Keywords

1 Introduction

The 19th National Congress of the Communist Party of China clearly established the concept of green development and promoted the development of new industrialization. Prefabricated buildings and green buildings are important carriers. However, there are still some problems restricting development in the process of popularization and practice of prefabricated buildings.

The prefabricated building is different from the cast-in-place structure, but the design principle adopted is “equivalent to cast-in-place” [1]. Therefore, the design idea of “first overall analysis and then split design” is often adopted, which easily leads to a wide variety of prefabricated components that are split. Recombination after splitting is also prone to problems such as steel bar collision. The core feature of prefabricated buildings is standardized design and assembly construction. However, experts and scholars have done a series of studies on the design and construction of prefabricated buildings and pointed out that the development of domestic prefabricated buildings lacks a standard system [2,3,4], and standardized design is not yet available. Mature, lack of innovation ability [5], and there are many problems in on-site construction, which unreflect the advantages of prefabricated buildings [6]. Therefore, the study of standardized design methods and technologies that are conducive to site simplification and high-efficiency assembly is of great significance to the development of prefabricated buildings.

This paper aims at the insufficient design standardization and construction conflicts in the development of prefabricated buildings. Based on the case of a prefabricated building project in a university in Hezhou, the prefabricated building and structure are analyzed from the aspects of building type selection, component optimization, structural calculation, and deepening design. The whole design process is analyzed, the key issues in the design and implementation of the prefabricated building project are comprehensively introduced, and the BIM forward design and some cutting-edge prefabricated new technologies are adopted, hoping to be useful for similar projects in the future.

2 Project Introduction

This project is located in a university in Hezhou City. The design service life is 50 years. The fire resistance rating of the building is Class II, the roof waterproof rating is Class I, and the seismic fortification intensity is 6 degrees. The building base has a total land area of 1138 m2, a total construction area of 3436 m2, a total of 3 floors, the first and second floors are 4.8 m high, the three floors are 3.9 m high, and the total height of the building is 13.5 m. It adopts a prefabricated integrated frame structure, the prefabrication rate of single buildings reached 62.9%, and the assembly rate reached 70.1%.

2.1 Architectural Design

The design of prefabricated buildings is different from cast-in-place structures. The design of prefabricated concrete buildings must not only meet the functional requirements of buildings in various parts of our country, but also meet the requirements of industrialized construction. Based on the principle of “less specifications, more combinations”, follow the modular coordination Standards, to carry out standardized, modular, and integrated architectural design.

2.1.1 Graphic Design

The nature of the building is a teaching building, and its main function is physical teaching and scientific research model display. Considering structural safety and economic rationality, it adopts a large bay and deep plan layout. The size of the column net is 7800 mm × 7800 mm, and the plan layout is regular and flat. It is more conducive to earthquake resistance and cost saving. In addition, the large-bay and large-depth plan layout reduces the types and quantity of prefabricated components, increases the repetition rate of components, and improves economic benefits. The graphic design is based on the different use functions of the room, and the depth is extended with the basic modulus. A variety of basic plane shapes are determined to form different personalized planes. However, the central location of the large bay and the deep depth is unfavorable for daylighting. Therefore, the internal atrium is opened, and the openings on each floor are gradually increased to increase the sense of hierarchy of the building. The atrium is equipped with a steel staircase and the top roof is a glass roof.

2.1.2 Facade Design

The building facade is a direct display of architectural artistic style. This project comprehensively coordinates the architectural effects and architectural functions for the facade design. The wall beams of this project are flat, highlight the column lines, and do not have too many external decorative components, which conforms to the industrial production characteristics of prefabricated building components with few specifications and high repetition rate. The façade wall adopts a new type of wall panel produced locally in Hezhou, and is designed as an integrated exterior wall. This wall panel integrates enclosure, waterproofing, heat preservation and heat insulation, and is assembled on site. The building façade enhances the façade effect through the combination of the spaced arrangement of doors and windows. The overall structure is simple and individual. The door and window openings and façade partitions all meet the requirements of modular design, presenting a standardized and diversified door and window enclosure system. And the component specifications are unified to meet the requirements of industrialization of prefabricated buildings.

This project is a regular symmetrical model, and the grid size is 7800 mm × 7800 mm. In order to increase the reuse rate and reduce the types of components, the project considers deepening design and component splitting in the architectural and structural design of the project, designing and merging components with similar sizes, reinforcements, and structures, and prefabricating components based on the standardization of component dimensions. Standardized and modular design. Finally, one column size is 600 mm × 600 mm, two main beam sizes (330 mm × 650 mm, 330 mm × 850 mm), one secondary beam size (250 mm × 600 mm), and one floor size (120 mm).

2.2 Structural Design

The project uses PKPM software for structural design, and the structural parameters are analyzed and calculated according to the actual situation of the project. Finally, the construction drawings of the structure were revised and perfected, considering the prominent problem of steel reinforcement collision in prefabricated buildings, and manual adjustments were made according to the results of the reinforcement calculated by the system. To reduce the collision problem of steel bars to a certain extent, and secondly, try to unify the reinforcement of the components as far as possible. The reinforcement should be controlled within ±5% according to the calculation area of the structural software, and the reinforcement area should be as large as possible to be larger than the calculation area to reduce the types of prefabricated components and improve the prefabrication of the factory. Production efficiency.

2.2.1 Checking Calculation of Bending Bearing Capacity

The basic components of prefabricated building beams and columns are prefabricated in the factory, transported to the construction site and then cast on site. According to the calculation results of reinforcement, many prefabricated beam components need to be equipped with multiple rows of steel bars at the bottom. When pouring on-site, it is easy to have too much steel bars in the node area, which is inconvenient for the connection, placement and pouring of the steel bars. The negative bending moment at the support is relatively large, and the lower bending moment is small, so consider a beam with multiple rows of steel bars. Under the effect of meeting the bearing capacity, part of the steel bars can be prevented from extending into the support. According to the concrete structure design specification GB50010-2010 [7], the flexural bearing capacity of the support can be checked. The check calculation results show that for beam members with multiple rows of steel bars, only the bottom row of steel bars can meet the requirements of flexural bearing capacity (Fig. 1). Therefore, the steel bars are cut according to the requirements of the plan atlas, and some steel bars do not extend into the support to reduce Anchor the steel bars at the cast-in-situ nodes to avoid serious collision problems caused by too dense steel bars in the node area, and the concrete is not densely poured.

Fig. 1.
figure 1

Schematic diagram of truncated longitudinal bars

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2.2.2 Checking Calculation of Shear Capacity

According to JGJ1-2014 [8] connection design requirements of the prefabricated concrete structure technical specification, the joint surface of the prefabricated component and the post-pouring concrete, grouting material, and setting material should be provided with a rough surface and a keyway. For the prefabricated slab components, it is sufficient to set the rough surface with a depth of not less than 4 mm and the post-cast concrete laminate layer to be combined. The rough surface should be less than 6 mm. The end face of the precast beam and the bottom of the precast column should also be designed with keyways according to the regulations, and their shear capacity should be checked. Therefore, the beam ends of this project are equipped with long keyways and checked according to the regulations. For parts that do not meet the requirements for shear resistance The required cross-section is added with shear-resistant steel bars in the post-cast part (Fig. 2).

Fig. 2.
figure 2

Additional short ribs on parts that do not meet the shear requirements

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3 Pilot Application of Prefabricated Technology

In order to improve the construction efficiency of prefabricated buildings and solve the quality problems of prefabricated buildings such as steel bar collision and improper concrete pouring, this project has applied many systematic comprehensive technologies of prefabricated concrete structures with high technical content and high economic benefits. The floor slab of the project adopts “reinforcement-free” laminated truss floor slabs. The columns in the dotted line range of the figure are prefabricated columns with corrugated pipes. The beam-column connection and the main and secondary beam connections adopt the new U-shaped steel bar ring buckle connection technology and steel card grooves.

3.1 Laminated Truss Floor Slab

Components of the laminated floor slab are made in the factory, prefabricated slabs are required to have lapped steel bars to be spliced on-site to form a force-bearing whole. The current conventional method is “beard rib” connection, but because of its “reinforced” structure that it has caused an increase in the process of opening holes, pouring and leak-proof grout sealing, etc. which is not convenient for industrial production. The “beard tendons” are also easy to bend and deform during transportation and hoisting, and even cause precast concrete damage and missing corners. Affecting the appearance and quality of the laminated board [9]. Formwork needs to be installed at the “beard ribs” of adjacent floor slabs on the construction site, and collisions also bring a lot of complicated procedures to the construction. Therefore, this project adopts the method of “no ribs”[10] in the joints between the laminated slabs (Fig. 3). At the same time, the composite wire mesh technology is used to improve the crack resistance of the slabs, so that the thickness of the precast laminated slabs is reduced to 30 mm, which is better than ordinary concrete. The laminated board is reduced by half, saving costs, and can be free of formwork and plastering, which greatly improves the efficiency of construction and installation.

Fig. 3.
figure 3

Closely assembled laminated floor slab

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3.2 U-Shaped Steel Bar Ring Buckle Connection Technology

On the third floor of the project, the ⑥ axis y direction AB and BC span reinforcements are 4 25, 2 25+2 22 respectively, and the left and right x direction beams are 2 25 reinforcements. At the intersection of the ⑥ axis and the B axis, the lower part of the joint is thick, and the amount is large, which is easy to produce Rebar collision problem. In order to reduce the collision problem of steel bars in the node area and ensure the effective transmission of node force, this project adopts U-shaped steel bar ring buckle connection technology. As shown in Fig. 4, the upper part of the laminated beam is the post-cast part, and the upper longitudinal steel bars of the beam can penetrate the nodes or supports, because the node reinforcement is too dense, the lower beams and the column reinforcements collide, and the lower longitudinal reinforcements of the beam undergo U-shaped bending. After folding, anchor into the beam concrete without extending into nodes or supports. Then arrange the closed circular steel bars and the U-shaped steel bars at the beam end between the longitudinal bars in the core area of the beam-column node to stagger each other, thereby forming a pair of U-shaped ring buckles at the beam end, and place four ring buckles at the four corners of the ring buckle. Root short-inserted steel bars, U-shaped steel bar ring buckle connection section stirrups are encrypted. The U-shaped steel bar ring buckle connection technology forms a “pin type” connection through the core area concrete surrounded by the ring buckle steel bar. The ring buckle steel bar can effectively transmit the tension and pressure required for the connection, ensuring the safety of the connection and effectively improving the on-site construction Efficiency [11].

The U-shaped steel ring buckle connection technology is simple in structure and convenient in construction. It can avoid the collision of the prefabricated beam steel bars with the joints or the steel bars in the supports, and solves the problem of complex and difficult anchoring of the steel bars in the core area of the beam-column of the traditional post-cast integral connection node. At the same time, the length of the post-cast section at the beam end can be shortened, and the standardization of prefabricated beam components can be realized.

Fig. 4.
figure 4

U-shaped node connection technology

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3.3 Overlap Connection of Primary and Secondary Beams and Steel Slot Connection Technology

The assembled monolithic concrete prefabricated secondary beam adopts the same superimposed form as the main beam, and the upper longitudinal steel bars are tied together on-site and poured together with the upper steel bars of the floor. For the connection of the lower reinforcement, that is, the connection of the primary and secondary beam nodes, the integral cast or shelving connection is often used [12]. Partially vacant sections are reserved in the prefabricated main girder using the cast-in-place method, which causes the main girder to be discontinuous and increases the difficulty of prefabrication and hoisting. However, the shelving type is not connected to the lower secondary beam steel bar, which belongs to the “hinged connection” and is not suitable for structures that bear dynamic loads and large spans. It can be seen that no matter whether the cast-in-place or shelving connection method is used, there are construction drawbacks. The design of this project The primary and secondary beam connections not only have no gaps in the primary beam but also connect the lower longitudinal ribs of the secondary beam. In addition to the U-shaped connection technology described above, there are also primary and secondary beam lap connections and steel slot connections.

The lap connection of the primary and secondary beams is to set a keyway on the side of the main beam and the part where the secondary beam is connected, and at the same time reserve the steel bars that overlap with the lower part of the secondary beam, and connect with the secondary beam reinforcement through the post-cast belt, as shown in Fig. 5. This lap method can not only avoid gaps in the main beam, but also realize the complete force of the primary and secondary beams, but the reserved lap steel bars for the main beam are not convenient for transportation and consume more steel. At the same time, the reserved post-pouring belt is longer and there are more wet operations.

In order to reduce the wet work of the post-cast belt, the project also adopted a new type of steel slot connection technology, that is, the steel slot is pre-embedded on the side of the main beam at the junction of the primary and secondary beams, and the stressed steel bars at the bottom of the secondary beams can be directly extended into the steel. The slot is anchored, mortar is poured, and concrete is finally poured to form a whole, as shown in Fig. 6. The construction difficulty of the steel slot connection is greatly reduced, and because of its special structure, when the connecting steel bar is stressed, the mortar or concrete poured in the gap between the steel slot and the connecting steel anchor head is in a three-way compression state. Its bearing capacity and ductility have been greatly improved to ensure that damage does not occur in the steel slot [13].

Fig. 5.
figure 5

Lap connection of main-secondary beams

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Fig. 6.
figure 6

Connection of main-secondary beam steel slots

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3.4 Bellows Through Hole Prefabricated Column

In prefabricated concrete buildings, in order to ensure the reliability of vertical force transmission, longitudinal prefabricated column reinforcement is generally connected by grouting sleeves, but the grouting sleeve connection not only requires high manufacturing precision of the prefabricated columns, but also detects the compactness of the mortar inside the sleeves. The difficulty is also higher, and the construction cost is also higher [14]. Therefore, this project uses a bellows through-hole precast column, that is, a metal corrugated pipe with the same height as the column is inserted at the position of the steel bar inside the precast column, and the steel wire rope is tied and fixed with the column stirrup skeleton, and then concrete is poured, as shown in Fig. 7. When the upper and lower columns are connected, the longitudinal ribs of the upper and lower columns are connected by mechanical connection or welding at the nodes first, and then the prefabricated columns with the through-hole of the bellows are hoisted to the installation position, and finally mortar or special purpose is poured along the through-holes of the bellows. Grout until it is below the height of the bellows.

Fig. 7.
figure 7

Bellows through hole prefabricated column

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The bellows through-hole prefabricated column has a large bellows cavity, which is much larger than the diameter of the column longitudinal ribs. When the beam-column steel bars collide, the longitudinal steel bars in the corrugated pipe have a larger space that can be moved, which greatly increases the fault tolerance rate, thereby reducing the precision requirements of the prefabricated column components and improving the construction efficiency. The longitudinal ribs of the upper and lower columns are welded or mechanically connected, and the integrity of the longitudinal ribs is good, which improves the load-bearing capacity of the precast column, and does not require the use of sleeves and high-strength mortar, which further reduces the cost.

4 Bim Collaborative Forward Design

The “language” of traditional building structure communication is mainly in the form of two-dimensional plan drawings, which has certain requirements for the space imagination ability of design and construction personnel. In addition, many problems cannot be seen on the plan drawings, but they cannot be carried out in actual construction, resulting in more on-site changes. In order to adapt to the increasing requirements of the construction industry for building structure design, BIM technology came into being. As a new comprehensive technology for building model design, BIM technology abstracts the specific data of architectural design through software Data processing is expressed in the form of 3D models [15]. Especially in prefabricated buildings, the components are prefabricated in the factory, and the joints are poured on site. The reinforcing bars of various components are overlapped or anchored at the joints, resulting in dense reinforcing bars at the joints and significant collision problems. If you continue to display with flat drawings, it will be difficult to deal with the collision problems Therefore, this project uses 3D modeling Rhino software to build 3D models of all components used in this project, especially for the node areas with dense steel bars and new connection technology. The modeling structure shows that all nodes using the new connection technology have not occurred. Joint collision problem, construction is convenient, and the joint steel bar with collision problem is adjusted, the steel bar is anchored by the combination of hook and anchor plate, and the colliding steel bar is bent, and the bending ratio is not more than 1:6. A structural reinforcement is added to the bent steel bar. The collision example is shown in Fig. 8. The two front and rear beams in figure a collide with the two outermost steel reinforcement bars as shown in Fig. 8 (a) (Beam End). The two ends of the steel bars are bent to the middle but collide with the column reinforcement as shown in Fig. 8 (b) (Middle bend). Therefore, the final solution is to bend upwards and add structural steel bars to the bending parts as shown in Fig. 8 (c)(Bends upwards).

Fig. 8.
figure 8

BIM technology application examples

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5 Conclusion

With the advancement of industrialization, the development of prefabricated buildings is an inevitable trend. However, there are still some problems that restrict development, mainly including insufficient standardization in design, various conflicts and collision problems in construction affect the integrity of structural connections, in order to promote the industrialization of prefabricated buildings, a standardized design system must be established, and research and development Various new connection technologies and new components optimize the structural system.

The design of prefabricated concrete buildings is integrated with the structural splitting. Under the premise of meeting the building functions, the principle of “less specifications, more combinations” and the modular coordination standard are followed to carry out standardized, modular and integrated architectural design. And the structure can also reduce the types of components through reinforcement merging, etc., and carry out standardized design of node connections.

The use of new connection technologies, such as U-shaped buckle steel bar connection technology and steel card slot connection technology, can effectively improve the problems of steel bar collision and construction inconvenience in the node area, and improve construction efficiency; at the same time, the use of new components can not only solve some of the current construction Problems, superior performance, low cost, easy to popularize and use, and meet the requirements of building industrialization.