The scale of China's building curtain wall industry exceeds 500 billion yuan, which has created irreplaceable value in achieving personalized expression of building appearance and functionality. The domestic building curtain wall has a development history of nearly 40 years since its application in the Great Wall Hotel project in 1984. Currently, it has entered a comprehensive development stage of widespread application of new technologies, materials, and processes, including intelligent informatization, green environmental protection, and factory assembly. Digitization, intelligence, and energy-saving technologies provide powerful tools for diversified innovation in curtain walls, and the diversity of shapes and functions increasingly meets the needs of modern people. Moreover, as a subsystem of architecture, the "personalization" of the curtain wall is not only inevitable but also a result of the owner's pursuit of commercial value, the architect's expression of personal expertise, and social evaluation in the process of meeting the requirements of customized design for buildings. In the process of industry development, the "style" of curtain walls continues to evolve, constantly improving around safety, comfort, aesthetics, and economy, and meeting this demand through continuous innovation. This precisely indicates that building curtain walls are closely linked to human sociality.
1. Analysis of the driving force behind innovation in curtain wall technology
The innovation of curtain wall technology initially focused on how to achieve safe and simple construction, so the emphasis was on construction and manufacturing installation processes [3]. For example, the hidden frame glass curtain wall of Harbin Senrong Building ensures high glass flatness and efficient installation through the design of fixed distance compression and positioning installation methods. In the past 15 years, energy-saving and green, assembly technology, intelligence, and personalized design have become the main demands for curtain wall innovation. Figure 1 summarizes the core driving force behind the innovative development of curtain wall technology, among which energy-saving and low-carbon requirements are global, and national and regional energy-saving standards are continuously improving year by year. As a representative variety of prefabricated curtain walls, unit curtain walls represent a high degree of industrialization, long retention time of original performance, and high-quality curtain walls. Against the backdrop of rapidly increasing manufacturing and installation labor costs, they conform to the direction of curtain wall implementation technology evolution. It is possible to reduce its overall cost by adopting a unit curtain wall form. Intelligence is a fundamental technological field for improving comfortable interaction and dynamic response, and is also a necessary condition for achieving low-carbon energy conservation at a higher level. By optimizing functions such as solar energy transmittance, visible light environment, and ventilation, comprehensive energy conservation and user comfort can be achieved. Digital design and process management have a decisive impact on improving curtain wall completion and reducing time costs. BIM and AI will play an important role in this regard.
The basic principles and policy requirements of architectural design have been subtly evolving. Before the Cultural Revolution, the "Party's architectural policy" was: applicable, economical, and beautiful when possible. Since the reform and opening up, there has been a renewed emphasis on the principles of applicability, economy, and aesthetics. But there have also been a batch of strange buildings and skins in the market, and people express their disgust towards strange buildings by criticizing ugly building activities. In response to social trends, the 2016 State Council document "Several Opinions on Further Strengthening Urban Planning, Construction and Management Work" proposed the importance of applicability, economy, green and aesthetics in architectural design, reiterated the importance of aesthetics, and added requirements for green and low-carbon. These architectural design concepts that carry social responsibility undoubtedly have a positive impact on the development trend of building facades.
2. Innovation and Development History of Curtain Wall Technology
From the perspective of architectural development, a certain style of curtain wall will generate an inherent demand for transformation after a period of prevalence. Therefore, it is possible to meet people's aesthetic positioning requirements of liking novelty and disliking old by changing the shape, structural system, or material technology, but the only constant is the demand for safety, economy, comfort, and aesthetics. For the past 30 years, domestic curtain walls have always adhered to technological innovation in order to meet this requirement. In terms of structural innovation, construction technology innovation, material and application innovation, functional integration innovation, and visual texture innovation, they have shown a positive response to diversified architectural creativity. Simultaneously developing diversified curtain wall systems to meet the diverse requirements of architectural design.
3. Innovative material applications have created diverse building curtain walls
The creativity of architects drives the continuous development of building curtain wall technology and the advancement of related materials. The emergence of new materials, in turn, has promoted innovation and development in curtain wall technology. Renowned Japanese architect Kengo Kuma mentioned at the beginning of his book "Kengo Kuma's Materials Research Laboratory" that "when new materials are encountered, a new era begins. From the curtain wall materials included in the national building materials industry's "14th Five Year Plan" for scientific and technological development, it can also be seen that the development of various new materials, such as structural bonding materials, high-strength linear metal materials, high-performance low radiation Low-E energy-saving glass, and diversified decorative panel materials, has received attention from all parties. In the field of artificial boards, materials such as porcelain board, ceramic board, glass fiber reinforced cement GRC board, ultra-high performance concrete UHPC board, reinforced glass fiber high polymerization hot pressed composite GRP board, sintered dense ceramic rock board, extruded concrete board ECP, and their composite application materials have been developed today. The rise of personalized shaped skins has provided application scenarios for GRC boards and UHPC boards. GRC boards are usually manufactured using the "injection molding" process, which is flexible, but the quality stability needs to be improved. GRC lightweight components are systematically solving material problems such as cracking, corner loss, and mold blackening around drainage holes by selecting high-quality raw materials, improving mold accuracy, ensuring an initial curing time of more than 7 days, surface waterproofing treatment, and optimizing drainage paths. UHPC has a strength three times higher than GRC, and its water absorption rate is only 1/5 or even lower than GRC. It also has the characteristics of lightweight, ultra-high strength, and moldability, and is highly favored by architects and owners in the field of structural decoration integration. And in the field of decoration without structural requirements, high-performance concrete (HPC) materials, combined with diversified fiber-reinforced materials for composite, are expanding their application scope.
4. Freeform surface curtain wall and cold bending technology
From the perspective of neuroaesthetics, curved architecture gives people a dynamic experience and a sense of closeness. So whether it's the curved elements in classic architecture or the recently digitized streamlined and irregular building curtain walls, they bring people a unique architectural aesthetic experience, such as the National Speed Skating Oval [5]. With the advancement of digital design methods and material processes, such as parameterized forward design and low-cost cold-formed metal sheet or glass and artificial board technology, favorable conditions have been created for the successful implementation of such projects. When the curvature of glass is generally greater than 1500 times the thickness, cold bending technology is suitable. According to experience, when the curvature value is ≤ L (short side)/60, due to the limitations of bending and tempering technology, cold bending technology can effectively ensure the building effect, high economic benefits, simple technology, and controllable safety. For example, the glass curtain wall project of the St. Petersburg Bank office building in Russia adopts the overall cold bending process of unit panels, and the exterior is in a hyperbolic shape. In this project, glass is assembled into flat unit curtain wall panels, and a micro curved curved curtain wall is achieved on site by applying pressure through pressure plates to maintain a low permanent additional stress on the glass surface. This project adopts AGC's three glass two cavity configuration with sunshade film, and the deflection value of one corner is about 50mm. Make a full-scale model in the factory, warp the glass by about 70mm through a pressure plate, conduct comprehensive testing for a period of time, verify the feasibility of the plan, and finally apply it to the project. The project has been completed and put into use for several years, and there has been no occurrence of glass breakage or other phenomena under long-term stress. There are many cases of using single insulated cold-formed glass in China, but there is no case of large-scale cold-formed double insulated glass.
5. Application and Prospect of Digital Design:
The digitalization of curtain walls originated from the application of BIM. The BIM digital platform plays an important role in the full information design of curtain walls, CAM manufacturing, and visualization of the entire lifecycle management process. Artificial intelligence will inject transformative power into the development of curtain walls, playing an important role in various fields such as automatic generation of building skin effects and curtain wall structures, detection of existing building defects, visual implementation and management of projects, and intelligent control and operation. The AI empowerment of architecture and skin applications currently mainly includes the following four areas: 1) Based on big data and machine learning, assisting in the design of pan building systems, and making efficient decisions on skin design schemes based on VR; 2) Collaborative intelligent manufacturing between robots and humans; 3) Based on real-time monitoring of real-life situations, achieve visualized full process management (integrating hardware and software); 4) Assist in the intelligent control operation after completion, and continuously optimize the operation algorithm and improve the operation effect based on big data deep learning.
The design assisted by big data and machine learning is still in its infancy. At present, BIM forward design technology is borrowed to establish a full information model, and then reverse correction modeling is carried out through on-site physical measurement to complete digital application. In the future, it will gradually shift to using AI to generate models, and then establish an efficient curtain wall design system by connecting with the component element library. The field of collaborative intelligent manufacturing between robots and humans is a rapidly developing area of AI technology. It can be foreseen that with the rapid increase in labor costs, factory based human-machine collaboration, as well as construction mechanization and intelligence, are the trend, and there is great room for upgrading intelligent manufacturing in this field
summary
After more than 30 years of rapid innovation and development, domestic building curtain walls have established a complete industry standard system, diversified material application system, and construction system. Through extensive curtain wall engineering practice, rich innovation and implementation experience have been accumulated, laying a solid foundation for high-quality development in the future. At the same time, there are also cases of short-lived curtain walls appearing after immature materials, processes, and technologies are put into the market, which affects the innovation value.
At present, "innovation systematization, design parameterization, management of physical virtual synchronization, and on-site construction mechanization" are effective means to improve the quality of curtain walls. It is necessary to take seriously the new materials, technologies, and processes used in each project, complete necessary experiments to reduce implementation risks, and improve the completion of building curtain walls.