Development status of safety performance testing t

Development status of safety performance testing technology of existing glass curtain wall

glass curtain wall was introduced into China from western countries in the 1980s, and reached a climax in the mid-1990s. It is a new modernist wall integrating architectural aesthetics, architectural functions, architectural energy conservation, architectural structure and other factors. At present, a large number of glass curtain walls have been built in large and medium-sized cities such as Beijing, Shanghai, Guangzhou and Shenzhen. The survey shows that at the end of 2017, China has built about 1billion square meters of various architectural curtain walls (including daylighting roofs), accounting for more than 90% of the world's total, of which glass curtain walls account for a large proportion. According to the requirements of the current national standards GB 16776-2005 silicone structural sealant for building and JGJ 102-2003 technical code for glass curtain wall engineering, the structural sealant needs to provide a 10-year quality assurance. However, it has been more than 20 years since the large-scale use of glass curtain walls in China, which has far exceeded the warranty period of structural sealant. In the past two years, governments across the country have attached great importance to various local building glass breakage accidents. The Ministry of housing and urban rural development issued another notice on strengthening the safety protection of glass curtain walls in August 2016

1 cause analysis of potential safety hazards of existing glass curtain walls

existing glass curtain walls should establish and improve the fixed-point procurement system of key raw and auxiliary materials, regular inspection and verification system and raw and auxiliary material change report system. The wall is a building glass curtain wall that has been completed, accepted and delivered for use. There are at least tens of thousands of safety accidents caused by the falling of building glass curtain walls in China every year, which is significantly more than similar accidents in foreign developed countries. Through research and analysis, most safety accidents are mainly caused by glass self explosion caused by internal stress and glass falling off caused by external factors

1 . 1 glass self explosion caused by internal stress

there are many reasons for glass curtain wall self explosion, but the fundamental reason is the local stress concentration in the stress layer inside the glass. In most cases, heterogeneous phases (such as nickel sulfide, elemental silicon, alumina, etc.) are the culprit of glass self explosion. For example, the phase transition of nickel sulfide causes a volume expansion of 2-4%, which makes the glass bear huge phase transition tensile stress; The physical and mechanical property parameters of some impurities do not match the glass, resulting in the tensile stress near the particles during the heating and cooling process of the glass, which will cause local stress concentration in the glass and lead to the self explosion of the glass

there are hidden dangers in the project quality during the installation of glass. For example, whether the perpendicularity of the glass plate, the perpendicularity and diagonal of the glass frame and other structural designs are reasonable, the connection and joint treatment of the nodes between the curtain wall components and the main structure, and whether the installation position deviation of the embedded parts exceeds the specification requirements may cause certain safety hazards to the project. If the installation design of the glass curtain wall is unreasonable, it cannot bear the gravity in the building, which may lead to the installation stress concentration of the curtain wall, such as extrusion deformation or bending, exceeding the stress limit of the glass, and finally leading to the glass bursting and falling

1 . 2 glass falling off caused by external factors

glass loosening caused by external factors. First of all, structural adhesive is one of the important materials in the structure of glass curtain wall. From the market survey, the failure of structural adhesive accounts for more than 40% of all quality problems of glass curtain wall. The quality of structural adhesive n-constant is not up to standard, and its compatibility with contact materials is poor; Affected by natural environment such as wind, rain, ultraviolet radiation, earthquake, etc. for a long time, colloid aging and loss of bonding ability will lead to curtain wall glass loosening and falling off. The glass supporting structure, namely the fixed device and frame structure, will also have structural failure or other unforeseen defects, which may lead to the fall of curtain wall glass

2 development status of safety performance testing technology of existing glass curtain wall

the safe service of glass materials, colloids and structural components of existing glass curtain wall is very important to ensure urban public safety. Through the analysis of glass self explosion and glass falling off, it lays a foundation for further technical research and practical application in the future. To obtain accurate data

2 . 1 glass self explosion safety detection technology

glass self explosion is mainly caused by local stress concentration caused by heterogeneity and improper installation design. Compared with the inspection of self explosion source, the inspection of glass stress concentration is more direct. Therefore, the detection of glass stress concentration is very important to prevent glass self explosion

the most basic means of glass appearance inspection in the project is to rely on empirical appearance inspection and touch inspection, that is, to observe whether there are defects on the glass surface under good natural light or scattered light conditions. This method is simple and easy, but only for the cases with obvious defects, human objective factors have a great impact, and there are high requirements for the working experience of inspectors

at present, most methods of glass stress test are only suitable for the inspection of glass before leaving the factory. There are four common methods: interference color method, namely simple polarizer method, quarter wave plate method, namely reading polarizer method, compensated glass stress test method, and penumbra polarizer method. These methods can detect small glass samples, which need to fix the detection instrument and move the glass samples, and are not suitable for detecting large existing glass curtain walls. In recent years, Wande Tian and others are developing a relatively new detection technology - photoelastic scanning method. According to the principle of transmission photoelasticity, the stress fringe image is obtained by using the change of transmission light intensity difference on both sides of the polarizer and the polarizer. Further amplification, analysis and processing of these areas can determine the type, size and location of impurities or defects. This principle can be used to detect the quality of tempered glass before leaving the factory, before installation or in service. The experimental device is simple, and the inspectors do not need to work at heights. Personnel and relatively expensive instruments and equipment can be carried out indoors, reducing personnel risks and saving detection costs. In addition, Liu Xiaogen and others can observe the stress spot caused by stress concentration at the vacuum glass support and the glass support point through the photoelastic scanning method. It is found that with the improvement of vacuum degree, the diameter of the stress spot also increases. This principle is mainly aimed at the detection of the actual engineering of vacuum glass, with simple operation and intuitive and reliable results

2 . 2 glass falling off safety performance detection technology

another form of curtain wall risk that leads to safety accidents is the falling of the whole curtain wall. The causes of such accidents are diverse, mainly including the aging and falling of structural glue and the loosening and falling of support structure. The following will outline its safety detection technology from these two aspects. See Table L for the comparison of main detection technologies

Table 1 Comparison of main detection techniques for safety performance of glass falling off

2 2 . The quality of structural adhesive is one of the important indexes to evaluate the overall safety and durability of glass curtain wall. The conventional detection methods of structural adhesive mainly include visual method, knock detection method, cutting and drawing method, shore hardness method, etc. Among them, the visual method and the knock detection method can only be used as the preliminary nondestructive testing of the quality testing of structural adhesive, which are greatly affected by human factors and have poor accuracy, and cannot meet the control requirements of glass safety testing; The cutting and drawing method is to send some samples cut on site back to the laboratory for testing, and carry out the drawing test on them, which can directly measure the maximum tensile strength, elongation at break, bonding failure form and bonding failure area of the colloid, and can be used as the detection method for arbitration. This method belongs to local damage detection, and it needs to be repaired and reinforced after detection; The shore hardness method is used to determine the hardening degree of structural adhesive. The method is simple and fast, and can quantify the quality of adhesive materials. However, hardness is only one of the indicators of structural adhesive, so it needs to be combined with conventional testing methods (such as visual inspection, knock inspection, etc.) or other methods to comprehensively determine

in recent years, other detection methods have appeared successively, such as push rod method, ultrasonic detection, acoustic emission detection, infrared detection and X-ray detection. In addition, the air bag method, sucker method and multi sucker method introduced from abroad are also applied in China. Push rod method and air bag method belong to indoor detection methods, Barium examination (the same scope is tested by simulating the wind pressure effect of glass curtain wall. The air bag method is closer to the wind pressure effect of glass curtain wall than the push rod method, but the displacement measurement device is needed. The push rod method device is simple and easy to carry, but it can only be used to detect the sealant used for the bonding between glass plate and aluminum profile subframe; the suction cup method and multi suction cup method are equivalent to the edge of glass panel by simulating the uniform wind load on the tested glass panel The multi suction cup method simulates the uniform wind pressure on the tested glass panel through multi-point loads. Compared with the suction cup method, it requires a tensile testing machine moving at a uniform speed, which is closer to the wind pressure on the glass curtain wall. These two methods belong to outdoor detection, and are commonly used to detect the adhesion of hollow glass curtain wall; Both ultrasonic testing and acoustic emission testing have high accuracy, but they are limited to flaw detection, the strength cannot be determined clearly, and the data processing is complex, and the acoustic emission testing method is only limited to the handover testing of aluminum components. Hu Shaohai and others used ultrasonic testing method to analyze the types and causes of bonding structure defects, and on this basis, they identified the defects of aluminum bonding structure. At the same time, Hu Shaohai and others also used acoustic emission detection method to stretch the bonding parts, which can detect the defect location and low bonding strength area. The results show that the defect identification of bonding structure by acoustic emission testing is more accurate than that by ultrasonic testing, and the estimation accuracy of bonding strength is slightly different from that by ultrasonic testing; X-ray detection method and infrared detection method have high defect recognition rate, high degree of automation, and can not determine the intensity. They are still in the initial stage in China, and the cost of relevant instruments and equipment is high

2 . 2 . 2 supporting structure safety performance testing technology

aluminum alloy profile is the main supporting structure of glass curtain wall, which bears various loads from the glass panel and plays a vital role in the safety of curtain wall. Film thickness is the basic item of quality inspection of aluminum alloy profiles, and the hardness of aluminum alloy profiles directly refers to the stress of materials, which is one of the important measurement indicators to test the structural safety. Therefore, the most important test indicators of the bearing capacity of aluminum alloy profiles include: film thickness and hardness of aluminum alloy profiles

at this stage, the hardness of aluminum alloy profile of glass curtain wall is usually measured by Wechsler hardness tester. There are many inspection methods of film thickness, including eddy current thickness gauge, cross-section measurement method and ultrasonic thickness gauge. Eddy current method is a conventional testing method, which has the characteristics of nondestructive, convenient and fast, and is suitable for rapid testing, production and acceptance testing at the construction site; The cross-section measurement method needs to take the sample back to the laboratory for detection, and the measurement is accurate. It is an arbitration detection method, but the sample preparation is complex and destroys the profile; Ultrasonic thickness gauge is widely used in field testing because of its high precision and automation

2 . 2 . 3 other related detection technologies

whether the supporting structure of the glass curtain wall is loose or damaged or the aging failure of the structural adhesive will lead to the loosening of the glass panel, resulting in the change of the vibration frequency of the glass. Based on this principle, Chen Zhenyu and others proposed a method to detect the debonding length of the structural adhesive based on FFT power spectrum, that is, through the separation of the excitation and the response signal of the measuring point in the glass panel