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Abstract: In recent years, structural glass directly bearing loads has become the latest trend in architectural glass applications. This article summarizes the important research progress of glass structure components and connecting nodes in developed and developing countries introduce some representative glass structure engineering applications in developed countries and developing countries and discuss the current research on glass structures. Key issues and prospects for future research work need to be focused on, to provide a reference for scientific research and engineering technicians in the field of glass structures.

Keywords: glass structure; laminated glass; buckling; member design; joint design

1. Introduction

Because of its characteristics of lighting, decoration, and durability, glass has been widely used in architecture for many years. However, for a long time, the application of architectural glass is mostly limited to the external protective components (the entry "component" is provided by Google) and the secondary stress components such as decorative components, such as doors and windows, curtain walls, etc. In recent years, with the improvement of glass processing technology, design technology, and the needs of social and economic development, the application of glass in buildings is not only limited to the envelope structure, and the glass structure that directly bears the load has become the latest trend of architectural glass application. Glass structural components (the entry "structural components" is provided by Google) such as glass beads and glass columns appear one after another in actual projects, and some buildings are even constructed entirely of glass structures, such as most of Apple's stores around the world. Glass structure buildings are transparent, bright and modern, and are loved by people. As a unique structural material, glass is increasingly valued by building owners and architects.

Developed countries began to put research results in the field of glass structures into engineering practice from the late 20th century, and built a series of well-known landmark buildings. In 2010, the European Committee for Standardization issued the future task of preparing the Eurocode and planned to incorporate the glass structure into the Eurocode. The design approach is expected to be incorporated into the new Eurocode after trial and evaluation. Although the research and engineering practice in the field of glass structure in my country is still in its infancy, in recent years, relevant scientific research institutes and universities have also carried out research on some components and nodes, and have achieved a series of important scientific research results and key technical reserves, housing urban and rural areas. In 2014, the Ministry of Construction officially incorporated the glass structure into the engineering construction standard system of the housing construction part, and China has initially met the conditions for carrying out the practice of glass structure engineering.

This article first reviews the important research progress in the field of external glass structures in some developing countries, including the mechanical properties of laminated glass under different stress conditions in the plane and outside the plane, as well as mechanical connection and structural adhesive bonding (the entry "adhesive" This paper briefly introduces several representative glass structure engineering applications such as the new entrance hall of the Van Gogh Museum and the Apple flagship store and discusses the future research work on glass structures. The key issues that still need to be paid attention to, provide a reference for scientific researchers and engineering technicians.

2. Research work on glass structure

2.1 Component research

Since the glass itself is a brittle material, to prevent the sudden brittleness of the glass components and the falling of fragments to hurt people, the glass structural components are generally made of laminated glass.

Some developed countries started the research on the mechanical properties of architectural laminated glass no later than the 1970s [2]. more comprehensive research. The study found the correlation between the mechanical properties of laminated glass and the shear properties of the interlayer film, established the analysis model of laminated glass components under different stress forms, and proposed the corresponding design methods. Wölfel[3] first proposed the concept of equivalent thickness in 1987, transforming the laminated glass into a single glass plate with equivalent bending (the entry "bending" is provided by Google) constant to analyze the out-of-plane Loaded laminated glass. In recent years, Galuppi[4] [5] proposed the enhanced equivalent thickness method using the energy variational principle to analyze and design multilayer laminated glass under different boundary and load conditions. Bedon, Amadio[6] studied the in-plane shear buckling behavior of four-sided supported laminated glass panels based on the equivalent thickness method and numerical simulation parameter analysis. Based on the Newmark composite beam theory, Amadio and Bedon[7][8][9] studied the influence of environmental temperature, loading time, initial defect, and other factors on the bearing performance of laminated glass in bending and compression, and gave a design suggestion. Machado-E-Costa [10] proposed the equivalent bending stiffness (the entry "stiffness" is provided by Google) and torsional stiffness calculation formulas based on the "sandwich" model. Bedon, Belis, and Luible[11] compared the existing analysis methods of laminated glass according to the test data, including the "sandwich" model, Newmark theory and equivalent thickness method formula, etc., and found that several calculation methods can be used at a given temperature and Under the condition of loading time, more accurate prediction values can be obtained. Bedon [12][13] used numerical simulation to study the lateral-torsional buckling behavior of elastically constrained monolithic glass beams subjected to in-plane bending according to the theory of elastically constrained lateral-torsional buckling of steel beams.

Some scholars in developing countries started relatively late in their research on laminated glass, but in recent years, with the increasing number of cases of direct stress on laminated glass in practical engineering, the research on its mechanical properties has gradually become hot spot. Some practitioners studied the load-displacement relationship of point-supported PVB laminated glass panels with finite difference method and suggested that the shear deformation of PVB (the entry "PVB" is provided by Google) laminated glass panels cannot be used in the design. neglect. Some practitioners conducted a 180-day load-holding test on the PVB laminated glass plate supported by four points at room temperature. Some practitioners found that the bearing capacity of PVB laminated glass panels simply supported on four sides decreased with the increase of temperature. Some practitioners found that the equivalent thickness of the four-sided simply supported PVB laminated glass plate decreased with the increase of the thickness of the adhesive layer. Some practitioners experimentally studied the out-of-plane bending properties of simply supported PVB and SGP laminated glass panels at both ends at different temperatures and loading times, as well as the bending properties of four-sided supported laminated glass panels. The test found that PVB The bending stress and deflection of the laminated glass plate are greatly affected by the temperature and the load holding time, and the creep effect is significant at room temperature, while the SGP laminated glass plate is less affected by the temperature and the load holding time. The deflection effect influences the load-deflection curve to change non-linearly, while the four-side-supported SGP laminated glass panel is less affected by the large deflection effect and both stiffness and bearing capacity increase with the thickness of the adhesive layer. Some practitioners conducted an axial compression test study on multi-layered glass columns. After the buckling failure of the laminated glass columns, there was no obvious residual bearing capacity. The analysis results establish the design curves of the laminated annealed glass column under short-term and long-term loading conditions, respectively.

2.2 Connection research

Typical connection methods in glass structures generally include mechanical connection, structural adhesive bonding, and hybrid connection using both at the same time. Among them, mechanical connections are generally connected by bolts, which are divided into pressure-bearing bolt connections and friction-type bolt connections; structural adhesive bonding mainly uses structural adhesive to bond glass and other materials to form connection nodes; hybrid connection nodes mainly It is a combination of mechanical connection and glue connection. The joint area is formed by metal parts and structural glue at the connection part.

At present, some scholars in developed countries have conducted comprehensive research on several typical connection methods in glass structures. Baitinger, Feldmann[22],[23] based on the theory of elasticity, studied the glass stress distribution at the pressure-bearing bolted joint (the entry "stress" is provided by the industry encyclopedia) and proposed a simplified pressure-bearing bolted joint Design formulas. Bernard and Vaudeville [24] studied the mechanical properties of friction bolted joints based on annealed glass and tempered glass, and found that the bolt torque and cushion friction coefficient have a significant impact on the stress distribution and failure mode of the joint. Campione [25] studied the mechanical properties of the joints of laminated glass components connected by the butt joints of angle steel and bolts. This joint can achieve a semi-rigid connection, but there is stress concentration near the bolt holes, which is prone to brittle failure. Bells [26] studied the influence of temperature and washer material on friction-type bolt joints used to connect laminated glass components and compared the effect of various factors on the preload force of bolts (the entry "preload" is provided by Google). The influence law on the friction force value at the node is discussed. Machalická and Eliášová[27] studied the influence of different substrates, different types of structural adhesives, different thicknesses of adhesive layers, different surface treatments, and aging effects on the bonding connection, and found that the glass surface frosting treatment can improve the bonding strength, The bond strength decreases with increasing bond line thickness. Overend [28] studied the properties of several structural adhesives such as silicone adhesive (the entry "silicone adhesive" is provided by the Encyclopedia of Industry), polyurethane adhesive, epoxy adhesive, two-component acrylate adhesive, and UV-curable acrylate adhesive. Mechanical properties such as stiffness and strength. Santarsiero [29][30] studied the tensile properties of two high-temperature adhesives (SGP, TSSA) and the joint shear properties of glass-stainless steel when considering temperature and strain rate, and obtained different temperatures, The engineering stress-strain curve of the rubber material at the strain rate, and the calculation formula of the shear capacity of the joint is given. Carvalho [31], Puller and Sobek [32], Zangenberg [33] studied the mechanical properties of metal implanted hybrid joints in laminated glass. The parameters studied include metal material type, cement material type, temperature, loading speed, force, etc.

At present, some scholars from developing countries have done little research on glass structure nodes, and the main research is still limited to the field of the glass curtain wall. Some practitioners discussed the design requirements of existing codes for the failure of silicone structural adhesives in point-supported curtain walls of glass rib support systems and proposed joint design formulas and structural design methods to avoid such hidden dangers. Some practitioners proposed a structural form of a glass skylight. Based on this structural system, he proposed a joint structural form in which steel spacers are set in the silicone structural sealant between the glasses to transmit pressure, reducing the amount of pressure at the joints. The phenomenon of force concentration or stress concentration is analyzed by numerical simulation. Some practitioners studied the effect of size and curing time on the tensile adhesion of silicone structural sealants, and proposed a re-bonding test method for the performance of existing curtain wall silicone structural sealants, which can judge its strength and reliability. (The entry "reliability" is provided by Google) and the degree of aging. [37] designed a connection node between the plexiglass and stainless steel for the structure of the neutrino detector, and studied the bearing capacity of the node and the mechanical performance under long-term load through experiments.

3. Engineering practice of glass structure

At present, structural glass is mainly used in floors, roofs, walls, stairs, canopies, and other components of housing construction, as well as facilities such as overpasses and sightseeing planks, and there are also some all-glass structures.

Some developed countries began to try the engineering application of glass structures in the 1980s, such as the load-bearing all-glass curtain wall structure built by Dutch Jan Benthem and Mels Crouwel in 1984 (the entry "glass curtain wall structure" is provided by the industry encyclopedia). Two sets of all-glass buildings (Fig. 3.1) designed by architectural and glass designers Santambrogio (Carlo Santambrogio and EnnioArosic) in Milan, Italy, consist of T-section columns composed of glass ribs and glass plates, and the glass beads and glass columns are connected by bolts to form a frame The structural system, the glass staircase, glass coffee table, glass cabinet, glass bed, etc. inside the building are all assembled by bolts.

Figure 1 All-glass building designed by Santambrogio: Three-story framed glass house

Figure 1 All-glass building designed by Santambrogio: Three-story framed glass house

The new entrance hall of the Van Gogh Museum opened in 2015 (Figure 3.2) was completed by Kisho Kurokawa Architects, Hans van Heeswijk Architects, and Octatube structural designers. It is currently the largest glass structure hall in the Netherlands. The structural design of the entrance hall is a mixed structure composed of the spherical glass roof, cold-formed (the entry "cold bending" is provided by the Encyclopedia of the Industry) glass curtain wall and steel structure (the entry "mixed structure" is provided by the Encyclopedia of the Industry), glass The staircase is supported by glass arches. The glass roof is composed of insulating glass and laminated glass units of different sizes and is supported by SG multi-laminated glass ribs. The maximum stiffener is 12m long and 700mm high. The glass beams are cemented to the main steel frame to provide support for the glass roof. The curved curtain wall is composed of cold-formed insulating glass units, which are fixed by 20 different triple-laminated glass stiffeners, the longest dimension of the stiffeners is 9.4m, and the minimum bending radius of the curtain wall is 11.5m [39].

The façade of the French Montpellier School of Medicine designed and constructed by François Fontès in 2017 adopts a glass curtain wall constrained by glass ribs (Figure 3.3). The maximum size of the glass plate is 3.8m*2.8m, and the glass rib is as high as 12.71m. Titanium metal is used. Implanted nodal connections are used to ensure lateral buckling and stability of the structure under seismic loads [40].

Apple has built a series of glass structures around the world (Figure 3.4). The entrance hall of the Apple flagship store on Fifth Avenue, New York, opened in 2006, is a pure glass cube with a side length of 10m, which is jointly supported by multi-laminated glass ribs and glass walls connected by metal implanted nodes. Opened in 2014, the Apple flagship store in Zorlu Center, Istanbul, Turkey, is equipped with a load-bearing building with all glass walls, which is composed of four SGP multi-layer tempered glass with a width of 10m and a height of 3m connected by structural silicone glue. The Kunming Apple flagship store, which opened in 2017, is supported by 8 groups of 5.4-meter-high U-shaped SGP multi-laminated glass columns supporting a carbon fiber-reinforced polymer roof with a diameter of 20.9 meters. The Steve Jobs Theater of the Apple headquarters building in the United States, which opened in 2017, has a carbon-fiber roof with a diameter of 47m supported by a cylinder composed of curved glass panels with a height of 7m. It is currently the largest glass support structure in the world. "Provided by Industry Encyclopedia), the glass plate is fixed in the steel channel that transmits seismic energy through structural silicone glue, so that the glass can adapt to the displacement caused by the earthquake, thus ensuring the integrity and robustness of the overall structure [41].

4. Outlook

In recent years, glass structure-related research has made a lot of important progress outside some developing countries, and the structural design methods have become increasingly rich and perfect, but there are still many problems in the engineering application of glass structure systems.

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