4.2 Prefabricated Concrete Frame Structure Technology
4.2.1 Technical Content
Prefabricated concrete frame structures include assembled integral concrete frames and other types of prefabricated frames. An assembled integral frame structure consists of frame beams and columns partially or fully made from prefabricated components, connected through reliable methods. The joints are then integrated on-site with cast-in-place concrete, cement-based grout, or similar materials. Other prefabricated frames generally feature various dry connections and are typically combined with shear walls or seismic supports.
Assembled integral frames can be analyzed using the same methods as cast-in-place concrete frames. Their bearing capacity and serviceability limit states are evaluated through elastic analysis. When calculating internal forces and displacements, both cast-in-place and composite floor slabs are assumed to be infinitely rigid within their planes. The design of prefabricated components and joints follows the same principles as those for cast-in-place concrete frames. Additionally, shear capacity must be checked at the vertical joints of composite beam ends and the horizontal joints at the base of prefabricated columns, especially under short-term design conditions. Structural arrangements should avoid tension at the horizontal joints of prefabricated columns during assembly.
There are two main categories for assembling integral frames: post-pouring at frame nodes and prefabricated frame nodes. The former involves connecting prefabricated components by casting concrete at beam-column joints, with components generally straight in shape. The latter places connection nodes within the middle of frame columns and beams, with prefabricated nodes shaped as crosses, T-shapes, or L-shapes. Due to challenges in manufacturing, transportation, and on-site installation, prefabricated frame nodes are rarely used in current projects.
When designing connection nodes for assembled integral frames, the cross-sectional dimensions of beams and columns, as well as the quantity, spacing, and positioning of reinforcing bars, must be carefully planned. Reinforcement anchorage and connections must comply with relevant national standards. It is important to avoid reinforcement conflicts and consider the installation sequence to facilitate construction. Longitudinal reinforcement in prefabricated columns is typically connected by sleeve grouting, mechanical cold extrusion, or similar methods. For cast-in-place beam-column joints, longitudinal tension reinforcement in composite beams should be anchored or connected within the post-pouring zone. Lower longitudinal bars may extend beyond the node into the post-pouring section. When composite beams are connected by butt joints, lower longitudinal bars should be mechanically connected, sleeve grouted, or welded within the post-pouring area. Stirrups in composite beams can be either integral enclosed or composite enclosed types.
4.2.2 Technical Indicators
The safety and quality requirements for prefabricated frame components and structures must meet national standards such as:
- “Technical Specification for Prefabricated Concrete Structures” JGJ 120-2014,
- “Technical Standard for Prefabricated Concrete Buildings” GB/T 51231,
- “Code for Design of Concrete Structures” GB50010,
- “Code for Construction of Concrete Structures” GB50666,
- “Code for Acceptance of Construction Quality of Concrete Structures” GB50204,
- “Technical Specification for Prefabricated Prestressed Concrete Assembled Integrated Frame Structures” JGJ 224.
Steel reinforcement mechanical connections must follow “Technical Specification for Application of Steel Reinforcement Mechanical Connection” JGJ 107. Steel sleeve grouting connections should comply with “Technical Specification for Application of Steel Sleeve Grouting Connection” JGJ 355, while anchor plate anchorage must meet “Technical Specification for Application of Steel Bar Anchor Plates” JGJ 256.
Key technical requirements for assembled integral frame structures include:
- The maximum allowable building height for assembled integral frames is essentially the same as for cast-in-place concrete frames.
- Prefabricated concrete frames should use high-strength concrete and reinforcing steel. Large diameter bars are preferred for longitudinal reinforcement in beams and columns to reduce bar quantity, increase spacing, improve assembly efficiency, ensure quality, and lower costs.
- For buildings taller than 12 meters or with more than three floors, prefabricated columns must use sleeve grouting connections, including fully and semi-grouted sleeves. The cross-sectional width of rectangular columns or diameter of circular columns should be at least 400mm, and no less than 1.5 times the beam width in the same direction. When sleeve connections are placed at column bases, the densified hoop reinforcement zone must be at least the length of the longitudinal stress bar connection zone plus 500mm. If the concrete cover of longitudinal bars exceeds 50mm, additional measures such as steel mesh reinforcement are needed to control cracking and prevent protective layer detachment during stress. For composite beams, the thickness of the post-poured concrete layer should be at least 150mm. For seismic grade I and II composite beams, integral enclosed stirrups should reinforce beam ends.
- In assembled integral frames with prefabricated columns and composite beams, column base joints are located at floor levels, with the concrete surface in post-pouring areas intentionally roughened. Column longitudinal bars must extend through the post-pouring nodes, and the base joint thickness should be 20mm, filled with grout. Prefabricated frame nodes—including middle and end nodes at intermediate and top floors—follow the same anchorage and connection methods as cast-in-place frame nodes. For top floor end nodes, columns may extend beyond the roof, with longitudinal reinforcement anchored in the extended section.
4.2.3 Scope of Application
Assembled integral concrete frame structures are suitable for public, residential, and industrial buildings in seismic zones with fortification levels from 6 to 8 degrees. Except for the highest seismic intensity of 8 degrees (0.3g), the maximum building height for prefabricated concrete structures is comparable to that of cast-in-place concrete buildings. Other types of prefabricated frames are mainly appropriate for low-rise residential, public, and industrial buildings.
4.2.4 Engineering Examples
Examples of projects using this technology include:
- China Construction International Hefei Residential Industrialization R&D and Production Base Project Supporting Comprehensive Building,
- Nanjing Vanke Shangfang Affordable Housing Project,
- Nanjing Vanke Jiuduhui,
- Leshan First Vocational High School Training Building,
- Shenyang Hunnan Twelve Transport Security Center,
- Shenyang Nanke Wealth Building,
- Haimen Elderly Apartment,
- Shanghai Zhuanqiao Wanda Plaza,
- Shanghai Lingang Heavy Equipment Industrial Zone H36-02 Plot Project, among others.
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