1 comparison of advantages and disadvantages of cast-in-place concrete rafters and prefabricated rafters
1.1 comparison of techniques
Rafters themselves are the supporting stress components of roof overhangs in ancient buildings, but in modern antique buildings with reinforced concrete, rafters have become an antique decorative structure. The modeling of concrete eaves of cast-in-place structure needs one-time casting. The design spacing of rafters is usually 1mm, and the gap between rafters is small and numerous, which leads to great difficulty in construction, high requirements on template flatness and size standard, complicated cutting and splicing of the original bamboo and wood plywood, and difficult construction in cracks if the cornice template is spliced, with large dimensional deviation, and difficult to grasp the appearance after formwork removal. The accuracy of woodworking operation is extremely high. And sporadic template fragments can not be removed, so they need to be polished and chiseled, otherwise the appearance will be affected. In the process of formwork construction, the requirements for supporting structures are also high, and the gaps of each rafter need to be reinforced. The steel bar installation of precast concrete rafters is also very complicated, and the position of each rafter needs to be reinforced separately, and then it is bound and connected with the overhanging steel bar into a whole, so the process is complicated. If the rafters are changed into prefabricated structures, they need to be installed and embedded before the cast-in-place structure of concrete roof overhangs is constructed, which is simple to operate, especially the accuracy requirements for woodworking templates are greatly reduced. Precast concrete rafters can process several templates at one time according to the drawing size and be reused. Before concrete pouring, steel bar installation and embedded parts installation can be carried out on the ground at one time, and the operation is simple. The wooden rafters can be processed according to the drawings, and the embedded parts can be fixed in place at the same time. Not only the overhanging part is easy to operate, but also the associated support and structural members can be restored to the conventional practice. It can greatly save the construction period and reduce the construction difficulty. It will be explained in the subsequent construction technology of this paper.
1.2 Economic comparison
The price of cast-in-place rafters, concrete and steel bars is similar to that of prefabricated wooden rafters. If precast concrete rafters are used, precast concrete rafters can save the amount of formwork and ensure the visual quality of concrete rafters. However, it is necessary to provide a larger site for prefabrication. There is no need for secondary grinding and correction, and the corners can be guaranteed to be beautiful. In the later stage of one-time cast-in-place concrete rafters, angle grinder and other machinery will be used to polish the concrete surface for the second time. All shuttering templates of cast-in-place rafters are one-time investment, and the labor and material construction costs are high. If prefabricated rafters are used, the above costs can be saved, and only the pre-processing of prefabricated wood can be considered. Using the advantages of moderate strength, convenient cutting, smooth and beautiful surface, the finished concrete sheet can be used instead of bamboo and wood plywood. Precast concrete sheet is cheap, far lower than bamboo and wood plywood formwork used in existing projects. The structural overhangs and precast wood rafters are poured together and molded at one time, without mechanical polishing and beautification of the concrete surface, and subsequent construction procedures such as sandpaper, painting and painting can be directly carried out. In the past practice, workers prefer to use this method, which can save labor and material economic costs and shorten the construction period.
1.3 safety comparison
The integrity of the spliced formwork is poor, and the protruding parts cannot be accessed. Later, it is necessary to construct steel bars and concrete pouring on it. There are difficulties in both steel binding and concrete vibrating. On the high edge of the roof, unstable roof formwork is a big safety hazard. The supporting structure of the rafters is connected with the roof as a whole, and the formwork removal of the cantilever members needs to meet the design requirements, which affects the formwork removal time. If prefabricated rafters are used, the size of the prefabricated rafters under the overhangs is 1mm×1mm, which can be used as the secondary keel supported by the formwork. The upper part of the formwork is made of concrete sheet, and the surface is flat, so that the constructors can directly stand on the surface of the formwork for construction, which provides favorable conditions for the safety of construction operations. The prefabricated rafters themselves can adopt independent and safe supporting structures, and can also be connected with beam and slab frames. It will not affect the subsequent construction and the formwork removal of the main beam slab.
2 construction process and operation points of prefabricated rafters
2.1 steel pipe scaffold with overhanging eaves for antique roof modeling
After computer drawing lofting, the elevation and position of the bottom of the rafters are determined. Steel pipe scaffold support shall be erected, and the quality standard of support erection shall meet the construction scheme and design requirements. Because it takes a long time to dismantle the cantilever members, it is suggested to adopt a separate supporting structure to avoid affecting the subsequent construction process.
2.2 Installation of elevation control formwork for wooden rafters (or concrete rafters)
Install the supporting formwork for fixing the probe of wooden rafters (or concrete rafters) on the steel pipe scaffold (as shown in Figure 1), nail the fixing baffle at the rafters, and determine the elevation and position to prevent wooden rafters (as shown in Figure 1). In this project, the steel pipe scaffold is used to support the beam and slab frame, which can improve the overall stability and save the rental materials.
2.3 wooden rafters (or concrete rafters) installation
Calculate the elevation and position of the tail of the wooden rafters (or concrete rafters), use bamboo and wood plywood templates, install them according to the design size, and process them into serrated grooves in advance to install the wooden rafters (or concrete rafters) in place at one time. Install the wooden rafters (or concrete rafters) into the groove of the existing formwork, and the tail will be 2mm into the beam. The top of the wooden rafters (or concrete rafters) is fixed on the control formwork. This picture shows the structural effect of wooden rafters, which are not anchored in concrete beams. If the concrete rafters are prefabricated, they can be inserted into the beam for 2mm according to the design size requirements (as shown in Figure 2).
2.4 Precast concrete sheet cutting and site installation
Use an angle grinder to cut the precast concrete sheet into pieces with the size of 12mm× the design length of the rafters, lay them on the gaps between wooden rafters (or concrete rafters), and use iron nails or steel nails to reinforce the concrete sheet with the wooden rafters (or concrete rafters). This practice is the essence that prefabricated rafters can replace concrete cast-in-place rafters. The author has used formwork in previous projects, but the cost of the new formwork is slightly higher, and the surface of the formwork is too smooth to be constructed during the subsequent putty or plaster scraping construction. There are hidden dangers of decay and peeling of wooden formwork under long-term weathering. Therefore, the finished concrete sheet was used in this construction. Its function is to replace the original wooden template and avoid the problem of cutting flat-fell seam of wooden template. No mold removal, low price and convenient operation, which can perfectly achieve the expected effect, and also avoid polishing process for later putty and paint construction. There will be no hidden dangers of decay and peeling. Avoid rework, and save the construction period and the economic cost of labor and materials. In the subsequent construction, overhanging steel bars and overhanging concrete can be directly installed on it.
2.5 cantilever formwork and steel bar installation
(1) When processing and installing cantilever formwork and steel bar, all indexes and dimensions should meet the design and specification requirements; (2) During formwork construction, the length, thickness and section size of overhangs shall be guaranteed; (3) Reinforcement processing shall ensure the design reinforcement length, hook position, size and spacing. Pay attention to the stirrup height and the number of cushion blocks, ensure the thickness of protective layer and meet the requirements of cantilever member design and steel bar structure. Make inspection records of concealed works, and notify the supervision representative to stand by when necessary.
2.6 concrete pouring with overhangs of antique roof modeling
When the overhangs are poured together with the roof concrete, attention should be paid to the protection of finished products during the pouring of overhangs, and it is forbidden for people to step on them. Pay attention to the fact that the vibrating time should not be too long when the internal vibrator is used for vibrating, so as not to affect the integrity of the concrete sheet, and it is forbidden to touch the concrete sheet to prevent the concrete sheet from cracking. Avoid touching the steel nails and hooks fixed on the wooden rafters, otherwise the wooden rafters will be displaced or loosened. If loose rafters are found later, they can be patched and installed from the bottom again. Other matters needing attention in construction are the same as "Floor Concrete Pouring Method".
2.7 form removal
After reaching 75% of the design strength of concrete, the formwork of overhanging side wall can be removed, and the subsequent putty and paste hanging construction can be carried out in advance. Cantilever is a cantilever member, and its support system needs to be removed after the concrete strength of cantilever reaches 1% of the design value. When dismantling, attention should be paid to the reinforced connecting members (such as spikes) between the wooden rafters (or concrete rafters) and the supporting formwork, and they should be dismantled in advance to avoid damage to the wooden rafters (or concrete rafters). After the formwork is removed, the next working procedure, such as putty and paint, can be directly carried out on the surface of the rafters. If the wooden rafters are found to be loose or falling, measures should be taken to install and repair them in time. See the attached figure for the effect after form removal. This project is an antique Ming and Qing traditional wooden rafters (as shown in Figure 4) or flying rafters, also known as overlapping rafters (as shown in Figure 5). Follow-up putty or gypsum construction can be directly carried out on its surface, and wooden rafters should be constructed in accordance with antique building practices, which is not described in this paper. For details, see Code for Construction and Quality Acceptance of Ancient Buildings (JGJ159-28)[1].
3 Other precautions
3.1 Main materials
Wood rafters (or concrete rafters, prefabricated in advance), spikes, finished concrete sheets with a thickness of 6 mm to 8 mm, steel pipe scaffolding, supporting fasteners, wooden formwork, commercial concrete, etc.
3.2 main machines and tools
chainsaws, angle grinders, cranes, etc.
3.3 working conditions
according to the requirements of drawings, specifications and construction scheme, steel pipes, templates, concrete and other materials are selected to meet the requirements. Prefabricated wooden rafters (or concrete rafters). Cut the concrete sheet for later use. Reasonable selection of concrete slump and technical disclosure. The equipment business such as vibrator is safe and reliable after trial operation. At the same time, it is equipped with full-time technicians for maintenance at any time. Before concrete pouring, the wooden pattern is evenly coated with release agent and wetted with water. Check the power supply and wiring, and get ready for lighting at night.
3.4 quality standards
The quality standards involved in this paper shall be followed: Code for Construction and Quality Acceptance of Ancient Building Construction (JGJ159-28)[2] and Code for Construction Quality Acceptance of Concrete Structure Engineering (GB524-215)[3].
3.5 finished product protection
During concrete pouring, finished products such as steel bars and concrete sheets shall be well protected. When the formwork is removed, the side formwork can be removed only when the concrete strength can ensure that its surface and edges and corners are not damaged due to the removal of the formwork. Demolition of bottom formwork and winter construction formwork can only be carried out after the compressive strength of the specimen cured under the same conditions reaches the schedule. The structure with removed formwork and support can bear all service loads only after the concrete reaches the design strength grade; When the effect of construction load is more unfavorable than that of use load, temporary support must be added after accounting. Template engineering operation organization, should follow the unity of formwork support and formwork removal by an operation team to perform operations. Its advantage is that formwork support only considers the convenience and safety of formwork removal. When formwork removal, personnel are familiar with the situation and easy to find the key points of formwork removal, which is beneficial to the progress, safety and protection of formwork and accessories.
4 Conclusion
In the modeling construction of the roof rafters of antique buildings, from the feasibility of technological operation, the economy of labor and materials, the construction period and the safety of construction, the prefabricated wooden rafters (or concrete rafters) are superior to the original construction method of cast-in-place rafters. It can be predicted that the design and construction of cast-in-place concrete rafters will eventually be replaced by prefabricated rafters. It is suggested that in the construction of antique rafters in the future, we should communicate well with the construction unit and design unit in advance, and apply this technology to the design in advance, so that the design unit can optimize the design of overhanging parts and save the economic cost of the construction and construction unit.
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