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Discussion on Prevention and Control of Curtain Wall Glass Breakage and Falling.

With the gradual application of new technologies and new technologies in construction engineering, high-rise buildings have a broader development space. The curtain wall, as the facade of a building, has gradually become a window to show the prosperity of the city. The application of architectural curtain walls began in the early 20th century. Since the first glass curtain wall was built, glass curtain walls have gradually been widely used in famous buildings all over the world.

It is estimated that the current construction area of glass curtain walls in the world exceeds 3 billion square meters. The huge stock of glass curtain walls and the trend of aging have led to frequent incidents of tempered glass breaking and falling and hurting people in recent years. The safety of glass curtain walls has begun to attract people's attention and attention.

However, the commonly seen safety problems of glass curtain walls do not mean that they are completely caused by the quality of glass. For example, the self-explosion of tempered glass reflects more the defects of the tempered glass itself, and cannot fully represent the safety problems of the existing curtain wall. In addition, low-price bids, raw material procurement quality issues, design and construction factors, random compression of construction schedules, and improper maintenance and management of existing curtain walls are also potential influencing factors for safety problems in glass curtain walls.

1. Cause Analysis of Curtain Wall Glass Breakage and Falling

1.1 Nickel sulfide impurities cause curtain wall glass to break

Building curtain wall glass requires the use of toughened glass. Tempered glass is a kind of safety glass, that has the characteristics of light weight and good mechanical properties, but the silver sulfide impurities inside the glass will cause the self-explosion of tempered glass. Over the years, studies have shown that the snail element is mainly brought in by the raw material quartz sand or sandstone, and the sulfur element is brought in by the fuel and auxiliary materials. In a high-temperature melting furnace at 1400°C~1500°C, silver and sulfur react chemically to form silver sulfide. When the temperature exceeds 1000°C, magnesium sulfide is randomly distributed in the molten glass liquid in the form of droplets; when the temperature drops to 797°C, the liquid state sulfide crystallizes and solidifies to form a high-temperature silver sulfide crystalline phase. That is, hexagonal crystal; when the temperature continues to drop to 379℃, it will further transform into B silver sulfide in a low-temperature state, that is, a trigonal crystal system, and the volume will expand by 2.38%. At this time, there is a huge phase transition tensile stress inside the glass, which is the internal reason for the self-explosion and rupture of the glass.

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1.2 Secondary processing leads to cracking of curtain wall glass

During the secondary processing, transportation, storage, and construction of the glass surface and edges, micro-cracks, scratches, cracks, burrs, and other defects will be left. These defects will form stress concentration and lead to the risk of self-explosion and rupture of tempered glass. Under the action of thermal stress and load, once it reaches or approaches the critical value of glass damage stress, the crack expands rapidly and eventually leads to glass rupture.

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1.3 Thermal stress leads to cracking of curtain wall glass

Thermal stress refers to the stress caused by uneven thermal expansion or cold contraction due to temperature differences in different parts of the material when heating or cooling. There are two common stresses in curtain wall glass: temporary stress and permanent stress. Temporary stress is caused by the poor thermal conductivity of the glass. When the glass is heated or cooled below the strain point temperature, a temperature gradient will be formed in each part, thereby generating thermal stress. The temporary stress is related to the temperature difference, the greater the temperature difference, the greater the temporary stress. When the temporary stress value exceeds the breaking limit strength of the glass, the glass will explode by itself. Permanent stress, that is, tempering stress, is the stress caused by the local temperature difference of the glass. Its size depends on the cooling rate of the finished product above the strain point temperature, the viscosity of the glass, and the thickness of the glass. The self-explosion of curtain wall glass due to thermal stress is mainly caused by temporary stress.

1.4 Improper design and construction lead to cracking of curtain wall glass

Glass breakage and falling caused by improper design and construction are mainly manifested in the following aspects:

1) Improper handling of the spacing and contact surface between glass and metal fixtures. The glass curtain wall specification has clear requirements on the embedded size, void space, and contact surface treatment. It is necessary to ensure that the glass has free expansion and contraction space under the influence of temperature changes and other factors and to avoid hard contact. Once there are factors that hinder the free expansion and contraction of the glass, the glass will be at risk of breaking. At the same time, the sides and bottom of the glass notch cannot be filled with hard materials, and the volume change of these materials will squeeze the glass and cause the glass to break. The installation of building curtain wall glass has strict technical requirements, and the "Technical Specifications for Glass Curtain Wall Engineering" must be followed during the installation process. In this specification, the length and thickness of the supporting components of the curtain wall glass are stipulated. It is required that each glass must be provided with two aluminum alloy or stainless steel brackets with a length greater than 100mm and a thickness greater than 2nlm at the lower end. To withstand the heavy load of the glass, the contact surface between the glass and the bracket must be separated by a film-molded neoprene pad to avoid hard contact.

2) Due to improper control of the size of the keel installation, glass processing errors, and other reasons, the glass and the keel or the fixed frame are not well fitted. The glass will warp under the action of external force on the plane. When the fixing piece hinders the warping change of the glass, the local stress will be too large, which will cause the glass to break.

3) For the interlayer of the glass curtain wall, the usual practice is to line the aluminum veneer behind the glass. When designing, if the distance between the rear liner and the glass is too small and there is no vent hole in the air layer between the two, the temperature of the air layer in the middle will rise sharply under direct sunlight, resulting in excessive glass temperature and The expansion of the gas causes the glass to burst.

4) The thickness, type, and plane size of the glass are unreasonably designed and cannot withstand the wind load, resulting in the overall fall of the glass window frame or the glass breaking.

5) The curtain wall structure design at the settlement joints and deformation joints is unreasonable, and the glass breaks due to the deformation of the main body.

6) With the promotion of the application of building curtain walls, a large number of curtain wall manufacturers have emerged, with mixed strengths. Market competition is not standardized, and construction supervision is not in place, resulting in material and construction quality not meeting standard specifications and design requirements. This is the most fundamental reason for the current glass curtain wall to fall and break. The reason for the cracking and falling of curtain wall glass is not only a single factor but usually a combination of multiple factors.

Cracks caused by glass slag inclusions, air bubbles, and other defects can only be controlled and reduced, and are inevitable, and the damage is relatively small.

Unreasonable design, improper secondary processing, inadequate transportation, and construction supervision are important reasons for the cracking and falling of curtain wall glass. External loads such as solar radiation, wind load, and earthquake load are also direct causes of curtain wall glass falling and cracking.

2. Prevention measures for curtain wall glass cracking and falling

2.1 Secondary heat treatment of tempered glass

Secondary heat treatment of tempered glass, that is, after the glass is tempered, it is subjected to a second heat soak treatment before leaving the factory. After the tempered glass is returned to the furnace, through a series of steps of heating to constant temperature and then cooling, the unstable crystalline magnesium sulfide inside the tempered glass is detonated or transformed and eliminated. After such treatment, the self-explosion rate of tempered glass can be controlled within 0.1%. The heat preservation time required for the secondary heat treatment of glass is long, and there is no mandatory requirement in the specification. Therefore, many projects have given up this reprocessing process to reduce the glass self-explosion rate due to deadlines or cost considerations. After the glass curtain wall is installed, hidden safety hazards are buried.

2.2 Preventive measures for processing defects

The glass edge has the largest tensile stress and the most serious processing defects. Improving the processing quality of the glass edge is the key to reducing glass breakage. First of all, in the process of glass processing, the glass should be well-protected to avoid scratches. The edge of the glass should be finely ground and chamfered. Secondly, in the process of glass transportation, attention should be paid to the packaging of the glass and soft isolation, and the sharp corners of the special-shaped glass should be protected. Defects caused by secondary processing are mainly caused by human factors. Therefore, strengthening the training of workers and standardizing the processing technology and procedures can effectively reduce the glass self-explosion caused by glass processing defects.

2.3 Strengthen the quality management of the design and construction process

1) Strictly review the design scheme of the new curtain wall.

To ensure the safety and reliability of curtain walls, the management of curtain wall design should be strengthened. Establish and improve the review system for new building curtain wall design schemes, and eliminate the possible safety risks of building curtain walls in the scheme design stage, to improve the safety and reliability of existing curtain walls. In addition, the current architectural design has broken through the current design codes and regulations in the design of the building appearance, such as novel and complex facade shapes, the design of super-large panel panels, and the emergence of new building materials. The current industry standard "Technical Specifications for Glass Curtain Wall Engineering" has made it difficult to meet the increasing safety requirements. According to the current design specifications, according to the conditions of different regions, stricter local standards and specifications are formulated and implemented, and the design scheme of the building curtain wall is reviewed based on this. The reasonable overall layout arrangement reduces the temperature gradient and the size of the glass grid. Reasonable glass fixing structures, the use of laminated glass, and other measures will greatly reduce the safety risk of new building curtain walls.

Figure 1 The installation processing of glass curtain wall 1

Figure 1 The installation processing of glass curtain wall 1

2) Strengthen quality management during construction.

Insufficient control of curtain wall glass installation process and construction procedures is one of the important reasons for curtain wall glass cracking and falling. Therefore, to avoid the risk of curtain wall glass breaking and falling, it is necessary to control the quality of the installation process and carry out detailed inspection and supervision of each process. Curtain wall construction enterprises should train and assess operators to ensure their professional technical level.

Figure 2 The installation processing of glass curtain wall 2

Figure 2 The installation processing of glass curtain wall 2

3) Improve the existing curtain wall inspection mechanism and develop new technologies for on-site inspection.

The safety inspection and appraisal work of the existing curtain wall is different from the general curtain wall performance inspection. The inspection of the existing curtain wall requires building curtain wall practitioners with high professional level and experience, including long-term engagement in curtain wall design, construction, and management, and can comprehensively Senior professionals who have mastered and familiarized with curtain wall specifications and testing methods shall be responsible, and conduct on-site inspections and project file checks on existing curtain walls by inspection procedures and inspection contents.

According to the inspection results, if necessary, further curtain wall performance testing requirements can be put forward, including testing items, testing schemes testing methods, etc. Strictly implement the daily safety inspection of the existing curtain wall, pass the special inspection, judge whether there is any risk in the existing curtain wall according to the test results, and issue a reliability appraisal report and corresponding maintenance or renovation plan. Since the safety inspection of the existing curtain wall is carried out on-site, especially for the on-site inspection of concealed works and structural sealants, there is no good detection method at present. To adapt to the characteristics of existing curtain wall inspection, glass curtain wall producers, installers, and practitioners must further research and develop some new inspection technologies, such as on-site non-destructive inspection technology, which can not only find the existing problems of the existing curtain wall but also maintain the existing Complete with curtain wall.

3. The epilogue

To improve the current situation of cracking and falling of existing curtain walls and improve the safety of our country's existing curtain walls, we should start with strengthening safety supervision, establishing a standardized and legalized architectural curtain wall market order and awareness, and strictly implementing the existing curtain wall safety management responsible person system and security checks. In particular, strengthen the quality control and safety management of the whole process of building curtain wall design, procurement, construction, and operation, and complete the formulation and implementation of relevant standards and specifications for existing curtain walls as soon as possible. At the same time, relevant departments should strengthen the construction of a professional talent team, so that personnel with a high level of professional knowledge can supervise the key nodes of the whole process and effectively improve the supervision level.

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