Current safety situation and Countermeasures of th

2022-09-19
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Existing curtain wall safety status and Countermeasures

existing curtain wall usually refers to the building curtain wall that has been completed and delivered for use, so the emergence of existing curtain wall is formed with the production and use of building curtain wall. From the structure and form of expression of similar building curtain walls, the existence of existing curtain walls can be traced back to the church lighting windows and various glass domes built of art glass in the west before the 19th century (Figure 1). The building curtain walls that initially have the structural form of building curtain walls should be the "Crystal Palace" built at the London Industrial Exposition in 1851, the glass curtain wall of the Harold building (Figure 2) built in San Francisco in 1917, the 381 meter high stone curtain wall of the Empire State Building in New York in 1831, the Lihua building in New York and the United Nations building built in the early 1950s, etc. Therefore, most of the existing curtain walls in the world have a history of hundreds of years. In terms of the real architectural curtain walls of the Lihua building and the United Nations building in New York, the existing curtain walls have existed in the world for more than half a century

with the development of China's reform and opening up, China also began to build glass curtain walls in the early 1980s. The front glass curtain wall completed in Guangzhou Trade Fair in 1981, the glass curtain wall of Beijing Great Wall hotel built in 1984, and the front full hidden frame glass curtain wall Shenzhen Development Center building built in 1986 have a history of more than 30 years. Therefore, in China, both curtain walls have been formed and existed for a long time

2. Current situation of existing curtain walls in China

the development and current situation of existing curtain walls in China are basically consistent with the speed and situation of economic development in China. Since the early 1980s, it has shown the momentum of rapid development. Its current basic status and characteristics can be summarized by large stock, aging and safety issues

2.1 existing curtain walls with huge stock

China's curtain wall industry started in 1983 and entered a period of rapid development in the 1990s. After nearly 30 years of development, it has become a leading curtain wall production and use country in the world, and is gradually moving towards a world-class curtain wall power. According to the industry statistics carried out by the aluminum doors, windows and curtain walls Committee of China Building Metal Structure Association, the annual output of building curtain walls in China has reached 1.05 million m2 by 1990 from 150000 m2 at the end of the sixth five year plan (1985), and 10 million m2 at the end of the Ninth Five Year Plan (2000), an increase of 70 times over the past 15 years and an average annual increase of 5 times. The area of building curtain walls completed and used during this period should be between 78.9 million square meters. From 2003 to 2013, the total output value of building curtain walls was 1028.87 billion yuan. If the average price was 1100 yuan per square meter, the curtain wall area completed during this period should be about 945 million square meters. Since 2014, with the deepening of China's economic transformation and the reduction of GDP growth, the annual completed area and output of building curtain walls have declined, but they have basically remained at about 100million square meters, with an output value of about 150billion yuan. For example, since 1983, the building curtain wall that has been built in China for more than 30 years, that is, the existing curtain wall should exceed 1.2 billion square meters, and its stock is huge

2.2 the aging existing curtain wall

building curtain wall belongs to the maintenance structure outside the building, and its design service life is 25 years according to the unified standard for reliability of building structure design GB (Figure 3). According to this standard, the building curtain wall built before 1990 in China has exceeded the design service life of the building. According to the above statistical results of the existing curtain wall stock, its area should be more than 10 million square meters. For example, considering the aging of some materials and the degradation of mechanical properties, most curtain walls before 2000 have experienced aging, and their area is even larger. It can be seen that with the passage of time, there will be more and more existing curtain walls in Figure 3, which is an irreversible trend

2.3 the safety of the existing curtain wall is of great concern

due to the huge stock of the existing curtain wall and the trend of aging, coupled with the frequent occurrence of incidents of self explosion and falling injury of tempered glass in recent years, the safety of the existing curtain wall has been extremely valued by the whole society, including the national high-level leaders and departments. According to the incomplete statistics reported in China in the past ten years, the main serious accidents of existing curtain walls in China are the falling off of the whole opening fan of the glass curtain wall, and the more safety accidents are the injury and damage caused by the scattering of glass fragments caused by the self explosion of tempered glass. Table 1 is the statistical data of glass curtain wall accidents in China in the past decade based on incomplete relevant reports

however, the problems of the glass curtain wall cannot represent all the problems existing in the existing curtain wall, especially the self explosion of tempered glass. It more reflects the defects of the tempered glass itself, and cannot fully represent the safety problems of the existing curtain wall. With the passage of time, the improper management of the existing curtain wall, the aging of materials, mechanical wear, metal corrosion and the repeated action of long-term load make the safety of the existing curtain wall have many uncertain potential factors that deserve our serious attention

3. Safety status of existing curtain walls

the safety status of existing curtain walls in China, through 30 years of use and testing under natural environmental conditions, as well as safety surveys carried out in different regions, has not yet seen the collapse of existing curtain wall structures. In several major earthquakes in recent years, the seismic performance and safety of existing curtain walls can also have good performance. It can be seen that the existing curtain walls in China are basically reliable in terms of safety performance. At the same time, we should also be sober to see that China's architectural curtain walls have developed from scratch, from non-standard to standard, and have experienced different stages of development. Some existing curtain walls have now exceeded the service life of architectural design, and different parts and materials of curtain walls have different degrees of aging, fatigue, wear and corrosion, which have an impact on the safety and reliability of existing curtain walls

3.1 the opening fan falls off

the opening part of the lighting and ventilation of the curtain wall is an essential component of the curtain wall. From the SARS infectious disease caused by SARS virus infection in 2003, as well as the needs of green building design, people have higher and higher voices for improving the ventilation capacity of building curtain walls, and have greater and greater requirements for the proportion of the curtain wall opening part in the curtain wall area, which is even listed as a mandatory provision in some local standards. However, from the perspective of safety, the openable part of the curtain wall, especially the openable fan, is a weak link in the safety of the curtain wall. From the recent survey of the safety status of existing curtain walls in Shenzhen for more than 25 years, it is found that nearly 30% of curtain walls have the phenomenon of opening fan falling off as a whole, especially in the typhoon season, the phenomenon is more serious, and some curtain walls have the phenomenon of opening fan falling off almost every year. In addition to unreasonable structural design, oversized window sashes, mismatched hardware fittings or poor quality, improper operation and use, especially the safety accidents caused by the failure of normal maintenance and repair of the open sash. This phenomenon is more common and dangerous in residential buildings. Figure 4 shows the opening fan of the damaged glass curtain wall, which has not been maintained and repaired in time

3.2 aging of structural sealant

hidden frame glass curtain wall uses silicone structural sealant to bond the glass to the supporting members of the curtain wall, and transmits the load of the glass panel to the main structure of the building through connectors. Due to its special assembly structure, the hidden frame glass curtain wall has a higher ability to reduce the stress damage and earthquake resistance caused by the glass mosaic structure than other kinds of glass curtain walls. The concealed frame glass curtain wall manufactured and installed in strict accordance with the specification requirements has good safety performance and use record. Because the hidden frame glass curtain wall adopts silicone structural sealant for structural assembly and the structural sealant bears the load transmission, the mechanical properties and aging resistance of silicone structural sealant have an important impact on the safety performance of the hidden frame glass curtain wall, so the mechanical properties and aging resistance of silicone structural sealant have attracted much attention from all walks of life, In particular, the performance of silicone structural sealant and its impact on the safety performance of glass curtain walls have attracted more attention for more than a decade

silicone structural sealant is a kind of polymer material with good aging resistance. It has been nearly half a century since silicone structural sealant was used to make and install hidden frame glass curtain walls in foreign countries in the early 1970s. In China, it has been 30 years since the mid-1980s. The test data in the aging resistance test of silicone structural sealant produced by a domestic silicone sealant manufacturer from the production date (1997) to 10 years (2007) and 15 years (2012) after use can be verified accordingly, as shown in Table 2. It can be seen from table 2 that the elasticity of silicone structural sealant decreased by 25% after 10 years and 35% after 15 years, but the tensile strength increased instead of decreasing

due to the need of engineering reconstruction, a recently suspended project in Shenzhen conducted on-site sampling and testing of silicone structural sealant (foreign products) that has been used for 20 years. It was found that although its elasticity has decreased significantly, which may affect the normal use of the sealant, its adhesion has not decreased significantly, and there are no defects that affect the bearing capacity of structural sealant, as shown in Table 3

silicone structural sealant, as a high polymer material, can be seen from the above test results that it has very good aging resistance. But so far, there is no accurate answer to its real service life limit in the world. In engineering use, at present, material suppliers generally only provide a 10-year commercial quality assurance period as usual, but this does not mean that silicone structural sealant has only 10 years of normal use performance. It is usually a misunderstanding that silicone structural sealant will be dangerous and there is no safety assurance after 10 years. China has recently completed the preparation of JGT construction industry product standard of silicone structural sealant for building curtain wall, and proposed that the design service life of silicone structural sealant for building curtain wall engineering should not be less than 25 years, that is, the service life of silicone structural sealant should be more than 25 years. It can also be proved that the service life of silicone structural sealant through strict production process control and testing should be more than 25 years, not the current 10-year shelf life. A large number of existing hidden frame glass curtain walls that have been in use for more than 30 years now are the result of natural aging resistance after a long time, which can reflect that the real aging resistance of silicone structural sealant can reach 25 years or more

in 2016, Shenzhen Municipal Bureau of housing and urban rural development organized curtain wall experts to conduct safety investigations on 16 curtain wall projects that have been built in Shenzhen for more than 20 years (before 2000). Among them, 10 projects are hidden frame and semi hidden frame glass curtain walls, which all use silicone structural sealant. So far, no safety accidents have occurred due to the aging of structural sealant. Through on-site key observation and manual inspection, it can be found that most of the structural sealants of the existing hidden frame glass curtain wall have aging signs of colloid hardening and surface chalking, especially the aging phenomenon of structural sealants over 25 years is more obvious, but the colloid that poses a danger to the safety of the curtain wall cracks itself, or has problems with the bonding surface

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