Glass curtain walls are widely used in modern buildings due to their bright and clean appearance, vibrant colors, and clear reflection effects. However, in recent years, with the continuous increase in the size of curtain wall glass, especially the increase in the use of coated glass, the problem of reflected image distortion has become increasingly prominent. This phenomenon is known as “light and shadow distortion of glass curtain walls” in the industry. The following Figure 1 shows a glass curtain wall with natural and clear light and shadow effects, while Figure 2 shows obvious distortion.
Figure 1
Figure 2
Actually, there is only one fundamental reason for light and shadow distortion. That is, glass deformation. When the surface of the glass curtain wall is no longer flat, the reflected image will be distorted. In actual projects, glass deformation may be caused by a variety of factors. This article will analyze these causes in detail. And then it will propose corresponding improvement measures to reduce or avoid the occurrence of light and shadow distortion problems.
The production of flat glass in China currently mainly adopts the float process. The process technology is mature, and the surface flatness of the produced glass is relatively high. Although float glass may also produce slight deformation during the production process, the deformation is usually small and generally does not cause obvious distortion of the reflected image.
Almost all glass used for curtain walls needs to undergo heat treatment, that is, tempering or semi-tempered process. However, whether it is tempered or semi-tempered, it will cause glass deformation to a certain extent, especially tempered glass, which has a more obvious degree of deformation. Heat treatment deformation is mainly divided into two categories: bow deformation and corrugated deformation. According to the current standard, the bow deformation of flat tempered glass shall not exceed 0.3% of the length of the glass. While the corrugated deformation shall not exceed 0.2%. Taking a piece of glass with a length of 2 meters and a width of 1 meter as an example, the maximum allowable bow deformation can reach 6 mm. This degree of deformation is enough to cause obvious distortion of the reflected image and is one of the main factors causing light and shadow distortion of glass curtain walls.
There are two main forms of deformation of insulating glass:
When there is a large altitude difference between the production site and the installation site of insulating glass, the air pressure in the hollow cavity of the glass will be unbalanced with the external air pressure, causing the glass panel to bulge outward or sink inward. This deformation can be alleviated by installing pressure balancing devices such as capillaries. But in actual projects, for the lack of corresponding measures, such deformation still occurs from time to time. And it is one of the occasional factors causing light and shadow distortion of glass curtain walls.
During use, insulating glass is affected by changes in ambient temperature. At the same time, its internal air pressure will also change accordingly. When the internal and external air pressures are inconsistent, the glass panel will also deform outward or inward. Generally speaking, the temperature is low in winter, and insulating glass is prone to inward concavity. In summer, the temperature is high, and it is easy to bulge outward. This deformation caused by temperature difference cannot be completely avoided, and the deformation amplitude is large, which is one of the main reasons for the light and shadow distortion of glass curtain walls.
Except for frameless glass curtain walls, most curtain wall systems adopt a bottom-supported (floor-mounted) installation method. This installation method makes it difficult to ensure that the glass panel is absolutely perpendicular to the ground. In fact, all curtain wall glass is tilted at a certain angle during installation, but the tilt angle is different.
When the glass is in installed at an inclination angle, its own weight will produce a component force in the horizontal direction. This puts constant pressure on the glass, which eventually leads to deformation. Especially for glass with larger vertical dimensions, its force deformation is more obvious. Although such deformation can be reduced by improving installation accuracy and ensuring verticality, it cannot be completely eliminated. Therefore, glass deformation caused by installation method is also one of the important factors leading to light and shadow distortion of glass curtain walls.
Curtain wall glass panel is usually connected and fixed to the frame by a pressure plate. Ideally, the pressure plate should be set up as a whole length to ensure uniform force. However, in actual projects, segmented pressure plates are very common for construction convenience or cost considerations.
Even if a full-length pressure plate is used, the inconsistent clamping force between the fixing screws can result in uneven force on the back of the glass, leading to deformation. This localized deformation is particularly obvious when using segmented pressure plates, or when the screws are not uniformly adjusted and the clamping force is uneven. This is one of the important reasons for the light and shadow distortion of the glass curtain wall.
In recent years, the size of curtain wall glass has continued to increase. The common vertical height has exceeded 2 meters, and some even reached 4 meters or 5 meters. The larger the glass area, the greater the deformation caused by heat treatment, installation, environmental impact, etc., which will eventually lead to more obvious light and shadow distortion.
Therefore, the trend of large-scale glass size is one of the main reasons for the increasingly prominent light and shadow distortion problem of curtain walls in recent years.
Although the area of curtain wall glass continues to increase, its thickness has not increased synchronously. In some projects, the glass thickness does not meet the standard requirements or just barely meets the standards. Insufficient glass thickness reduces structural rigidity and is prone to large deformation under external forces, which in turn exacerbates light and shadow distortion.
On the contrary, the thicker the glass, the stronger its ability to resist deformation, and various types of distortion will be relatively reduced. Therefore, when selecting large-area glass, you should consider increasing the thickness simultaneously to improve overall stability and visual effects.
In recent years, due to the risk of self-explosion of tempered glass, more and more curtain wall projects have chosen to use laminated glass for the outer sheet and tempered glass for the inner sheet in the construction of insulating glass. This practice is reasonable in terms of safety, but there are often inappropriate glass thickness combinations.
For example, some projects use the glass configuration of 6+1.52PVB+6+12A+8 (or 10) mm. That is, the outer sheet is 6+1.52PVB+6 laminated glass, and the inner sheet is 8mm or 10mm tempered glass. In this combination, the effective thickness of 6+1.52PVB+6 laminated glass is about 7.5mm. And as we know, the overall stiffness of the glass plate is directly related to its effective thickness. In other words, the stiffness of this laminated glass is lower than that of 8mm or even 10mm single-piece tempered glass, which is relatively “softer” and has weaker deformation resistance.
When the insulating glass is affected by temperature difference, the deformation is mainly borne by the outer laminated glass. The 6+1.52PVB+6 is located on the outside of the curtain wall glass. Once it is deformed, it will directly affect the overall flatness and reflection effect of the curtain wall, and easily cause obvious light and shadow distortion. Therefore, unreasonable glass configuration, especially the insufficient rigidity of the outer laminated glass, is one of the important factors leading to light and shadow distortion.
Although the coating on the glass surface will not increase its actual deformation, it will significantly increase its visible light reflectivity, making the light and shadow distortion more obvious visually. At present, curtain wall glass generally uses Low-E coated glass. The high reflective properties of this type of glass can easily magnify the reflection distortion caused by slight deformation of the glass surface.
The visual effect of light and shadow distortion of glass curtain wall is also closely related to the distance of the observer. When standing close to the glass curtain wall, it is usually difficult to detect obvious distortion, and the light and shadow effect is even very natural. However, when the observation distance increases to tens of meters, the originally tiny distortion will become more obvious under the effect of perspective magnification. Then, the degree of light and shadow distortion will increase accordingly.
This means that even if with the same deformation degree of glass surface, the visual distortion effect will vary significantly for the different observation position. Therefore, in the process of curtain wall design and construction, in addition to controlling the actual deformation, the visual distance and main observation angle of the building should also be fully considered to improve the overall visual quality.
For the main factors that cause the deformation of curtain wall glass mentioned above, a series of targeted measures can be taken to reduce the overall and local deformation of the glass, thereby effectively improving the light and shadow distortion problem of the glass curtain wall. Specific optimization suggestions are as follows:
Strictly control the flatness of tempered glass in the processing link, minimize the bow or wave deformation caused by heat treatment, and reduce light and shadow distortion from the source.
During the installation process, the pressure distribution of the pressure plate around the glass should be consistent. Especially when using segmented pressure plates, the clamping force of the fixing screws should be uniformly adjusted to avoid local stress concentration causing glass deformation.
Optimize the installation accuracy, ensure that the glass surface is as vertical as possible to the ground, and reduce the long-term deformation risk caused by the lateral force generated by gravity.
On the premise of meeting the function and aesthetics of the building, the use of ultra-large area glass should be limited, and glass of sufficient thickness should be selected to improve the overall rigidity and resist external force deformation.
When there is a large altitude difference between the production site and the use site of insulating glass, a capillary tube should be installed in the hollow cavity or other pressure balancing technology should be used to prevent convex or concave deformation caused by the internal and external pressure difference.
For insulating glass with laminated glass as the outer sheet, its effective thickness should be appropriately increased to improve rigidity, thereby slowing down the deformation of the outer layer caused by temperature difference.
The light and shadow effect of glass curtain wall is one of its most visually impactful decorative features. When the light and shadow are clear and real, it can significantly enhance the beauty of the building. Once distortion occurs, it is easy to cause visual discomfort and even affect the overall appearance.
In fact, as long as scientific and reasonable design and control measures are taken in the glass configuration, processing and installation, the light and shadow distortion problem can be effectively improved, so that the glass curtain wall can present a satisfactory and even amazing visual effect.
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