time.
③ Other preparatory work.
After the pouring and tensioning of the last section of each T-structure is completed, the unnecessary construction load on the T-structure is removed, and other construction loads are moved to the 0 # section, so that the construction load on the T-structure is in a relatively balanced state, and the relative deformation and "shear difference" displacement at the end of the closure section are avoided, which will affect the closure accuracy. At the same time, the elevation of the whole bridge deck and the axis of the bridge are jointly measured to observe the temperature change. The observation time is not less than 48 hours, and the observation interval is 1 hour. The relationship curves between horizontal deformation and vertical deformation of beam end and temperature are drawn.
5] Pouring and curing of closed concrete
① Apply counterweight.
In order to make the structural system in a stable state during the pouring of closed concrete, a counterweight is applied to the cantilever end after the rigid support is locked, and the counterweight adopts the water pressure of the water tank, which is equivalent to the weight of closed concrete. When pouring concrete, gradually reduce the counterweight according to the pouring speed.
⑵ Complete the closed concrete pouring within the time specified by the supervisor and design institute that the temperature is low and the temperature difference does not change much, and the temperature will start to rise after the concrete pouring is completed.
(3) The mixture ratio test of concrete in closed section should be carried out in advance. Concrete with small water cement ratio and a certain proportion of micro-expansive agent has early strength on the premise of ensuring the design strength of concrete. During construction, we should strengthen construction management, strengthen vibration, pay attention to watering and prevent cracks. The grade of concrete should be one grade higher than that of beam concrete. Concrete should be cured. The part of the light beam exposed to the sun must be shielded.
[6] Guarantee measures for winter construction of closure section
In the closing section of low-temperature construction, it takes a long time to complete the system transformation by applying prestress because of the slow solidification speed of concrete. In this long construction project, the concrete that has not reached the strength in the closed section is greatly influenced by temperature change, shrinkage and creep of completed structural concrete and construction load. In order to ensure the construction quality, the following measures are taken during the construction:
(1) Construction shall be carried out in strict accordance with the design procedure;
(2) To strengthen the structure with rigid supports inside and outside the closure section, it is required that the hanger used for pouring has sufficient vertical rigidity, and at the same time, 2-4 temporary prestressed tendons are tensioned to ensure that the concrete in the closure section is not bent and stretched before the system conversion.
(3) Strengthen the formwork manufacturing process, require the formwork surface to be flat, reduce the longitudinal constraints of the formwork on the beam body, and remove the temporary anchorage facilities on the pier in time to ensure that one side of the box girder can freely expand and contract during the concrete solidification of the closure section.
(4) Concrete with early strength, high strength, small shrinkage or slight expansion is used in the closure section, water consumption is strictly controlled, concrete strength is accelerated, prestress is applied as soon as possible, and system transformation is completed.
⑤ Take effective measures to strengthen maintenance, to ensure that concrete is not damaged by freezing in hardening and curing engineering, to ensure that concrete is in a state of heating and compression in early curing engineering, and to reduce early shrinkage cracks of concrete.
3. 1.2.3.5. System conversion
The main span of this bridge is designed as a 48+80+48m continuous box girder with three holes and one connection, and the piers and beams are hinged. Temporary consolidation measures are taken at the top of pier 1# and pier 2# in suspension casting construction to form a temporary T-shaped structure, and the T-shaped beam is always in a state of negative bending moment change during suspension casting. With the closure of T-shaped structures in turn, the beam is also transformed into the form of alternating distribution of positive and negative bending distances when the bridge is completed. During this period, the distribution of prestress and the transformation of beam structure system are realized in turn by timely lifting the consolidation support. The beam-span structural system transformation should be carried out after the longitudinal continuous prestressed tendons in the disturbed section are tensioned and grouted. Support reaction adjustment should meet the design requirements.
(1) 1 # pier and 2# pier are suspended;
(2) ① and ③ span beam pier segments are closed in situ, and the consolidation measures of ① and ② segments 1# and 2# prestressed tendons are lifted to complete the structural system transformation;
(3) The beam span (2) is closed, and the continuous reinforcement of the span (3) is tensioned to form the design structure of the main girder.
3. 1.2.3.6. Linear control
Basic principle of (1) linear control
Linear control refers to the use of control software to correct the geometric deformation of bridge-span structure in the cantilever construction stage of prestressed concrete continuous steel beam, so as to make it reach the ideal state of design.
The basic principle of linear control is: according to the calculation, provide the final deflection change value (that is, vertical deformation) of each segment of the beam, set the construction pre-camber, and adjust the front elevation of formwork installation of each beam segment accordingly.
Expressed by the following formula:
Hi=f
Where: Hi- the actual formwork elevation of the I-th beam section.
Hi '- the design elevation of the first I-beam section
F comprehensive consideration of the influence of various factors and increase the construction camber (positive, negative downward).
The key to the linear control of cantilever beam construction is to analyze the changing state of structural deflection in each construction stage and step and determine the gradually completed deflection curve. According to the construction process, the main factors affecting the deflection are as follows:
(1) Deflection caused by the following factors in the formation stage of T.
A beam section concrete weight;
B. Hanging basket on the beam and other construction loads;
C the role of tensioning cantilever prestressed tendons.
(2) In the closing phase, the deflection caused by the following factors will continue to occur.
Closed section of concrete weight and counterweight function;
B. Dismantle formwork hanger or beam section installation equipment;
C the role of tensioning continuous prestressed tendons.
(3) In the above process, there will be deflection caused by prestress loss caused by elastic compression, shrinkage, creep, prestress tendon relaxation and channel friction.
⑵ Pre-camber calculation
① Basic assumptions
Concrete is a homogeneous material.
B during construction and operation, the stress Δ h of the beam section is less than 0.5 ra, and within this stress range, it can be considered that creep, strain and stress are linear.
C superposition principle is suitable for creep calculation, that is, creep deformation caused by stress increment can be accumulated and summed.
D ignore the influence of prestressed reinforcement and ordinary reinforcement on the stress and deformation of concrete.
② Pre-camber calculation
On the basis of the above assumptions, considering the difference of shrinkage and creep caused by different concrete ages of each segment, the shrinkage and creep process of continuous steel beam construction is divided into the same time periods as the construction process, that is, pouring new beam segments, tensioning prestressed tendons, moving hanging baskets, system closure and so on. The number of structural units in each time period is consistent with the actual number of structural beam segments. In each time period, the structure is comprehensively analyzed, and all the node displacement increments generated in this time period are obtained. After analyzing all time periods, the final pre-camber value can be obtained by superposition.
(3) Determination of construction elevation of segment leading edge
① Determination of construction elevation
The elevation Hi of vertical formwork for segment leading edge construction consists of two parts (design elevation Hi' and comprehensive pre-camber fi), namely: design elevation Hi'= H0+δHi.
Where H0 is the elevation of 0# section of pier top.
Δ hi is the increment caused by the slope of the beam.
Comprehensive camber fi=fi 1+fi2+fi3
Where fi 1 is an arch.
Fi2 increment value reserved for basket deformation
Fi3 is the influence value of foundation settlement.
Therefore, the construction elevation of segment leading edge is:
Hi = Hi `+ fi = H0+δHi+fi 1+fi2+fi3
② Deformation calculation of hanging basket
The self-designed cable-stayed light hanging basket is used in the construction of the main span, and its deformation includes: elastic deformation of truss, elastic and inelastic deformation of front sling.
Calculation of primary truss deformation
The truss is simplified as a hinged form, and its elastic deformation is calculated according to the different weight of each beam segment.
B deformation calculation of front sling
Simplify the front beam of the bottom die into a continuous beam with elastic supports, calculate the stress of each support according to the actual load of each beam section, and then calculate the deformation of the sling according to the stress situation.
Inelastic deformation test
Inelastic deformation of hanging basket is measured by hanging basket pressure test. For hanging baskets that have not been subjected to pressure test, the deformation value of hanging baskets that have been subjected to pressure test (each set of hanging baskets is manufactured by the same factory and process) is set during the first hanging basket construction. For the hanging basket that has passed the pressure test, inelastic deformation is considered to have been eliminated and will not be considered during construction.
(4) Site construction control
Invite the design unit to guide the linear observation of cantilever beam construction to ensure the accuracy of linear control.
① Construction control block diagram
② Construction lofting
During the construction of the beam section, the center line control survey shall be carried out according to the control points provided by the design, the setting-out measuring point shall be located at the front of the bottom formwork beam section, and the template elevation shall be converted into coordinate elevation when the formwork is erected. During the construction process, the center line and temporary leveling points of the whole bridge should be checked regularly to ensure the accuracy of the construction survey of each T-structure.
③ Material parameter measurement
A. Measure the raw material performance, mixture ratio, slump and bulk density of concrete in each beam section.
B. Measure the strength value Rba of elastic modulus Eh, and estimate the creep coefficient φ and prestress age of concrete for 7d and 28d.
C the elastic modulus Ey and standard modulus Rby of prestressed materials are measured.
Measure the construction load value and the center of mass.
④ Construction observation
According to the construction sequence, each suspension casting section shall be observed for 5 times, that is, after the hanging basket is in place and before pouring concrete; After pouring the concrete of the beam section; Before tensioning the longitudinal prestressed tendons; After the longitudinal prestress is established; Before moving the hanging basket (that is, before the next operation). Every observation should record the elevation change. The measurement results are timely fed back to the linear control team in the form of tables (uniformly formulated during construction), and some unexpected situations are reflected in the remarks column. The linear control team will feed back the computer calculation results and template elevation to the technicians in time.
⑤ Precautions
A construction should be carried out in strict accordance with the requirements of balanced construction to avoid inaccurate measurement data caused by unbalanced construction load and bridge deck debris.
B. Construction observation shall be carried out before sunrise every morning, and observation shall not be carried out under the conditions of high temperature, strong light and strong wind.
C to a person, instrument for observation, to avoid the error caused by the measurement on the high pier.
D. Observe frequently, record frequently and give timely feedback.
E strictly control the performance of beam-making raw materials, and basically achieve the unity of the whole bridge.
5] cantilever pouring beam quality control standard
(1) The allowable deviation of cantilever casting beam section construction shall meet the following requirements:
A concrete strength: meet the design requirements;
Axis deviation of bridge b:10mm;
C bridge top elevation:10mm;
D the allowable deviation of fabrication and installation of steel skeleton shall be handled according to the provisions in the table below.
3. 1.2.3.7. Construction of bridge deck system
Install the sidewalk guardrail so that it is unified on a plane within the full length of the straight line.
The elevation at the top of the guardrail is consistent with the slope of the line, and the inside of the guardrail column is consistent.
The sidewalk board is firmly placed, the asphalt hemp reinforcement is tightly packed, and there is no three-legged phenomenon in the board. The slab joints of the whole straight line segment shall be consistent.
The railing metal shall be derusted according to the specification requirements, and the antirust paint, topcoat and spray paint shall be uniform, and the paint surface shall be bright without flowing and leakage.
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