(1) Construction of 0# section of continuous rigid frame beam
1 and 0# section construction method
0# section adopts pier top bracket platform for construction. The 0# section is long and the concrete volume is large, so it is planned to be poured in two layers. The outer formwork is a large steel formwork, and the corner is filled with composite steel formwork; Internal model adopts composite steel formwork, with a proper amount of wood mold; The end formwork adopts steel-wood composite formwork. When the 0# section is poured in layers, the horizontal construction joints should be chiseled, and high-grade cement slurry should be sprinkled before the upper concrete pouring to improve the cohesive force of the two layers of concrete. Formwork and reinforcement are hoisted by tower crane, and pump pipe is installed on derrick. Concrete is pumped into the formwork and vibrated by an internal vibrator.
2, 0# section construction technology
(1) construction process
Installation of pier top bracket platform and bottom formwork → Installation and unloading of 0 # section side formwork → Installation and unloading of wooden wedge or jack → Adjustment of 0 # section side formwork in place → Pre-pressing and unloading of support platform → Adjustment of formwork position and elevation → Binding of bottom plate reinforcement and web plate protruding reinforcement → Installation of bottom plate prestressed steel pipe and prestressed reinforcement → Acceptance by supervision engineer → Installation of longitudinal and transverse webs. Vertical prestressed pipes and tendons → Installation of web and diaphragm formwork → Supervision engineer's acceptance → Installation of roof bottom formwork → Binding of roof bottom tendons and pipeline positioning tendons → Installation of roof longitudinal prestressed pipes and transverse prestressed pipes and tendons → Installation of roof steel mesh → Supervision engineer's acceptance → Pouring 0# segmental concrete → Concrete curing → Dismantling end formwork → Chiseling concrete connection surfaces at both ends and washing with high-pressure water → Strength, elastic modulus and longitudinal tension of concrete after age meet the design requirements.
(2) Description of construction points
After the pier column construction is completed, steel plates with the thickness of δ= 20mm are used as the bottom supports between the pier walls of the rectangular hollow pier, and the transverse spacing of the steel plates is1.0m. After the cross-arm I-beams are installed on the steel plates, I-beams are laid longitudinally, and transverse I-beams 22 are set on the I-beams with the spacing of 0.5m The universal reinforcement connecting plates are embedded outside the rectangular hollow pier. After assembling the universal bar bracket and the cross arm I-beam, install the wooden bent frame to form the supporting surface of the bottom die of section 0#, and the bottom die of section 0# adopts the combined steel formwork. Install angle steel fence around the support and hang up the protective net.
After the scaffold is assembled, sand bag method or water tank method shall be used for preloading, and the preloading load shall be considered as 1.25 times of the total weight of section 0 concrete and other related construction loads.
During the construction of 0# section, according to the requirements of installing the hanging basket, all kinds of reserved channels and embedded steel bars are reserved, so that the hanging basket can be accurately positioned when assembled.
0# pipeline is dense. After concrete pouring, flush the pipeline with high-pressure water pipe. The vertical prestressed grouting hole is located on the inner side of the box girder web, and the grouting hole is set on the vertical corrugated pipe and sealed with sealant tape.
0# steel bars and pipelines are dense, and the steel beam pipeline position is fixed by positioning steel mesh, which is welded firmly on the steel skeleton with a spacing of 0.5m The steel skeleton welded by positioning steel mesh is spot-welded with horizontal steel bars to prevent the pipeline position from moving. When the position of prestressed pipeline conflicts with the skeleton reinforcement, keep the position of pipeline unchanged and move the position of ordinary reinforcement appropriately.
When pouring the 0# section web, concrete side windows and tamping holes should be left at the internal mold to reduce the free falling height of concrete, prevent concrete segregation and excessive impact on the pipeline, and avoid the violent collision between the tamping rod and the pipeline. After the concrete is poured to the reserved hole position, close the side window and reinforce it, and continue the upward construction.
(3) Installation of steel bars and prestressed pipes
Steel bars, prestressed pipes and embedded parts are manufactured centrally in the workshop. Steel bars are made into steel mesh and steel skeleton, which are transported to the construction site by car, hoisted by tower crane and installed manually. After the installation of steel bars, prestressed pipes and embedded parts is completed, install concrete funnels and cylinders. Self-check the position of steel bars, prestressed pipes and related embedded parts, and report to the supervision engineer for inspection after it is correct. Concrete pouring can only be carried out after it is confirmed to be qualified.
(4) Concrete pouring
Concrete is mixed centrally in the mixing station, transported horizontally to the construction site by the concrete mixer, and pumped vertically and horizontally into the mold by the concrete delivery pump. The concrete conveying pipeline is laid along the tower crane frame, and the pipeline outlet adopts artificial auxiliary cloth.
In order to prevent the increase of energy consumption in the process of concrete pipe plugging and transportation, and make the workability, aggregate gradation and sand ratio of concrete meet the technical requirements of pumping, compound admixture is added.
When pouring, it should be carried out in layers, evenly and symmetrically to prevent the pier from bearing eccentric pressure. Firstly, the bottom plate is poured, the concrete at both ends of the bottom plate is directly pumped into the mold, and the skylight in the middle is opened by the top plate and enters the mold through a series of cylinders; Pump the web concrete into the mould with skylight pump and tamp it. After pouring to a certain height, the skylight is closed, and the concrete is pumped into the mold from the top plate. Finally, the roof concrete is poured. When pouring concrete, strictly control the thickness of layered pouring and tamping quality, avoid directly tamping prestressed bellows, and prevent pipeline displacement and slurry leakage. After concrete pouring, it should be cured in time. When the concrete strength and corresponding elastic modulus meet the design requirements, the prestressed tendons shall be tensioned and grouted according to the design requirements and the principle of symmetry and synchronization. The vertical prestressed reinforcement used as the rear anchor of the hanging basket is not tensioned for the time being, and then tensioned after the hanging basket moves forward.
(2) Construction of continuous beam suspension irrigation section
1, hanging basket design scheme
The suspended irrigation construction adopts triangular vertical light hanging basket, which is driven by hydraulic pressure, moves forward as a whole, and the main girder formwork is in place at one time. The structure is simple, the force is clear, safe and reliable, and the operation is convenient. Two sets of hanging baskets shall be installed for each of the two main piers, and the construction shall be synchronized.
The hanging basket consists of the following systems: main beam system, external formwork system, bottom formwork system, internal formwork system, suspension system, anchoring system and walking system. The anchoring system includes a main anchor and a bottom anchor. The main anchor is to anchor the end of the main girder to the top surface of the box girder by using the vertical prestressed reinforcement of the box girder web. Bottom anchor refers to anchoring the back joist of bottom formwork to the bottom plate of box girder with back anchor rod. The traveling system comprises a traction oil cylinder, a track beam, traveling wheels and anti-falling wheels. The track beam is anchored on the top plate of the box girder through vertical steel bars, and moves forward step by step, and the traveling wheel and the anti-buckle wheel move forward along the track beam. When pouring concrete, support the main girder so that the walking wheels are not stressed. In order to ensure the smooth and beautiful appearance of concrete in each beam section after formwork removal, all external formwork adopts new steel plates.
2, hanging basket assembly and test
0# section after the completion of the cast-in-place, hanging basket assembly and test work.
Hanging basket assembly: During hanging basket assembly, small components are hoisted by tower cranes, and the procedures are as follows: install track beams at predetermined positions and adjust and fix them; Hoisting the main girder, installing anti-buckle wheels and walking wheels to anchor the main girder; Assemble the main beam-column truss, install the cable-stayed belt of the main beam, and tension the cable-stayed belt according to the pre-camber of the main beam; Hoisting the rear crossbeam, the front crossbeam and the walking truss to form a stable system for the main girder; Install the front sling; Assemble the bottom die. After the front and rear beams, longitudinal beams and formwork form an integral bottom formwork system, two 10t winches respectively lift both ends of the front and rear beams, and lift them in place at one time. The front sling is connected with the front joist, and the rear joist is anchored on the bottom plate of the box girder (bottom anchor). After demoulding of 0# section, the outer die moves forward. Install the internal model and its sliding beam, adjust the template and complete the installation of the whole hanging basket.
Static load test method of hanging basket: the static load test of hanging basket adopts the method of loading bottom formwork and hanging water tank step by step, and the sides of pier top are symmetrical. Select multiple stress points and use the wire rope pulley mechanism to distribute the load. The total load is 65438+ 0.25 times of the maximum segment weight. Load 50% for the first time, 75% for the second time, and finally 100%. Hold the load for 20 minutes after each level of loading, conduct a comprehensive inspection of the hanging basket and make records. When the situation is normal, load it at the lower level. The loading method is to pump water into the water tank. Testers are responsible for the division of labor, unified coordination, in case of emergency, to ensure timely drainage of water tanks and unloading of hanging baskets. The purpose of the test is to detect the stress and deformation of the hanging basket, eliminate the inelastic deformation of the hanging basket, and measure the elastic deformation, so as to provide a basis for the formwork elevation of 1# section. During the test, the strain gauge is used to measure the stress and the level is used to observe the deformation. The results meet the design requirements of hanging basket before use.
3. Building technology
Install the hanging basket in the 0# section and use it as the main anchor and bottom anchor → Support the main girder (unload the walking wheel) → Do the static load test of the hanging basket → Adjust the formwork pouring 1# section concrete → Stretch the longitudinal prestress → Unload the front sling and bottom anchor → Dismantle the formwork → Lay the track beam → Drop the main girder bracket → Lift the front and rear joists with the walking sling → Remove the main anchor → Check the walking wheel, buckling-resistant wheel and hydraulic traction. The two hanging baskets at both ends of the T-shaped structure move forward symmetrically → the hanging baskets walk in place → the main anchor and bottom anchor are used first → the main girder is supported → the formwork is adjusted in place, steel bars and pipes are bound, concrete is poured → prestressed tensioning → the next cycle is entered.
4, suspension irrigation construction process
The fabrication and binding of steel bars shall be carried out in strict accordance with the requirements of drawings and specifications. Steel bars shall be free from corrosion and pollution, welded firmly and installed accurately. The thickness of protective layer of reinforcement in each part shall meet the design requirements to prevent the reinforcement from being exposed. The reserved length of each longitudinal reinforcement shall not be less than 45d (reinforcement diameter), and all summer-planting reinforcement shall be welded and the joints shall be staggered.
Longitudinal bellows shall be installed in sections. When installing, the joint should be sealed tightly. Within the width range of 20cm around the joint, first wrap a layer of black tape, wrap three layers of plastic film on the black tape, and then wrap a layer of black tape, and fasten it with 22# iron wire.
After the construction of steel bars and prestressed pipes in each beam section is completed and passed the inspection, the hanging basket should be inspected again, especially the connection of each part, and the elevation and center line should be rechecked, and concrete pouring should be carried out after the rechecking is qualified.
In order to ensure the quality of the beam, each beam segment is poured at one time. Two concrete delivery pumps are used for balanced pouring construction, and the unbalanced weight at both ends of the T-shaped structure is strictly controlled within 25%. Before pouring the concrete of beam section, the concrete surface at the summer-planting site should be chiseled and washed with high-pressure water, and the formwork of suspension casting section should be closely combined with the completed beam section. When pouring concrete, it should be poured from the front end and connected with the completed beam section concrete at the root. And according to the bottom plate first, then the web plate and the top plate in order, left and right symmetrical casting. The top surface of the bottom plate shall be plastered before initial setting, and the concrete of the top plate shall be painted before initial setting.
Strictly control the water-cement ratio of concrete, the cement dosage per cubic meter of concrete is not more than 500kg, and the slump is not more than 16cm. In the process of concrete pouring, strictly implement the concrete pouring technology.
The concrete of the bottom plate is thick, and it is poured and compacted in two layers. The concrete of web and diaphragm is thin and high, and the steel bars are dense, so it is difficult for concrete to be vibrated into the mold. The chord pipe or pump pipe shall be placed close to the concrete surface and the inner window of the web, so as to ensure that the free pouring height of concrete shall not exceed 1.5m and the vibrating thickness shall not exceed 30cm；; Roof concrete is thin and large, and it is poured in blocks.
In the process of concrete molding and vibrating, operators are not allowed to walk on the concrete surface before the concrete is vibrated, so as to avoid the phenomenon of "empty" and "false compaction" of concrete caused by pipe sinking.
Concrete curing of beam section is covered with plastic film and tarpaulin, and cured by sprinkling water.
Install the embedded anchor bolt hole of the hanging basket and keep the position accurate to ensure the accurate positioning of the hanging basket in the next cycle.
5, longitudinal and transverse prestressed construction
When the strength, elastic modulus and age of the concrete in the beam section meet the design requirements, the longitudinal prestressing tendons are tensioned in the order of transverse, longitudinal and vertical; Symmetrical midline tension of longitudinal cable; First web, then roof, then down, then up, then long cable, short cable. The anchorage and fixture of prestressed tendons must meet the design requirements. Before use, the appearance, hardness and static anchorage performance should be checked, which should comply with the current national standard "Anchors, Fixtures and Connectors for Prestressed Concrete". Machines, equipment and instruments should be regularly maintained and calibrated. The calibration period of tensioning equipment should not exceed 6 months, the oil gauge should not exceed 1 week, and the jack should not exceed 1 month. Re-check if there is any abnormality during use. The blanking length of the steel strand is determined according to the design, and the steel strand is cut by a grinding wheel cutter, and then combed and bundled. Before the longitudinal prestressed steel beam passes through the beam, the impurities in the channel are washed with high-pressure water, and the moisture in the channel is dried by an air compressor. The longitudinal prestressed steel beam is pulled by winch and cooperated manually.
Tensioning process of longitudinal prestressed tendons: hole cleaning → threading → installing anchorage → placing tensioning equipment → tensioning the whole beam to 5% to control the tensioning stress → unloading the whole beam → tensioning to 10% to control the tensioning stress → unloading again → tensioning the whole beam to 10% to control the tensioning stress (measuring the initial reading) → dividing it into 50% and 75%.
According to the different stretching time, longitudinal prestressed tendons are divided into pre-straight beams, pre-bent beams and post-bent beams. Tension the pre-straight beam and pre-bent beam when pouring the T-structure, and tension the back beam when the main bridge is closed.
After the concrete of transverse prestressed beam of box girder reaches the designed strength, elastic modulus and age, it will be tensioned in one direction and staggered, and then grouted in time.
When prestressing, the prestressing tendon, anchorage and jack are located on the same axis. Prestress tensioning adopts "double control" tensioning method, with tension as the main force and elongation as the check. When the difference between the actual elongation and the calculated elongation is more than 6%, the reason should be found out and handled in time. In the anchoring stage, ensure that the internal shrinkage of prestressed tendons at the tensioning end meets the design requirements, and the number of broken wires and sliding wires of prestressed tendons does not exceed the specified value.
After the prestressing tendons are tensioned, the anchorage and tendon shall not be impacted, and the pipeline shall be grouted immediately.
In order to ensure the prestressed construction quality of the bridge, the construction technology, positioning steel bars and pipeline line type are strictly controlled during the construction. Before installing the pipeline, the burrs at both ends of the pipeline should be removed, and the quality and cross-sectional shape of the pipeline should be checked. When the pipeline may leak slurry, take measures to cut it off and use it after the cross sections at both ends of the pipeline are shaped. The joint between nozzle and pipeline and horn tube is sealed with adhesive tape to prevent slurry leakage. Prestressed pipelines shall be provided with positioning steel mesh pieces every 50 cm, and the deviation of pipeline axis after positioning shall not be greater than 0.5 cm. At the junction of the pipe and the horn, the pipe is perpendicular to the anchor pad. The clamp uses an open handle to drive two clips into the anchor ring evenly at the same time, so that the outer end faces of the two clips are in the same plane. When the height difference between the two clips is greater than 2mm, take it out and reinstall it. Check whether the inner wall of the anchor ring is rusted before use, and use it after rust removal. The concrete under the anchor shall be vibrated strictly, and the mortar and concrete in the anchor bell tube shall be removed before crossing the beam. Reasonably control the limit amount of the limit plate, so that the steel strand has no scratches and slight scratches. All kinds of prestressed tendons are cut by grinding wheel cutter.
According to the characteristics of the curved channel, in principle, a tee pipe is set in the middle of each steel beam. When the length of the steel beam is more than 60m, a drain pipe shall be set at the highest point of each wave crest of the corrugated pipe according to the principle of 20m apart. The drain pipe is made of steel pipe, which is 200mm above the concrete surface.
6, vertical prestressed construction
When the concrete in the beam reaches the designed strength, elastic modulus and age, the vertical prestressed tendons can be tensioned. The vertical prestressed steel bar adopts cold-drawn grade Ⅳ thick steel bar. The vertical prestressing tendon is perforated by galvanized double-wave metal corrugated pipe, one end of the jack is tensioned, the upper end is used as the tensioning end, the lower end is anchored in the beam body, and grouting holes are set as required. During construction, the quality shall be strictly controlled to ensure the accurate location of the pipeline, and the tensioning shall be carried out in strict accordance with relevant regulations to prevent the thick steel bars from being cut by electric welding and oxyacetylene. Before use, check to make sure there is no rust and bump. After blanking, both ends of the cap are pre-tensioned one by one on the tensioning platform, and the pre-tensioning is 100% of the design tension. Before tensioning, remove the cement slurry on the end rod and the base plate, check whether the base plate is horizontal, screw the nut to the root after it is qualified, and tighten the connecting nut. Then put the jack in place and center it, and connect the finish rolled threaded steel bar. The tensioning head of the jack is screwed into the rebar thread with a length of not less than 40 mm When the jack is opened, the prestressed rebar is tensioned. Due to the great loss of vertical prestress, it is tensioned to 100% control force for several times, and the load is 1 ~ 2 min. After the nut is tightened, measure the elongation. Compared with the calculated value, the deviation is within 6%.
7. Grouting
Grouting pump is used for grouting, and mortar mixer is used for mixing cement slurry. After mixing, the cement slurry conforms to the current relevant regulations, and the fluidity and bleeding performance tests are carried out to make cement slurry strength test blocks. After the stretching of longitudinal prestress and transverse prestress is completed, the redundant steel strand is removed, and the anchor sealing concrete is poured with formwork. At the same time, PVC pipes are used to reserve channels for installing grouting valves. Before grouting, clean the pipeline with high-pressure water to keep the pipeline dense. The cement slurry label is 40Mpa, the water cement ratio is not more than 0.4, and the cement is R42.5 ordinary silicon cement. The slurry conveying pipe is made of high-strength rubber pipe, with compressive capacity ≥ 1MPa, which is not easy to break during pressure grouting, and the connection is firm, so as to avoid falling off the pipe.
The cement slurry is filtered through a1.2mm screen before entering grouting pump.
Grouting should be carried out before the fluidity of mortar decreases (about 30 ~ 45 minutes), and the channel should be continuously poured at one time. In order to ensure the grouting quality of the suspension grouting section, the grouting of the upper section should be carried out after the concrete pouring of the lower section is completed. Install screw pipe and high-pressure hose on the grouting hole of anchor pad to lead out the concrete surface, and install valve on the high-pressure hose. Close the slurry outlet valve only after the thick slurry is drained, and keep the pressure at 0.6 ~ 0.7 MPa for 2 minutes. Close the slurry inlet valve only after there is no water leakage or slurry leakage.
When laying the track beam, find the center line and lay it flat, and check the anchors before walking to ensure that it is in normal working condition.
Before each concrete pouring, the main stress components such as main anchor, bottom anchor, sling and support shall be inspected to ensure their working reliability and construction safety.
When the hanging baskets at both ends move forward, they should be synchronized to prevent large unbalanced bending moment.
Strictly measure and monitor the hanging basket during construction, and fasten the safety net around the hanging basket to ensure the safety of bridge construction.
After the concrete strength, elastic modulus and age of the beam meet the design requirements and the longitudinal prestressed tendons are tensioned, the hanging basket can be transferred to the next stage.
(3) side span cast-in-place section construction
1 side span construction method
Across the cast-in-place section of T-shaped side span beam of 3 14 provincial highway bridge, a bracket is set on the abutment part, and the pier part is embedded by corbel method. After the formwork assembly of the cast-in-place section at the tail end is completed, the supporting part is pre-pressed by sandbag method, the bracket part is pre-pressed by hanging water tank, and sandbags are piled on the other side as counterweight. The preloading weight is 1.25 times of the total weight of beam section and other loads. After the preloading inspection is qualified, the formwork shall be reserved for settlement, and the plug formwork shall be installed for terminal construction. Before the bearing is installed, the bearing crosshair should be set out first (according to the pre-deviation measurement of the design bearing center line). Before lifting the bearing, clean the bearing bottom plate, and brush a layer of 1mm thick epoxy resin on the cushion stone at the same time, and then lift the bearing in place. After confirmation, pour epoxy mortar into the hole and insert the anchor bolt. After the anchor bolt is reinforced, clean the relative sliding surface of the bearing with acetone and install the bearing top plate. At this time, the whole bracket is installed.
2, T beam side span cast-in-place section construction technology
The construction technology of cast-in-place T-shaped side beam adopts bracket and embedded bracket. Construction process of cast-in-place section: leveling site → assembling support → adjusting bottom formwork elevation → installing bottom formwork and outer formwork → binding beam reinforcement, installing corrugated pipe and vertical prestressed coarse reinforcement → installing internal formwork → pouring beam concrete and curing → dismantling internal formwork → dismantling support after closing and tensioning.
Key points of construction: In order to ensure the construction quality of cast-in-place beam section, internal formwork shall be installed before pouring the bottom plate of box girder, and continuous pouring shall be conducted in the order of "bottom plate → web plate → top plate".
3, prestressed concrete continuous T structure closure section construction points
(1) The mid-span is closed on the hanging basket, and the side span is closed on the hanger between the bracket and the cantilever end.
(2) Closure section is the key to continuous rigid frame construction. Strictly control the construction process to ensure that the centerline and elevation error of the closure mouth are within the allowable range of the specification.
(3) Before closure, the elevation and axis of the top surface of the box girder shall be jointly measured, and the temperature change, relative elevation change and axis deviation of the beam body shall be continuously observed, and the length change of the beam body under the influence of temperature shall be observed. The continuous observation time is not less than 48h, and the observation interval depends on the temperature change and beam structure.
(4) The design of closed rigid supports and the tensioning of temporary beams must be carried out in strict accordance with the design requirements. The locking time of rigid support is determined according to the continuous observation results, which requires symmetrical, balanced and synchronous locking within the time interval of minimum relative deformation and minimum temperature change of the beam. In order to reduce the locking time, before locking, prepare the work (such as putting the jack in the appropriate position, etc.). ) should complete the tension of the closed temporary beam. After the welding of rigid support is completed, all closed temporary beams shall be tensioned within 1h according to the design requirements.
(1) The concrete in the closure section should be one grade higher than that in the beam, and the early strength is required, so micro-expansive concrete should be adopted.
(2) Construction sequence of the closure section of the whole bridge: first close the side span, then close the middle span (3). One end of the side span is erected on the cast-in-place front pier beam, and the other end is supported by the embedded suspenders on the side cantilever box girder of the side span. * * * Set four suspenders, two of which are anchored on the bottom plate of box girder, and the other two are anchored at the wing root of box girder. Lock the beam when it is closed to bear the compressive stress caused by the longitudinal extension of the cantilever due to temperature rise. At the same time, some longitudinal prestressed continuous beams are used as temporary beams for tension, and prestress is used to offset the tension caused by the shortening of both ends due to cooling. In order to make the locking structure achieve the same deformation as the materials piled on the cantilever and the cantilever, the locking stress of the temporary locking device should be greater than the friction resistance of the movable supports of the piers on both sides.
(4) Construction procedure of mid-span closure section: Complete each suspension pouring section of T-shaped structure → A hanging basket moves forward and is anchored on the opposite T-shaped structure → Adjust the center line and elevation of hanging basket formwork → Bind bottom web reinforcement according to design requirements → Install prestressed pipes and vertical prestressed tendons → Bind roof reinforcement and install horizontal and vertical prestressed tendons pipes → Apply jacking force → Install closure bracket and lock box girder → Complete concrete pouring of closure section.
(5) The closing section is supported by equal strength welding with the lowest temperature on that day, the box girder is locked as required, and the concrete pouring of the closing section is completed within 2h. At the same time, when pouring, the heavy objects will be lifted gradually, and the water tank will be selected as the heavy objects, so that the concrete pouring can be completed in the closed section under constant load.
(6) When the concrete strength of the closed section meets the design requirements, the second batch of prestressed beams will be tensioned in the order of first long and then short.
(4) Deflection control of cantilever casting beam
1, the basic principle of deflection control
Linear control refers to modifying the geometric deformation of the bridge span structure by using control software in the cantilever construction stage of prestressed concrete continuous rigid frame bridge, 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=Hi 1+f where:
Hi-actual formwork support elevation of I-beam section hi1-design elevation of I-beam section f-additional construction camber (positive and negative downward) considering various factors.
The key to linear control of cantilever beam construction is to analyze the structural deflection change state 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:
Cantilever deflection caused by the following factors in the formation stage of single T:
Self-weight of beam concrete;
Hanging baskets on beams and other construction loads;
Function of tensioning cantilever prestressed tendons.
During the closing phase, continuous deflection caused by the following factors will continue to occur:
The role of concrete weight and counterweight in closed section;
Dismantle formwork hanger or beam section installation equipment;
Function of tensioning continuous prestressed tendons.
In the above process, there will be deflection caused by prestress loss caused by elastic compression, shrinkage, creep, prestress tendon relaxation, pore friction and other factors.
2. Pre-camber calculation
Basic assumption: concrete is a homogeneous material. In the process of construction and operation, the stress Δ h of beam section is less than 0.5 ra, and it can be considered that creep, strain and stress are linear within this stress range; The superposition principle is suitable for creep calculation, that is, the creep deformation caused by stress increment can be accumulated and summed; Ignore the influence of prestressed reinforcement and ordinary reinforcement on the stress and deformation of concrete.
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 rigid frame bridge 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
The elevation Hi of the segment leading edge construction formwork consists of two parts (design elevation Hi 1 and comprehensive camber fi), namely: design elevation hi 1 = ho+δ hi, where:
Ho is the elevation Δ h of the pier top 0# section, and the comprehensive pre-camber increment caused by the slope of the beam is fi=fi 1+fi2+fi3, where:
Fi 1 is the pre-camber of the segment, and the incremental value fi3 reserved for the deformation of the hanging basket is the influence value of foundation settlement.
Therefore, the construction elevation of segment leading edge is:
Hi = hi1+fi = ho+Δ hi+fi1+fi2+fi3 The self-designed unbalanced self-propelled hanging basket is used in the construction of the main span, and its deformation includes elastic deformation of truss, elastic deformation of front sling and inelastic deformation.
Calculation of truss deformation: simplify the truss into hinged form, and calculate its elastic deformation according to the different weight of each beam section.
Deformation calculation of the front sling: simplify the front joist 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 untested hanging baskets, the deformation value of tested hanging baskets (each set of hanging baskets is manufactured by the same factory and process) is set at the first hanging basket construction. For the hanging basket that has been tested, it is considered that inelastic deformation has been eliminated and will not be considered during construction.
Construction lofting: During the construction of the beam section, the midline control survey shall be carried out according to the control points provided by the design, and the measuring point of formwork lofting shall be located at the front edge of the bottom formwork beam section. When the formwork is erected, the above template elevation is converted into coordinate elevation. 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.
Material parameter measurement: measure the raw material performance, mixture ratio, slump and bulk density of concrete in each beam section; The elastic modulus Eh, strength value Rba and creep coefficient φ of concrete at 7d, 28d and prestress age were measured. Pre-stressed materials with measured elastic modulus Ey and standard strength Rhy; Measure the construction load value and the center of mass.
Construction observation: According to the construction sequence, each suspension casting section shall be observed for 5 times, that is, before pouring concrete after the hanging basket is in place, before pouring concrete in the beam section, before stretching the longitudinal prestressing tendons, after stretching the longitudinal prestressing tendons, and before moving the hanging basket (that is, before the next operation).
Each observation shall record the elevation change, measured temperature, settlement of pile caps, etc. 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.
For more information about project/service/procurement bidding, and to improve the winning rate, please click on the bottom of official website Customer Service for free consultation:/#/? source=bdzd