A brief discussion on the deformation of energy-saving insulating glass in curtain walls

Energy-saving hollow glass curtain walls can be seen almost everywhere in modern cities. As long as you have paid attention to the image reflected by the curtain wall, you must be familiar with the following situation.

1. What's going on?

The perfectly good building is distorted in the "eyes" of the glass curtain wall.

According to the law of reflection of light, the shape of the reflection image formed by an object through a mirror depends on the surface shape of the mirror. The image formed by a plane mirror is the same size and direction as the object. The closer the mirror surface shape is to a plane, the closer the image is to the object. On the contrary, the image becomes more distorted.
Most of the architectural glass used in curtain wall construction is flat glass. However, factors such as the shape of the glass substrate and the installation process cause the surface of the curtain wall to be not particularly smooth, so the reflected image of the surrounding scenery on its surface will produce a certain degree of distortion. For irregularly shaped scenery such as white clouds, it is not easy for us to notice the distortion of the reflected image. But for regular-shaped scenery, such as straight tall buildings or flagpoles, the distortion of the reflected image is particularly obvious.

2. Why is this happening?

The "Regulations on the Management of Safety Glass in Buildings" stipulates that when glass is used as a building material, safety glass must be used for exterior windows of buildings with 7 floors or above, window glass with an area greater than 1.5m², floor-to-ceiling windows with the bottom edge of the glass less than 500mm from the final decoration surface, and curtain walls (except full glass curtains).

The safety glass mentioned here refers to tempered glass, laminated glass and other glass products processed by a combination of tempered glass or laminated glass, such as safety insulating glass, etc. that comply with current national standards.

On the one hand, improper temperature control of the glass during the tempering process will lead to temperature differences between different locations on the glass surface, causing the glass to bend. On the other hand, the deformation and wear of the tempering furnace rollers will also affect the flatness of the glass surface. In actual projects, some customers require that the glass enter the tempering furnace in a uniform direction, so that the tempered glass installed on the wall will only deform in the same direction. Small details of process control during the tempering process may affect the flatness of tempered glass.

Therefore, it is very necessary to strictly control the flatness of the glass before joining the insulating glass sheets.

GB 15763.2-2005 "Safety Glass for Buildings Part 2 Tempered Glass" stipulates the curvature of flat tempered glass, requiring that the bow curvature should not exceed 0.3% and the corrugated curvature should not exceed 0.2%.

T/ZBH 001-2017 "Architectural Glass Appearance Quality Requirements and Assessment" stipulates that for tempered glass, laminated glass and insulating glass with a nominal thickness of not less than 6mm, the bow curvature should not be greater than 0.15%, the wave curvature should not be greater than 0.12mm, and the difference between two adjacent wave crests in the middle should not be greater than 0.076mm.

Insulated glass may also deform due to factors such as ambient temperature, altitude, wind pressure, etc.

As we all know, gas will expand and contract as the ambient temperature changes. For well-sealed insulating glass, the thickness of different positions of the hollow cavity will be different when the ambient temperature changes, and the thickness difference between the middle and the edges is particularly obvious.

A piece of 6Low-E+12A+6 insulating glass with a size of 1200×700mm was used as a test sample. It was placed indoors and outdoors. After standing for a period of time, the thickness of the middle, left and right parts of the cavity was measured with a GlassMeter100 insulating glass thickness gauge. The data are shown in the table below.

It can be seen that under this test condition, the thickness of different positions of the hollow cavity is different, and the thickness of the middle part is smaller than the edge part, that is, there is a certain degree of depression on the surface of the insulating glass. When the temperature is 17°C, the thickness of the middle part of the hollow cavity is about 0.7mm smaller than the thickness of the two sides; when the temperature is minus 4°C, the overall thickness of the hollow cavity is reduced, and the thickness of the middle part is about 2.2mm smaller than the thickness of the two sides, that is, the surface of the insulating glass is more concave.
In addition, improper installation process and positional deviation control will cause the reflection image of the entire curtain wall to be fragmented and lose the beauty that the glass curtain wall should have. JGJ 139-2001 "Quality Inspection Standard for Glass Curtain Wall Projects" stipulates parameters such as the flatness of curtain wall glass and the height difference of the joints between two adjacent glasses.

Of course, there are also some curtain wall designs that can cause this situation.

First of all, it must be clear that the building's curtain wall cannot be made into an ideal mirror, and the reflected image will inevitably be more or less distorted. In addition, the deformation of curtain wall glass after installation is the result of a combination of factors.

In actual production, the flatness of the building glass substrate itself can be improved by improving the glass tempering process, installing breathing tubes or capillary tubes on insulating glass, and standardizing factory quality inspection methods.

When designing the curtain wall, in order to reduce deformation, if the strength of the hardware allows, the thickness of the outdoor glass can be thickened or semi-tempered laminated glass can be used. During installation, factors such as structure and stress are taken into consideration, and the installation process of the curtain wall is strictly controlled to reduce the "distortion" of the surrounding scenery to a certain extent and reflect the beauty that the glass curtain wall should have.