Keywords: cantilever casting; Temporary consolidation; System transformation; Temporary locking; collect
The closure section usually connects the two sides by concrete pouring. After concrete pouring, it takes a certain time from initial setting to steel beam tensioning. During this period, due to the influence of day and night temperature changes and uneven sunshine, the early shrinkage of newly poured concrete, the shrinkage and creep of the completed structure, the change of structural system, the change of construction load and external force, etc., will cause the deformation and internal force of the structure. Connecting with the engineering practice of the closure construction of prestressed continuous beam of Jidong Bridge in Shi Wa, this paper puts forward the problems that should be considered in the closure section of continuous beam, the key technical measures and main quality control methods in the closure construction.
1 general situation of the project: the 48m+80m+48m continuous beam structure across Xinpu Highway is a three-span prestressed concrete continuous box beam with a total length of177.5m. The calculated span arrangement is that the middle fulcrum beam is 6.65m high, and the middle 9m section of the 13.25m straight line section and the side span beam are 3.85m high. The hanging basket suspension casting method is adopted for construction. 1 1# block is the closure section, and the closure sequence is side span first and then middle span. After the concrete is poured in the closed section of the side span, the prestressed tendons of the side span are tensioned to relieve the temporary consolidation of the top of the main pier, so that the cantilever T-shaped structure becomes a simply supported structure; After the mid-span closure, two simply supported structures form a continuous beam, completing two system transformations.
2 closure scheme
2. 1 Construct T-structure from pier 2 and pier 3 to the maximum cantilever end 10, set up side span 1 and cast-in-place section support of pier 2, and pour cast-in-place section concrete at the same time, then dismantle No.2 hanging basket, install hanging baskets of side span closure section 1 4, pour steel beams of side span closure section, and dismantle cast-in-place slab.
2.2 pouring midspan closed section; Tension the steel beam of the closed section in the middle of the span and dismantle the hanger; Transfer the bearing of pier 3 in the middle span to complete the closure of the whole bridge, as shown in Figure 2.
The transformation of the main girder system of the bridge is from the rigid frame state before closure to the statically indeterminate structure after closure. After folding, the cable tension completes the system conversion.
3 problems to be solved in the closure section
3. 1 hanger installation problem
After the construction of the cast-in-place section and the middle section of the side span is completed 1 1, install the hanger of the side span closure section, lock the side span closure section, pour the concrete of the side span closure section, tension the T 12, F 16 and the lateral and vertical prestressing tendons of the side span, and dismantle the hanger of the side span closure section. After tensioning the side spans T 12, F 16 and the lateral and vertical prestressing tendons of the side spans, the top of the main pier is temporarily consolidated, so that the permanent support of the main pier is stressed, and the side spans T 14, b1-b14 are tensioned. In addition to the hanger of the mid-span closure section, the suspension bracket should be removed.
After concrete pouring, the change of temperature will cause the expansion and deformation of the beam, and the different sunshine temperatures on the left and right sides of the beam will also cause the distortion of the beam. It is necessary to temporarily lock the closed part to prevent its relative deformation. The temporary locking of the closure section should resist various external forces such as temperature stress and unbalanced bending moment at both ends of the T-shaped structure to ensure the construction safety of the cantilever T-shaped structure and no cracks in the closure section.
3.2 temporary locking
3. 1. 1 before the closure construction, strengthen meteorological observation, choose the closure construction when the weather is stable, and arrange to weld the rigid skeleton of the closure section at the lowest temperature in a day (2:00~4:00 am, 20~25℃), and the welding should be completed quickly to form a rigid joint. According to the calculation, the force caused by temperature stress is relatively large, and only the side span formwork is binding on the beam at this time. If the binding force is less than the temperature stress of the beam, the beam will definitely move. As long as the temporary locking force of the closed section is greater than the binding force of the formwork and support, the closed section can be relatively non-deformed.
3. 1.2 According to the construction experience of cantilever casting box girder bridge in our department, combined with the characteristics of box girder segment of Shiwu Passenger Dedicated Line, the concrete method of rigid skeleton is: embedding 10mm thick steel plate at the bottom of box girder segment, setting φ25U anchor bar at the bottom of plate, and laying local reinforcement under embedded steel plate. The connecting material is 30a channel steel, and every two steel plates are welded into four groups. The embedded steel plate on the roof is flush with the top surface of the box girder, that is, it is located below the heightening platform. After the rigid skeleton is removed, concrete is poured to make up for it, so as to ensure the flatness of the heightening platform. Before pouring concrete, the rigid skeleton should be welded quickly, and the welding should be carried out at the lowest temperature in a day. Before welding, the beam at the welding place should be covered with geotextile and wetted with water to prevent the beam concrete from being burned during welding.
3. 1.3 Strictly control the deflection of cantilever construction, so that the height difference between the two cantilever ends is not more than 5mm and the center line deviation is not more than 10mm when closed. If the rigid connecting rod is not closely attached to the embedded steel plate due to the longitudinal slope of the bridge, it can be plugged with a thin steel plate to ensure the welding quality. After the closure section of the side span is locked by rigid skeleton, under the action of temperature change, the slight sliding caused by the change of beam body is mainly offset by the friction between the concrete and the formwork in the closure section of the side span because the temporary support of the middle pier has not been removed. After the construction of the side span cast-in-place section is completed, the square timber supporting the side bracket shall be removed to enable the bracket to move.
3. 1.4 Temporary tensioning beam. According to the fact that the steel strand is temporarily stretched to straighten the longitudinal prestressed beam to reduce the deflection and deformation of the beam, and the prestressed beam with shorter length is stretched to better restrain the closure section, 2T 13 and 2B8 longitudinal prestressed beams are temporarily tensioned to offset the tensile stress of the beams at both ends of the newly poured concrete in the closure section when the temperature drops.
3.3 Concrete pouring in closed section
The day before concrete pouring, the whole beam surface and box body should be watered to cool down. According to the weather forecast, when there is strong wind to cool down within 3 days, concrete pouring will not be carried out in the closure section. Concrete connection surfaces at both ends shall be completely chiseled, wetted and washed clean. The newly developed micro-expansive concrete is used in the closure section, and the site technical director confirms that the slump of concrete entering the mould is not more than 20cm through automobile pumping. When pouring concrete, the commercial concrete factory and the site must have the person in charge of the project department present for on-site command. After concrete pouring, special personnel shall spray water for curing, after initial setting, cover plastic film and geotextile for moisture curing, and special personnel shall record the concrete temperature.
3.3. 1 side span closure
When the side span is closed, the temporary consolidation of the top of the main pier can be satisfied without the unbalanced bending moment generated by the counterweight. According to the elevation measurement results, if the elevation deviation at both ends of the closure section is within 15mm, there is no need for counterweight leveling; If the height difference is above 15mm, press the counterweight on both sides of the T-shaped structure to make the end elevation of 1 1 block equal to the end elevation of the cast-in-place section 12 block.
In order to reduce the influence of deflection and deformation caused by weight change on both sides of T-shaped structure on newly poured concrete during concrete pouring, it is suggested to speed up the pouring, so that the deformation can be completed before the initial setting of concrete, or add the same counterweight on both sides of T-shaped structure, and cancel the side span counterweight simultaneously with concrete pouring. Water tanks are placed on both sides of the T-shaped structure for counterweight, and the weight of water in the water tank on the side of the side span is reduced synchronously with the concrete pouring weight of the side span closure section, so as to keep the moment balance on both sides of the T-shaped structure.
3.3.2 Mid-span closure If the height difference between the two sides of the mid-span closure section is within 15mm, the mid-span will not be weighed; If the deviation is greater than 15mm, use a water tank to press heavy objects on one side of the mid-span closure section to make the elevation of both ends of the mid-span closure section consistent. After the concrete strength reaches 50MPa, the counterweight can be cancelled.
3.3 Tension of closed section
After the concrete strength of the closed section meets the design requirements, relax the side formwork and internal formwork of the hanger. Then the other prestressed tendons are tensioned in turn, and the bottom die can be removed after tensioning is completed. Before concrete pouring in the closure section, all prestressed steel strands should be inserted into the corrugated pipe. In order to prevent the corrugated pipe from blocking during concrete pouring, besides sealing the interface with twisted tape, it is necessary to check whether there are holes opened by electric spark one by one. At the same time, when pouring concrete, the concrete mouth should avoid the position of corrugated pipes as much as possible, and three-way exhaust holes should be buried at the top of each longitudinal corrugated pipe in the closed section to prevent the pipeline from being blocked in the closed section and affect the grouting quality. The tensioning sequence of prestressed tendons shall be carried out according to the design requirements.
4 closed construction quality control method
4. 1 must be balanced and symmetrical. Unnecessary construction load on the cantilever beam is removed before closure, so as to avoid relative deformation and "scissors difference" displacement during closure construction and affect closure accuracy.
4.2 The concrete in the closure section should be one grade higher than that in the beam section, and it should be strong early. It is best to use micro-expansive concrete, and do a good job of tamping and curing.
4.3 Before closure, the elevation and axis of the top surface of box girder shall be jointly measured, and the elevation of the center line shall be gradually adjusted to ensure smooth closure.
4.4 After the concrete pouring of the closure section is completed, strengthen the maintenance and keep the web, bottom plate and top plate of the box girder moist. At the same time, the adjacent sections on both sides of the closure section are sprinkled with water to reduce the box girder deformation caused by temperature difference.
4.5 The tensioning sequence of prestressed tendons in the closure section is first top plate, then bottom plate, first long beam, then short beam, and tensioning is symmetrical.
5 conclusion
The continuous beam adopts the scheme of external temporary locking steel, which has been proved by practice to ensure the safety of engineering construction, and there is no crack on the surface concrete of the beam body in the closed section, which proves the correctness of the closure scheme.
References:
[1] Design and construction drawing of prestressed concrete double-track 48+80+48m continuous beam for ballastless track.
[2] Technical Guide for Construction of Railway Bridges and Culverts on Passenger Dedicated Lines.
(3) Interim Standards for Quality Acceptance of Railway Bridges and Culverts for Passenger Dedicated Lines.
Wu Wenqing. Lecture on cantilever pouring construction knowledge of prestressed concrete continuous beam bridge
[5] Li Youming. Construction of continuous beam closure section and system conversion.