Curtain wall glass image distortion problem and solution.

1. Preface

The glass curtain wall has been widely used in modern buildings for its advantages of beauty, energy saving, durability, and light weight. In recent decades, a large number of glass curtain wall buildings have been built around the world. When it comes to the shortcomings of curtain wall glass, the image distortion of curtain wall glass architectural glass has always been an insurmountable problem. Almost every piece of curtain wall glass has more or less distortion of the reflected image, which will make high-altitude birds confuse their flight direction and cause Produce some unnecessary photochemical smog, etc.

The image distortion of curtain wall glass is usually caused by the unevenness of the outer surface of the glass. In recent years, scholars from all walks of life have also conducted a series of studies on the problem of image distortion of curtain wall glass. 

① Some glass deep-processing practitioners have analyzed the factors that affect the image deformation of curtain wall insulating glass, and proposed to improve the flatness of tempered glass itself, optimize glass design, Strictly control the installation accuracy, and take a series of measures such as installing capillary tubes and selecting coated glass with appropriate reflectivity in consideration of the use environment. 

② Some architectural designers put forward corresponding control measures for various factors to reduce the visual deformation of urban curtain wall glass, to enhance the aesthetic effect of the building, 

③ The curtain wall glass constructor adopted the measure of "high-altitude vehicle + crane" to install the curtain wall glass, and applied the total station positioning re-measurement to ensure the accurate installation of the curved large plate inverted glass,

④ LIJIANG Glass researched tempered glass waviness and stress spot phenomenon. These studies are all aimed at affecting the flatness of the glass surface and lack a description of the main factors affecting the glass image distortion. 

This article analyzes the various factors affecting glass image distortion one by one, finds out the main factors affecting glass image distortion, and controls them.

2. The Project Overview

Shanghai World Financial Center project, as shown in Figure 1, the glass is made of semi-tempered laminated insulating glass (6HS+1.52PVB+6HS+12A+6TP), = the distortion effect is more obvious, as shown in Figure 2.

^{Figure 1 Shanghai World Financial Center Project}

^{Figure 2 Shanghai World Financial Center Project}

^{Figure 3 Shanghai IFC Project}

^{Figure 4 Shanghai IFC Project}

For the Shanghai International Finance Center project, as shown in Figure 3, the glass is made of tempered insulating glass (12TP+12A+8TP), and the distortion effect is relatively obvious, as shown in Figure 4.

The Pearl River City project in Guangzhou is shown in Figure 5. The height of the building is 309m. The glass is made of toughened insulating glass (8TP+12A+6TP), and the image is distorted.

Nanchang Greenland Center, as shown in Figure 6, has a building height of 289m. The glass is made of toughened insulating glass (8TP+12A+8TP), and the image is distorted.

^{Figure 5 Guangzhou Pearl River City Project}

^{Figure 6 Nanchang Greenland Center}

^{Figure 7 Bulgari Hotel Beijing}

^{Figure 8 Guangzhou TaiKoo Hui}

The Bulgari Hotel Beijing is shown in Figure 7. The glass is made of semi-tempered double-laminated insulating glass (8HS+1.52PVB+8HS+12A+8HS+1.52PVB+8HS), and the image distortion is obvious.

As shown in Figure 8, Guangzhou Taikoo Hui has a building height of 212m. The glass is tempered insulating glass (10TP+12A+8TP), and the image distortion is obvious.

From the above cases of high-end curtain walls in first-tier cities such as Beijing, Shanghai, and Guangzhou, it can be seen that reflection image distortion cannot be avoided. The degree of distortion is related to the flatness of the glass surface, the distance of the projected object, the texture of the projected object, and the distance and Angle matter a lot. Of course, what these factors can control is the flatness of the glass surface.

3. The Reason analysis

3.1 Characterization Indicators and requirements

Architectural glass/safety glass is generally used. Tempered glass and semi-tempered laminated glass are all safety glasses. There are two indicators to characterize the surface flatness of tempered glass and semi-tempered glass: bow deformation and wave deformation.

During the test, the sample needs to be kept at room temperature for at least 4 hours, the sample needs to be measured in a vertical cube, and two pads are placed under the sample. Use a straight edge or metal wire to be horizontally attached to both sides of the diagonal direction of the product, use a feeler gauge to measure the gap between the straight edge and the glass, and use the percentage of the height of the arc to the length of the chord to express the bending of the bow Spend. In the case of local waveform measurement, the measurement method is that the line is parallel to the edge of the glass by 25mm, and the length is required to be 300mm. The height of the trough or crest is measured with a feeler gauge, and the measured result is divided by 300mm, and the percentage obtained is used to represent the curvature of the waveform, as shown in Figure 9 in detail.

1. Bow deformation;
2. Glass side length or diagonal length;
3. Waveform deformation;
4. 300mm

^{Figure 9 Schematic diagram of bow and wave curvature}

For tempered glass: "Safety Glass for Buildings Part 2: Tempered Glass") Article 5.4 requires that the curvature of flat tempered glass should be less than or equal to 0.3% when bowed, and less than or equal to 0.2% when waved.

For semi-tempered glass: Article 6.5 of "Semi-Tempered Glass" requires that the curvature of flat tempered glass should be less than or equal to 0.4% when bowed, and less than or equal to 0.167% when waved.

3.2 Optical deformation analysis of float glass

Tempered glass and semi-tempered glass are made by secondary heat treatment of float glass. For float glass: Article 5.6 of "Flat Glass" requires that the curvature of flat glass should not exceed 0.2%. (i.e. the bow and wave bend of the glass are not more than 0.2%). In addition, another very important index to characterize the surface flatness of float glass is optical deformation, as shown in Figure 10. Place the sample vertically at a distance of 4.5m from the screen. The screen has black and white diagonal stripes with uniform brightness.

The distance between the observer and the sample is 4.5m, and the observer observes the stripes on the screen through the sample. First, the stripes are deformed, and then the sample is slowly rotated until the deformation disappears, and the incident angle at this time is recorded.

^{Figure 10 Schematic diagram of detecting optical deformation}

In "Flat Glass", float glass is divided into qualified products, first-class products, and excellent products according to appearance quality. The incident angles corresponding to the required optical deformation are 50°, 60°, and 60°. The qualified products, first-class products, and excellent products here correspond to the architectural grade, automotive grade, and mirror-making grade in the early "Float Glass". At present, the original sheets (float glass) of tempered glass and semi-tempered glass used on curtain walls are mostly qualified products. The incident angle corresponding to the optical deformation is mostly 50°. At present, the incident angle of optical deformation of foreign high-grade float glass has reached 70°. To improve the flatness of tempered glass, the appearance quality of float glass must be improved first, and it can be suggested that relevant parties use first-class and high-quality float glass. This can greatly improve the image distortion of the curtain wall glass.

3.3 Analysis of glass tempering and semi-tempering

The process of secondary heat treatment on the original sheet is called tempering and semi-tempering. The factor that has a greater impact on imaging in the tempering process is distortion. If distortion occurs, it can be considered a serious problem with process control. The wave deformation in the middle of the tempered glass parallel to the direction of the glass tempered silicon rod can generally be controlled below 1/3 of the international standard 2‰, that is, the wave deformation is controlled within 0.2mm, and a better tempering furnace can be controlled within 0.15mm. If the bow deformation is controlled within the range of 1‰ and the waveform deformation is controlled within 0.15mm during the second heat treatment, and then the wide-side furnace is adopted, the final deformation of the finished product will be controlled in a relatively ideal state.

3.4 Analysis of deformation of insulating glass cavity

To save energy, the curtain wall mostly uses insulating glass. The gas in the cavity is located in a closed environment, which will cause deformation of the glass due to temperature changes, as shown in Figure 11. Compared with the temperature when the glass is combined when the temperature rises, the glass is convex, and when the temperature is lowered, the glass is concave.

^{Figure 11 Schematic diagram of temperature difference deformation of insulating glass}

• Air conditioner height 20 degrees - convex 1.05mm
• The air cavity is raised by 40 degrees - convex 226ee
• The air cavity is raised by 60 degrees - convex 3.46mm

Symmetry

• Asymmetric Airlift 20 degrees - convex 1.14mm
• The opening height of the air cavity is 40 degrees - the outer convex is 2.57mm
• The height of the air is 60 degrees - convex 3.79mm

^{Figure 12 Temperature difference deformation data of insulating glass}

Using professional software analysis, for 3m*1.5m, 10mm thick tempered glass, when the temperature changes, the deformation is as follows, see Figure 12, it can be seen that when the temperature of the cavity increases by 20°, the glass protrudes 1mm to 1.4mm. Of course, the image distortion caused by this deformation is symmetrical and changes with temperature. Compared with the bow deformation of the glass itself, it is still relatively small.

However, it is necessary to control the temperature and humidity of the environment during the lamination stage of the insulating glass. In general, the higher the humidity, the more water vapor content. After the insulating glass is produced, the molecular sieve absorbs most of the water vapor sealed in the cavity, and the insulating glass often presents a concave shape. Therefore, it is necessary to ensure that the gluing of insulating glass is carried out in a suitable processing environment and control the temperature of the processing workshop at 20°C~30°C, and the humidity at 50%~60%. In addition, when the glass is combined horizontally, the glass will be concave under the action of its weight.

A piece of glass with a width of 1.5m, a length of 2m, and a thickness of 8 mm will have a concave center of 2.2 mm under the action of its weight lying flat. For a glass with the same size and a thickness of 6 mm, under the action of its weight lying flat, the concavity in the center may even reach 3.9 mm. The glass seals the cavity in a concave state. Once the glass stands up, this concave state cannot be restored to flatness under the action of atmospheric pressure. Therefore, if conditions permit, the insulating glass should still be combined vertically, and an automated vertical insulating glass production line should be used as much as possible.

Atmospheric pressure changes with altitude. Every time the height increases by 500m, the atmospheric pressure decreases by 5500N/㎡; correspondingly, for the insualting glass with a cavity thickness of 12mm, the outer glass will protrude by 1mm. The method of effectively controlling the deformation of insulating glass caused by air pressure changes If the altitude difference between the manufacturing site and the use site is greater than 500m, add a capillary to the insulating glass.

To improve the energy saving of glass, now the insulating glass cavity is mostly filled with air. To ensure the balance of the air pressure difference between the inside and outside of the insulating glass cavity, it is necessary to control the amount and air pressure of the filled air, especially the need to use an externally displayed insulating glass argon inflator, or an automatic insulating glass inflating production line that can be digitally displayed to prevent Excessive filling of argon causes excessive internal pressure and causes the glass to bulge.

3.5 Analysis of glass installation deviation

The glass is fixed to the main body of the structure through aluminum alloy profiles. Deviations in the main structure, embedded parts, curtain wall keels, and glass group frames may lead to uneven outer surfaces of individual glass and the entire curtain wall. This requires strict control of material processing and installation quality throughout the process. "Building Curtain Wall" mentioned in Article 6.4.2 that the flatness of the curtain wall should not be greater than 2.5mm (measured with a 2m ruler and steel ruler).

3.6 Image distortion weight analysis

To find out the main cause of glass image distortion, the American Standard is used to calculate the image distortion index, as shown in Table 1, it can be seen that the most important factor causing glass image distortion is the tempered bow deformation of the glass, which accounts for 72%, which is the main cause.

Table 1 Glass image loss of frame weight factor analysis table

4. The Conclusion

4.1 The image distortion of curtain wall glass is mainly caused by the bow deformation of tempered or semi-tempered glass.

4.2 Improve the quality grade of the original glass sheet (float glass). For example, the use of first-class or superior products can effectively increase the incident angle of its optical deformation, which is beneficial to control the bow deformation during later glass tempering or semi-tempering.

4.3 Take positive measures to improve the quality of the insulating glass. For example, to ensure reasonable temperature and humidity during lamination; to adopt vertical plate lamination insulating glass production line; to add capillary devices when there is a significant height difference between the place where the insulating glass is manufactured and the place where it is used.

4.4 Improve the installation quality of curtain wall glass to ensure that the flatness of the curtain wall meets the specification requirements.

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